![Comparison of the radial component at 30 km altitude with the previous study. PN2010: radial components at 30 km altitude reproduced from the grid data by Purucker and Nicholas [2010]. PN2010-SVM30: difference of radial components at 30 km altitude between PN2010 and SVM30. Lambert equal area projection is adopted. Grid lines are at 30° intervals of latitude and longitude.](https://www.researchgate.net/publication/276453683/figure/fig6/AS:267615633408064@1440815947953/Comparison-of-the-radial-component-at-30km-altitude-with-the-previous-study-PN2010_Q320.jpg)
![Power spectra of the spherical harmonic expansion on the lunar magnetic anomalies. The horizontal and vertical axes are degree n of the spherical harmonic and the Lowes spectra Rn [Lowes, 1974] on a logarithmic scale, respectively. Thick line: SVM power spectra (n = 1–450) based on the radial component by the SVM method in the present study; thin line: PN2010 power spectra (n = 1–178) by Purucker and Nicholas [2010].](https://www.researchgate.net/publication/276453683/figure/fig2/AS:267506757664795@1440789989949/Power-spectra-of-the-spherical-harmonic-expansion-on-the-lunar-magnetic-anomalies-The_Q320.jpg)
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Surface vector mapping of magnetic anomalies over the Moon using Kaguya and Lunar Prospector observations: SVM of magnetic anomalies over the Moon
Journal of Geophysical Research: Planets
Abstract and Figures
We have provided preliminary global maps of three components of the lunar magnetic anomaly on the surface applying the surface vector mapping (SVM) method. The data used in the present study consist of about 5 million observations of the lunar magnetic field at 10–45 km altitudes by Kaguya and Lunar Prospector. The lunar magnetic anomalies were mapped at 0.2° equi-distance points on the surface by the SVM method, showing the highest intensity of 718 nT in the Crisium antipodal region. Overall features on the SVM maps indicate that elongating magnetic anomalies are likely to be dominant on the Moon except for the young large basins with the impact demagnetization. Remarkable demagnetization features suggested by previous studies are also recognized at Hertzsprung and Kolorev craters on the farside. These features indicate that demagnetized areas extend to about 1–2 radii of the basins/craters. There are well-isolated central magnetic anomalies at four craters: Leibnitz, Aitken, Jules Verne, and Grimaldi craters. Their magnetic poles through the dipole source approximation suggest occurrence of the polar wander prior to 3.3–3.5 Ga. When compared with high-albedo markings at several magnetic anomalies such as the Reiner Gamma anomalies, three-dimensional structures of the magnetic field on/near the surface are well correlated with high-albedo areas. These results indicate that the global SVM maps are useful for the study of the lunar magnetic anomalies in comparison with various geological and geophysical data.
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... These strong anomalies under Mare Crisium are 17 supposed to have magnetized magma intrusion as the source. 18 In contrast to the strong anomaly, these weak fields have been rarely 19 studied, and their origin is even more controversial (Hood and Spudis, 2016; 20 Wieczorek, 2018). Since the intensities of magnetic anomalies varies 21 significantly, there is a wrong impression that only a small portion of the lunar 22 crust was magnetized. ...
... 12 Element in matrix could be calculated by equation (2) underground space is divided equally into cuboid cells . 18 In equation (2), i=1, 2, 3, and j=1, 2, 3. S1, S2, S3 represent , , in the 19 cartesian coordinate. 20 According to equation (2), the measured magnetic anomaly vectors at sites in 1 a line could be calculated by the cuboid cells in a line underground. ...
... The result of the 16 inversion is the magnitude of magnetization of each cube cell. Considering the 17 physical properties, the magnitude of magnetization is always larger than 0. 18 The initial model of magnetization underground m0 is set to be 10 −4 A/m that is 19 default in the used software. It should be mentioned that m0 could be any 20 small value larger than 0 and smaller than 1. ...
... However, there is incontrovertible evidence suggesting that it once had a dynamo field, initially indicated through paleomagnetic analyses of the lunar rocks returned by Apollo 11. Subsequent orbital measurements from Apollo 15 and 16, along with recent probes like Lunar Prospector and Kaguya, have highlighted significant heterogeneity in the distribution of lunar crustal magnetic fields, with the strongest magnetic field area mainly concentrated in the vicinity of Salton Crater in the South Pole (e.g., [3]). Surface magnetic measurements in low-latitude regions such as the 4 Apollo missions [4] and Lunokhod 2 rover [5] have revealed substantial variations in field directions across scales ranging from hundreds of meters to kilometers. ...
... Tsunakawa et al. [3,54] have provided global maps of the 3 components of the lunar surface magnetic anomaly using the surface vector mapping (SVM) method based on data at 10-to 45-km altitudes from LP and Kaguya satellites. The SVM method is effective in creating a global magnetic field map when the regional magnetic anomalies are connected across the Moon [55]. ...
... A large concentration of magnetic anomalies is located in the farside highlands of the southern hemisphere, with the highest intensity (~719 nT inferred from Tsunakawa et al. 's [3] model) located in the Crisium antipodal region. This phenomenon is interpreted as the amplification of the magnetic field, resulting from either the solar wind or lunar crust due to the impact-related plasma processes at the antipodes of the impact basins [8,72,73]. ...
The Moon currently lacks a global magnetic field; however, both paleomagnetic analyses of lunar rock samples and orbital magnetic measurements indicate that it once possessed a core dynamo. Magnetic field measurements of some datable impact basins suggest that the lunar core dynamo persists to the Nectarian period (~3.9 to 3.8 billion years ago Ga). Investigations of the Apollo samples using modern methods demonstrate that the field overall was active between 4.25 and 1.92 Ga. During the period prior to 3.56 Ga, the field was sometimes comparable to Earth’s but subsequently declined dramatically and ultimately ceased. Several hypotheses have been proposed to explain the dynamo generation and duration. Thermal convection in the lunar core could have provided dynamo energy for the first several hundred million years while core crystallization could have sustained the dynamo for up to 1.5 Ga. Other mechanisms, such as mantle and/or inner core precession, changes in the rotation rate of the lunar mantle caused by impacts, and a basal magma ocean, also hold the potential to power the dynamo during some time of lunar evolutionary history. Impacts related to plasmas are believed to be insufficient for crustal magnetization though they can amplify the pre-existing magnetic field before the impacts. This paper summarizes and reviews the current understanding of lunar magnetic field evolution, including paleomagnetic studies that quantify the timing of the lunar surface strength, global crustal magnetization features derived from recent global magnetic field models based on orbital magnetic measurements, and various models explaining the powering of a lunar dynamo, which can account for most observations. Finally, we propose the outstanding questions and offer guidance for future lunar exploration such as the Chang’E series and lunar scientific observatories.
... Orbital spacecraft and ground magnetic field observations, together with paleomagnetic analysis of Apollo samples, have demonstrated that the crust of the Moon is, at least, partially magnetized (Fuller & Cisowski, 1987;Hood et al., 1981;Lin, 1979;Mitchell et al., 2008;Purucker, 2008;Purucker & Nicholas, 2010;Richmond & Hood, 2008;Takahashi et al., 2014;Tsunakawa et al., 2015). Magnetic anomalies, and the corresponding magnetization in the crust, are found to be heterogeneously distributed over the lunar surface (Carley et al., 2012;Richmond & Hood, 2008;Tsunakawa et al., 2015;Wieczorek, 2018). ...
... Orbital spacecraft and ground magnetic field observations, together with paleomagnetic analysis of Apollo samples, have demonstrated that the crust of the Moon is, at least, partially magnetized (Fuller & Cisowski, 1987;Hood et al., 1981;Lin, 1979;Mitchell et al., 2008;Purucker, 2008;Purucker & Nicholas, 2010;Richmond & Hood, 2008;Takahashi et al., 2014;Tsunakawa et al., 2015). Magnetic anomalies, and the corresponding magnetization in the crust, are found to be heterogeneously distributed over the lunar surface (Carley et al., 2012;Richmond & Hood, 2008;Tsunakawa et al., 2015;Wieczorek, 2018). However, the origin of lunar crustal magnetism is still not fully understood (for a review, see Wieczorek et al. (2022)). ...
... As input to these two inversions, we use the lunar magnetic field maps of Tsunakawa et al. (2015), which are based on Lunar Prospector and Kaguya vector magnetic field measurements. In particular, we use their maps of the radial magnetic field component at 30 km altitude, with a 0.5°spacing. ...
Spacecraft measurements show that the crust of the Moon is heterogeneously magnetized. The sources of these magnetic anomalies are yet not fully understood, with most not being related to known geological structures or processes. Here, we use an inversion methodology that relies on the assumption of unidirectional magnetization, commonly referred to as Parker's method, to elucidate the origin of the magnetic sources by constraining the location and geometry of the underlying magnetization. This method has been used previously to infer the direction of the underlying magnetization but it has not been tested as to whether it can infer the geometry of the source. The performance of the method is here assessed by conducting a variety of tests, using synthetic magnetized bodies of different geometries mimicking the main geological structures potentially magnetized within the lunar crust. Results from our tests show that this method successfully localizes and delineates the two‐dimensional surface projection of subsurface three‐dimensional magnetized bodies, provided their magnetization is close to unidirectional and the magnetic field data are of sufficient spatial resolution and reasonable signal‐to‐noise ratio. We applied this inversion method to two different lunar magnetic anomalies, the Mendel‐Rydberg impact basin and the Reiner Gamma swirl. For Mendel‐Rydberg, our analysis shows that the strongest magnetic sources are located within the basin's inner ring, whereas for Reiner Gamma, the strongest magnetic sources form a narrow dike‐like body that emanates from the center of the Marius Hills volcanic complex.
... An alternative method for producing global and regional maps of the lunar crustal magnetic field was developed by Tsunakawa et al. (2010Tsunakawa et al. ( , 2014Tsunakawa et al. ( , 2015, who used an inverse boundary-value approach. This method combined both the Kaguya and Lunar Prospector magnetometer datasets and used only data collected between altitudes of 10 and 45 km within the lunar wake and geomagnetic tail. ...
... More recently, Ravat et al. (2020) have performed global inversions using magnetic monopoles, combined with along-track magnetic field gradients and an L1-norm model regularization criterion. Their models have a similar spatial resolution as those of Tsunakawa et al. (2015), but have comparatively lower signal strengths beyond about spherical harmonic degree 200. ...
... The total magnetic field strength of the Tsunakawa et al. (2015) model is presented in Fig. 4 at an altitude of 30 km above the mean planetary radius. In the upper row, the field strength is plotted using a linear scale for both the near and farside hemispheres of the Moon, and in the lower row the field strengths are plotted using a logarithmic scale. ...
... If magnetic connection existed, a determination was made of the crustal magnetic field strength (|B|) of the region which contained the footpoint of the approximated field line. By using the lunar crustal magnetic field model of Tsunakawa et al. (2015) at an altitude of 30 km above the lunar surface, magnetic field magnitude was obtained. If the field line was connected to a region with a field strength larger than 0.5 nT, designated as B weak in Figure 4, ETSI was flagged as a probable wave generation mechanism. ...
... Map showing the change in (a) perpendicular temperature, (b) parallel temperature. Thin (thick) contours correspond to the 10 nT (30 nT) crustal magnetic field magnitude at the surface from the(Tsunakawa et al., 2015) crustal magnetic field model. Arrows correspond to events covered in the event analysis. ...
... Map showing (a) cross-field drift of the ions perpendicular to the local magnetic field, that is, the perpendicular component of Vi_reflected-Vi_solar wind; (b) the electrostatic wave energy density, corresponding to electrostatic waves greater than 10s of Hz; (c) the electromagnetic wave energy density, corresponding to electromagnetic waves below 10s of Hz. Thin (thick) contours correspond to the 10 nT (30 nT) crustal magnetic field magnitude at the surface from the(Tsunakawa et al., 2015) crustal magnetic field model. Arrows correspond to events covered in the event analysis. ...
Interactions between the incident solar wind plasma and lunar crustal magnetic fields can lead to modifications in electron and ion dynamics that can result in the generation of electrostatic and electromagnetic waves. The resulting waves can then interact with the ambient electrons, leading to perpendicular and/or parallel electron heating. We analyze 10 years of data from the Acceleration, Reconnection, Turbulence, and Electrodynamics of Moon's Interaction with the Sun mission, when the spacecraft was within 200 km of the dayside lunar surface and within the solar wind, in order to characterize the near‐Moon plasma environment. We find that as the reflected ion density increases, electrostatic waves associated with plasma conditions favorable for the electron cyclotron drift instability play an increasingly important role in perpendicular electron heating, while electromagnetic interactions display the opposite trend. Additionally, we find that electrostatic waves associated with parallel heating exhibit plasma characteristics that are consistent with both the electron two‐stream instability and the modified two‐stream instability.
... We mapped the total magnetic field strength at the lunar surface based on Kaguya and Lunar Prospector magnetometer data. The Chang'E-5 landing site exhibits a relatively low intensity with an estimated maximum magnetic field strength of 1.18 nT (Supplementary Fig. 7) 55,56 . Our observations showed that there were very few spherical iron-sulfide grains in the Chang'E-5 lunar fines, and only two magnetite-bearing spherical iron-sulfide grains were found in the studied samples. ...
... The data of the total field intensity on the lunar surface were derived from the results of Tsunakawa et al. (2015) and Ravat et al. (2020) 55,56 . Originally, the magnetic field was obtained at altitudes of 10-45 km by the Kaguya and Lunar Prospector missions. ...
... The data of the total field intensity on the lunar surface were derived from the results of Tsunakawa et al. (2015) and Ravat et al. (2020) 55,56 . Originally, the magnetic field was obtained at altitudes of 10-45 km by the Kaguya and Lunar Prospector missions. ...
Ferric iron as well as magnetite are rarely found in lunar samples, and their distribution and formation mechanisms on the Moon have not been well studied. Here, we discover sub-microscopic magnetite particles in Chang’E-5 lunar soil. Magnetite and pure metallic iron particles are embedded in oxygen-dissolved iron-sulfide grains from the Chang’E-5 samples. This mineral assemblage indicates a FeO eutectoid reaction (4FeO = Fe3O4 + Fe) for formation of magnetite. The iron-sulfide grains’ morphology features and the oxygen’s distribution suggest that a gas–melt phase reaction occurred during large-impact events. This could provide an effective method to form ubiquitous sub-microscopic magnetite in fine lunar soils and be a contributor to the presentation of ferric iron on the surface of the Moon. Additionally, the formation of sub-microscopic magnetite and metallic iron by eutectoid reaction may provide an alternative way for the formation of magnetic anomalies observed on the Moon. Magnetite is rarely present on the Moon. Here the authors report the magnetite formed by eutectic reaction during the impact process in Chang’E-5 lunar soil, and the potential contribution of this magnetite formation to magnetic anomalies on the Moon.
... Figures 7-9 show the occurrence rates of the selenographic coordinates in the solar wind, magnetosheath, and magnetotail. The contours in solid black and magenta lines indicate a crustal field strength of 2 nT at a 30 km altitude evaluated by the lunar magnetic anomaly model of Tsunakawa et al. (2015). Note that the observation points in these distributions are heavily biased in longitude owing to the geometrical constraints of the lunar orbit (panels a and f in Figures 7-9). ...
... In other cases, the occurrence rates generally decrease near the magnetic anomalies, as suggested by Harada et al. (2014). However, we also find two features that contradict this trend: (a) the occurrence rates tend to be relatively high regardless of the Moon's position, day or night, or spectral shapes at latitudes from −20° to 20° and longitudes from −90° to −60°, where Reiner Gamma and other isolated magnetic anomalies are located (Kurata et al., 2005;Tsunakawa et al., 2015); and (b) for the lower band only events on the night side in the solar wind and magnetosheath (Figures 7 and 8g), high occurrence rates are seen in several longitude and latitude bands with no apparent association with magnetic anomalies. ...
... However, we pointed out the two notable exceptions. The first exception is the enhanced occurrence rate near latitudes from −20° to 20° and longitudes from −90° to −60°, where several isolated magnetic anomalies exist, including a strong magnetic anomaly called Reiner Gamma around latitude 8° and longitude −58° (Kurata et al., 2005;Tsunakawa et al., 2015). This may imply that the spatial extent of the magnetic anomalies could play a role in the wave excitation, but it remains unclear why the magnetic connection to these isolated magnetic anomalies is favorable for wave occurrence. ...
We present statistical analyses of whistler‐mode waves observed by Acceleration, Reconnection, and Turbulence and Electrodynamics of the Moon's Interaction with the Sun (ARTEMIS). Although some observations showed that the lunar whistler‐mode waves have similarities to the terrestrial chorus emissions, it remains unknown whether the banded structure typically seen in chorus is common to the lunar waves. In this study, we automatically detected whistler‐mode waves from 9 years of ARTEMIS data and classified them into four types of spectral shapes: lower band only, upper band only, banded, and no‐gap. We first show that a magnetic connection to the lunar surface is a dominant factor in the wave generation. The occurrence rate of whistler‐mode waves is 10 times larger on the magnetic field line connected to the Moon. Then we compared the field line connected events according to the position of the Moon and the condition of the field‐line foot point (day/night and existence of magnetic anomalies). The results show that (a) almost no banded event is observed in any circumstances, suggesting that generation mechanisms for the two band structure of the terrestrial chorus are largely ineffective around the Moon and (b) the wave occurrence rate depends on the foot point conditions, presumably affected by electrostatic/magnetic reflections deforming the velocity distribution of the resonant electrons. Thus, our results provide implications for the two band structure formation and new insights into fundamental processes of the Moon‐plasma interaction.
... The diffuse waves were not detected above intense magnetic anomalies. Figure 5 shows the Kaguya position during the detection of the diffuse wave projected onto a color-coded map of the crustal magnetic field magnitude at the lunar surface calculated from the Kaguya observation (Tsunakawa et al., 2015). No intense magnetic field was found at the Kaguya position. ...
... The orange line extending from the equator to the lower left direction indicates the Sun's direction. The lunar surface is color-coded with the magnitude of the lunar crustal field at 0 km altitude (Tsunakawa et al., 2015). ...
... Kaguya's position during the diffuse ELF wave event on 8 March 2008 projected onto the map of the lunar crustal magnetic field. The colors indicate the magnetic field magnitude at 0 km altitude(Tsunakawa et al., 2015) on the Lambert azimuthal equal area projection. Kaguya's position (red crosses) is plotted every 1 min from 5:11 to 15:17 and 7:03 to 07:13. ...
The solar wind particles reflected by the lunar magnetic field are the major energy source of electromagnetic wave activities, such as the 100 s magnetohydrodynamic waves and the 1 Hz whistler‐mode waves generated by protons and the non‐monochromatic whistler‐mode waves generated by mirror‐reflected electrons. Kaguya found a new type of whistler‐mode waves at 100 km altitude above the polar regions of the Moon with a broad frequency range of 1–16 Hz. The waves appear diffuse in both the time and frequency domains, and their occurrence is less sensitive to the magnetic connection to the lunar surface. The polarization is right‐handed with respect to the background magnetic field, and the wave number vector is nearly parallel to the magnetic field perpendicular to the solar wind flow. The diffuse waves are thought to be generated by the solar wind ions reflected by the lunar magnetic field through cyclotron resonance. The resonant ions are expected to have a velocity component parallel to the magnetic field larger than the solar wind bulk speed; however, such ions were not always simultaneously detected by Kaguya. The waves may have been generated above the dayside of the Moon and then propagated along the magnetic field being convected by the solar wind to reach the polar regions to be detected by Kaguya.
... Figure 3d shows total magnetic field intensity of the lunar magnetic anomalies at 30 km altitudes. The field intensity is obtained from the surface vector mapping (SVM) method (Tsunakawa et al., 2015). The Kaguya orbits measuring the Moon-originating ions are distributed between 345°and 0°in longitude and over a wide latitude range from 80°to 50°. ...
... This motional electric field drives the ions on the lunar surface and in the exosphere in the direction perpendicular to the magnetic field at a convection speed of 15 km/s. If we assume that the tail magnetic field lines are frozen to ions born at zero velocity via photoionization of neutrals on the lunar surface and in the exosphere, their trajectories are (Tsunakawa et al., 2015). The white box in (d) indicates the region where the rapid/significant energy change events were detected. ...
We analyze data acquired by the Kaguya satellite on 14 October 2008 when the Moon was in the terrestrial magnetotail lobe to gain new insight into the energization of ions originating from the Moon. The Moon‐originating ions were detected over a broad range of latitudes from −80° to 50° above the Moon's dayside at ∼100 km altitude. The fluxes of the Moon‐originating ions were observed at energies from ∼50 to ∼1,000 eV. Additionally, these ions exhibited a wide distribution pitch angle spanning from ∼45 to 90°. The energy levels of ions originating from the Moon show rapid changes, either increasing or decreasing by a factor of ∼10 within 8 min without the solar zenith angle dependence. Such rapid energy changes were observed over the highland regions. These observations are discussed in light of possible acceleration mechanisms of Moon‐originating ions, including temporal and spatial effects.
... Recent observations show that the Moon has a large number of local crustal magnetic fields, known as magnetic anomalies (Mitchell et al. 2008;Tsunakawa et al. 2015). Some strong magnetic anomalies can stand off or deflect the incoming solar wind and reduce the solar wind flux on the lunar surface. ...
... The lunar body is treated as an insulated sphere for its low conductivity, and an absorbing or float boundary condition is applied at the inner boundary, which means that all the variables of the solar wind (both the plasma and the field) can pass the lunar surface freely. The lunar crustal fields are obtained by a 450°spherical harmonic model (Tsunakawa et al. 2015). A locally refined spherical grid is used, in which the smallest cell near the surface has a size of about 8 km in the radial direction and about 16 km in the horizontal direction. ...
The solar wind can directly interact with the lunar surface and provide an important source for surface space weathering and water generation. Here we study the solar wind implantation flux on the lunar surface with global Hall MHD simulations. The shielding effects of both the Earth’s magnetosphere and lunar magnetic anomalies are considered. It is found that a large-scale lunar mini-magnetosphere can be caused by the solar wind interaction with the magnetic anomalies on the lunar far side, which causes a large shielding area on the surface. In addition, the Earth’s magnetosphere brings a longitudinal variation in the implantation flux, with minimum fluxes at 0° longitude. With the integrated flux over a lunation, we find that there are some local cavities on the implantation flux map, which are colocated with both the magnetic anomalies and the lunar swirls. Further studies show that there is a south–north asymmetry in the implantation flux, which can be used to explain the lower water content observed in the southern hemisphere. Our results provide a global map of the solar wind implantation flux on the lunar surface and are useful for evaluating the large-scale effect of solar wind implantation and sputtering on the space weathering and the water or gas generation of the surface.
... The scale of the crustal magnetic eld ranges from a few thousand kms down to less than 1 km (Dyal et al., 1974;Hood et al., 2001;Mitchell et al., 2008;Tsunakawa et al., 2015). It is expected that small-scale crustal elds also interact with solar wind, but observational studies have been di cult because small-scale events are often masked by large-scale phenomena. ...
... It is smaller than the longitudinal shift of approximately 1 of Kaguya orbit per 1 revolution, which corresponds to 29 km at Kaguya latitude 14.1 at the detection of this limb compression. This may account for the nonrecurrent detection of compression.The spatial extent 0.36 is close to the spatial resolution of 0.2 of the Lunar Surface Vector Mapping of Kaguya LMAG(Tsunakawa et al., 2015). The scale size was much smaller than the ion inertia length of 180 km calculated from the Alfvén speed of 52 km s -1 and the proton gyrofrequency of 0.29 rad/s. ...
Short-period magnetic enhancements were detected by the MAP-LMAG magnetometer onboard Kaguya orbiting the moon in solar wind at an altitude of 100 km. The duration was typically 10 seconds, which corresponds to 0.5 degrees in latitude along the Kaguya orbit and a scale size of 15 km. The magnitude of the magnetic field was enhanced up to 1.5 to 3.6 times as large as that of the preceding quiet periods. No such magnetic enhancements were found in the upstream solar wind magnetic field. The short-period magnetic enhancements were categorized into 2 groups. One is the smallest scale limb compression detected at the terminator region of the moon in a nearly constant solar wind magnetic field. The magnetic field flared away from the moon consistently with the previously known limb compressions. It was detected in a low dynamic pressure of the solar wind, which is a favorable condition for the detection of limb compressions, with no recurrence. No intense local magnetic field was identified at the foot of the magnetic field line of the limb compression. The scale size deduced from the duration was as small as 11 km, 85 times as small as that of previously reported limb compressions. The other types of magnetic enhancements appeared at the magnetic discontinuities of the solar wind magnetic field, similar to the hot flow anomalies observed at the Earth’s bow shock. A similar high-pressure structure that compresses the ambient magnetic field can be generated by the solar wind ions reflected at the moon channeled back along the current sheet of a tangential discontinuity when the motional electric field points toward the current sheet. The hot ions themselves were not detected on the nightside of the moon, while the magnetic field compressed by the expanding region can penetrate through the moon to be detected as magnetic field enhancements on the nightside of the moon.
... H 2 O/proton (Johnson, 1990), relatively short lifetimes against ion sputtering losses for a 500 nm thick ice sheet within PSRs were estimated (10 2 -10 4 years; see their Figure 6). However, previous mapping has shown evidence for non-negligible magnetic anomalies over the south polar region (e.g., Tsunakawa et al., 2015). Because of the low angle of solar wind incidence near the lunar poles, even moderate magnetic anomalies may significantly deflect the ion bombardment. ...
... Peak amplitudes of these anomalies at 30 km altitude are ∼5 nT on some orbit tracks but are ∼2.5 nT after two-dimensional filtering. A previous crustal field map that is most nearly comparable to the present mapping approach is that of Tsunakawa et al. (2015), who analyzed a combination of LP and KG data but using a surface vector mapping (SVM) method. Their map at one-degree resolution is available from https://data.darts.isas. ...
Plain Language Summary
Significant amounts of water ice are believed to be trapped in permanently shadowed regions (PSRs) near the lunar south pole. The extent to which water ice can survive in PSRs is limited by several factors, including the rate of sputtering losses from the solar wind ion bombardment. This loss rate has previously been estimated considering only the ambient solar wind flow and effects of topography. However, due to the low angle of solar wind incidence near the poles, even moderate magnetic anomalies may strongly deflect the ion bombardment within PSRs, helping to shield their interiors from ion sputtering losses. Here, new maps of crustal fields in the lunar polar regions are presented, confirming that more strong anomalies are present near the south pole than near the north pole, including over at least two permanently shadowed craters. These anomalies are about one‐quarter of the strength of the prototypical Reiner Gamma anomaly on the near side and should therefore be effective in shielding the interiors of these PSRs from the ion bombardment. The presence of stronger anomalies near the south pole than near the north pole may therefore help explain why more water ice is found near the south pole.
... More recently, the Lunar Prospector orbiting spacecraft recorded the distribution and strengths of magnetic anomalies on the lunar surface (Binder, 1998;Ravat et al., 2020;Tsunakawa et al., 2010Tsunakawa et al., , 2015. Many of these anomalies are suspected to be associated with impactor or ejecta material (Hood et al., 2001;Wieczorek et al., 2012). ...
... Despite these differences, it is expected that the remanent magnetic signature of lunar lava flows can provide sufficient anomalies to detect candidate locations to search for lava tubes, as well as bound their internal geomorphology. For surveys conducted on the lunar surface, within 2 m of the ground, magnetic anomalies are known to range from 5 to 313 nT from Apollo mission stationary and portable magnetometer observations, and surfacelevel anomalies are estimated to be >500 nT, in select regions, from Kaguya and Lunar Prospector observations (Dyal & Gordon, 1973;Tsunakawa et al., 2015). Assuming a lunar lava tube magnetic anomaly is of the same relative order of magnitude with respect to the background variability as seen at Skull Cave (signal to background variation ∼1.4), the resulting magnetic anomaly would be in the range of 10-100s of nT, which is well within the resolution capability of the current state of the art for spaceborne magnetometers (Connerney et al., 2015(Connerney et al., , 2017. ...
Lava tubes are a commonplace feature on the terrestrial planets, and knowledge of tube size and location informs lava flow processes. Future exploration of lava tubes on the Moon can provide access to geologic environments that likely remain unaltered from their emplacement billions of years ago. Lunar lava tubes may also provide astronauts protection from thermal extremes, meteoroid impacts, and radiation. High‐resolution magnetic identification and characterization of lava tubes can be used to help inform future scientific investigations of lava tubes for human exploration and utilization. We demonstrate how magnetometry is useful for determining the geometry and extent of lava tubes on the Earth and, by proxy, the Moon, by relating the magnetic anomalies produced by lava tubes to their location and geomorphology. Using a proton‐precession total field magnetometer, we surveyed an area of more than 100,000 m², with cross‐tube linear traverses spaced at 3–5 m, perpendicular to an approximately 1,000 m length of the Modoc Crater lava tube complex, within the Lava Beds National Monument (California, USA). The observed magnetic anomalies of the sections known as Incline, Skull, and Ship Caves are compared against synthetic predictions, and the sensitivity of the magnetic anomalies to the tube geometry used to derive a basic relationship between the two. We use our model of terrestrial lava tube magnetic anomalies and adjust for the lunar magnetic environment to predict the signature of anomalies resulting from tubes on the Moon.
... The magnetic field strength decreases rapidly with distance from the source as a factor of 1/r 3 for a dipole in free space, due to the inherent behavior of potential fields. These attenuations make it challenging for orbital data to accurately measure magnetic intensities and vector component strength at the surface (e.g., Richmond & Hood 2008;Purucker & Nicholas 2010;Tsunakawa et al. 2015;Ravat et al. 2020). As noted by Hemingway & Tikoo (2018), current inversion techniques can underestimate crustal field intensity by more than an order of magnitude. ...
The formation mechanisms, extent, and compositions of red spots on the lunar surface have intrigued the lunar community for decades. By identifying a new dome and another silicic crater in the highlands nearby, we find that the silicic volcanism in the Gruithuisen region extends beyond the three major domes. Our observations indicate that the Gruithuisen domes have low iron and titanium contents. They are enveloped by ejecta from surrounding regions and host silica-rich material excavated by the young craters consistent with previous work. Our boulder maps of the Gamma dome display a high boulder count and indicate that the Diviner rock abundance maps are only sensitive to boulders larger than ∼2 m. The H-parameter values are sensitive to presence of rocks and may be a better indicator of rocks at submeter scales. The Delta dome has gentle slopes, lower rock abundance, and one young crater, and it could serve as a safe and scientifically valuable site for landing and exploration of the domes and nearby region. The dome also displays anomalously high H-parameter in the same region as the crater, indicating the potential presence of pyroclastic materials. We observe up to 200 ppm of OH/H 2 O on the domes and nearby mare despite the presence of a weak magnetic field to the south of Delta dome, further supporting the potential presence of pyroclastics in the region. This study could potentially aid in logistical and scientific decisions of the future NASA missions in the region. Unified Astronomy Thesaurus concepts: Lunar composition (948); The Moon (1692); Volcanism (2174)
... Since most crustal magnetic field models only provide two-dimensional intensity maps as a function of longitude and latitude at a given altitude, we can only project each source location vertically onto a map of crustal magnetic field intensities to estimate its corresponding intensity. The map used in the present study was modeled by Tsunakawa et al. (2015), which shows the intensities at the lunar surface. For each mass species, we calculate the median crustal magnetic field intensity at each flux bin containing at least 20 data points. ...
The Moon is enveloped in an exosphere, which is comprised of a variety of neutral atoms and molecules. Once exospheric neutrals are ionized by photons, protons, or electrons from the Sun, the resulting ions are accelerated by the electromagnetic fields of their surroundings and can thereby travel away from their source locations. These ions are the so-called pickup ions and are frequently observed by the two Acceleration, Reconnection, Turbulence, and Electrodynamics of Moon’s Interaction with the Sun (ARTEMIS) spacecraft. In this study, we identify 115 events from an 11 yr period of ARTEMIS observations, which contain a total of 11,987 samples for our statistics. By using analytical ion trajectory calculations, we trace the source location of each pickup ion observation. Most pickup ion trajectories originate near the subsolar point, consistent with the efficiency of sputtering. We find that the flux of pickup ions strongly anticorrelates with the source altitude, providing indirect evidence of decreasing exospheric ion flux with increasing altitude. We also find that the flux of pickup ions does not show a significant relationship with the crustal magnetic field intensity. This implies that a depression of sputtering efficiency or the trapping of near-surface freshly born ions by a crustal magnetic anomaly may not reduce the subsequent pickup ion flux as effectively as expected. In summary, the present paper provides a statistical view of lunar pickup ion fluxes in association with the altitude, local time, and local crustal magnetic field of their source locations.
... Global maps of the lunar vector magnetic field on the surface derived from LP and Kaguya/SELENE observations are applied in this study to investigate the distribution of magnetic anomalies (e.g., swirl) and their effects on the space weathering processes of the CE-4/CE-6 landing region. The vector magnetic field datasets consist of total field strength and three components of different directions, i.e., B t (total force), B e (east component), B n (north component), and B r (radial component) (Tsunakawa et al., 2015). ...
... It is a simple crater of 3 km diameter, and its ejecta boulders have been excavated from a depth of about 600 m, reasonably assuming a simple crater excavation mode (Melosh, 1989). The impact occurred about 70 km away from the magnetic anomaly of Reiner Gamma, where a magnetic flux density of >100 nT at the surface level has been derived from orbital measurements (e.g., Tsunakawa et al., 2015). The source of the magnetic field is potentially a magnetized body, for example, a melt sheet of impact ejecta from a large basin that incorporated highly magnetic projectile material (e.g., Wieczorek et al., 2012). ...
In a database of lunar fractured boulders (Rüsch & Bickel, 2023, https://doi.org/10.3847/psj/acd1ef), we found boulders with reflectance features dissimilar to previously known morphologies. We performed a photo‐geologic investigation and determined that the features correspond to a dust mantling on top of boulders with a unique photometric behavior. We next performed a photometric model inversion on the dust mantling using Bayesian inference sampling. Modeling indicates that the dust photometric anomaly is most likely due to a reduced opposition effect, whereas the single scattering albedo is not significantly different from that of the nearby background regolith. This implies a different structure of the dust mantling relative to the normal regolith. We identified and discussed several potential processes to explain the development of such soil. None of these mechanisms can entirely explain the multitude of observational constraints unless evoking anomalous boulder properties. Further study of these boulders can shed light on the workings of a natural dust sorting process potentially involving dust dynamics, a magnetic field, and electrostatic dust transport. The presence of these boulders appears to be limited to the Reiner K crater near the Reiner Gamma magnetic and photometric anomaly. This close spatial relationship further highlights that poorly understood processes occur in this specific region of the Moon.
... Lunar crustal magnetic fields play a crucial role in the incidence and reflection of the solar wind ions on the lunar surface (e.g., Lue et al. 2011;Saito et al. 2012). Observations have shown that on average between 5-10% of the solar wind proton flux is reflected from lunar crustal magnetic fields at an altitude between the surface and the location of the spacecrafts (Lue et al. 2011;Saito et al. , 2012Poppe et al. 2017;Tsunakawa et al. , 2015. However, depending on the strength and location of the crustal magnetic fields and the energy and angle of the incident solar wind ions, up to 50% reflection over strong crustal fields have been also reported (Lue et al. 2011). ...
Prebiotic chemical evolution that led to the emergence of life on primitive Earth is interlinked with the delivery of organic material through the impact of comets, asteroids and meteorites. The catastrophic nature of impact leads to significant damage to planetary bodies. The high pressure and temperature can cause molecules to break apart and they may not survive in such extreme conditions. Also, impact-induced shock can cause impacted molecules to undergo vibration, dissociation, deformation, depending on their chemical properties and thus can offer enormous potential for the synthesis of building blocks of life. Novel experimental and theoretical approaches are required to simulate the phenomena that occur during impacts. In this brief review, we discuss impacts and related processes through laboratory experiments and simulations that study the impact-shock chemistry and its role in the Origins of Life.
... Lacking a geodynamo, the moon has a very small magnetic field which varies widely across the surface but is typically of order a few tens of nT, roughly 1000 times weaker than the Earth's field [24]. Hence, there would be no need for magnetic shielding or compensation for a typical ultracold atom experiment on the lunar surface. ...
Existing space-based cold atom experiments have demonstrated the utility of microgravity for improvements in observation times and for minimizing the expansion energy and rate of a freely evolving coherent matter wave. In this paper we explore the potential for space-based experiments to extend the limits of ultracold atoms utilizing not just microgravity, but also other aspects of the space environment such as exceptionally good vacuums and extremely cold temperatures. The tantalizing possibility that such experiments may one day be able to probe physics of quantum objects with masses approaching the Plank mass is discussed.
... Particles from the interplanetary space, such as solar wind ions, galactic cosmic rays, and micrometeoroids, can directly bombard the lunar surface, resulting in a relatively harsher space environment. However, it has been found that the Moon possesses a large number of local crustal magnetic fields, known as magnetic anomalies (Hood et al. 2001;Mitchell et al. 2008;Tsunakawa et al. 2015), which can deflect the solar wind and form a smallscale structure with lower solar wind flux. Previously, both the Lunar Prospector (LP) and the ARTEMIS spacecraft observed shock-like structures near some strong magnetic anomalies (Lin et al. 1998;Halekas et al. 2006Halekas et al. , 2014. ...
A shock or a mini-magnetosphere was once thought to be formed by the solar wind interaction with strong lunar magnetic anomalies. However, the full structure of a mini-magnetosphere has never been verified and whether a mini-magnetosphere can be completely formed remains a controversy. In this work, we present a unique multipoint observation of such an interaction by the ARTEMIS spacecraft and the Chang'E-4 rover. Both solar wind deceleration and penetration are observed by the Chang'E-4 rover on the lunar surface near the magnetic anomaly. Meanwhile, a shock is observed by the ARTEMIS spacecraft downstream from the magnetic anomaly. It is suggested that the magnetic anomaly cannot stand off the solar wind, and there is no shock but just a boundary layer near the magnetic anomaly. Accordingly, a mini-magnetosphere is not completely formed and the downstream shock observed the ARTEMIS spacecraft just corresponds to a trailing shock.
... Due to the inherent behavior of potential fields, magnetic field strength decreases rapidly with distance from the source (a factor of 1/r 3 for a dipole source in free space), and this attenuation is difficult to overcome in analysis of orbital data to obtain an accurate view of magnetic intensities and vector component strength at the surface (e.g., Richmond & Hood, 2008;Mitchell et al., 2008;Purucker & Nicholas, 2010;Tsunakawa et al., 2015;Ravat et al., 2020;Hood et al., 2021). What is seen in orbit is effectively a fraction of the crustal field intensity, so in this paper we model magnetic intensity and structure in a three-dimensional perspective from a normalized spacecraft orbital altitude to the surface, to capture detail not possible in the use of orbital data alone. ...
With the wealth of missions selected to visit the lunar surface in the decade ahead, preparatory investigations into surface conditions are underway to explore potential challenges and science returns during these missions. One such mission, Lunar Vertex, is slated to explore a much-anticipated region–the lunar swirl and magnetic anomaly known as Reiner Gamma. Lunar swirls are unique natural laboratories for exploring solar wind interactions with partially magnetized rocky bodies, and possess characteristics that have not yet been observed on any other body in the Solar System. This work aims to combine current magnetic mapping of Reiner Gamma with ultraviolet wavelength datasets, towards further understanding the sensitivities of ultraviolet measurements in regions that may be partially magnetically shielded from solar wind weathering and magnetospheric plasma populations. Observations and models herein are collected and derived from orbital sources and will be used for comparison to future orbital and surface observations of Reiner Gamma by Lunar Vertex.
... While stronger magnetic anomalies are generally not clearly associated with known geological processes, association with denser lunar crust and magnetic anomalies has been observed (Gong and Wieczorek, 2020). We used the recent model of the lunar magnetic anomaly on the surface provided by Tsunakawa et al. (2015). It is based on observations of the missions Lunar Prospector (Binder, 1998) and Kaguya (Shimizu et al., 2008). ...
The gravity aspects for the Moon (the gravity disturbance, the Marussi tensor, two gravity invariants, dimensionality ratio, the strike angles, and the virtual deformations), all combined with magnetic anomalies and detailed surface topography, allow new views of specific locations on the Moon. Using these new gravity quantities, we hypothesize the following for several features on the Moon. A dike-like intrusion (exceeding ~100 km in length) from inside to outside of the Clavius crater likely solidified at the time of the existence of lunar dynamo. Mare Crisium analyses show a specific distribution of faulting across the mare. The same size impacts, Crisium and Clavius, present the dilatational deformation that is smoother for Crisium, while Clavius is under variable concentric compression due to an uplift of denser rock. Mare Orientale deformation not only confirmed the prior finding of the near surface faults, but also reveals a nature of the faulting (expansion vs compression blocks). Magnetic analyses of related lunar anomalies constrain mascon extent under the Copernicus structure and outline contraction areas from cooling of the upwelled mantle material. Mare Imbrium impact event has demagnetized regolith along with the Copernicus crater using a novel mechanism of shock propagation while plasma demagnetization. Clavius' magnetic field reveals magnetization that is likely more than four billion years old. Mare Crisium impact has a unique magnetization signature by impact related transient field. Mare Orientale showed, for the first-time, rippling-like effect of the Moon's mantle. This process of upwelled rippled mantle allows efficient demagnetization of the Orientale basin. For the first time, the application of the gravity aspects has been extended from the Earth to the Moon. This approach opens a new and inspiring field of planetary studies and point to otherwise hardly detectable phenomena. More detailed studies should follow.
... Lunar crustal magnetic fields play a crucial role in the incidence and reflection of the solar wind ions on the lunar surface (e.g., Lue et al. 2011;Saito et al. 2012). Observations have shown that on average between 5-10% of the solar wind proton flux is reflected from lunar crustal magnetic fields at an altitude between the surface and the location of the spacecrafts (Lue et al. 2011;Saito et al. , 2012Poppe et al. 2017;Tsunakawa et al. , 2015. However, depending on the strength and location of the crustal magnetic fields and the energy and angle of the incident solar wind ions, up to 50% reflection over strong crustal fields have been also reported (Lue et al. 2011). ...
The Moon and Mercury are airless bodies, thus they are directly exposed to the ambient plasma (ions and electrons), to photons mostly from the Sun from infrared range all the way to X-rays, and to meteoroid fluxes. Direct exposure to these exogenic sources has important consequences for the formation and evolution of planetary surfaces, including altering their chemical makeup and optical properties, and generating neutral gas exosphere. The formation of a thin atmosphere, more specifically a surface bound exosphere, the relevant physical processes for the particle release, particle loss, and the drivers behind these processes are discussed in this review.
... There are three swirls (Figure 14, identified by [19]) on the highland at the southwest side of Mare Fecunditatis with a low optical maturity (OMAT, Figure 14b, [40]), and the surface vector mapping (SVM, [114]) of magnetic field (B-flied) shows this area has a magnitude of up to 100 nT (Figure 14d). The MI mineral data (Figure 14c, [40]) of the swirl show consistent plagioclase with the surrounding area, which contradicts hypothesis (3). ...
Mare Fecunditatis is a ~310,000 km2 flat basalt plain located in the low-latitude area of the Moon. Plenty of volcanic features (multiple episodes of mare basalts, sinuous rilles, lava tubes, pyroclastic deposits, domes, irregular mare patches (IMP), ring-moat dome structures (RMDS), floor-fractured craters), tectonic features (grabens and wrinkle ridges), impact-related features, and other features (swirls, pit craters) are identified in Mare Fecunditatis. An in-situ mission to Mare Fecunditatis is scientifically significant to better understand the lunar thermal histories and other questions. All previous in-situ and human missions (Apollo, Luna, Chang’E) were limited to small areas, and no traverse longer than 40 km has been made yet. With the development of technology, long-distance movement will be possible in the future on the lunar surface, providing opportunities to explore multiple sites at one mission with complete documentation of the regional geology. Eight high-value targets (pit crater, IMPs, RMDSs, young basalts, high-Al basalts, pyroclastic deposits, swirls, and fresh craters) were found in Mare Fecunditatis, and a ~1400 km-traverse in 5 years is proposed to explore them to solve the most fundamental lunar questions.
... The second simulation used a proposed field geometry for the Reiner Gamma swirl resulting from 3D fully kinetic modeling of the solar wind interaction with a Surface Vector Mapping model based on quiet solar wind and night-side magnetic field observations from the Lunar Prospector and Kaguya spacecraft (Deca et al., 2018;Tsunakawa et al., 2015). The simulation by Deca et al. (2018) included a solar wind velocity of 350 km s −1 streaming perpendicular to the lunar surface and a 3 nT interplanetary magnetic field at a 45° angle toward the lunar surface. ...
The scattering of light off the bright regions of lunar swirls suggests a relatively higher fraction of fine‐grained dust compared to dust with similar optical maturity values found elsewhere on the lunar surface when assuming an isotropically oriented dust distribution, without preferred grain orientation. Here we propose a mechanism by which the ferromagnetic lunar fines, lofted by electrostatic effects or by meteoroid impacts, may rotate in a lunar magnetic anomaly and, upon landing, produce patches of organized alignments, generating anisotropic surface structures. We simulate these rotations for Reiner Gamma and provide a proof of concept for such a dust rotation model. While the magnetic forces remain negligibly small to influence the trajectories of the lofted particles, the magnetic torques have a significant effect on the rotation of magnetized dust, and can generate an organized landing pattern for elongated lunar fines, which offers a possible explanation for the lunar albedo patterns.
... Nevertheless, it possesses a series of mini-magnetospheres associated with the numerous magnetic anomalies spread over the surface (Coleman et al. 1972;Hood et al. 2001). A detailed map of magnetic anomalies over the Moon was produced from Kaguya and Lunar Prospector and can be found on Tsunakawa et al. (2015). Magnetic anomalies produce a shielding of specific surface regions, acting on all ionized particles present in the close lunar environment; as a consequence, they cause the double effect of deflecting impinging particles over the core region of the anomaly and of intensifying the flux impinging on the surrounding region (in an annular shape) Saul et al. 2013). ...
Sodium and, in a lesser way, potassium atomic components of surface-bounded exospheres are among the brightest elements that can be observed from the Earth in our Solar System. Both species have been intensively observed around Mercury, the Moon and the Galilean Moons. During the last decade, new observations have been obtained thanks to space missions carrying remote and in situ instrumentation that provide a completely original view of these species in the exospheres of Mercury and the Moon. They challenged our understanding and modelling of these exospheres and opened new directions of research by suggesting the need to better take into account the relationship between the surface-exosphere and the magnetosphere. In this paper, we first review the large set of observations of Mercury and the Moon Sodium and Potassium exospheres. In the second part, we list what it tells us on the sources and sinks of these exospheres focusing in particular on the role of their magnetospheres of these objects and then discuss, in a third section, how these observations help us to understand and identify the key drivers of these exospheres.
... The lunar exosphere is rather tenuous at around 10 −9 mbar: This was first detected by in situ measurements of the Apollo missions and found to consist of atoms and light molecular species, such as Ar, He, Ne, Na, K, and H (29). Dust grains above the surface are regarded to come from ejecta produced by the continual bombardment of the Moon by interplanetary micrometeoroids and small grains lifted by plasma-induced near-surface electric fields (30). The existence of lunar magnetic anomalies, i.e., regions of local magnetization, has been confirmed (31)(32)(33). Some magnetic anomalies have surface fields of up to hundreds of nanoteslas. ...
Human activities on the lunar surface are severely constrained by the space radiation dominated by cosmic rays (CRs). Here, we report the first measurements of the low-energy (about 10 to 100 MeV/nuc) CR spectra on the lunar surface from China's Chang'E-4 (CE-4) mission around the solar minimum 24/25. The results show that for the proton, helium, CNO, and heavy-ion groups, the ratios (ratio errors) of the CE-4 fluxes to those from the near-earth spacecraft are 1.05 (0.15), 1.30 (0.18), 1.08 (0.16), and 1.24 (0.21), respectively, and to those predicted by the models [CRÈME96 and CRÈME2009] are instead [1.69 (0.17), 2.25 (0.23)], [1.66 (0.17), 1.76 (0.18)], [1.08 (0.11), 1.07 (0.11)], and [1.33 (0.18), 1.17 (0.15)]. Moreover, a notable enhancement of 3 He/ 4 He ratio is observed at ~12 MeV/nuc, and the CR dawn-dusk symmetry is confirmed. These results provide valuable insights into the CRs on the lunar farside surface and will benefit future lunar exploration.
... By linearly extrapolating the magnetic field observed by Kaguya/LMAG at 04:18:33 UT, we estimated the magnetic footprint to be at (45 • N, 42 • E) in the ME coordinate system. At this location on the lunar surface, the crustal magnetic field strength reproduced by the surface vector mapping (SVM) method (Tsunakawa et al. 2015) is as weak as 0.6 nT. Based on the simple sum of the crustal magnetic field of 0.6 nT and external magnetic field of ∼3 nT observed at Kaguya's location, the magnetic field strength at the footprint is estimated to be at most ∼ 3.6 nT. ...
Wave–particle interactions are fundamental processes in space plasma, and some plasma waves, including electrostatic solitary waves (ESWs), are recognised as broadband noises (BBNs) in the electric field spectral data. Spacecraft observations in recent decades have detected BBNs around the Moon, but the generation mechanism of the BBNs is not fully understood. Here, we study a wake boundary traversal with BBNs observed by Kaguya, which includes an ESW event previously reported by Hashimoto et al. Geophys Res Lett 37:L19204 10.1029/2010GL044529 (2010). Focusing on the relation between BBNs and electron pitch-angle distribution functions, we show that upward electron beams from the nightside lunar surface are effective for the generation of BBNs, in contrast to the original interpretation by Hashimoto et al. Geophys Res Lett 37:L19204 10.1029/2010GL044529 (2010) that high-energy electrons accelerated by strong ambipolar electric fields excite ESWs in the region far from the Moon. When the BBNs were observed by the Kaguya spacecraft in the wake boundary, the spacecraft’s location was magnetically connected to the nightside lunar surface, and bi-streaming electron distributions of downward-going solar wind strahl component and upward-going field-aligned beams (at $$\sim$$ ∼ 124 eV) were detected. The interplanetary magnetic field was dominated by a positive $$B_Z$$ B Z (i.e. the northward component), and strahl electrons travelled in the antiparallel direction to the interplanetary magnetic field (i.e. southward), which enabled the strahl electrons to precipitate onto the nightside lunar surface directly. The incident solar wind electrons cause negative charging of the nightside lunar surface, which generates downward electric fields that accelerate electrons from the nightside surface toward higher altitudes along the magnetic field. The bidirectional electron distribution is not a sufficient condition for the BBN generation, and the distribution of upward electron beams seems to be correlated with the BBNs. Ambipolar electric fields in the wake boundary should also contribute to the electron acceleration toward higher altitudes and further intrusion of the solar wind ions into the deeper wake. We suggest that solar wind ion intrusion into the wake boundary is also an important factor that controls the BBN generation by facilitating the influx of solar wind electrons there.
Graphical Abstract
... Moreover, the MSA observation in the QSO-Ls will provide the distribution maps which divide the Phobos surface to several areas because the spatial resolution of scattering and sputtering points reduces to around the spacecraft's altitude. If there are electromagnetic effects similar to those of the Earth's Moon, such as magnetic anomalies (e.g., Tsunakawa et al. 2015) or surface charging (e.g., Stubbs et al. 2014), low-altitude observation of ions and magnetic field with the MSA will reveal their associated phenomena (Saito et al. 2012). The situation of the MSA observation in the 3D-QSO is the same as that in the QSO-M because the ion observation is mostly determined by the spacecraft' altitude and solar wind conditions. ...
The science operations of the spacecraft and remote sensing instruments for the Martian Moon eXploration (MMX) mission are discussed by the mission operation working team. In this paper, we describe the Phobos observations during the first 1.5 years of the spacecraft’s stay around Mars, and the Deimos observations before leaving the Martian system. In the Phobos observation, the spacecraft will be placed in low-altitude quasi-satellite orbits on the equatorial plane of Phobos and will make high-resolution topographic and spectroscopic observations of the Phobos surface from five different altitudes orbits. The spacecraft will also attempt to observe polar regions of Phobos from a three-dimensional quasi-satellite orbit moving out of the equatorial plane of Phobos. From these observations, we will constrain the origin of Phobos and Deimos and select places for landing site candidates for sample collection. For the Deimos observations, the spacecraft will be injected into two resonant orbits and will perform many flybys to observe the surface of Deimos over as large an area as possible.
Graphical Abstract
... Since the Moon has large and wide magnetic anomalies (e.g., Richmond and Hood 2008;Tsunakawa et al. 2015), which deflect and/or reflect the solar wind (Saito et al. 2012), some areas are substantially shielded from the solar wind by an induced electric field (Futaana et al. 2013). Such a shielding from the solar wind can reduce the progression of the space weathering (Kramer et al. 2011;Bamford et al. 2016). ...
The mass spectrum analyzer (MSA) will perform in situ observations of ions and magnetic fields around Phobos as part of the Martian Moons eXploration (MMX) mission to investigate the origin of the Martian moons and physical processes in the Martian environment. MSA consists of an ion energy mass spectrometer and two magnetometers which will measure velocity distribution functions and mass/charge distributions of low-energy ions and magnetic field vectors, respectively. For the MMX scientific objectives, MSA will observe solar wind ions, those scattered at the Phobos surface, water-related ions generated in the predicted Martian gas torus, secondary ions sputtered from Phobos, and escaping ions from the Martian atmosphere, while monitoring the surrounding magnetic field. MSA will be developed from previous instruments for space plasma missions such as Kaguya, Arase, and BepiColombo/Mio to contribute to the MMX scientific objectives.
The Moon generated a long‐lived core dynamo magnetic field, with intensities at least episodically reaching ∼10–100 μT during the period prior to ∼3.56 Ga. While magnetic anomalies observed within impact basins are likely attributable to the presence of impactor‐added metal, other anomalies such as those associated with lunar swirls are not as conclusively linked to exogenic materials. This has led to the hypothesis that some anomalies may be related to magmatic features such as dikes, sills, and laccoliths. However, basalts returned from the Apollo missions are magnetized too weakly to produce the required magnetization intensities (>0.5 A/m). Here, we test the hypothesis that subsolidus reduction of ilmenite within or adjacent to slowly cooled mafic intrusive bodies could locally enhance metallic FeNi contents within the lunar crust. We find that reduction within hypabyssal dikes with high‐Ti or low‐Ti mare basalt compositions can produce sufficient FeNi grains to carry the minimum >0.5 A/m magnetization intensity inferred for swirls, especially if ambient fields are >10 μT or if fine‐grained Fe‐Ni metals in the pseudo‐single domain grain size range are formed. Therefore, there exists a possibility that certain magnetic anomalies exhibiting various shapes such as linear, swarms, and elliptical patterns may be magmatic in origin. Our study highlights that the domain state of the magnetic carriers is an under‐appreciated factor in controlling a rock's magnetization intensity. The results of this study will help guide interpretations of lunar crustal field data acquired by future rovers that will traverse lunar magnetic anomalies.
The paper presents the results of laboratory experiment modeling the interaction between Lunar magnetic anomalies and Solar wind. To model the LMA we use quadrupole magnetic field. The main dimensionless parameter of the problem, the ion inertia length relative to the mini-magnetosphere size, well corresponds between experiment and LMA conditions. The main result is measurement of the magnetically reflected proton fluxes, which show qualitative agreement to available satellite data.
The Moon of our Earth has a tenuous atmosphere, known as an exosphere. The ionization of this exosphere is speculated to possibly form a weak ionosphere. Some radio occultation (RO) experiments have suggested the presence of a dense ionosphere with an electron density on the order of hundreds of cm −3 near the surface. Using in situ measurements from the ARTEMIS mission during 2012-2021, we conduct statistical analyses and case studies to investigate the plasma density at near-surface altitudes. ARTEMIS measurements reveal no plasma densities at altitudes between 10 and 50 km that exceed 35 cm −3 , and therefore they provide no evidence for a steady-state or global lunar ionosphere at the level suggested by some RO observations. Density profiles with local time and altitude show higher density in the sunlit sector than in the shadowed sector. These observations suggest that the natural variation of solar wind plasma flux with solar zenith angle plays a critical role in controlling the plasma population near the surface. This research provides a reference for a comparison with RO observations and a statistical view of the low-altitude plasma environment near the lunar surface. Unified Astronomy Thesaurus concepts: The Moon (1692); Lunar science (972); Solar wind (1534); Space plasmas (1544)
The near lunar surface contains small‐scale magnetic field structures that provide a natural test bed for observing plasmas with a non‐zero Hall electric field, as well as potentially facilitating electron‐only reconnection. This study presents observational evidence of magnetized electrons as well as demagnetized ions when THEMIS‐ARTEMIS probe B reached an altitude of ∼15 km above the lunar surface. Additionally, observations suggest the presence of a field line topology change and traversal of a closed magnetic field structure containing solar wind electrons, suggestive of magnetic reconnection having occurred at some point between the solar wind interplanetary magnetic field and a lunar crustal magnetic field. Thus, the observations presented here are consistent with previous studies that predict prominent Hall electric fields near lunar crustal magnetic fields and further suggest that the solar wind interplanetary magnetic field may reconnect with lunar crustal magnetic fields, most likely via electron‐only reconnection.
Context. Lunar swirls are bright albedo features only found on the Moon that are still not entirely understood. It is commonly accepted that reduced space weathering plays a role in explaining the origins of lunar swirls because the local magnetic fields that are typically associated with these albedo anomalies are effective in reducing the solar wind influx. However, additional processes are required to fully explain the spectral, photometric, and polarimetric properties of the swirls.
Aims. In this study, we compare the photometric properties of the Chang’e-5 landing site to those of the Reiner Gamma swirl. Because the physical effects of a landing rocket jet on the lunar regolith are relatively well known, these observations can provide important insights into the physical properties of lunar swirls.
Methods. We determined the single scattering albedo, opposition effect strength, and surface roughness of the Reiner Gamma swirl and the Chang’e-5 landing site with their respective statistical uncertainties based on the Hapke model and Bayesian inference sampling.
Results. The Chang’e-5 landing site and the Reiner Gamma swirl exhibit similar photometric properties, in particular: an increased albedo and a reduced opposition effect strength. Additionally, the landing site is about 20% less rough compared to the surrounding area.
Conclusions. These findings suggest that the swirl surface is less porous compared to the surrounding surface, similarly to a landing site where the top layer of the regolith has been blown away effectively so that the compactness was increased. We conclude that external mechanisms that are able to compress the uppermost regolith layer are involved in lunar swirl formation, such as interactions with the gaseous hull of a passing comet.
While the lunar magnetic field has been measured to a high degree of resolution, its origin and the mechanism of its generation are not well understood. The Apollo sample returned showed that the crust of the Moon was magnetized by a magnetic field of variable intensity and at variable times during the Moon’s history. Magnetic data suggest that the Moon contained its own magnetic field in its early history.
The theory of quantities derived from the gravitational potential—gravity aspects (descriptors)—is recalled but not repeated in detail. It has already been summarized in our two books (Klokočník et al. 2017b, 2020a) together with a physical explanation and many examples. Our sources of gravitational, magnetic and topographic data are described.
We show maps with the gravity aspects, topography from LOLA, and magnetic intensities. First, we present global views on the Moon (Sect. 8.1), and then closer views by segments of the lunar surface with more details.
The combination of gravity aspects with magnetic field intensities and LOLA topography provides a unique description of structural features on the Moon (impact craters, mascons, maria (seas), catenae, ghost craters, polygonal structures, rifts, radial structures, intrusions, or crustal fractures). For this section, we selected unique features from Chap. 8 to show more details.
The work attempts to understand the mineralogy of the reported geochemical anomaly located in the north – northeast region of the Korolev basin using Moon Mineralogy Mapper (M³) onboard Chandrayaan -1 and other lunar datasets. To understand the mineralogy, colour composite images using integrated band depth parameters and mineral indices were prepared, and the M³ spectral signatures corresponding to the unique colours of these colours composites were investigated. Further, Modified Gaussian Model (MGM) deconvolution was applied to these spectra. The results of spectra studies reveal that the area is made up of heterogeneous lithologies, predominantly composed of anorthosite along with minor occurrences of pyroxene-bearing (both low-calcium and high-calcium variety) and spinel-bearing lithologies. Correlation of spectral studies with the morphology revealed that pyroxene was typically associated with fresh craters and their ejecta. Spinel was found to be ubiquitous and is well-dispersed, possibly distributed along with the ejecta blanket of large impact craters. This association hints that the pyroxene-hosted layer most likely occurs beneath the spinel-bearing layer. Such observed assemblages may have resulted from physical mixing during the impact cratering events. This mixed lithology could arise due to the mafic mineral-bearing ejecta of the South-Pole Aitken (SPA) basin and spinel-bearing Orientale ejecta. Korolev most likely impacted on a thick layer of SPA ejecta, and impact basins such as Hertzsprung and Orientale have occurred post-Korolev formation. Orientale being the youngest of the large impact basins, its ejecta carrying the light plain material would have overprinted the older SPA ejecta. Smaller impact craters would have churned the ejecta so that presently we observe a composite lithology with a variable abundance of pyroxene and spinel.
Different from the Earth, the solar wind can directly impact the lunar surface, and partly be scattered as Energetic Neutral Atoms (ENAs). However, the lunar magnetic crustal fields in some regions, called magnetic anomalies, can deflect the solar wind to form a mini-magnetosphere, shielding the surface. All previous understandings about these processes are obtained from orbit, and the truth on the lunar surface is still unknown. The Advanced Small Analyzer for Neutrals (ASAN) onboard Chang’E-4 mission can detect the reflected Energetic Neutral Atoms (ENAs) from the lunar surface, which will provide new perspectives to study the solar wind interaction with the Moon. Here is a review on the recent works with the ENA data from ASAN, focusing on introducing some new discoveries by ASAN, such as a higher ENA reflection ratio, more ENAs gathered at lower energies, and some heavier ENAs other than the H ENA. Compare with the upstream solar wind data, it is found that the ENAs in the energy range of 105~523 eV are closely related with the solar wind. Moreover, the ENA fluxes downstream from the magnetic anomalies are generally smaller. Combined with the global Hall MHD simulation, reduction in the ENA flux is confirmed to be caused by a mini-magnetosphere. Meanwhile, it is found the formation of the mini-magnetosphere is determined by the solar wind dynamic pressure and the ion inertia length, and the mini-magnetosphere brings a deceleration to the solar wind, by a differential electrostatic potential of 50~260 V.
This paper considers the problem of efficient computation of the spherical harmonic expansion, or Fourier transform, of functions defined on the two dimensional sphere, S2. The resulting algorithms are applied to the efficient computation of convolutions of functions on the sphere. We begin by proving convolution theorems generalizing well known and useful results from the abelian case. These convolution theorems are then used to develop a sampling theorem on the sphere. which reduces the calculation of Fourier transforms and convolutions of band-limited functions to discrete computations. We show how to perform these efficiently, starting with an O(n(log n)2) time algorithm for computing the Legendre transform of a function defined on the interval [-1,1] sampled at n points there. Theoretical and experimental results on the effects of finite precision arithmetic are presented. The Legendre transform algorithm is then generalized to obtain an algorithm for the Fourier transform, requiring O(n(log n)2) time, and an algorithm for its inverse in O(n1.5) time, where n is the number of points on the sphere at which the function is sampled. This improves the naive O(n2) bound, which is the best previously known. These transforms give an O(n1.5) algorithm for convolving two functions on the sphere.
An objective scheme is presented for estimating the lunar crustal magnetic field from the LMAG (Lunar MAGnetometer) data of the SELENE ("KAGUYA") spacecraft. Our scheme improves the equivalent source method in three respects. The first improvement is that the source calculation is performed simultaneously with detrending. The second is that a great number of magnetic charges (magnetic monopoles) are used as the equivalent sources. The third is that the distribution of the magnetic charges is detremined by the damped least squares method, and the optimum smoothness is determined objectively by minimizing Akaike's Bayesian Information Criterion (ABIC). For testing the scheme, we apply it to the Lunar Prospector magnetometer data in the region centered at the Reiner Gamma magnetic anomaly. The magnetic field map at an altitude of 20 km is stably drawn from datasets for different altitudes (18 km and 34 km). The ABIC minimizing criterion successfully controls the smoothness due to the numerical damping and extracts as much information as possible from the given data. This scheme will help produce a coherent lunar magnetic anomaly map by integrating the observations from various altitudes of the SELENE and previous missions.
Despite extensive study, we do not yet fully understand the origins of the unique lunar crustal magnetism. The strength of surface fields and their relation to local geology are crucial pieces of the puzzle. However, only a few surface measurements exist, and spacecraft magnetometers cannot detect magnetization with wavelengths much smaller than the orbital altitude. Meanwhile, electron reflectometry (ER) enables a remote measurement of surface fields, but its sensitivity to magnetization with different spatial scales is not well understood. In this paper, we report on new simulations of the ER technique and its sensitivity to magnetic fields produced by simulated crustal magnetization with various strengths and spatial distributions, utilizing full particle tracing simulations and the same data analysis techniques used for space data. We find that the ER technique reliably detects surface fields from magnetization with wavelengths larger than ˜10 km but has increasingly less sensitivity to smaller wavelengths. Since the few surface measurements we have imply very incoherent near-surface magnetization, this implies that the ER technique may seriously underestimate the strength of lunar fields in some areas. Our results imply that small-scale impact-related crustal magnetization may prove even more important than previously thought.
We show presence of a mini-magnetosphere above the Reiner Gamma magnetic anomaly (RGA) region in the solar wind, using Lunar Prospector magnetometer (MAG) measurement data. RGA is one of the strongest magnetic anomalies on the Moon. Two magnetic anomalies are found from six MAG datasets at 17–40 km altitudes in the lunar wake or the geomagnetic tail lobe and are well explained by a two-dipole model. When RGA was exposed to the solar wind plasma, two MAG datasets were obtained at 27–29 km altitudes. Although the magnetic anomalies survived against the plasma pressure, they were heavily distorted in comparison with the magnetic field of the two-dipole model. Flow directions and dynamic pressures of the solar wind plasma at those periods indicate that the distortions were caused by forming a mini-magnetosphere over the RGA region in the solar wind.
Lunar Prospector (LP) electron reflectometer measurements show that
surface fields are generally weak in the large mare basalt filled impact
basins on the near side but are stronger over highland terranes,
especially those lying antipodal to young large impact basins. Between
the Imbrium and Nectaris basins, many anomalies correlate with the
Cayley and Descartes Formations. Statistical analyses show that the most
strongly magnetic nearside terranes are Cayley-type light plains, terra
materials, and pre-Imbrian craters. Light plains and terrae include
basin impact ejecta as a major component, suggesting that magnetization
effects from basin-forming impacts were involved in their formation. The
magnetization of pre-Imbrian craters, however, may be evidence of early
thermal remanence. Relatively strong, small-scale magnetic anomalies are
present over the Reiner Gamma feature on western Oceanus Procellarum and
over the Rima Sirsalis rille on the southwestern border of Procellarum.
Both Apollo subsatellite and LP data show that the latter anomaly is
nearly aligned with the rille, though LP magnetometer and reflectometer
data show that the anomaly peak is actually centered over a light plains
unit. This anomaly and the Reiner Gamma anomaly are approximately
radially aligned with the center of Imbrium, suggesting an association
with ejecta from this basin.
The high-sensitivity fluxgate Lunar MAGnetometer (LMAG) is mounted on SELENE (KAGUYA) to investigate the near-surface electromagnetic environment and the evolution of the Moon through magnetic field observation. To avoid possible electromagnetic interferences, a triaxial fluxgate sensor (MGF-S) is installed at the far end of a 12-m-long mast. It is critical for the accurate observation to monitor MGF-S alignment in orbit, and thus we have calibrated the sensor alignment by measuring the known magnetic fields generated by the sensor alignment monitor coil (SAM-C) wound onto the mast canister. In-orbit calibration of the MGF-S alignment was performed twice each revolution during the initial check-out phase of the satellite. It is concluded that there is no systematic difference in the sensor alignment between the day-side and night-side. Applying a new technique based on the Davis-Smith method to the observed magnetic field data when KAGUYA was exposed to the solar wind, a zero offset of each axis was quickly and stably determined every month. As a result, LMAG has been calibrated with an accuracy that is sufficient for detection of the lunar magnetic anomaly at an altitude of 100~km and for high-resolution electron reflectometry.
Key words: SELENE (KAGUYA), lunar magnetic field, magnetometer, in-orbit calibration.
Large amplitude, monochromatic ultra low frequency (ULF) waves were detected by MAP/LMAG magnetometer onboard Kaguya during the period from 1 January 2008 to 30 November 2008 on its orbit 100 km above the lunar surface. The dominant frequency was 8.3 × 10−3 – 1.0 × 10−2 Hz, corresponding to the periods of 120 s – 100 s. The amplitude was as large as 3 nT. They were observed in 10% of the time when the moon was in the solar wind far upstream of the Earth's bow shock. They were detected only by Kaguya on the orbit around the moon, but not by ACE in the upstream solar wind. The occurrence rate was high above the terminator and on the dayside surface. The direction of the propagation was not exactly parallel to the interplanetary magnetic field, but showed a preference to the direction of the magnetic field and the direction perpendicular to the surface of the moon below the spacecraft. The sense of rotation of the magnetic field was left-handed with respect to the magnetic field in 53% of the events, while 47% showed right-handed polarization. The possible generation mechanism is the cyclotron resonance of the magnetohydrodynamic waves with the solar wind protons reflected by the moon. The energy of the reflected protons can account for the energy of the ULF waves. The propagation direction which are not parallel to the incident solar wind flow can explain the observed frequency and the nearly equal percentages of the left-handed and right-handed polarizations.
The Holy GRAIL?
The gravity field of a planet provides a view of its interior and thermal history by revealing areas of different density. GRAIL, a pair of satellites that act as a highly sensitive gravimeter, began mapping the Moon's gravity in early 2012. Three papers highlight some of the results from the primary mission. Zuber et al. (p. 668 , published online 6 December) discuss the overall gravity field, which reveals several new tectonic and geologic features of the Moon. Impacts have worked to homogenize the density structure of the Moon's upper crust while fracturing it extensively. Wieczorek et al. (p. 671 , published online 6 December) show that the upper crust is 35 to 40 kilometers thick and less dense—and thus more porous—than previously thought. Finally, Andrews-Hanna et al. (p. 675 , published online 6 December) show that the crust is cut by widespread magmatic dikes that may reflect a period of expansion early in the Moon's history.
1] Interaction between the solar wind and objects in the solar system varies largely according to the settings, such as the existence of a global intrinsic magnetic field and/or thick atmosphere. The Moon's case is characterized by the absence of both of them. Low energy ion measurements on the lunar orbit is realized more than 30 years after the Apollo period by low energy charged particle analyzers MAP-PACE on board SELENE(KAGUYA). MAP-PACE ion sensors have found that 0.1%$1% of the solar wind protons are reflected back from the Moon instead of being absorbed by the lunar surface. Some of the reflected ions are accelerated above solar wind energy as they are picked-up by the solar wind convection electric field. The proton reflection that we have newly discovered around the Moon should be a universal process that characterizes the environment of an airless body. Citation: Saito, Y., et al. (2008), Solar wind proton reflection at the lunar surface: Low energy ion measurement by MAP-PACE onboard SELENE (KAGUYA), Geophys. Res. Lett., 35, L24205, doi:10.1029/ 2008GL036077.
Ground calibration experiments of the SELENE high sensitivity fluxgate Lunar Magnetometer (LMAG) have been performed in order to determine the alignment, sensitivity, and offset of the sensors (MGF-S). It is checked out that the sensors are orthogonal to each other within 0.4 degrees, and the linearity of the ambient magnetic field and the output from the sensors are confirmed. Also, the temperature dependences of the offset and sensitivity are examined but no clear signatures of temperature dependencies can be seen. SELENE has an in-flight calibration system in order to determine the direction of the magnetometer routinely. The magnetic fields generated by the sensor alignment monitor coil (SAM-C) system are used for the in-flight calibration. The magnetic field distributions generated by SAM-C are determined and the accuracy of determination of the magnetometer position and direction is estimated. Multiple measurements will allow us to determine the direction of MGF-S with about 0.1-degree accuracy. Appropriate corrections from the results of the ground and in-flight calibrations will allow us to recover the magnetic field near the moon with accuracy about 0.1 nT.
Plasma signature around crustal magnetic fields is one of the most important topics of the lunar plasma sciences. Although recent spacecraft measurements are revealing solar–wind interaction with the lunar crustal fields on the dayside, plasma signatures around crustal fields on the night side have not been fully studied yet. Here we show evidence of plasma trapping on the closed field lines of the lunar crustal fields in the solar–wind wake, using SELENE (Kaguya) plasma and magnetic field data obtained at 14–15 km altitude from the lunar surface. In contrast to expectation on plasma cavity formation at the strong crustal fields, electron flux is enhanced above Crisium Antipode (CA) anomaly which is one of the strongest lunar crustal fields. The enhanced electron fluxes above CA are characterised by (1) occasional bi-directional field-aligned beams in the lower energy range (<150 eV) and (2) a medium energy component (150–300 eV) that has a double loss-cone distribution representing bounce motion between the two footprints of the crustal magnetic fields. The low-energy electrons on the closed field lines may come from the lunar night side surface, while supply mechanism of medium-energy electrons on the closed field line remains to be solved. We also report that a density cavity in the wake is observed not above the strongest magnetic field but in its vicinity.
The inductive generation of magnetic fields in fluid planetary interiors is known as the dynamo process. Although the Moon today has no global magnetic field, it has been known since the Apollo era that the lunar rocks and crust are magnetized. Until recently, it was unclear whether this magnetization was the product of a core dynamo or fields generated externally to the Moon. New laboratory and spacecraft measurements strongly indicate that much of this magnetization is the product of an ancient core dynamo. The dynamo field persisted from at least 4.25 to 3.56 billion years ago (Ga), with an intensity reaching that of the present Earth. The field then declined by at least an order of magnitude by ∼3.3 Ga. The mechanisms for sustaining such an intense and long-lived dynamo are uncertain but may include mechanical stirring by the mantle and core crystallization.
Copyright © 2014, American Association for the Advancement of Science.
As first noted 200 years ago by Laplace, the Moon's rotational and tidal bulges are significantly larger than expected, given the Moon's present orbital and rotational state. This excess deformation has been ascribed to a fossil figure, frozen in when the Moon was closer to the Earth. However, the observed figure is only consistent with an eccentric and non-synchronous orbit, contrary to our understanding of the Moon's formation and evolution. Here, we show that lunar mascons and impact basins have a significant contribution to the observed lunar figure. Removing their contribution reveals a misaligned fossil figure consistent with an early epoch of true polar wander (driven by the formation of the South Pole-Aitken Basin) and an early low-eccentricity, synchronous lunar orbit. This new self-consistent model solves a long-standing problem in planetary science, and will inform future studies of the Moon's dynamical evolution and early dynamo.
The origin of the Moon's large-scale topography is important for understanding lunar geology(1), lunar orbital evolution(2) and the Moon's orientation in the sky(3). Previous hypotheses for its origin have included late accretion events(4), large impacts(5), tidal effects(6) and convection processes(7). However, testing these hypotheses and quantifying the Moon's topography is complicated by the large basins that have formed since the crust crystallized. Here we estimate the large-scale lunar topography and gravity spherical harmonics outside these basins and show that the bulk of the spherical harmonic degree-2 topography is consistent with a crust-building process controlled by early tidal heating throughout the Moon. The remainder of the degree-2 topography is consistent with a frozen tidal-rotational bulge that formed later, at a semi-major axis of about 32 Earth radii. The probability of the degree-2 shape having both tidal-heating and frozen shape characteristics by chance is less than 1%. We also infer that internal density contrasts eventually reoriented the Moon's polar axis by 36 +/- 4 degrees, to the configuration we observe today. Together, these results link the geology of the near and far sides, and resolve long-standing questions about the Moon's large-scale shape, gravity and history of polar wander.
Palaeomagnetic measurements suggest that an active core dynamo operated on the Moon from 4.2 to 3.56 billion years ago(1-3). Since the Apollo era, many magnetic anomalies have been observed on the Moon. The magnetization of the lunar crust in some of these regions could preserve the signature of an early dipolar magnetic field generated by a core dynamo. Thus, the magnetic anomalies may yield information about the position of the palaeomagnetic pole during the time that the dynamo operated. Here we present a comprehensive survey of magnetic anomalies on the lunar surface using magnetometer data(4,5) obtained by the Lunar Prospector and Kaguya lunar orbiters. We extract magnetization vectors from 24 magnetic anomalies using an iterative inversion method and derive the palaeomagnetic poles. We find that the north poles, as well as the antipodal south poles, cluster in two distinct locations: one near the present rotation axis and the other at mid-latitude. The clustering is consistent with a dipole-dominated magnetic field generated in the lunar core by a dynamo that was reversing, much like that of Earth. Furthermore, the two pole clusters imply that the Moon experienced a polar wander event during its ancient history due to the reorientation of the Moon with respect to its spin axis by 45 degrees-60 degrees.
We investigate the possibility that a strong core dynamo of the Moon has magnetized the lunar crust. The magnetic data from two missions, Lunar Prospector and Kaguya, are used and the magnetic fields of two different features are examined: The isolated small magnetic source bodies with almost no topographic signatures, and the impact craters with diameters larger than 100 km. Five data sets are examined separately for each of the isolated magnetic anomalies: the r, θ, and φ components of the Lunar Prospector data, the r component of a 150-degree spherical harmonic model of the lunar magnetic field, and the r component of the Kaguya data. The r component of the Lunar Prospector data is also used to derive the magnetic field over the impact craters. We conclude that most of the ancient lunar far side crust is heterogeneously magnetized with coherency wavelength about a few hundred km. The paleomagnetic north poles determined from modeling the magnetic field of both features show some clustering whereas the source bodies are widely distributed, suggesting that the magnetizing field may have been a core dynamo field. Paleointensity data suggest that the core field intensity was at least 1 mT at the core mantle boundary. There is also evidence for core field reversals, because further clustering occurs when the south poles of some features are considered.
We investigate Kaguya observation of ion acceleration around a lunar crustal magnetic anomaly located in the South Pole-Aitken basin at an altitude of 100 km. The accelerated ions in the 230 eV to 1.5 keV energy range were identified by a characteristic dispersion signature in the energy-time spectrogram that appeared repeatedly upon Kaguya's approach to the magnetic anomaly. The interplanetary magnetic field was almost parallel to the solar wind velocity and thus the electric field was very small. The results of our analysis show that ions with energies below 230 eV were accelerated up to 1.5 keV by an electric field produced by the interaction between the solar wind and the magnetic anomaly. We argue that the low-energy ions mainly originated from the solar wind ions with energies of 450 eV that were backscattered on the lunar surface.
Previous work has shown that the strongest concentrations of lunar
crustal magnetic anomalies are located antipodal to four large,
similarly aged impact basins (Orientale, Serenitatis, Imbrium, and
Crisium). Here, we report results of a correlation study between
magnetic anomaly clusters and geology in areas antipodal to Imbrium,
Orientale, and Crisium. Unusual geologic terranes, interpreted to be of
seismic or ejecta origin associated with the antipodal basins, have been
mapped antipodal to both Orientale and Imbrium. All three antipode
regions have many high-albedo swirl markings. Results indicate that both
of the unusual antipode terranes and Mare Ingenii (antipodal to Imbrium)
show a correlation with high-magnitude crustal magnetic anomalies. A
statistical correlation between all geologic units and regions of medium
to high magnetization when high-albedo features are present (antipodal
to Orientale) may suggest a deep, possibly seismic origin to the
anomalies. However, previous studies have provided strong evidence that
basin ejecta units are the most likely sources of lunar crustal
anomalies, and there is currently insufficient evidence to differentiate
between an ejecta or seismic origin for the antipodal anomalies. Results
indicate a strong correlation between the high-albedo markings and
regions of high magnetization for the Imbrium, Orientale, and Crisium
antipodes. Combined with growing evidence for an Imbrian age to the
magnetic anomalies, this supports a solar wind deflection origin for the
lunar swirls.
We have developed a new method for regional mapping of the lunar magnetic anomalies as the vector field at the surface using the satellite observation, that is the surface vector mapping (SVM). The SVM is based on the inverse boundary value problem with a spherical boundary surface. There are two main procedures for reducing effects of bias and noise on mapping: (1) preprocessing the data to provide first derivatives along the pass, and (2) the Bayesian statistical procedure in the inversion using Akaike’s Bayesian Information Criterion. The SVM was applied to two regions: the northwest region of the South Pole-Aitken basin as a strong magnetic anomaly region, and the southeast region of the lunar near side as a weak magnetic anomaly region. Since the results from the different datasets of the Kaguya and Lunar Prospector observations show good consistency, characteristic features of the lunar magnetic anomalies at the surface are considered to be well estimated except for components of wavelength shorter than about 1°. From the results by the SVM, both of the regions show elongation patterns of the lunar magnetic anomalies, suggesting lineated structures of the magnetic anomaly sources.
"A comprehensive review of lunar science and evolution from the
viewpoint of historical geology, based on data from both photogeologic
observations and lunar-sample analysis."
The existence of magnetization signatures and landform modification
antipodal to young lunar impact basins is investigated further by (a)
producing more detailed regional crustal magnetic field maps at low
altitudes using Lunar Prospector magnetometer data; and (b) examining
Lunar Reconnaissance Orbiter Wide Angle Camera imagery. Of the eight
youngest lunar basins, five are found to have concentrations of
relatively strong magnetic anomalies centered within 10° of their
antipodes. This includes the polar Schrödinger basin, which is one
of the three youngest basins and has not previously been investigated in
this context. Unusual terrain is also extensively present near the
antipodes of the two largest basins (Orientale and Imbrium) while less
pronounced manifestations of this terrain may be present near the
antipodes of Serenitatis and Schrödinger. The area near the Imbrium
antipode is characterized by enhanced surface thorium abundances, which
may be a consequence of antipodal deposition of ejecta from Imbrium. The
remaining three basins either have antipodal regions that have been
heavily modified by later events (Hertzsprung and Bailly) or are not
clearly recognized to be a true basin (Sikorsky-Rittenhouse). The most
probable source of the Descartes anomaly, which is the strongest
isolated magnetic anomaly, is the hilly and furrowed Descartes terrain
near the Apollo 16 landing site, which has been inferred to consist of
basin ejecta, probably from Imbrium according to one recent sample
study. A model for the origin of both the modified landforms and the
magnetization signatures near lunar basin antipodes involving shock
effects of converging ejecta impacts is discussed.
A new magnetic map of the Moon, based on Lunar Prospector magnetometer
observations, sheds light on the origin of the South Pole-Aitken basin
(SPA), the largest and oldest of the recognized lunar basins. A set of
WNW-trending linear to arcuate magnetic features, evident in both the
radial and scalar observations, covers much of a 1000 km wide region
centered on the NW portion of SPA. The source bodies are not at the
surface because the magnetic features show no first-order correspondence
to any surface topographic or structural feature. Patchy mare basalts of
possible late Imbrian-age are emplaced within SPA and are inferred to
have been emplaced through dikes, directly from mantle sources. We infer
that the magnetic features represent dike swarms that served as feeders
for these mare basalts, as evident from the location of the Thomson/Mare
Ingenii, Van de Graaff, and Leeuwenhoek mare basalts on the two largest
magnetic features in the region. Modeling suggests that the dike zone is
between 25 and 50 km wide at the surface, and dike magnetization
contrasts are in the range of 0.2 A/m. We theorize that the basaltic
dikes were emplaced in the lunar crust when a long-lived dynamo was
active. Based on pressure, temperature, and stress conditions prevalent
in the lunar crust, dikes are expected to be a dominantly subsurface
phenomenon, consistent with the observations reported here.
Magnetic fields measured by the satellite Lunar Prospector show large
scale features resulting from remanently magnetized crust. Vector data
synthesized at satellite altitude from a spherical harmonic model of the
lunar crustal field, and the radial component of the magnetometer data,
have been used to produce spatially continuous global magnetization
models for the lunar crust. The magnetization is expressed in terms of
localized basis functions, with a magnetization solution selected having
the smallest root-mean square magnetization for a given fit to the data,
controlled by a damping parameter. Suites of magnetization models for
layers with thicknesses between 10 and 50 km are able to reproduce much
of the input data, with global misfits of less than 0.5 nT (within the
uncertainties of the data), and some surface field estimates. The
magnetization distributions show robust magnitudes for a range of model
thicknesses and damping parameters, however the magnetization direction
is unconstrained. These global models suggest that magnetized sources of
the lunar crust can be represented by a 30 km thick magnetized layer.
Average magnetization values in magnetized regions are 30-40 mA/m,
similar to the measured magnetizations of the Apollo samples and
significantly weaker than crustal magnetizations for Mars and the Earth.
These are the first global magnetization models for the Moon, providing
lower bounds on the magnitude of lunar crustal magnetization in the
absence of multiple sample returns, and can be used to predict the
crustal contribution to the lunar magnetic field at a particular
location.
Many of the Moon's crustal magnetic anomalies are accompanied by high
albedo features known as swirls. A leading hypothesis suggests that
swirls are formed where crustal magnetic anomalies, acting as mini
magnetospheres, shield portions of the surface from the darkening
effects of solar wind ion bombardment, thereby leaving patches that
appear bright compared with their surroundings. If this hypothesis is
correct, then magnetic field direction should influence swirl
morphology. Using Lunar Prospector magnetometer data and Clementine
reflectance mosaics, we find evidence that bright regions correspond
with dominantly horizontal magnetic fields at Reiner Gamma and that
vertical magnetic fields are associated with the intraswirl dark lane at
Airy. We use a genetic search algorithm to model the distributions of
magnetic source material at both anomalies, and we show that source
models constrained by the observed albedo pattern (i.e., strongly
horizontal surface fields in bright areas, vertical surface fields in
dark lanes) produce fields that are consistent with the Lunar Prospector
magnetometer measurements. These findings support the solar wind
deflection hypothesis and may help to explain not only the general form
of swirls, but also the finer aspects of their morphology. Our source
models may also be used to make quantitative predictions of the near
surface magnetic field, which must ultimately be tested with very low
altitude spacecraft measurements. If our predictions are correct, our
models could have implications for the structure of the underlying
magnetic material and the nature of the magnetizing field.
A re-examination of all available low-altitude LP magnetometer data confirms that magnetic anomalies are present in at least four Nectarian-aged lunar basins: Moscoviense, Mendel–Rydberg, Humboldtianum, and Crisium. In three of the four cases, a single main anomaly is present near the basin center while, in the case of Crisium, anomalies are distributed in a semi-circular arc about the basin center. These distributions, together with a lack of other anomalies near the basins, indicate that the sources of the anomalies are genetically associated with the respective basin-forming events. These central basin anomalies are difficult to attribute to shock remanent magnetization of a shocked central uplift and most probably imply thermoremanent magnetization of impact melt rocks in a steady magnetizing field. Iterative forward modeling of the single strongest and most isolated anomaly, the northern Crisium anomaly, yields a paleomagnetic pole position at 81° ± 19°N, 143° ± 31°E, not far from the present rotational pole. Assuming no significant true polar wander since the Crisium impact, this position is consistent with that expected for a core dynamo magnetizing field. Further iterative forward modeling demonstrates that the remaining Crisium anomalies can be approximately simulated assuming a multiple source model with a single magnetization direction equal to that inferred for the northernmost anomaly. This result is most consistent with a steady, large-scale magnetizing field. The inferred mean magnetization intensity within the strongest basin sources is ∼1 A/m assuming a 1-km thickness for the source layer. Future low-altitude orbital and surface magnetometer measurements will more strongly constrain the depth and/or thicknesses of the sources.
Lunar swirls are albedo anomalies associated with strong crustal magnetic fields. Swirls exhibit distinctive spectral properties at both highland and mare locations that are plausibly explained by fine-grained dust sorting. The sorting may result from two processes that are fairly well established on the Moon, but have not been previously considered together. The first process is the vertical electrostatic lofting of charged fine dust. The second process is the development of electrostatic potentials at magnetic anomalies as solar wind protons penetrate more deeply into the magnetic field than electrons. The electrostatic potential can attract or repel charged fine-grained dust that has been lofted. Since the finest fraction of the lunar soil is bright and contributes significantly to the spectral properties of the lunar regolith, the horizontal accumulation or removal of fine dust can change a surface's spectral properties. This mechanism can explain some of the spectral properties of swirls, accommodates their association with magnetic fields, and permits aspects of weathering by micrometeoroids and the solar wind.
Three new models of the global internal magnetic field of the Moon based on Lunar Prospector (LP) fluxgate magnetometer observations are developed for use in understanding the origin of the Moon's crustal magnetic field and for modeling its interaction with the solar wind. The models are at spherical harmonic degree 170, corresponding to 64 km wavelength resolution, from 30 km mean altitude LP observations. Coverage is complete except for a few areas near each pole. Original signal amplitudes are best preserved in the sequential approach map, whereas feature recognition is superior in the coestimation and harmonic wave number correlation maps. Spherical harmonic degrees less than 15, corresponding to 666 km wavelength, are largely absent from the Moon's internal magnetic field. We interpret this bound in terms of the Moon's impact history. A derived magnetization map suggests magnetizations may locally exceed 0.2 A/m in the lunar crust at the survey resolution if the magnetic crust is as thick as 40 km.
We investigate the crustal magnetic signatures of lunar craters using Lunar Prospector (LP) electron reflectometer data. Craters of all ages often have associated magnetic lows, showing that crustal fields were present even in pre-Nectarian times (>~3.9 Ga). Magnetic lows extend to ~2-4 crater radii, suggesting shock rather than thermal demagnetization. Younger craters are more likely to have clear and complete demagnetization signatures, suggesting that many older magnetic lows have been subsequently obscured. No size dependence is found for craters larger than 50 km in diameter, suggesting that demagnetization effects for all craters in this size range completely penetrate the magnetized layer. If shock demagnetization is responsible, this suggests an upper limit of ~50 km for the depth of magnetization. Evidence of edge effects due to magnetization contrasts may show that strong far-side crustal fields are coherent at scales of ~25 km.
Unraveling the timing and duration of mare volcanism on the Moon is essential for understanding its thermal evolution. The end of mare volcanism is poorly constrained, because mare basalts are incompletely sampled. In this study, employing SELENE (Kaguya) high-resolution images, we performed new crater size-frequency measurements for 49 young mare units (<~3.0Ga) in the Procellarum KREEP Terrane (PKT), in which the latest magma eruption of the Moon occurred. Mare volcanism in this region continued until ~1.5Ga, suggesting that volcanic activity in this region ceased ~1.0Ga after the magma eruption had globally ceased 2.5-3.0Ga. Volcanic activity may have peaked 1.8-2.2Ga ago. The youngest basalts occur around the Aristarchus plateau and the Kepler crater, which are located in the central region of the PKT. It is likely that heating in the crust due to the concentration of heat-producing elements in the PKT delayed cooling of a partial-melting zone in the underlying mantle. In contrast with previous basalt dating in this region, our results indicate a higher correlation between ages and spectral types of mare basalts; the young mare units in the PKT tend to have spectral types corresponding to high titanium contents, while low titanium basalts occur mainly in the early stage. The titanium variation in mare basalts may reflect vertical heterogeneity in TiO2 content in the upper mantle beneath the PKT.
The highly magnetic (field magnitudes of 50 nT at 18 km altitude) Reiner Gamma albedo feature on the near side of the moon has been explained in terms of differential space weathering of an old feature, or a recent cometary impact. We investigated this feature using magnetometer data from Lunar Prospector. The minimum magnetization necessary to explain the magnetic field observations varies from 100 A/m for a 10 m thick layer, to 1 A/m for a 1 km thick layer. Magnetic sources appear to lie within a few km of the surface, and be magnetized in a north-south direction. The strength of the magnetization appears spatially related to the albedo of the feature. These constraints point towards an ancient origin for the magnetic field signal (possibly due to basin impact ejecta), and the origin of the albedo feature as a consequence of retarded ageing under the umbrella of the Reiner Gamma mini-magnetosphere.
A theoretical model suggests a short-lived lunar dynamo driven by a mantle overturn forming crustal thickness dichotomy, while the lunar paleomagnetic data and crustal magnetic fields suggest both of presence and absence of a global magnetic field of the Moon ˜4 billion years (Gyr) ago. Here we carry out numerical simulations of a possible lunar dynamo, including effects of mantle overturn on heat flux heterogeneity at the core-mantle boundary. As a result, the surface field intensity of the lunar dynamo is about 100 nT. In this case, a lunar magnetosphere could be present with a sunward size of 1.4 times the Moon's radius. Considering the 4.6 Gyr orbit evolution of the Moon, the magnetic field intensity and the magnetosphere size could be larger, suggesting the lunar crustal magnetization acquired in the ancient dynamo field.
Previous processing of the Lunar Prospector magnetometer (LP-MAG) data has yielded ~40% coverage of the Moon. Here, new mapping of the low-altitude LP-MAG data is reported with the goal of producing the first global vector map of the lunar crustal magnetic field. By considering all data regardless of the external plasma environment and using less restrictive editing criteria, 2360 partial and complete passes have been identified that can be used to investigate the lunar crustal magnetic anomalies. The cleanest global coverage is provided using 329 low-altitude nightside and terminator passes. An inverse power method has been used to continue the final mapping data to constant altitude. Using the 329 optimal passes, global maps of the lunar crustal magnetic field are constructed at 30 and 40 km. Consistent with previous studies: (1) the largest concentrations of anomalies are mapped antipodal to the Crisium, Serenitatis, Imbrium, and Orientale basins and (2) isolated anomalies at Reiner Gamma, Rima Sirsalis, Descartes, and Airy are mapped. Anomalies previously unmapped by the LP-MAG experiment include (1) isolated anomalies near the craters Abel and Hartwig, (2) weak magnetization within the Nectarian-aged Crisium and Moscoviense basins, and (3) a relatively weak anomaly in an area dominated by crater chains associated with the formation of Nectaris. Future work with the new low-altitude data set is discussed and will include determining whether the lunar anomalies are capable of deflecting the solar wind and investigating directions of magnetization to evaluate a possible former core dynamo.
The direction of propagation of magma-filled cracks was theoretically examined for a two-dimensional model. The analytical result indicates that magma-filled cracks, which have magmatic pressures larger than a critical value, propagate in parallel with the maximum principal direction of far-field stress. This may give one of theoretical grounds to the dike method by which the regional stress field is estimated.
We have used multispectral images from Clementine and data from Lunar Prospector's magnetometer to conduct a survey of lunar crustal magnetic anomalies, prominent lunar swirls, and lesser known swirl markings to provide new information on the nature of swirls and their association with magnetic anomalies. We find that all swirls and swirl-like albedo patterns are associated with areas of magnetized crust, but not all areas of magnetized crust are colocated with swirl-like albedo anomalies. All observed swirls exhibit spectral characteristics similar to immature material and generally have slightly lower FeO values compared with their surroundings as determined with a multispectral iron-mapping method. We discuss these results in relation to the various hypotheses for swirl formation. The comet impact hypothesis for lunar swirls would not predict a difference in the spectrally determined FeO content between swirls and nearby ordinary surfaces. The compositional difference could be explained as a consequence of (1) magnetic shielding of the surface from the solar wind, which could produce anomalous space weathering (little darkening with limited reddening) and potentially alter the predictions of the multispectral iron-mapping algorithm while the compositional contrast could be enhanced by delivery of lower-FeO ejecta from outside the swirl; and (2) accumulation of fine plagioclase-rich dust moving under the influence of electric fields induced by solar wind interactions with a magnetic anomaly. Therefore, we cannot at present clearly distinguish between the solar wind shielding and electrostatic dust accumulation models for swirl formation. We describe future measurements that could contribute to solution of the puzzle of swirl origin.
Twenty-one orbits of Lunar Prospector magnetometer data obtained during an extended passage of the Moon through a lobe of the geomagnetic tail in April 1998 are applied to estimate the residual lunar induced magnetic dipole moment. Editing and averaging of individual orbit segments yields a negative induced moment with amplitude −2.4 ±1.6 × 1022 Gauss-cm³ per Gauss of applied field. Assuming that the induced field is caused entirely by electrical currents near the surface of a highly electrically conducting metallic core, the preferred core radius is 340±90 km. For an iron-rich composition, such a core would represent 1 to 3% of the lunar mass.
We report on ages derived from impact crater counts for exposed mare basalt units in the northern part of the lunar nearside hemisphere (Mare Frigoris), the eastern and northeastern part of the nearside hemisphere (Lacus Temporis, Joliot, Hubble, Goddard, Mare Marginis, and Mare Smythii), the central part of the nearside hemisphere (Palus Putredinis, Mare Vaporum, and Sinus Medii), and the southwestern part of the nearside hemisphere (Grimaldi, Crüger, Rocca A, Lacus Aestatis, and Schickard). In Mare Frigoris, we dated 37 basalt units, showing ages from 2.61 to 3.77 Gyr, with most units being formed in the late Imbrian period between 3.4 and 3.8 Gyr ago. In Mare Vaporum we dated six spectrally homogeneous units that show model ages of 3.10 to 3.61 Gyr. Our model ages of basalts in Mare Marginis range from 3.38 to 3.88 Gyr and are mostly older than basalts in Mare Smythii (3.14-3.48 Gyr). The model ages of four units in Sinus Medii indicate that the basalts in this region formed 3.63 to 3.79 Gyr ago. We find an excellent agreement of our crater size-frequency model ages of the Palus Putredinis area, which contains the Apollo 15 landing site, with the radiometric ages of Apollo 15 samples. According to our crater counts, basalts in Palus Putredinis are 3.34 Gyr old and this compares favorably with the radiometric ages of 3.30-3.35 Gyr of the olivine-normative and quartz-normative basalts of the Apollo 15 landing site. Lacus Aestatis is a small irregular-shaped mare patch in the southwestern nearside and shows an Imbrian age of 3.50 Gyr; basalts in Lacus Temporis in the northeastern nearside formed between 3.62 and 3.74 Gyr ago and are, therefore, older than the basalts in Lacus Aestatis. We found that basalts in craters of the southwestern nearside (Schickard, Grimaldi, Crüger, and Rocca A) are also mostly younger than basalts in craters of the northeastern nearside (Hubble, Joliot, and Goddard). While basalt ages vary between 3.16 and 3.75 Gyr in the southwest, basalts in the northeast are 3.60-3.79 Gyr old. These results confirm and extend the general distribution of ages of mare basalt volcanism and further underline the predominance of older mare basalt ages in the eastern and southern nearside and in patches of mare peripheral to the larger maria, in contrast to the younger basalt ages on the western nearside (Oceanus Procellarum).
Maps of relatively strong crustal magnetic field anomalies detected at low altitudes with the magnetometer instrument on Lunar Prospector are presented. On the lunar nearside, relatively strong anomalies are mapped over the Reiner Gamma Formation on western Oceanus Procellarum and over the Rima Sirsalis rille on the southwestern border of Oceanus Procellarum. The main Rima Sirsalis anomaly does not correlate well with the rille itself but is centered over an Imbrian-aged smooth plains unit interpreted as primary or secondary basin ejecta. The stronger Reiner Gamma anomalies correlate with the locations of both the main Reiner Gamma albedo marking and its northeastward extension. Both the Rima Sirsalis and the Reiner Gamma anomalies are extended in directions approximately radial to the center of the Imbrium basin. This alignment suggests that Imbrium basin ejecta materials (lying in many cases beneath the visible mare surface) are the sources of the nearside anomalies. If so, then the albedo markings associated with the stronger Reiner Gamma anomalies may be consistent with a model involving magnetic shielding of freshly exposed mare materials from the solar wind ion bombardment. Two regions of extensive magnetic anomalies are mapped in regions centered on the Ingenii basin on the south central farside and near the crater Gerasimovic on the southeastern farside. These regions are approximately antipodal to the Imbrium and Crisium basins, respectively. The Imbrium antipode anomaly group is the most areally extensive on the Moon, while the largest anomaly in the Crisium antipode group is the strongest detected by the Lunar Prospector magnetometer. A consideration of the expected antipodal effects of basin-forming impacts as well as a combination of sample data and orbital measurements on the nearside leads to the conclusion that the most probable sources of magnetic anomalies in these two regions are ejecta materials from the respective impacts. In both regions the strongest individual anomalies correlate with swirl-like albedo markings of the Reiner Gamma class visible on available orbital photography.
In previous theories concerning auroras and magnetic storms,
interplanetary space was assumed to be void. Because of the recent accrual of
evidence, which shows that there exists an interplanetary gas with densities of
100 to 1000 particles/cc, theories are proposed for the commencement and for the
first phase of a magnetic storm. The only mcdification necessary for correcting
a previous theory is that the geomagnetic impulse caused by the induced currents
in the stream is propagated, not with the speed of light as was originally
supposed, but with a finite speed which in the neighborhood of the earth
approximates to the Alfven velocity. The original theories are outlined along
with the modifications to the theory of the first phase of a magnetic storm.
Consideration is given for the motion of the quasi-shock wave in the neighborhood
of the earth caused by the rapid increase of the earth's magnetic field.
Illustrations are given from which it is seen that the disturbance is propagated
through the interplanetury gas with the local Alfven velocity. The time of
propagation of the disturbance through the gas is derived. (B.O.G.)
We have attempted to constrain the lunar core size from electrical conductivity sounding by using magnetic field data from the Kaguya (KG) and Lunar Prospector (LP) satellites. As suggested by previous studies, the signature of induction in the core can sometimes be detected by the satellites as an internal dipole field when the Moon enters from the magnetosheath to the tail lobe of the Earth’s magnetosphere. Since the magnetic anomaly field is up to about 2 nT at the orbital altitude (∼100 km), we removed the anomaly field from the observed magnetic field on the basis of the magnetic anomaly analysis. A spherical harmonic analysis tuned for the satellite observations was applied for separation of the internal and external fields, and the internal to external dipole ratio was used to estimate the size of the core. We estimated the effect of lunar mantle induction due to the external field fluctuation in the tail lobe region for two typical conductivity models of the lunar interior. Simulation results show that mantle induction fields seriously disturb the core signal. However, data selection made with reference to statistical distribution of the dipole ratio is effective to minimize the influence of the mantle conductivity on the core size estimates. The internal to external dipole ratio obtained for the selected period of KG observation was 0.0023 ± 0.0019. The mean value corresponded to a lunar core size of 290 km, and the upper bound of the lunar core size was estimated to be about 400 km with a 95% confidence limit. On the other hand, it was not possible to select a time interval that was suitable for successful application of the method adopted in the present work to the LP dataset. Larger external magnetic field variation during the period of LP observation caused this, although the stronger magnetic field at the LP observation period was expected to be suitable for the core size estimation. The larger external field variation during the period of LP observation was a consequence of moderately active solar conditions in 1998 relative to the deep solar minimum conditions in 2008.
The concept of lunar paleomagnetism is reviewed and its implications for
lunar science is discussed. The topics considered include the remanent
magnetism of the moon, the paleointensities of the lunar field, the
existence of a lunar core, paleomagnetic directions, reorientation of
the moon, paleopoles and paleoequators, primeval satellites of the moon,
and energy sources of the primeval moon.
The magnetic field around the Moon has been successfully observed at a nominal altitude of ∼100 km by the lunar magnetometer (LMAG) on the SELENE (Kaguya) spacecraft in a polar orbit since October 29, 2007. The LMAG mission has three main objectives: (1) mapping the magnetic anomaly of the Moon, (2) measuring the electromagnetic and plasma environment around the Moon and (3) estimating the electrical conductivity structure of the Moon. Here we review the instrumentation and calibration of LMAG and report the initial global mapping of the lunar magnetic anomaly at the nominal altitude. We have applied a new de-trending technique of the Bayesian procedure to multiple-orbit datasets observed in the tail lobe and in the lunar wake. Based on the nominal observation of 14 months, global maps of lunar magnetic anomalies are obtained with 95% coverage of the lunar surface. After altitude normalization and interpolation of the magnetic anomaly field by an inverse boundary value problem, we obtained full-coverage maps of the vector magnetic field at 100 km altitude and the radial component distribution on the surface. Relatively strong anomalies are identified in several basin-antipode regions and several near-basin and near-crater regions, while the youngest basin on the Moon, the Orientale basin, has no magnetic anomaly. These features well agree with characteristics of previous maps based on the Lunar Prospector observation. Relatively weak anomalies are distributed over most of the lunar surface. The surface radial-component distribution estimated from the inverse boundary value problem in the present study shows a good correlation with the radial component distribution at 30 km altitude by Lunar Prospector. Thus these weak anomalies over the lunar surface are not artifacts but likely to be originated from the lunar crustal magnetism, suggesting possible existence of an ancient global magnetic field such as a dynamo field of the early Moon. The possibility of the early lunar dynamo and the mechanism of magnetization acquisition will be investigated by a further study using the low-altitude data of the magnetic field by Kaguya.
Bayesian spline regression for 1-D time series is useful for smoothing palaeointensity data. This method, however, is not applicable to directional data composed of inclination and declination. In the present study a new spline regression, based on Bayesian statistics, is developed for smoothing a time series of unit vectors subjected to Fisher distribution. An optimal smoothing factor is determined by minimizing a Bayesian information criterion (ABIC) in this new method. These methods are applied to archaeomagnetic data in Japan, and the X, Y and Z components are estimated in the absolute scale for the time span of 200–2000 yr BP.
We investigate the energy of arrangements of N points on the surface of a sphere in R 3 , interacting through a power law potential V = r α , −2 < α < 2, where r is Euclidean distance. For α = 0, we take V = log(1/r). An area-regular partitioning scheme of the sphere is devised for the purpose of obtaining bounds for the extremal (equilibrium) energy for such points. For α = 0, finer estimates are obtained for the dominant terms in the minimal energy by considering stereographical projections on the plane and analyzing certain logarithmic potentials. A general conjecture on the asymptotic form (as N → ∞) of the extremal energy, along with its supporting numerical evidence, is presented. Also we introduce explicit sets of points, called "generalized spiral points", that yield good estimates for the extremal energy. At least for N ≤ 12, 000 these points provide a reasonable solution to a problem of M. Shub and S. Smale arising in complexity theory.
Abstract— We survey the magnetic fields of lunar multi-ring impact basins using data from the electron reflectometer instrument on the Lunar Prospector spacecraft. As for smaller lunar craters, the primary signature is a magnetic low that extends to ˜1.5–2 basin radii, suggesting shock demagnetization of relatively soft crustal magnetization. A secondary signature, as for large terrestrial basins, is the presence of central magnetic anomalies, which may be due to thermal remanence in impact melt rocks and/or shock remanence in the central uplift. The radial extent of the anomalies may argue for the former possibility, but the latter or a combination of the two are also possible. Central anomaly fields are absent for the oldest pre-Nectarian basins, increase to a peak in early Nectarian basins, and decrease to a low level for Imbrian basins. If basin-associated anomalies provide a good indication of ambient magnetic fields when the basins formed, this suggests the existence of a “magnetic era” (possibly due to a lunar core dynamo) similar to that implied by paleointensity results from returned lunar samples. However, the central basin anomalies suggest that the fields peaked in early Nectarian times and were low in Imbrian times, while samples provide evidence for high fields in Nectarian and early Imbrian times.