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Global spherical harmonic models of the internal mgnetic field of the Moon based on sequential and coestimation approaches
Abstract
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.
... • Figure S2 • Figure S3 • Figure S4 • Figure S5 • Figure S6 • Figure S7 samples, form the basis for contemporary global analysis of lunar magnetism. Analysis of these data sets using advanced data reduction and modeling techniques (Purucker & Nicholas, 2010;Tsunakawa et al., 2015) have led to numerous regional studies and interpretations (e.g., Arkani-Hamed & Boutin, 2014Hemingway & Garrick-Bethell, 2012;Nayak et al., 2017;Purucker et al., 2012;Wieczorek, 2018;Wieczorek et al., 2012). ...
... The PDS data were converted to latitude; longitude; altitude; and B r , B θ , B ϕ components (r = outward; θ = southward; and ϕ = eastward as in the usual spherical coordinate system). We processed these data in multiple ways, and eventually settled on data sets either from the lunar wake with respect to the solar wind or in the Earth's magnetotail when the spacecraft was within 20°with respect to the opposite side of the Sun (similar to Purucker & Nicholas, 2010). We also used their procedure to fit and remove lunar internal and external field dipole terms (Purucker & Nicholas, 2010). ...
... We processed these data in multiple ways, and eventually settled on data sets either from the lunar wake with respect to the solar wind or in the Earth's magnetotail when the spacecraft was within 20°with respect to the opposite side of the Sun (similar to Purucker & Nicholas, 2010). We also used their procedure to fit and remove lunar internal and external field dipole terms (Purucker & Nicholas, 2010). The wake/tail selection is important because crustal magnetic field lines are significantly compressed due to solar wind pressure (similar to pressure balance at the bow shock, de Pater & Lissauer, 2015; Hood & Schubert, 1980) for data taken directly in the solar wind. ...
We use L1‐norm model regularization of |Br| component at the surface on magnetic monopoles bases and along‐track magnetic field differences alone (without vector observations) to derive high quality global magnetic field models at the surface of the Moon. The practical advantages to this strategy are the following: monopoles are more stable at closer spacing in comparison to dipoles, improving spatial resolution; L1‐norm model regularization leads to sparse models which may be appropriate for the Moon which has regions of localized magnetic field features; and along‐track differences reduce the need for ad‐hoc external field noise reduction strategies. We examine also the use of Lunar Prospector and SELENE/Kaguya magnetometer data, combined and separately, and find that the Lunar Prospector along‐track vector field differences lead to surface field models that require weaker regularization and, hence, result in higher spatial resolution. Significantly higher spatial resolution (wavelengths of roughly 25–30 km) and higher amplitude surface magnetic fields can be derived over localized regions of high amplitude anomalies (due to their higher signal‐to‐noise ratio). These high‐resolution field models are also compared with the results of Surface Vector Mapping approach of Tsunakawa et al. (2015, https://doi.org/10.1002/2014JE004785). Finally, the monopoles‐ as well as dipoles‐based patterns of the Serenitatis high amplitude magnetic feature have characteristic textbook patterns of Br and Bθ component fields from a nearly vertically downwardly magnetized source region and it implies that the principal source of the anomaly was formed when the region was much closer to the north magnetic pole of the Moon.
... While the Moon is to first order a simple plasma absorbing object with no global upstream bow shock, lunar crustal magnetic fields can significantly perturb this picture up to regional scales. Lunar crustal magnetic fields are heterogeneously dispersed across the lunar surface on scales ranging from one to thousands of kilometers with strengths up to at least hundreds of nanoTesla [Hood et al., 1981;Halekas et al., 2001;Mitchell et al., 2008;Purucker and Nicholas, 2010]. Previous in situ measurements by spacecraft have observed a wealth of plasma phenomena associated with lunar crustal magnetic anomalies, including reflected proton populations [Futaana et al., 2003;Saito et al., 2010;Lue et al., 2011;Halekas et al., 2013], ambipolar electrostatic fields [Saito et al., 2012], limb compressions and/or shocks [Russell and Lichtenstein, 1975;Halekas et al., 2006aHalekas et al., , 2014, and a variety of electromagnetic waves [e.g., Halekas et al., 2006bHalekas et al., , 2008Tsugawa et al., 2011;Harada et al., 2015]. ...
... where we have integrated over the scattered proton velocity vector, v. Figure 3 shows the results of the backward Liouville tracing algorithm for the single P1 lunar flyby shown in Figure 1, including (a) the lunar surface crustal magnetic field strength [Purucker and Nicholas, 2010], and the 5° × 5° spatial distribution of (b) the total number of particle trajectories backtraced from ARTEMIS P1 to the lunar surface and (c) the reflected proton flux relative to the incident solar wind flux. Figure 3b shows that for the 2 July 2014 flyby, particle trajectories traced back from ARTEMIS P1 to a broad swath of the moon spanning longitudes between 50° and 180° and latitudes between −70° and 40° with the highest concentration of locations centered under the ARTEMIS periselene location near selenographic latitude/longitude of [−10°, 140°]. ...
... Backtracing results for ARTEMIS P1 observations on 2 July 2014 (Figure 1): (a) The surface crustal magnetic field strength[Purucker and Nicholas, 2010], (b) the number of observations traced back to each 5 × 5° latitude and longitude bin, and (c) the reflected proton flux relative to the incident solar wind flux. Red labels inFigure 3adenote prominent magnetic anomalies: Mare Marginis (MM), South Pole/Aitken Basin (SPA), Gerasimovich (GER), and Reiner Gamma (RG). ...
Despite their small scales, lunar crustal magnetic fields are routinely associated with observations of reflected and/or back-streaming populations of solar wind protons. Solar wind proton reflection locally reduces the rate of space weathering of the lunar regolith, depresses local sputtering rates of neutrals into the lunar exosphere, and can trigger electromagnetic waves and small-scale collisionless shocks in the near-lunar space plasma environment. Thus, knowledge of both the magnitude and scattering function of solar wind protons from magnetic anomalies is crucial in understanding a wide variety of planetary phenomena at the Moon. We have compiled 5.5 years of ARTEMIS observations of reflected protons at the Moon and used a Liouville tracing method to ascertain each proton's reflection location and scattering angles. We find that solar wind proton reflection is largely correlated with crustal magnetic field strength, with anomalies such as South Pole/Aitken Basin (SPA), Mare Marginis, and Gerasimovich reflecting on average 5-12% of the solar wind flux while the un-magnetized surface reflects between 0.1-1% in charged form. We present the scattering function of solar wind protons off of the SPA anomaly, showing that the scattering transitions from isotropic at low solar zenith angles to strongly forward-scattering at solar zenith angles near 90° . Such scattering is consistent with simulations that have suggested electrostatic fields as the primary mechanism for solar wind proton reflection from crustal magnetic anomalies.
... Based on data from the above 2 lunar orbital magnetic satellites, several lunar magnetic field models have been constructed using different techniques. Purucker and Nicholas [53] developed a spherical harmonic crustal model up to degree 170, corresponding to a wavelength resolution of 64 km utilizing LP measurements with a mean altitude of 30 km. ...
... The contribution from the external field often surpasses that of the internal field in many parts of the Moon in the orbital magnetic measurements. At present, the most commonly employed method to remove external field interference is to select satellite measurements within the lunar wake, occurring when the Moon is in the solar wind or within Earth's magnetotail [52,53]. However, this approach limits the spatial coverage of available data, and despite its implementation, the interference of the external field cannot be completely eliminated when constructing an accurate lunar magnetic field model. ...
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.
... Two polar orbital missions, Lunar Prospector (LP) launched in 1998 and the SELENE (Kaguya, KG) mission launched in 2007, provide a nearly global set of vector magnetometer measurements at relatively low altitudes Lin et al., 1998;Tsunakawa et al., 2010). While valuable large-scale maps of the crustal field have already been produced using these data (e.g., Purucker & Nicholas, 2010;Ravat et al., 2020;Richmond & Hood, 2008;, it is argued here that maps of improved accuracy over individual regions can be constructed by selecting only the best magnetometer measurements (lowest altitude with least amount of external field contamination) over those specific regions. Once the best measurements are identified, an equivalent source dipole (ESD) technique (e.g., Mayhew, 1979;von Frese et al., 1981) can be applied to normalize the measurements to a constant altitude. ...
... Most general characteristics of the field maps in Figures 5 and 6 are consistent with those of previous largescale maps of the lunar crustal field (e.g., Purucker & Nicholas, 2010;Ravat et al., 2020;. For example, fields are relatively weak across the north-central near side in an area occupied by Mare Imbrium and Oceanus Procellarum. ...
A new large‐scale map of the lunar crustal magnetic field at 30 km altitude covering latitudes from 65°S to 65°N has been produced using high‐quality vector magnetometer data from two complementary polar orbital missions, Lunar Prospector and SELENE (Kaguya). The map has characteristics similar to those of previous maps but better resolves the shapes and distribution of weaker anomalies. The strongest group of anomalies is located on the northwest side of the South Pole‐Aitken basin approximately antipodal to the Imbrium basin. On the near side, both strong isolated anomalies and weaker elongated anomalies tend to lie along lines oriented radial to Imbrium. These include named anomalies such as Reiner Gamma, Hartwig, Descartes, Abel, and Airy. The statistical significance of this tendency for elongated anomalies is verified by Monte Carlo simulations. Great circle paths determined by end points of elongated anomaly groups and the locations of five individual strong anomalies converge within the inner rim of Imbrium and intersect within the Imbrium antipode zone. Statistically significant evidence for similar alignments northwest of the Orientale basin is also found. The observed distribution of anomalies on the near side and the location of the strongest anomaly group antipodal to Imbrium are consistent with the hypothesis that iron from the Imbrium impactor was mixed into ejecta that was inhomogeneously deposited downrange in groups aligned radial to the basin and concentrated antipodal to the basin.
... LMAs on the dayside with greater reflection of the solar wind ions (Lue et al. 2011) may be the source regions of the plasma cloud ions. We selected two LMAs as the Figure 3 shows the lunar map of contours of the 2 nT magnetic field at an altitude of 30 km (Purucker & Nicholas 2010) in the selenographic (SEL) frame. The two triangles at (lon: 210°, lat: −15°) and (lon: 166°, lat: −2.5°) denote the selected source regions of reflected ions in cases one and two. ...
... On the lunar magnetic map(Purucker & Nicholas 2010), the potential source regions of cases one and two are shown as red and blue triangles, respectively. ...
Because of the plasma pressure gradient across the lunar wake boundary, the interplanetary magnetic field is
enhanced in the lunar wake. In previous works, the solar wind ions that enter into the near lunar wake are found to
have an unimportant influence on the magnetic field within the lunar wake. In this study, two cases of a 22%–70%
reduction of the magnetic field in the near lunar wake are first observed by the Acceleration, Reconnection,
Turbulence and Electrodynamics of the Moon’s Interaction with the Sun mission (ARTEMIS). The magnetic field
depressions are caused by the refilling of dense plasma clouds (with densities of 0.20–0.47 cm−3
) into the near
lunar wake. The ions of the plasma clouds that originate from the reflected solar wind ions in the lunar dayside are
accelerated into the near lunar wake by the solar wind convection electric field. The source regions of the reflected
ions can be traced back to the lunar magnetic anomalies over the sunlit surface. Pressure (magnetic pressure +
thermal pressure) balances are roughly maintained for both cases at the boundary between the plasma cloud and the
ambient plasma. Our results imply that the interaction between the solar wind and lunar magnetic anomalies
drastically disturbs the near-Moon electromagnetic environment
... 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)
... However, this lowest-order description is enriched by the presence of lunar crustal magnetic anomalies (LCMAs), which are distributed in a complex and multipolar pattern over the Moon's surface (Coleman et al. 1972;Purucker & Nicholas 2010). The scale size of these anomalies varies between 1 and 1000 km (Dyal et al. 1974) with surface magnetic field strengths of up to a few hundred nT, falling to a few nT at 100 km altitude. ...
Lunar crustal magnetic anomalies (LCMA) are sub-ion-gyroradius structures that have been shown to stand off the solar wind (SW) plasma from the Moon’s surface, forming shock-like discontinuities and reflecting incident SW protons. In this Letter, the results of high-resolution, 2D fully kinetic simulations show a bursty electron-only magnetic reconnection in the SW-LCMA interaction region, characterized by the quasiperiodic formation and ejection of magnetic islands and strong parallel electron flows along the X-point separator lines. The islands are observed to modify the magnetic pressure pileup and Hall electric field above the LCMA, leading to sharp increases in reflected protons that drive electromagnetic fluctuations downstream and short distances upstream in the SW.
... 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). ...
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.
... In one technique (Richmond and Hood 2008;Hood 2011;Hood et al 2013;Hood and Spudis 2016) the field measurements are extrapolated to a constant altitude and data are retained only where the external fields are slowly varying and of low amplitude. Purucker (2008) and Purucker and Nicholas (2010) alaternatively used two different modeling techniques to remove the external fields in order to create global models from the Lunar Prospector magnetometer data. The final models were expressed using a spherical harmonic representation with a maximum spherical harmonic degree of 170, which resolves wavelengths as small as 64 km. ...
... Spherical harmonic models have previously been used in studying small particles' shapes, particularly aggregates for use in concretes (e.g., Garboczi 2002), without the need to record individual locations for each point on the particle surface. They have also been used extensively in a range of planetary applications, particularly in modeling small body shapes (e.g., Barnouin et al. 2019;Zuber et al. 2000), planetary topography (Wieczorek 2015), and gravitational (e.g., Zuber et al. 2013) and magnetic fields (e.g., Purucker 2008; Purucker & Nicholas 2010). The use of the spherical harmonic series allows for an arbitrary grid to be chosen for evaluating the surface's shape, and the geodesic grid helps correct any sampling irregularities that may be present in the original scans owing to complex, difficult-to-scan surfaces or other inherent difficulties with the 3D scanner. ...
In this study, we set out to explore the relationship between fracture roughness and sample strength. We analyze 45 fragments of Aba Panu, Allende, and Tamdakht, three meteorites that have been strength-tested to disruption, to determine whether their shape or texture is correlated with measured compressive strength. A primary goal is to understand whether these exterior properties correlate with more challenging strength-related measurements. We first scan the samples and construct high-fidelity 3D models. The gradient-based angularity index AI g and the rms slope roughness metric θ rms are applied to all nine samples, and their validity and any correlation between them are analyzed. We find that different sample subsets show significant variation in both correlation strength and direction. We also find AI g to be of questionable validity in its application to highly angular samples. Based on our methodology and results, we do not find sufficient separation between the roughness values of samples to allow distinct identification of the three meteorites based on roughness alone. Additionally, neither metric shows a strong correlation with the strength of individual fragments. We do find, however, that the spread of the fragment strength distribution within a given meteorite has some correlation with its average roughness metric. Increased fragment roughness may imply greater structural heterogeneity and therefore potentially weaker behavior at larger sizes. We only have significant data sets for two meteorites, however, which are insufficient to correlate meteorite fracture roughness to meteorite strength in any simple way.
... 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.
... However, remanent magnetization measured in the crust by satellites [e.g. Mitchell et al., 2008;Purucker and Nicholas, 2010] and on lunar rock samples collected during the Apollo missions [e.g. Weiss and Tikoo, 2014] both suggest that a dynamo was operating in the past. ...
The Cassini state equilibrium associated with the precession of the Moon predicts that the mantle, fluid core and solid inner core precess at different angles. We present estimates of the dissipation from viscous friction associated with the differential precession at the core-mantle boundary (CMB), $Q_{cmb}$, and at the inner core boundary (ICB), $Q_{icb}$, as a function of the evolving lunar orbit. We focus on the latter and show that, provided the inner core was larger than 100 km, $Q_{icb}$ may have been as high as $10^{10}-10^{11}$ W for most of the lunar history for a broad range of core density models. This is larger than the power required to maintain the fluid core in an adiabatic state, therefore the heat released by the differential precession at the ICB can drive a past lunar dynamo by thermal convection. This dynamo can outlive the dynamo from precession at the CMB and may have shutoff only relatively recently. Estimates of the magnetic field strength at the lunar surface are of the order of a few $\mu$T, compatible with the lunar paleomagnetic intensities recorded after 3 Ga. We further show that it is possible that a transition of the Cassini state associated with the inner core may have occurred as a result of the evolution of the lunar orbit. The heat flux associated with $Q_{icb}$ can be of the order of a few mW m$^{-2}$, which should slow down inner core growth and be included in thermal evolution models of the lunar core.
... They have provided excellent lunar magnetic field data at the various altitudes during their mission phase. Two global magnetic spherical harmonics models of the lithospheric field were produced from the observational data (Purucker & Nicholas 2010;Tsunakawa et al. 2015). KMAG may find it difficult to detect the lunar lithospheric magnetic field because the two models and past observation data suggest the presence of less than 1 nT of the lunar magnetic field at an altitude of 100 km, which may experience interference from the interplanetary magnetic field (IMF). ...
Kplo-MAGnetometer (KMAG) is one of the scientific instruments of Korea Pathfinder Lunar Orbiter (KPLO) set to be launched in 2022. Its objectives are magnetic field investigation and technical demonstration near the surface of the Moon. Specifically, it will investigate the lithospheric magnetism of the Moon and measure the electromagnetic wave properties near the lunar surface. It consists of three fluxgate magnetometers on a 1.2 m long boom, which is relatively shorter than the boom used in other missions. The three magnetometers are included for scientific measurements, redundancy checks, and multi-sensor technical investigation. The magnetometers and an inner Anisotropic Magneto-Resistive sensor perform simultaneous sampling to correct for the magnetic field interference caused by the spacecraft. The fully integrated flight model assembly showed that the magnetometer noise level was less than 30 pT Hz−1/2 at 1 Hz and stability was within ±0.2 nT at the 10 Hz sampling rate. This paper describes the configuration and performance of the KMAG using the multi-sensing method. KPLO, THEMIS-ARTEMIS spacecraft, and Commercial Lunar Payload Service modules will be in their operational phase simultaneously. Therefore, the KMAG will be able to contribute to multi-site in-situ measurements of the lunar magnetic field. We expect that the KMAG will provide an up-to-date lunar observation data set and an opportunity to perform the multi-sensor observation. © 2021. The Astronomical Society of the Pacific. All rights reserved.
... Understanding these lunar magnetic anomalies could provide clues as to (i) geodynamic history and existence of a lunar core dynamo; (ii) the magnetic effects of large-scale impacts; and (iii) the role of solar wind ion bombardment in producing space weathering on minerals. Recent magnetometer data has been acquired by the Lunar Prospector (1998)(1999) and Kaguya (SELENE) (2008-2009) (Mitchell et al. 2008;Purucker & Nicholas 2010;Tsunakawa et al. 2010). However, penetrators with magnetometer instruments could be useful with respect to localized magnetic anomalies, and to better understand magnetic-oriented mineralogy. ...
Fundamental scientific objectives concerning the surface and subsurface material and dynamics of the Moon are the drivers for the use and advancement of penetrators, which emplace a suite of scientific instruments by impact into a planetary surface, typically at velocities of dozens to hundreds of meters per second. Small lunar penetrators are poised to become a valuable new tool for lunar science and exploration during the next decade. These low-cost ballistic probes can be deployed in large numbers from orbit, or from descending robotic or crewed vehicles, in order to explore and characterize the diversity of extreme lunar shallow subsurface environments. In this paper, we describe the general overview of penetrator objectives, potential instrumentation, and how these would benefit the advancement of lunar science at various extreme environments.
... Using this method in practice at lunar magnetic anomalies, we first obtain a best-fit model from the Tsunakawa et al. (2015) lunar magnetic field model (surface vector mapping [SVM] method). Other magnetic field maps have been published (e.g., Purucker and Nicholas 2010;Ravat et al., 2020), but we expect the SBR values for each of these datasets to be similar, such that our uncertainty estimates will also be similar. We then create synthetic random background-field-added datasets (such that the SBR is similar to the real-world SBR, within ± 1 SBR units) in the manner described in Section 2.1. ...
Studies of lunar paleopoles have been used to make a variety of inferences about past episodes of true polar wander and the orientation of the ancient dynamo field. However, the large and variable uncertainties commonly reported for such studies make robust conclusions difficult. To make further progress, we used synthetic magnetic anomalies to assess a common method to estimate magnetization direction uncertainty. We find that with this method, magnetic anomalies with higher inclinations have systematically higher uncertainties than lower inclination anomalies. We call this effect inclination bias. A similar effect is found for declination, but it is weaker. We also find that this method often produces overly conservative uncertainty estimates. To avoid these effects, we use Monte Carlo methods to determine magnetization direction uncertainty. We apply our methods to five lunar magnetic anomalies with a wide range of reported magnetization directions and paleopole locations. We find that inclination bias partly explains the previously reported anomalously high and low direction uncertainties for two of these anomalies: Reiner Gamma and Airy. Our more robust uncertainties allow us to conclude that four paleopoles are located near the equator. Such low latitudes cannot be explained by true polar wander inferred from other independent datasets, such as the lunar gravity field and the polar hydrogen distribution. This in turn implies that the dynamo axis was once offset from the spin axis.
... The Moon does not currently possess a global magnetic field generated by dynamo action. However, remanent magnetization measured in the crust by satellites (e.g., Mitchell et al., 2008;Purucker & Nicholas, 2010) and on lunar rock samples collected during the Apollo missions (e.g., both suggest that a dynamo was operating in the past. Paleomagnetic analyses on Apollo samples indicate that a dynamo characterized by high surface intensities of several tens of μT to perhaps as high as 120 μT operated early in the lunar history between about 4.25 and 3.56 Ga (Cournède et al., 2012;Garrick-Bethell et al., 2009Tikoo et al., 2012;Shea et al., 2012;Suavet et al., 2013), although the accuracy of the very high paleointensity values have been called into question (e.g., Lepaulard et al., 2019). ...
The Cassini state equilibrium associated with the precession of the Moon predicts that the mantle, fluid core, and solid inner core precess at different angles. We present estimates of the dissipation from viscous friction associated with the differential precession at the core‐mantle boundary (CMB), Qcmb, and at the inner core boundary (ICB), Qicb, as a function of the evolving lunar orbit. We focus on the latter and show that, provided the inner core was larger than 100 km, Qicb may have been as high as 10¹⁰–10¹¹ W for most of the lunar history for a broad range of core density models. This is larger than the power required to maintain the fluid core in an adiabatic state; therefore, the heat released by the differential precession at the ICB can drive a past lunar dynamo by thermal convection. This dynamo can outlive the dynamo from precession at the CMB and may have shut off only relatively recently. Estimates of the magnetic field strength at the lunar surface are of the order of a few μT, compatible with the lunar paleomagnetic intensities recorded after 3 Ga. We further show that it is possible that a transition of the Cassini state associated with the inner core may have occurred as a result of the evolution of the lunar orbit. The heat flux associated with Qicb can be of the order of a few mW m⁻², which should slow down inner core growth and be included in thermal evolution models of the lunar core.
... The Moon possesses small-scale crustal magnetic fields dispersed widely across its surface with sizes ranging from less than one to thousands of kilometers and magnitudes up to at least hundreds of nanotesla at the surface (e.g., Halekas et al., 2001;Hood & Schubert, 1980;Mitchell et al., 2008;Purucker & Nicholas, 2010). Despite small scales, these crustal magnetic fields can reflect incident solar wind protons from the dayside hemisphere of the Moon (e.g., Futaana et al., 2003;Lue et al., 2011;Poppe et al., 2017;Saito et al., 2008), locally reducing the space weathering rates on the lunar regolith (e.g., Hood & Williams, 1989). ...
The refilling of the lunar wake is facilitated by the wake ambipolar electric potential arising from the electron pressure gradient. Incident solar wind protons can be reflected by the lunar crustal magnetic fields and the lunar surface on the dayside and repicked up, entering the lunar wake due to their large gyroradii. This burst of positive charges can cause the lunar wake potential to be reduced by hundreds of volts. We utilize over 7 years of ARTEMIS (Acceleration, Reconnection, Turbulence, and Electrodynamics of the Moon's Interaction with the Sun) measurements to systematically investigate how the reflected protons affect the lunar wake potential structure when the Moon is immersed in the solar wind. RPs have a peak occurrence rate of ∼20% for downstream distances from the Moon at N×2πRg and a preference of high occurrence rates and high densities in the direction of the motional electric field of the solar wind. We show that reflected protons in the lunar wake can significantly change the electrostatic ambipolar potentials in the wake, leading in turn to the formation of field‐aligned, accelerated electron beams. Our case study also suggests a nonmonotonic field‐aligned potential structure in the presence of reflected protons in the wake. Lastly, our results show that when the reflected proton density is larger than ∼30% of the local proton density from refilling solar wind protons, the wake potential scales as the logarithmic density of reflected protons, which can be explained by the Boltzmann relation.
... If the thickness of magnetized crust is D C = 10 km, then a detection from an orbiter (O − E1 50 km) or aerial platform (O − E~50 km) mandates M ≥ 0.18 or 0.06 A/m, respectively. These minimum values are similar to the actual magnetization intensities derived for Mercury (Johnson et al., 2015) and Moon (Purucker & Nicholas, 2010) where metallic iron and/or iron alloys and sulfides-not magnetite-are the dominant magnetic carriers. Magnetization intensities as high as~20 A/m are possible on Mars if single-domain magnetite and/or hematite is the dominant carrier (Dunlop & Arkani-Hamed, 2005) and surface strengths of a few hundred nanotesla are predicted in the vicinity of the NASA InSight lander (e.g., Johnson et al., 2019). ...
Observations of planetary magnetic fields provide fundamental insights into the origin and evolution of terrestrial planets. However, whether Venus ever hosted a dynamo is unknown. Here we show that crustal remanent magnetism is a potentially observable consequence of an ancient Venusian dynamo, in contrast to previous studies that dismissed this possibility. Past spacecraft measurements only exclude crustal magnetization near the Venera 4 landing site and northward of 50° South latitude for >150‐km coherence scales and strong magnetization intensities. Magnetite grains with sizes commonly observed in volcanic rocks can retain thermoremanent magnetism at Venusian conditions for >1 billion years. Depths to the Curie temperature of magnetite are ~5–40 km and typically less than predicted crustal thicknesses at our analyzed localities. Aerial platforms could detect expected magnetizations at horizontal scales similar to the ~50‐km operating altitude. Any detection would validate models of planetary accretion, geologic processes, and climate history.
... Instead, it possesses very localized magnetic field anomalies. Such anomalies are also found on the Earth (e.g., Lesur et al., 2016), the Moon (e.g., Purucker & Nicholas, 2010), and Mercury (Hood, 2016;Johnson et al., 2015), but the Martian anomalies are 1 or 2 orders of magnitude more intense than on these bodies. ...
While devoid of an active magnetic dynamo field today, Mars possesses a remanent magnetic field that may reach several thousand nanoteslas locally. The exact origin and the events that have shaped the crustal magnetization remain largely enigmatic. Three magnetic field data sets from two spacecraft collected over 13 cumulative years have sampled the Martian magnetic field over a range of altitudes from 90 up to 6,000 km: (a) Mars Global Surveyor (MGS) magnetometer (1997–2006), (b) MGS Electron Reflectometer (1999–2006), and (c) Mars Atmosphere and Volatile EvolutioN (MAVEN) magnetometer (2014 to today). In this paper we combine these complementary data sets for the first time to build a new model of the Martian internal magnetic field. This new model improves upon previous ones in several aspects: comprehensive data coverage, refined data selection scheme, modified modeling scheme, discrete‐to‐continuous transformation of the model, and increased model resolution. The new model has a spatial resolution of ∼160 km at the surface, corresponding to spherical harmonic degree 134. It shows small scales and well‐defined features, which can now be associated with geological signatures.
... Other maps considered here include the LROC WAC color (Boyd et al., 2012;Denevi et al., 2016;Sato et al., 2014) and stereo topography (Scholten et al., 2012), Diviner Christiansen Feature (CF; emissivity maximum; Greenhagen et al., 2010), Mini-RF radar (Cahill et al., 2014), and Lunar Prospector fluxgate magnetometer (Purucker & Nicholas, 2010) data. ...
The Lyman Alpha Mapping Project has detected five discrete low‐albedo anomalies in Lyman‐α (Ly‐α; 121.6 nm) nighttime reflectance maps. These anomalies reside on the nearside of the Moon within the southeastern Oceanus Procellarum and northwestern Mare Nubium, coincident with regions that have been observed to be photometrically anomalous at visible wavelengths. Some of the spectral properties of these regions within near ultraviolet (NUV) to visible (VIS) wavelengths are consistent with lunar swirls, and they have been reported as “probable swirls” in at least one of these studies. However, while these regions have low Ly‐α normal albedo values relative to nearby highlands regions and are consistent with Ly‐α albedo observations of other swirls, they do not spectrally redden at wavelengths >160 nm as other swirls have been shown to do and they are not associated with regions of higher magnetic intensity. Interestingly, while these anomalies are not easily discerned in NUV single‐band images, the anomalies can be discerned in NUV color ratio composites of the Lunar Reconnaissance Orbiter Camera Wide‐Angle Camera. At least one anomaly observed in the far ultraviolet is not easily observed in the NUV. Additional analyses of their thermal emission Christiansen Feature values, empirically corrected for maturity, suggest that regolith maturity is what differentiates them from their surroundings. While these anomalies may not be swirls, they may represent an “endmember” to swirls minus the influence magnetic materials, which have been hypothesized to provide solar wind shielding. If true, they may provide additional insight into the generation of lunar swirls.
... (1) Mare Imbrium, (2) Mare Serenitatis, (3) Mare Crisium, (4) Mare Humorum, (5) Mare Humboldtianum, (6) Mendel-Ryberg zone, (7) Mare Korolev, (8) MareMoscoviense, (9) Mare Nectaris. Th e fi gure is plotted in Lambert equal are projection(Purucker, Nicholas 2010) ...
The paper discusses the results of the magnetic anomalies of the Earth and inner planets measured by the satellites. It illustrates the magnetic anomalies of the African continent, the global magnetic anomalies of the Earth, the magnetic anomalies of the Kursk and the Pannonian Basin and other magnetic anomalies which are produced by the impact structures. It summarizes the anomalies measured by the spacecrafts around the Mercury, Venus, Mars and the Moon.
... Magnetic field data acquired from orbit show that the Moon possesses many magnetic anomalies that are scattered across its surface (e.g., Hood & Schubert, 1980;Purucker & Nicholas, 2010;Takahashi et al., 2014). The observed anomalies are enigmatic in that the vast majority do not unambiguously correlate with known geologic structures or processes. ...
Magnetic field data acquired from orbit shows that the Moon possesses many magnetic anomalies. Though most of these are not associated with known geologic structures, some are found within large impact basins within the interior peak ring. The primary magnetic carrier in lunar rocks is metallic iron, but indigenous lunar rocks are metal poor and cannot account easily for the observed field strengths. The projectiles that formed the largest impact basins must have contained a significant quantity of metallic iron, and a portion of this iron would have been retained on the Moon's surface within the impact melt sheet. Here we use orbital magnetic field data to invert for the magnetization within large impact basins using the assumption that the crust is unidirectionally magnetized. We develop a technique based on laboratory thermoremanent magnetization acquisition to quantify the relationship between the strength of the magnetic field at the time the rock cooled and the abundance of metal in the rock. If we assume that the magnetized portion of the impact melt sheet is 1 km thick, we find average abundances of metallic iron ranging from 0.11% to 0.45 wt %, with an uncertainty of a factor of about 3. This abundance is consistent with the metallic iron abundances in sampled lunar impact melts and the abundance of projectile contamination in terrestrial impact melts. These results help constrain the composition of the projectile, the impact process, and the time evolution of the lunar dynamo.
... Magnetic field measurements obtained from orbit also show that there are strong magnetic anomalies of crustal origin distributed heterogeneously across the lunar surface. Though the initial orbital measurements obtained by Apollo only covered a small swath across the equator [Lin, 1979;Hood et al., 1981], more recent missions with polar orbits such as Lunar Prospector [Richmond and Hood, 2008;Mitchell et al., 2008;Purucker and Nicholas, 2010] and Kaguya Tsunakawa et al., 2015] have extended these measurements globally. ...
Orbital magnetic field data show that portions of the Moon's crust are strongly magnetized, and paleomagnetic data of lunar samples suggest that Earth-strength magnetic fields could have existed during the first several hundred million years of lunar history. The origin of the fields that magnetized the crust are not understood and could be the result of either a long-lived core generated dynamo or transient fields associated with large impact events. Core-dynamo models usually predict that the field would be predominantly dipolar, with the dipole axis aligned with the rotation axis. We test this hypothesis by modeling the direction of crustal magnetization using a global magnetic field model of the Moon derived from Lunar Prospector and Kaguya magnetometer data. We make use of a model that assumes that the crust is unidirectionally magnetized. The intensity of magnetization can vary with the crust, and the best-fitting direction of magnetization is obtained from a non-negative least squares inversion. From the best fitting magnetization direction we obtain the corresponding North magnetic pole predicted by an internal dipolar field. Some of the obtained paleopoles are associated with the current geographic poles, while other well-constrained anomalies have paleopoles at equatorial latitudes, preferentially at 90° East and West longitudes. One plausible hypothesis for this distribution of paleopoles is that the Moon possessed a long-lived dipolar field, but that the dipole was not aligned with the rotation axis as a result of large scale heat flow heterogeneities at the core-mantle boundary.
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.
This study presents a new spherical harmonic (SH) model of the crustal magnetic field of Mars, based on the magnetic field data set measured by the Mars Global Surveyor (MGS) and the Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft. To minimize the influence of external fields due to solar wind interaction with Mars, we rejected data that were observed dayside and above an altitude of 500 km. The data points of MAVEN were reduced by using a proxy of solar wind activity that identified and rejected any data measured during magnetically disturbed intervals. We used a conventional least squares technique to estimate the Gauss coefficients fitted to the reduced data set and made a compromise between model misfit and model roughness by truncating the SH model at degree 110. This model is capable of representing crustal fields with a spatial resolution approaching ∼200 km at 120 km altitude and ∼260 km at the Martian surface. Since our model fits MAVEN's observational data better than previous models, especially the data obtained during MAVEN's low altitude periapsis passes, we conclude that it may more accurately approximate the low‐altitude crustal field. We calculate the crustal field power spectrum of various models and find that small‐scale fields at low altitudes were underestimated by most previous models. This new model could benefit future studies associated with the Martian crustal field and its interaction with the solar wind.
Reiner Gamma is a prime target for low‐orbiting spacecraft or even surface‐landed missions in the near future. The region hosts a prominent lunar swirl that is co‐located with a strong and well‐structured magnetic anomaly. We simulate and discuss Reiner Gamma's near‐surface plasma environment at different altitudes above the lunar surface using the fully kinetic particle‐in‐cell code iPIC3D. The input magnetic field model is based on orbital‐altitude observations from the Kaguya and Lunar Prospector missions. We develop eight simulation cases, representing the distinct plasma regimes. Reiner Gamma is exposed to along a typical orbit, including different solar wind incidence angles and the magnetosheath crossing. We show that the plasma environment is vastly different at different altitudes and depends critically on the upstream plasma parameters, consistent with the predictions of the solar wind standoff model. Our work helps to define measurement requirements for a possible future low‐orbiting or lander mission to the Reiner Gamma area or similarly magnetized regions of the lunar surface.
Determining the presence or absence of a past long-lived lunar magnetic field is crucial for understanding how the Moon's interior and surface evolved. Here, we show that Apollo impact glass associated with a young 2 million-year-old crater records a strong Earth-like magnetization, providing evidence that impacts can impart intense signals to samples recovered from the Moon and other planetary bodies. Moreover, we show that silicate crystals bearing magnetic inclusions from Apollo samples formed at ∼3.9, 3.6, 3.3, and 3.2 billion years ago are capable of recording strong core dynamo-like fields but do not. Together, these data indicate that the Moon did not have a long-lived core dynamo. As a result, the Moon was not sheltered by a sustained paleomagnetosphere, and the lunar regolith should hold buried 3 He, water, and other volatile resources acquired from solar winds and Earth's magnetosphere over some 4 billion years.
Planetary magnetic fields are prime objects of comparative planetology. Planetary magnetic fields usually indicate the transformation of kinetic energy into magnetic energy. Their study allows deep insights into physical processes in the environment, at the surface, and in the interior of planetary bodies. From a surface perspective, external as well as internal sources are important and are used as a guideline of this review. External sources are rooted in the interaction of the planetary environment with the body under consideration. The major internal source is the dynamo process. Magnetic induction is a mediator between both processes. Starting in 1919 with Joseph Larmor's pioneering suggestion on dynamo action being the source of planetary magnetic fields, the past hundred years have seen a revolutionary increase of our knowledge on the various source processes and their dynamics. Appreciation of our current understanding requires the presentation of the history of ideas in concert with up‐to‐date descriptions of the latest findings, concepts, and future observational needs.
The magnetometer instrument MPO-MAG on-board the Mercury Planetary Orbiter (MPO) of the BepiColombo mission en-route to Mercury is introduced, with its instrument design, its calibration and scientific targets. The instrument is comprised of two tri-axial fluxgate magnetometers mounted on a 2.9 m boom and are 0.8 m apart. They monitor the magnetic field with up to 128 Hz in a $\pm 2048$ ± 2048 nT range. The MPO will be injected into an initial $480 \times 1500$ 480 × 1500 km polar orbit (2.3 h orbital period). At Mercury, we will map the planetary magnetic field and determine the dynamo generated field and constrain the secular variation. In this paper, we also discuss the effect of the instrument calibration on the ability to improve the knowledge on the internal field. Furthermore, the study of induced magnetic fields and field-aligned currents will help to constrain the interior structure in concert with other geophysical instruments. The orbit is also well-suited to study dynamical phenomena at the Hermean magnetopause and magnetospheric cusps. Together with its sister instrument Mio-MGF on-board the second satellite of the BepiColombo mission, the magnetometers at Mercury will study the reaction of the highly dynamic magnetosphere to changes in the solar wind. In the extreme case, the solar wind might even collapse the entire dayside magnetosphere. During cruise, MPO-MAG will contribute to studies of solar wind turbulence and transient phenomena.
We performed an analysis of spacecraft multispectral images for lunar swirls in order to gain an improved understanding of optical space weathering on the Moon and its causes. LROC WAC data provide information on the slope of the spectrum in the near-UV (NUV), as measured by the 321-nm/415-nm or 321-nm/360-nm reflectance ratios. Kaguya MI data were used to assess the near-infrared (NIR) continuum slope (1548-nm/749-nm reflectance ratio). Context for interpreting the spectral variations found in the remotely observed regions of the lunar surface is provided by laboratory reflectance spectra of lunar rocks and soils, as well as spectra for transparent silica gel analogs (Noble et al., 2007) containing different sizes and abundances of nanometer-sized iron (nsFe) particles. We gain additional insights into the spectral effects of sample maturity by considering the ferromagnetic resonance parameter (Is) values for mare and highland soils, as well as the number density of nsFe particles in the silica gels.
We examined a set of three mare swirls (Reiner Gamma, Ingenii, and Mare Marginis) and three highland swirls (Airy, Descartes, and Gerasimovich). The NIR continuum slopes of both mare and highland swirls are shallower than those of the nearby normal mature regolith. Bright swirl surfaces have higher NIR slopes than normal fresh material of the same albedo. The NUV ratios within mare swirls are lower than in the mature background, but for highland swirls, the NUV ratios are approximately the same as the mature background. We do not see definitive evidence for "over-maturation" (excessive darkening and reddening beyond that found in the normal background surfaces) in dark lanes at the swirls we examined, although saturation of weathering effects at a high-iron location like Reiner Gamma could prevent over-maturation from appearing – even if enhanced solar-wind bombardment related to deflection by local magnetic fields is taking place.
Evaluation of the NIR character of swirls and comparison with lab spectra of lunar soils and nsFe-bearing silica gel analogs leads to the conclusion that swirl materials contain abundances of nsFe that are lower than that of normal non-swirl background surfaces; the nsFe content of swirls corresponds to immature (though not pristine) or submature soils. However, the size distribution of nsFe in swirls is anomalous compared with normal lunar surfaces, with a deficiency in the smaller size range (< ~15 nm), as inferred from the NUV character of swirls. Because the flux of solar-wind ions reaching the surface in swirls is attenuated by shielding by crustal magnetic fields, we conclude that solar-wind exposure is the primary agent for production of small nsFe in normal lunar space weathering. Micrometeoroid bombardment, which is unimpeded by the presence of magnetic fields, is mainly responsible for production of larger nsFe in space weathering.
Paleomagnetic studies of Apollo samples indicate that the Moon generated a core dynamo lasting for at least 2 billion years. However, the geometry of the lunar magnetic field is still largely unknown because the original orientations of essentially all Apollo samples have not been well-constrained. Determining the direction of the lunar magnetic field over time could elucidate the mechanism by which the lunar dynamo was powered and whether the Moon experienced true polar wander. Here we present measurements of the lunar magnetic field 3.7 billion years (Ga) ago as recorded by Apollo 17 mare basalts 75035 and 75055. These samples formed as part of basalt flows in the Taurus-Littrow valley that make up wall-rock within Camelot crater, now exposed at the rim of the crater. Using apparent layering in the parent boulder for 75055, we inferred its original paleohorizontal orientation on the lunar surface at the time of magnetization. We find that 75035 and 75055 record a mean paleointensity of ~50 µT. Furthermore, 75055 records a paleoinclination of 34 ± 11°. This inclination is consistent with, but does not require, a selenocentric axial dipole field geometry (i.e., a dipole in the center of the Moon and aligned along the spin axis). Additionally, although true polar wander is also not required by our data, true polar wander paths inferred from some independent studies of lunar hydrogen deposits and crustal magnetic anomalies are consistent with our measured paleoinclination.
Processes associated with the presence of magnetic fields, which can be important in dusty plasmas on the Moon, are considered. Lower-hybrid wave processes under interaction of the magnetotail of the Earth with dusty plasma near the surface of the Moon are described. Lower-hybrid waves are excited due to the relative motion of magnetospheric ions and charged dust grains, which leads to the establishment of a well developed lower hybrid plasma turbulence. The effective collision frequency characterizing the anomalous loss of ion momentum due to ion-wave interaction, as well as the electric fields arising in the system are found. It is shown that the electric fields excited due to the development of lower-hybrid turbulence are somewhat weaker than those arising due to the charging of the lunar surface under the action of solar radiation. Nevertheless, they are quite significant to affect the electric field pattern above the Moon. The obtained effective collision frequencies should be taken into account when deriving hydrodynamic equations for dusty plasma ions with allowance for their turbulent heating. Problems related to the consideration of magnetic fields, which can be important for detailed study of the dusty plasmas at the Moon, are stated. The possibility of generation of wave motions in the near-surface lunar plasma should be taken into consideration when interpreting the observational data.
The Advanced Small Analyzer for Neutrals (ASAN) is an international payload onboard the Chinese Chang’E-4 rover. It performed for the first time measurements of energetic neutral atoms (ENAs) on the lunar surface. We show a typical mass separated ENA energy spectrum measured by the ASAN. After normalizing to the impinging solar wind proton energy, good agreement to previous measurements from Chandrayaan-1 and IBEX is found. The hydrogen ENA albedo is estimated to be 32% for energy above 30 eV, comparable to the estimates obtained from Chandrayaan-1 and IBEX. Particle fluxes at lower energies are generally higher than those observed by Chandrayaan-1 and IBEX, and possible reasons are discussed.
The global electromagnetic induction response of the Moon has been solved numerically for several electrical conductivity profiles using the finite-element method. Here we demonstrate the capability and applicability in both two and three spatial dimensions for any input magnetic field time series measured at the Moon. We discuss the applicability of a vacuum approximation to the induced magnetic field response to the lunar plasma environment and the challenges of isolating geomagnetic induced fields including the interaction with the lunar wake structure. We perform three validation analyses comparing our vacuum model response to analytic solutions: (1)the time domain response to a step impulse or tangential discontinuity within the solar wind, (2)the time domain response to a ramp driving function, and (3)the broadband frequency domain response. We fit the analytic solutions to a root-mean-square error of better than 1% for all cases. We analyze the accuracy range and demonstrate our model's capability of resolving interior structure from Apollo magnetometer data. We present the first time domain numerical solution of the induced magnetic field response of the Moon in vacuum for any driving input signal and any interior conductivity profile, building on previous Apollo era work. Lastly, we discuss the trade-offs between model accuracy and performance, which is of particular concern for large datasets and iterative optimizations. The transfer function method developed here is applicable to other airless body two-point magnetometer measurements including Apollo, ARTEMIS, and future lunar geophysical networks.
The Moon does not currently possess a dynamo, but its crust contains numerous magnetic anomalies detected from orbit. The geologic origins of these anomalies are still unknown, including the archetypal Reiner Gamma magnetic anomaly. To gain insight, we study a small magnetic anomaly, herein called the octopus, which is possibly associated with Reiner Gamma. The octopus has curving bright albedo patterns characteristic of features known as swirls. We use high cadence 9 Hz Lunar Prospector magnetometer data, along with constraints provided by this swirl's albedo pattern, to perform inversions for the swirl's magnetic source body characteristics. We use three different inversion methods, and they all return similar results. We also estimate the depth of magnetization from characteristics of the horizontal component of the magnetic field and the albedo pattern. We find that performing inversion for source body properties at small swirls has advantages compared to larger anomalies, or anomalies without albedo markings. We find the octopus is magnetized in the same direction as the main Reiner Gamma anomaly (within 1σ uncertainties), suggesting they formed contemporaneously. The large spatial distance between these coeval anomalies and their inferred shallow source body depths are compatible with formation in a hot ejecta deposit that cooled in the presence of a dynamo field, as suggested by Hood et al. (2001). However, a key remaining enigma is why the northeastern Reiner Gamma “tail” formation has a magnetization direction ∼60° different from the main body and octopus.
We report the first observational evidence for the transport of the solar wind protons scattered from the lunar magnetic anomaly (LMA) into the near wake region from SWIM/Sub-keV Atom Reflecting Analyzer (SARA) aboard Chandrayaan-1. These protons with high angular spread are observed in the near wake region for specific orientations of interplanetary magnetic field. The typical energy range is 600–1,000 eV, which is either smaller or comparable to that of solar wind. Using our backtracing model, the source region of these protons is found to be the large LMA at South Pole-Aitken basin on the dayside, suggesting that these are solar wind protons forward scattered from LMA at the South Pole-Aitken. The flux of these protons is ∼5 × 10⁻⁴ of the solar wind proton flux, which is comparable to the proton population in near wake due to other known processes. Such protons can significantly affect the electromagnetic environment in near wake region.
Lunar swirls are collections of finely structured bright and dark surface markings, alternating over length scales of typically 1–5 km. If swirls are the result of plasma interactions with crustal magnetic anomalies or electrostatic or magnetic sorting of fine materials, the magnetic field orientation must vary over similar length scales. This requires that the associated source bodies be both shallow and narrow in horizontal extent. The correspondingly restricted volume of the source bodies in turn implies strong rock magnetization. Here we show that if ∼300-nT surface fields are necessary to produce observable swirl markings, the required rock magnetization must be >0.5 A/m, even for very shallow sources and likely closer to ∼2 A/m or more. This strong source rock magnetization, together with the geometric constraints that favor magmatic structures such as dikes or lava tubes, requires a mechanism to enhance the magnetic carrying capacity of the rocks. We propose that heating associated with magmatic activity could thermochemically alter host rocks and impart them with magnetizations an order of magnitude stronger than is typical of lunar mare basalts. Our results both place constraints on the geometry and magnetization of the source bodies and provide clues about the possible origins of the Moon's crustal magnetic anomalies.
We study the scattering of solar wind protons off the lunar surface, using ion observations collected over 6 years by the ARTEMIS satellites at the Moon. We show the average scattered proton energy spectra, directional scattering distributions, and scattering efficiency, for different solar wind incidence angles and impact speeds. We find that the protons have a scattering distribution that is similar to existing empirical models for scattered hydrogen energetic neutral atoms, with a peak in the backward direction (toward the Sun). We provide a revised model for the scattered proton energy spectrum. We evaluate the positive to neutral charge state ratio by comparing the proton spectrum with existing models for scattered hydrogen. The positive to neutral ratio increases with increasing exit speed from the surface but decreases with increasing impact speed. Combined, these counteracting effects result in a scattering efficiency that decreases from ~0.5% at 300 km/s solar wind speed to ~0.3% at 600 km/s solar wind speed.
This chapter describes the magnetic fields of the planets, from Mercury to Neptune, including the large satellites (Moon and Ganymede) that have or once had active dynamos. The chapter describes the spacecraft missions and observations that, along with select remote observations, form the basis of our knowledge of planetary magnetic fields. The methods of analysis used to characterize planetary magnetic fields are discussed as well as the models used to represent the main field (due to dynamo action in the planet's interior) and/or remanent magnetic fields locked in the planet's crust, where appropriate. These observations provide valuable insights into dynamo generation of magnetic fields, the structure and composition of planetary interiors, and the evolution of planets.
Spacecraft observations show that weak magnetic fields of crustal origin are ubiquitous across the surface of the Moon. To investigate the origin of these magnetic anomalies, a model was developed for the magnetic power spectrum that consists of ensembles of randomly magnetized sills or prisms. Localized spectrum analyses constrained how the parameters of this model vary with position, including the size of the sources, a quantity proportional to their mean-squared dipole moment, and the depth to the top and bottom of the magnetized region. The depth to the top of the magnetized region varies from the surface to about 25 km. The magnetic carriers in the deep crust likely formed at the same time as the crust itself, implying that a core-generated dynamo field must have existed when the crust was cooling during the first 100 million years of lunar evolution. The parameter related to the strength of magnetization shows the existence of a prominent region on the nearside hemisphere that is largely unmagnetized and that correlates with a region of extremely low surface field strengths. This region lies entirely within a geological province that is highly enriched in heat-producing elements (the Procellarum KREEP Terrane), suggesting that this region escaped being magnetized because of prolonged high crustal temperatures. The nearside magnetic low may be representative of the size of that portion of the crust that is highly enriched in heat producing elements, which is almost one third the size of the Procellarum KREEP Terrane based on surface thorium abundances.
We developed a spinner magnetometer to measure the natural remanent magnetization of large Apollo lunar rocks in the storage vault of the Lunar Sample Laboratory Facility (LSLF) of NASA. The magnetometer mainly consists of a commercially available three-axial fluxgate sensor and a hand-rotating sample table with an optical encoder recording the rotation angles. The distance between the sample and the sensor is adjustable according to the sample size and magnetization intensity. The sensor and the sample are placed in a two-layer mu-metal shield to measure the sample natural remanent magnetization. The magnetic signals are acquired together with the rotation angle to obtain stacking of the measured signals over multiple revolutions. The developed magnetometer has a sensitivity of 5 × 10⁻⁷ Am² at the standard sensor-to-sample distance of 15 cm. This sensitivity is sufficient to measure the natural remanent magnetization of almost all the lunar basalt and breccia samples with mass above 10 g in the LSLF vault.
Wave processes occurring under the interaction of the Earth's magnetosphere with dusty plasma near the lunar surface are studied. Ion-acoustic waves are shown to be excited in some regions of the magnetosphere due to the development of a linear hydrodynamic instability. This results in the excitation of ion-acoustic turbulence in these regions. Dust-acoustic waves are demonstrated to be generated due to the development of linear kinetic instability in the entire region of magnetotail interaction with dusty plasma near the Moon. Correspondingly, dust-acoustic turbulence can be excited in the entire region of the interaction of the Earth's magnetosphere with dusty plasma near the lunar surface. We discuss magnetic reconnection processes, which are related to the development of plasma turbulence at the Moon.
Approximate maps of the lunar crustal magnetic field at low altitudes in the vicinities of the three Imbrian-aged impact basins, Orientale, Schrödinger, and Imbrium, have been constructed using Lunar Prospector and Kaguya orbital magnetometer data. Detectable anomalies are confirmed to be present well within the rims of Imbrium and Schrödinger. Anomalies in Schrödinger are asymmetrically distributed about the basin center, while a single isolated anomaly is most clearly detected within Imbrium northwest of Timocharis crater. The subsurface within these basins was heated to high temperatures at the time of impact and required long time periods (up to 1 Myr) to cool below the Curie temperature for metallic iron remanence carriers (1043 K). Therefore, consistent with laboratory analyses of returned samples, a steady, long-lived magnetizing field, i.e., a former core dynamo, is inferred to have existed when these basins formed. The asymmetrical distribution within Schrödinger suggests partial demagnetization by later volcanic activity when the dynamo field was much weaker or nonexistent. However, it remains true that anomalies within Imbrian-aged basins are much weaker than those within most Nectarian-aged basins. The virtual absence of anomalies within Orientale where impact melt rocks (the Maunder Formation) are exposed at the surface is difficult to explain unless the dynamo field was much weaker during the Imbrian period.
A number of magnetic anomalies are present along the northern edge of the lunar South Pole-Aitken (SPA) basin. A variety of hypotheses for their formation have been proposed, but an in-depth study of their properties has not been performed. Here we use two different methods to invert for their source body characteristics: one that completely searches a small parameter space of less than ten uniform-strength dipoles per anomaly, and another that uses grids of hundreds of dipoles with variable magnetization strengths. Both methods assume uniform magnetization directions at each anomaly and with one exception, produce nearly the same results. We introduce new Monte Carlo methods to quantify errors in our inversions arising from Gaussian time-dependent changes in the external field and the uncertain geometry of the source bodies. We find the errors from uncertainty in source body geometry are almost always higher. We also find a diverse set of magnetization directions around SPA, which we combine with other physical arguments to conclude that the source bodies were likely magnetized in a dynamo field. Igneous intrusions are a reasonable explanation (Purucker et al., 2012) for the directional variability, since they could be intruded over different magnetic epochs. However, the directional variability also implies either surprisingly large amounts of true polar wander or a dynamo not aligned with the lunar spin axis. We also explore the possibility that true polar wander caused by the SPA impact could allow iron-rich SPA ejecta to record a diverse set of magnetic field directions. Some of this material may have also become sesquinary ejecta and re-impacted across the Moon on 10⁴-10⁶ year timescales to capture such changes. No completely satisfactory answer emerges, except that the dipole-axis of the lunar dynamo may have been variable in direction.
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.
Version 3.1 of the Generic Mapping Tools (GMT) has been released. More than 6000 scientists worldwide are currently using this free, public domain collection of UNIX tools that contains programs serving a variety of research functions. GMT allows users to manipulate (x,y) and (x,y,z) data, and generate PostScript illustrations, including simple x-y diagrams, contour maps, color images, and artificially illuminated, perspective, and/or shaded-relief plots using a variety of map projections (see Wessel and Smith [1991] and Wessel and Smith [1995], for details.). GMT has been installed under UNIX on most types of workstations and both IBM-compatible and Macintosh personal computers.
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.
The Decade of Geopotential Field Research, inaugurated in 1999 with the launch of the Danish satellite Ørsted on 23 February, was designed as an international effort to promote and coordinate continuous monitoring of geopotential field variability in the near-Earth environment. Since 1999, the Challenging Minisatellite Payload (CHAMP), the Gravity Recovery and Climate Experiment (GRACE), the Satélite de Aplicaciones Científicas-C (SAC-C), and most recently, the Gravity field and steady-state Ocean Circulation Explorer (GOCE) satellites have combined with Ørsted to generate an unprecedented wealth of data on Earth's magnetic and gravity fields. Interpretation of the new magnetic data from the Decade has led to improvements in scientists' knowledge of the fast changing small scales of the Earth's magnetic field, providing details of magnetic field generation within the Earth's core. The new magnetic data have also been used in the World Digital Magnetic Anomaly Map (WDMAM) project, which ``images'' the lithosphere's igneous and metamorphic rocks. Such data, associated theory, and modeling work also led to the discovery of previously undetected processes with magnetic signatures that can be observed by satellites, including oceanic tides, ionospheric pressure gradient currents and ionospheric plasma irregularities, and serpentinized mantle overlying subduction zones. Knowledge of the magnetic properties of these processes provides scientists with a new perspective of the physics involved in the phenomena.
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.
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.
The spatial ‘power’ spectrum of the main geomagnetic field has been estimated for harmonics up to n= 500. It is shown to consist of two components, long wavelengths being dominated by fields originating in the core, and short wavelengths by fields originating in the crust; the cross-over occurs at n≥ 11, a wavelength ⩽ 3600 km.
The core field is often approximated by a set of spherical harmonic coefficients. It is shown that at present main field coefficients for n≿ 9, and secular variation coefficients for n≿ 6, are not known with significant accuracy. Estimates are made of the standard deviations of the IGRF coefficients, and the standard deviation of the IGRF field deduced. This field is known to about 0.5 per cent at the surface but only to about 10 per cent at the core. Its time variation is known only to about 20 per cent at the surface, and is very uncertain at the core.
The lithospheric contribution to the Earth’s magnetic field is concealed in magnetic field data that have now been measured
over several decades from ground to satellite altitudes. The lithospheric field results from the superposition of induced
and remanent magnetisations. It therefore brings an essential constraint on the magnetic properties of rocks of the Earth’s
sub-surface that would otherwise be difficult to characterize. Measuring, extracting, interpreting and even defining the magnetic
field of the Earth’s lithosphere is however challenging. In this paper, we review the difficulties encountered. We briefly
summarize the various contributions to the Earth’s magnetic field that hamper the correct identification of the lithospheric
component. Such difficulties could be partially alleviated with the joint analysis of multi-level magnetic field observations,
even though one cannot avoid making compromises in building models and maps of the magnetic field of the Earth’s lithosphere
at various altitudes. Keeping in mind these compromises is crucial when lithospheric field models are interpreted and correlated
with other geophysical information. We illustrate this discussion with recent advances and results that were exploited to
infer statistical properties of the Earth’s lithosphere. The lessons learned in measuring and processing Earth’s magnetic
field data may prove fruitful in planetary exploration, where magnetism is one of the few remotely accessible internal properties.
KeywordsLithospheric magnetic field-Modelling-Regional modelling-Planetary magnetic field-Satellite
Many geological features of the Earth's lithosphere create variations in the Earth's magnetic field that can be detected by satellites. The resulting magneti anomaly maps can provide new insights into the tectonic features and broad structures of the lithosphere. This book documents the acquisition, reduction and analysis of satellite magnetic field data in the study of the Earth's lithosphere. The isolation of the lithospheric field from fields originating in the Earth's core, ionosphere and magnetosphere is discussed in detail, and a summary of the characteristics of each field is included. This work also provides a complete summary of published maps, and the methods used to create them. Rock magnetism concepts and sources of variation in magnetization are discussed to help the reader understand the issues in interpreting the data, and the various interpretation methods, such as forward modeling, are summarized. Results of data from North America, Africa, Australia, Europe, and the oceans illustrate the methodologies of interpretation, and the anomalies associated with particular geologic and tectonic settings. Mapping and interpreting lithospheric fields from satellite magnetic data calls for the solution of a new set of problems and the development of a new complement of analytic tools, and has resulted in the new sub-discipline of geomagnetism. Advances students and researchers will find that The Magnetic Field of the Earth's Lithosphere provides a much needed review of this important topic.
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.
Conjugate gradient and sparse matrix techniques are utilized in the
solution of a geomagnetic inverse problem. Global crustal data sets
collected from low-earth orbit are quickly inverted (using a design
matrix approach) or continued to a common altitude (using a normal
matrix approach) even when using parameterizations of 10,000 or more
dipoles. The sparsity results from the rapid decay of the magnetic field
with distance from the dipole. Iterative techniques such as the
conjugate gradient save computer time and space when compared to more
direct approaches using the Householder transformation, thus allowing
problems that were intractable to all but the largest supercomputers to
be performed on workstations of only moderate power.
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.
Equivalent point source inversion in the rectangular coordinate system has been widely used to reduce satellite magnetic data collected at different altitudes to a common elevation over small areas. This method is based on the expression of the magnetic anomaly caused by a magnetic dipole. Such an expression derived in a spherical coordinate system by von Frese et al. [1981] is found erroneous. We point out the errors in von Frese et al.'s [1981] formulas and present the correct expression for the magnetic field of a magnetic dipole in a spherical coordinate system.
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.
Many geological features of the Earth's lithosphere create variations in
the Earth's magnetic field that can be detected by satellites. The
resulting magnetic anomaly maps can provide new insights into the
tectonic features and broad structures of the lithosphere. This book
documents the acquisition, reduction and analysis of satellite magnetic
field data in the study of the Earth's lithosphere. The text considers
issues of interpreting data, summarizes various interpretation methods,
considers rock magnetism concepts and sources of variation in magnetism,
and provides a complete summary of published maps and the methods used
to create them. Mapping and interpreting lithospheric fields from
satellite magnetic data has resulted in the new subdiscipline of
geomagnetism. Advanced students and researchers will find that The
Magnetic Field of the Earth's Lithosphere provides a much needed review
of this important topic.
We present new paleomagnetism results for some of the oldest rocks in
the Apollo collection. We find evidence for strong magnetic fields
before 4.1 Ga, making this the oldest paleointensity determination for a
large, differentiated body.
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.
To achieve the scientific objectives related to the lunar magnetic field measurements in a polar orbit at an altitude of 100km,
strict electromagnetic compatibility (EMC) requirements were applied to all components and subsystems of the SELENE (Kaguya)
spacecraft. The magnetic cleanliness program was defined as one of the EMC control procedures, and magnetic tests were carried
out for most of the engineering and flight models. The EMC performance of all components was systematically controlled and
examined through a series of EMC tests. As a result, the Kaguya spacecraft was made to be very clean, magnetically. Hence
reliable scientific data related to the magnetic field around the Moon were obtained by the LMAG (Lunar MAGnetometer) and
the PACE (Plasma energy Angle and Composition Experiment) onboard the Kaguya spacecraft. These data have been available for
lunar science use since November 2009.
KeywordsElectromagnetic compatibility (EMC)-Magnetic cleanliness-SELENE (Kaguya)-LMAG-Moon
The Lunar Prospector Electron Reflectometer has obtained the first global map of lunar crustal magnetic fields, revealing that the effects of basin-forming impacts dominate the large-scale distribution of remanent magnetic fields on the Moon. The weakest surface magnetic fields (<0.2 nT) are found within two of the largest and most recent impact basins, Orientale and Imbrium. Conversely, the largest concentrations of strong surface fields (>40 nT) are diametrically opposite to these same basins. This pattern is present though less pronounced for several other post-Nectarian impact basins larger than 500 km in diameter. The reduced strength and clarity of the pattern for older basins may be attributed to: (1) demagnetization from many smaller impacts, which erases antipodal magnetic signatures over time, (2) superposition effects from other large impacts, and (3) variation in the strength of the ambient magnetizing field. The absence of fringing fields stronger than 1 nT around the perimeter of the Imbrium basin or associated with craters within the basin implies that any uniform magnetization of the impact melt must be weaker than ∼10−6 G cm3 g−1. This limits the strength of any steady ambient magnetic field to no more than ∼0.1 Oe at the lunar surface while the basin cooled for tens of millions of years following the Imbrium impact 3.8 billion years ago.
It is proved that if a spherical shell is magnetized in the direction of and proportional to a magnetic field of origin internal to the shell and the magnetizing field later disappears, no magnetic field exists external to the shell. Similarly if a spherical shell is magnetized parallel to and proportional to a magnetic field of external origin and this magnetizing field later disappears, the magnetic field internal to the shell is zero. These theorems are true only if these ideal conditions are met, but are applicable to the interpretation of the natural remanent magnetization of the lunar crust. It is shown that the present absence of a magnetic dipole field of the Moon supports the hypothesis that the magnetizing field was of internal origin but does not distinguish whether this was due to a dynamo in the lunar core or to a primaeval magnetization of its interior. Local magnetic fields around the Moon are interpreted as arising from the departure from sphericity of the shell and large craters.
3D simulations of basin-scale lunar impacts are carried out to investigate: (a) the origins of strong crustal magnetic fields and unusual terrain observed to occur in regions antipodal to young large basins; and (b) the origin of enhanced magnetic and geochemical anomalies along the northwest periphery of the South Pole-Aitken (SPA) basin. The simulations demonstrate that a basin-forming impact produces a massive, hot, partially ionized cloud of vapor and melt that expands thermally around the Moon, converging near the basin antipode approximately 1 h after the impact for typical impact parameters. In agreement with previous work, analytic calculations of the interaction of this vapor–melt cloud with an initial ambient magnetic field predict a substantial temporary increase in field intensity in the antipodal region. The time of maximum field amplification coincides with a period when impacting ejecta also converge near the antipode. The latter produce antipodal shock stresses within the range of 5–25 GPa where stable shock remanent magnetization (SRM) of lunar soils has been found experimentally to occur. Calculated antipodal ejecta thicknesses are only marginally sufficient to explain the amplitudes of observed magnetic anomalies if mean magnetization intensities are comparable to those produced experimentally. This suggests that pre-existing ejecta materials, which would also contain abundant metallic iron remanence carriers, may be important anomaly sources, a possibility that is consistent with enhanced magnetic anomalies observed peripheral to SPA. The latter anomalies may be produced by amplified secondary ejecta impact shock waves in the thick SPA ejecta mantle occurring near the antipodes of the Imbrium and Serenitatis impacts. Together with converging seismic compressional waves, these antipodal impact shocks may have produced especially deep fracture zones along the northwest edge of SPA near the Imbrium antipode, allowing the ascent of magma with enhanced KREEP concentrations.
The rock magnetism and paleomagnetism of the Apollo samples is reviewed and evidence is presented for an era of strong lunar magnetic fields between 3.9 and 3.6 By. The most plausible model for these fields is a short lived lunar dynamo, which may have been driven by compositional convection associated with the freezing of a lunar core.
A preliminary model of the internal magnetic field of the Moon is developed using a novel, correlative technique on the low-altitude Lunar Prospector magnetic field observations. Subsequent to the removal of a simple model of the external field, an internal dipole model is developed for each pole-to-pole half-orbit. This internal dipole model exploits Lunar Prospector's orbit geometry and incorporates radial and theta vector component data from immediately adjacent passes into the model. These adjacent passes are closely separated in space and time and are thus characteristic of a particular lunar regime (wake, solar wind, magnetotail, magnetosheath) or regimes. Each dipole model thus represents the correlative parts of three adjacent passes, and provides an analytic means of continuing the data to a constant surface of 30 km above the mean lunar radius. The altitude-normalized radial field from the wake and tail regimes is used to build a model in which 99.2% of the 360 by 360 bins covering the lunar surface are filled. This global model of the radial magnetic field is used to construct a degree 178 spherical harmonic model of the field via the Driscoll and Healy sampling theorem. Terms below about degree 150 are robust, and polar regions are considered to be the least reliable. The model resolves additional detail in the low magnetic field regions of the Imbrium and Orientale basins, and also in the four anomaly clusters antipodal to the large lunar basins. The model will be of use in understanding the sources of the internal field, and as a first step in modeling the interaction of the internal field with the solar wind.
It is uncertain whether the Moon ever formed a metallic core or generated a core dynamo. The lunar crust and returned samples are magnetized, but the source of this magnetization could be meteoroid impacts rather than a dynamo. Here, we report magnetic measurements and 40Ar/39Ar thermochronological calculations for the oldest known unshocked lunar rock, troctolite 76535. These data imply that there was a long-lived field on the Moon of at least 1 microtesla approximately 4.2 billion years ago. The early age, substantial intensity, and long lifetime of this field support the hypothesis of an ancient lunar core dynamo.
Lunar Prospector is providing a global map of the composition of the moon and analyzing the moon's gravity and magnetic fields. It has been in a polar orbit around the moon since 16 January 1998. Neutron flux data show that there is abundant H, and hence probably abundant water ice, in the lunar polar regions. Gamma-ray and neutron data reveal the distribution of Fe, Ti, and other major and trace elements on the moon. The data delineate the global distributions of a key trace element-rich component of lunar materials called KREEP and of the major rock types. Magnetic mapping shows that the lunar magnetic fields are strong antipodal to Mare Imbrium and Mare Serenitatis and has discovered the smallest known magnetosphere, magnetosheath, and bow shock complex in the solar system. Gravity mapping has delineated seven new gravity anomalies and shown that the moon has a small Fe-rich core of about 300 km radius.
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