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Aerial electromagnetic sounding of the lithosphere of Venus
Abstract
Electromagnetic (EM) investigation depths are larger on Venus than Earth due to the dearth of water in rocks, in spite of higher temperatures. Whistlers detected by Venus Express proved that lightning is present, so the Schumann resonances ∼10–40Hz may provide a global source of electromagnetic energy that penetrates ∼10–100km. Electrical conductivity will be sensitive at these depths to temperature structure and hence thermal lithospheric thickness. Using 1D analytic and 2D numerical models, we demonstrate that the Schumann resonances—transverse EM waves in the ground-ionosphere waveguide—remain sensitive at all altitudes to the properties of the boundaries. This is in marked contrast to other EM methods in which sensitivity to the ground falls off sharply with altitude. We develop a 1D analytical model for aerial EM sounding that treats the electrical properties of the subsurface (thermal gradient, water content, and presence of conductive crust) and ionosphere, and the effects of both random errors and biases that can influence the measurements. We initially consider specified 1D lithospheric thicknesses 100–500km, but we turn to 2D convection models with Newtonian temperature-dependent viscosity to provide representative vertical and lateral temperature variations. We invert for the conductivity-depth structure and then temperature gradient. For a dry Venus, we find that the error on temperature gradient obtained from any single local measurement is ∼100%—perhaps enough to distinguish “thick” vs. “thin” lithospheres. When averaging over thousands of kilometers, however, the standard deviation of the recovered thermal gradient is within the natural variability of the convection models,
... Electromagnetic methods, for instance magnetotellurics, provide a tool to constrain water, but also melt and carbon dioxide content (e.g., Sifré et al. 2014). Such measurements are best made from the surface, but aerial sounding is a possibility and can still achieve exploration depths exceeding the lithospheric thickness with sufficiently low electromagnetic frequencies (Grimm et al. 2012). If Venus' crust is dry, this method could provide an independent measurement of crustal and lithospheric thickness. ...
... An issue for such a measurement is the availability of electromagnetic sources. Lightning in Venus' atmosphere (e.g., Russell et al. 2007)-if found to be present-could provide one such source at frequencies that allow for wave penetration as deep as 100 km into Venus' interior (Grimm et al. 2012). ...
The dynamics and evolution of Venus’ mantle are of first-order relevance for the origin and modification of the tectonic and volcanic structures we observe on Venus today. Solid-state convection in the mantle induces stresses into the lithosphere and crust that drive deformation leading to tectonic signatures. Thermal coupling of the mantle with the atmosphere and the core leads to a distinct structure with substantial lateral heterogeneity, thermally and compositionally. These processes ultimately shape Venus’ tectonic regime and provide the framework to interpret surface observations made on Venus, such as gravity and topography. Tectonic and convective processes are continuously changing through geological time, largely driven by the long-term thermal and compositional evolution of Venus’ mantle. To date, no consensus has been reached on the geodynamic regime Venus’ mantle is presently in, mostly because observational data remains fragmentary. In contrast to Earth, Venus’ mantle does not support the existence of continuous plate tectonics on its surface. However, the planet’s surface signature substantially deviates from those of tectonically largely inactive bodies, such as Mars, Mercury, or the Moon. This work reviews the current state of knowledge of Venus’ mantle dynamics and evolution through time, focussing on a dynamic system perspective. Available observations to constrain the deep interior are evaluated and their insufficiency to pin down Venus’ evolutionary path is emphasised. Future missions will likely revive the discussion of these open issues and boost our current understanding by filling current data gaps; some promising avenues are discussed in this chapter.
... Lightning produced by either volcanic or atmospheric processes would provide a probe into Venus' current and past evolution. Lightning strikes could also excite global Schumann resonances at frequencies of tens of Hz, enabling electromagnetic sounding of Venus's lithosphere from an aerial platform (e.g., Grimm et al., 2012). Finally, lightning on Venus would create unique chemical environments in the atmosphere (e.g., Delitsky & Baines, 2015;Krasnopolsky, 2006). ...
Lightning in the atmosphere of Venus is either ubiquitous, rare, or non‐existent, depending on how one interprets diverse observations. Quantifying when and where, or even if lightning occurs, would provide novel information about Venus' atmospheric dynamics and chemistry. Lightning is also a potential risk to future missions, which could float in the cloud layers (∼50–70 km above the surface) for up to an Earth‐year. Over decades, spacecraft and ground‐based telescopes have searched for lightning at Venus using many instruments, including magnetometers, radios, and optical cameras. Two optical surveys (from the Akatsuki orbiter and the 61‐inch telescope on Mt. Bigelow, Arizona) observed several flashes at 777 nm (the unresolved triplet emission lines of excited atomic oxygen) that have been attributed to lightning. This conclusion is based, in part, on the statistical unlikelihood of so many meteors producing such energetic flashes, based in turn on the presumption that a low fraction (<1%) of a meteor's optical energy is emitted at 777 nm. We use observations of terrestrial meteors and analogue experiments to show that a much higher conversion factor (∼5%–10%) should be expected. Therefore, we calculate that smaller, more numerous meteoroids could have caused the observed flashes. Lightning is likely too rare to pose a hazard to missions that pass through or dwell in the clouds of Venus. Likewise, small meteoroids burn up at altitudes of ∼100 km, roughly twice as high above the surface as the clouds, and also would not pose a hazard.
... Recent work also suggests that valuable geoscience studies can be performed from the aerostat itself. Infrasound signatures of earthquakes can be detected in the atmosphere [18], and natural-source electromagnetic sounding can probe the upper mantle [19]. Together, these techniques can constrain the geodynamics of Venus without ever touching the surface. ...
... Measurements of changes in the magnetosphere induced by the solar wind enable probing of the planetary core [13]. Aerial electromagnetic sounding [14] can characterize regional variations in lithospheric thickness. The detection of lightning via electromagnetic and infrasound signatures enables determination of its occurrence rate, and assessment of its effect on atmospheric chemistry [15]. ...
... Kiefer (1991), however, suggests that upper mantle viscosities are greater on Venus than for the present-day Earth and similarly, Smrekar et al. (2007) propose a stronger lithosphere for Venus based on the low water content of its interior. Until further exploration is undertaken that includes surface seismometers capable of withstanding the high temperatures (as proposed by Crisp et al. 2002) and/or balloon EM sounding (Grimm et al. 2012), Venus' crustal thickness and rheological profile will remain the subject of debate. Even for Earth where so much data exists, 'crème brûlée' and 'jelly sandwich' strength profiles are still debated (e.g. ...
Evidence for modern plate tectonics in the Archaean is equivocal to absent, and alternative environments for formation and deformation of greenstone sequences are summarized. We focus on proposals for an unstable stagnant lid basaltic plateau crust, with cratonization occurring initially above major mantle plumes. Archaean continental drift initiated as a result of mantle traction forces acting on newly-formed subcontinental mantle keels, with further cratonic growth occurring as a result of terrane accretion to the leading edges of the migrating cratonic nuclei.
... Furthermore, these TEM waves can be measured from a balloon. Using a long traverse, the mean thermal gradient can be recovered to within 25% [14]. (2) Ref [5], from Tx power and theoretical noise floor only. ...
EM sounding formally encompasses both propagative (high-frequency) and
inductive (low-frequency) methods. Orbital, surface-penetrating radars
have been shown to be effective to depths of kilometers on the Moon and
in icy parts of Mars, but have been ineffective over the most of Mars'
silicate surface. Fixed, surface-based radars will likely improve
penetration to no more than several hundred meters. The optimum future
application of surface-penetrating radar on Mars is to provide
subsurface geological context for rovers. Deep sounding can be
accomplished using natural-source induction, and has been used to probe
the Moon and Galilean satellites. Applications include Mars groundwater,
Venus geodynamics, and the interior of the Moon from crust to core.
Magnetotellurics has the advantage of performing a complete sounding
from a single station.
Surface geologic features form a detailed record of Venus’ evolution. Venus displays a profusion of volcanic and tectonics features, including both familiar and exotic forms. One challenge to assessing the role of these features in Venus’ evolution is that there are too few impact craters to permit age dates for specific features or regions. Similarly, without surface water, erosion is limited and cannot be used to evaluate age. These same observations indicate Venus has, on average, a very young surface (150–1000 Ma), with the most recent surface deformation and volcanism largely preserved on the surface except where covered by limited impact ejecta. In contrast, most geologic activity on Mars, the Moon, and Mercury occurred in the 1st billion years. Earth’s geologic processes are almost all a result of plate tectonics. Venus’ lacks such a network of connected, large scale plates, leaving the nature of Venus’ dominant geodynamic process up for debate. In this review article, we describe Venus’ key volcanic and tectonic features, models for their origin, and possible links to evolution. We also present current knowledge of the composition and thickness of the crust, lithospheric thickness, and heat flow given their critical role in shaping surface geology and interior evolution. Given Venus’ hot lithosphere, abundant activity and potential analogues of continents, roll-back subduction, and microplates, it may provide insights into early Earth, prior to the onset of true plate tectonics. We explore similarities and differences between Venus and the Proterozoic or Archean Earth. Finally, we describe the future measurements needed to advance our understanding of volcanism, tectonism, and the evolution of Venus.
Lightning in the atmosphere of Venus is either ubiquitous, rare, or non-existent, depending on
how one interprets diverse observations. Quantifying when and where, or even if lightning
occurs would provide novel information about Venus’s atmospheric dynamics and chemistry.
Lightning is also a potential risk to future missions, which could float in the cloud layers (~50–
70 km above the surface) for up to an Earth-year. Over decades, spacecraft and ground-based
telescopes have searched for lightning at Venus using many instruments, including
magnetometers, radios, and optical cameras. Two optical surveys (from the Akatsuki orbiter and
the 61-inch telescope on Mt. Bigelow, Arizona) observed several flashes at 777 nm (the
unresolved triplet emission lines of excited atomic oxygen) that have been attributed to lightning.
This conclusion is based, in part, on the statistical unlikelihood of so many meteors producing
such energetic flashes, based in turn on the presumption that a low fraction (< 1%) of a meteor’s
optical energy is emitted at 777 nm. We use observations of terrestrial meteors and analogue
experiments to show that a much higher conversion factor (~5–10%) should be expected.
Therefore, we calculate that smaller, more numerous meteors could have caused the observed
flashes. Lightning is likely too rare to pose a hazard to missions that pass through or dwell in the
clouds of Venus. Likewise, small meteors burn up at altitudes of ~100 km, roughly twice as high
above the surface as the clouds, and also would not pose a hazard.
One objective of a lander mission to Jupiter's icy moon Europa is to detect liquid water within 30 km as well as characterizing the subsurface ocean. In order to satisfy this objective, water within the ice shell must also be identified. Inductive electromagnetic (EM) methods are optimal for water detection on Europa because even a small fraction of dissolved salts will make water orders of magnitude more electrically conductive than the ice shell. Compared to induction studies by the Galileo spacecraft, measurements of higher-frequency ambient EM fields are necessary to resolve the shallower depths of intrashell water. Although these fields have been mostly characterized by prior missions, their unknown source structures and plasma properties do not allow EM sounding using a single surface magnetometer or the orbit-to-surface magnetic transfer function, respectively. Instead, broadband EM sounding can be accomplished from a single surface station using the magnetotelluric (MT) method, which measures horizontal electric fields as well as the three-component magnetic field. We have developed a prototype Europa Magnetotelluric Sounder (EMS) to meet the measurement requirements in the relevant thermal, vacuum, and radiation environment. EMS comprises central electronics, a fluxgate magnetometer on a mast, and three ballistically deployed electrodes to measure differences in surface electric potential. In this paper, we describe EMS development and testing as well as providing supporting information on the concept of operations and calculations on water detectability. EMS can uniquely determine the occurrence of intrashell water on Europa, providing important constraints on habitability.
The lunar exosphere is produced by a combination of processes including thermal desorption, micrometeoroid bombardment, internal gas release, photon‐stimulated desorption, and charged‐particle sputtering. Here we investigate an additional mechanism not previously considered for the Moon, namely the role that newly born ions from the exosphere itself play in sputtering additional neutrals from the lunar surface, known as self‐sputtering. Our calculations suggest that this process may sputter neutrals into the lunar exosphere at a rate equal to or greater than charged‐particle sputtering due to passage through the Earth's plasma sheet when spatially averaged over the lunar dayside, while locally, self‐sputtering may equal or exceed solar wind charged‐particle sputtering and micrometeoroid bombardment. We use known or modeled densities and distributions of exospheric neutrals, laboratory‐derived values for the photoionization rates and neutral sputtering yields, and knowledge of the ambient electromagnetic environment at the Moon to derive estimates of the self‐sputtered neutral flux. We present the spatial variation of the self‐sputtered neutral flux and discuss the implications thereof.
Electromagnetic (EM) sounding of the Earth has been used for decades for science and resource exploration, but has seen limited planetary application. The inductive foundation of low-frequency electromagnetics has much greater investigation depth than surface-penetrating radar, at the expense of poorer resolution. Ambient EM energy from the solar wind, ionosphere, magnetosphere, or lightning provides natural sources for deep sounding, and can be measured using simple magnetometers and/or electrometers. Artificial sources (transmitters) yield higher SNR in shallower investigations. Planetary applications include crust and mantle structure, temperature, and composition, characterization of groundwater or interior oceans, mapping of shallow ice deposits, and potentially even detection of extant biosignatures.
Electromagnetic (EM) sounding of the Moon, largely performed during the Apollo program, provided constraints on core size, mantle composition, and interior temperature. We present new analytical and numerical models that demonstrate the abilities of a next generation of EM sounding to (1) determine the electrical structure of the outermost 500 km and its lateral variability, specifically to understand the extent of upper-mantle discontinuities and the structure of the Procellarum KREEP Terrane; (2) determine the temperature and composition of the lower mantle; and (3) better constrain core size. New EM sounding need not rely on the Apollo methodology, which analyzed the magnetic transfer function between a surface station and a distantly orbiting satellite. Instead, a network of magnetometers (as few as two) can be used, or a complete sounding can be performed from a single station by measuring both electric and magnetic fields. Furthermore, in the magnetotail or lunar wake, sensors can operate from orbit, at altitudes up to the desired investigation depth. The twin-spacecraft ARTEMIS mission will test these methods and a lunar geophysical network will provide definitive results.
The propagation of extremely low frequency (ELF) waves in the Earth surface-ionosphere cavity and the properties of the related Schumann resonances have been extensively studied in order to explain their relation with atmospheric electric phenomena. A similar approach can be used to understand the electric environment of Venus and, more importantly, search for the evidence of possible atmospheric lightning activity, which remains a controversial issue. We revisit the available models for ELF propagation in the cavity of Venus, recapitulate the similarities and differences with other planets, and present a full wave propagation finite element model with improved parameterization. The new model introduces corrections for refraction phenomena in the atmosphere; it takes into account the day-night asymmetry of the cavity and calculates the resulting eigenfrequency line splitting. The analytical and numerical approaches are validated against the very low frequency electric field data collected by Venera 11 and 12 during their descents through the atmosphere of Venus. Instrumentation suitable for the measurement of ELF waves in planetary atmospheres is briefly addressed.
The European Venus Explorer (EVE) mission described in this paper was proposed in December 2010 to ESA as an ‘M-class’ mission
under the Cosmic Vision programme. It consists of a single balloon platform floating in the middle of the main convective
cloud layer of Venus at an altitude of 55km, where temperatures and pressures are benign (∼25°C and ∼0.5 bar). The balloon
float lifetime would be at least 10 Earth days, long enough to guarantee at least one full circumnavigation of the planet.
This offers an ideal platform for the two main science goals of the mission: study of the current climate through detailed
characterization of cloud-level atmosphere, and investigation of the formation and evolution of Venus, through careful measurement
of noble gas isotopic abundances. These investigations would provide key data for comparative planetology of terrestrial planets
in our solar system and beyond.
KeywordsVenus–Planetary mission–Cosmic vision–Superpressure balloon–Geochemistry–Dynamics
The paper investigates the possible parameters of global ULF and VLF resonances on the terrestrial planets, Jupiter, and Io. Schumann resonance frequencies are determined for these planets. In addition, the eigenfrequencies and q-factors of ULF and VLF (transverse) resonances are calculated for Venus in the framework of an exponential vertical profile of ionization in the lower ionosphere.
The effect of strongly temperature-dependent viscosity on convection in the interior of Venus is studied systematically with the help of finite element numerical models. For viscosity contrasts satisfying experimental constraints on the rheology of rocks, Venus is likely to be in the regime of stagnant lid convection. This regime is characterized by the formation of a slowly creeping, very viscous lid on top of the mantle-Venusian lithosphere and is in agreement with the tectonic style observed on Venus. Stagnant lid convection explains large geoid to topography ratios on Venus by the thermal thinning of a thick lithosphere. The thickness of the lithosphere can be as large as 400-500 km for Beta Regio and 200-400 km on average. Geoid and topography data and experimental data on the rheology of rocks provide constraints on the viscosity of the mantle, 1020-1021 Pa s; the convective stresses in the interior, 0.2-0.5 MPa; the stresses in the lid, 100-200 MPa; the velocity in the interior, 0.5-3 cm yr-1 and the heat flux beneath the lithosphere, 8-16 mW m-2. Parameterized convection calculations of thermal history of Venus are difficult to reconcile with a thick present-day lithosphere. However, a sufficiently thick lithosphere can be formed if a convective regime with mobile plates was replaced by stagnant lid convection around 0.5 b.y. ago. One of the possible explanations for the cessation of Venusian plate tectonics is that during the evolution of Venus, stresses in the lid dropped below the yield strength of the lithosphere. This model predicts a drastic drop in the heat flux, thickening of the lithosphere, and suppression of melting and is consistent with the hypothesis of cessation of resurfacing on Venus around 0.5 b.y. ago.
The propagation of extremely low frequency (ELF) waves in the Earth surface-ionosphere cavity and the properties of the related Schumann resonances have been extensively studied in order to explain their relation with atmospheric electric phenomena. A similar approach can be used to understand the electric environment of Venus and, more importantly, search for the evidence of possible atmospheric lightning activity, which remains a controversial issue. We revisit the available models for ELF propagation in the cavity of Venus, recapitulate the similarities and differences with other planets, and present a full wave propagation finite element model with improved parameterization. The new model introduces corrections for refraction phenomena in the atmosphere; it takes into account the day-night asymmetry of the cavity and calculates the resulting eigenfrequency line splitting. The analytical and numerical approaches are validated against the very low frequency electric field data collected by Venera 11 and 12 during their descents through the atmosphere of Venus. Instrumentation suitable for the measurement of ELF waves in planetary atmospheres is briefly addressed.
A large coupling between the interior and the atmosphere is shown, leading to an escape of 15% of the energy of seismic surface waves in the atmosphere. Such coupling can be used to detect seismic signals from future Venus orbiter and must also be taken into account in Venus' spin evolution theories.
Previous experimental studies of convection in fluids with temperature-dependent viscosity reached viscosity contrasts of the order of 105. Although this value seems large, it still might not be large enough for understanding convection in the interiors of Earth and other planets whose viscosity is a much stronger function of temperature. The reason is that, according to theory, above 104–105 viscosity contrasts, convection must undergo a major transition—to stagnant lid convection. This is an asymptotic regime in which a stagnant lid is formed on the top of the layer and convection is driven by the intrinsic, rheological, temperature scale, rather than by the entire temperature drop in the layer. A finite element multigrid scheme appropriate for large viscosity variations is employed and convection with up to 1014 viscosity contrasts has been systematically investigated in a 2D square cell with free-slip boundaries. We reached the asymptotic regime in the limit of large viscosity contrasts and obtained scaling relations which are found to be in good agreement with theoretical predictions.
Presents results of a long-period MT investigation of the electrical structure beneath the eastern North Pacific. The electric field data consist of ~2 years of continuously recorded voltages across an unpowered, ~4000-km-long submarine telephone cable (HAW-1) extending from Point Arena, California, to Oahu, Hawaii. Inversion of the MT response reveals a conductive zone (0.05-0.1 S/m) between 150 and 400 km depth and a positive gradient below 500 km. This upper mantle conductivity is too high to be explained by solid-state conduction in dry olivine using reasonable mantle geotherms. Calculations based on measurements of hydrogen solubility and diffusivity in olivine indicate that H⁺ dissolved in olivine, possibly combined with a lattice preferred orientation consistent with measured seismic anisotropy, provide sufficient conductivity enhancement to explain the inversion results. The high conductivity may also be explained by the presence of gravitationally stable partial melt. -from Authors
THE high abundance ratio of deuterium to hydrogen in the atmosphere of Venus (120 times that on Earth) can he interpreted either as the signature of a lost primordial ocean1, or of a steady state in which water is continuously supplied to the surface of Venus by comets or volcanic outgassing, balancing loss through hydrogen escape2,3. New observations4–6 of a water concentration of only 30 parts per million in Venus' atmosphere imply that the residence time of water in the atmosphere, before it escapes to space, is short compared with the age of the Solar System, casting doubt on the primordial ocean hypothesis. But a recent theoretical reanalysis of collisional ejection7 has increased estimates of the deuterium escape efficiency by a factor of 10: this means that if the venusian water budget is in steady state, the D/H ratio of the source water must be 10–15 times higher than that on Earth, ruling out cometary water, whose D/H ratio is thought to be lower than this8. Here I suggest that these observations can be understood either as the result of continuous outgassing from a highly fractionated mantle source (such as might result from severe dessication of the mantle, or massive hydrogen escape early in the planet's history) or Rayleigh fractionation after massive outgassing from catastrophic resurfacing of the planet in the past 0.5–1 Gyr.
1] The fluxgate magnetometer on Venus Express samples the magnetic field near periapsis at 128 Hz. Bursts of plane-polarized magnetic waves in the vicinity of 100 Hz are observed propagating at small angles to the magnetic field. The magnetic field is generally horizontal in the region around periapsis, located at high northern latitudes. When the magnetic field remains within 15° of horizontal during the 2-min periapsis pass, no such waves are observed; but when there are brief periods during which the local magnetic field dips into the atmosphere by more than 15°, the bursts begin to appear. Such radial excursions of the magnetic field occur 25% of the time in the region around periapsis. The bursts are seen only on passes with these excursions. We interpret this magnetic control in terms of the coupling between the electromagnetic wave from lightning discharges refracted vertically by the increasing electron density and the nearly horizontal ionospheric magnetic field along which the energy is guided to the spacecraft. The inferred rate of electric discharges in the Venus atmosphere is about 20% of that seen in the Earth's atmosphere.
Hydrogen1 in Earth’s interior is known to play a key role in a number of processes. Consequently, inferring the distribution of hydrogen is a critical step in our study of dynamics and evolution of Earth. Usually, hydrogen distribution is inferred from two types of samples at the surface. First, a magma may contain hydrogen (water), and under some conditions the water content of the magma can be quenched upon cooling. In these cases, measurements of the water content of the magma provide us with some constraints on the hydrogen (water) content of the source region (if we know the partitioning of hydrogen between magmas and the source rocks, and the degree of melting). Second, hydrogen content of some xenoliths transported by magma can be measured. They provide a direct clue as to the hydrogen content in a region from which a xenolith has been carried. However, these direct, petrologic approaches have two major problems. Firstly, the sampling is limited by the distribution of volcanoes, and even if there are volcanoes that carry rocks from Earth’s interior, the depth extent that volcanoes sample rocks is limited (usually <200 km). Secondly, there is no guarantee that the hydrogen content that one measures on these samples actually represents the hydrogen content in a region where these samples came from. For example, hydrogen is known to diffuse very easily so hydrogen dissolved in minerals could diffuse out during the transport of a rock, or conversely, a piece of rock may acquire extra hydrogen during its ascent. Also hydrogen atoms dissolved in minerals may precipitate in the mineral to form fluid inclusions or micro-scale hydrous minerals. In summary, the direct method to infer the distribution of hydrogen from rock samples has major limitations, and an alternative approach, i.e., remote sensing hydrogen content from geophysical …
The magnetotelluric (MT) method, a technique for probing the electrical conductivity structure of the Earth, is increasingly used both in applied geophysics and in basic research. This is the first book on the subject to go into detail on practical aspects of applying the MT technique. Beginning with the basic principles of electromagnetic induction in the Earth, this introduction to magnetotellurics aims to guide students and researchers in geophysics and other areas of earth science through the practical aspects of the MT method: from planning a field campaign, through data processing and modelling, to tectonic and geodynamic interpretation. The book will be of use to graduate-level students and researchers who are embarking on a research project involving MT; to lecturers preparing courses on MT; and to geoscientists involved in multi-disciplinary research projects who wish to incorporate MT results in their interpretations.
THEMIS instruments incorporate a tri-axial Search Coil Magnetometer (SCM) designed to measure the magnetic components of waves associated with substorm breakup and expansion. The three search coil antennas cover the same frequency bandwidth, from 0.1 Hz to 4 kHz, in the ULF/ELF frequency range. They extend, with appropriate Noise Equivalent Magnetic Induction (NEMI) and sufficient overlap, the measurements of the fluxgate magnetometers. The NEMI of the searchcoil antennas and associated pre-amplifiers is smaller than 0.76 pT
\(/\sqrt{\mathrm{Hz}}\)
at 10 Hz. The analog signals produced by the searchcoils and associated preamplifiers are digitized and processed inside the Digital Field Box (DFB) and the Instrument Data Processing Unit (IDPU), together with data from the Electric Field Instrument (EFI). Searchcoil telemetry includes waveform transmission, FFT processed data, and data from a filter bank. The frequency range covered depends on the available telemetry. The searchcoils and their three axis structures have been precisely calibrated in a calibration facility, and the calibration of the transfer function is checked on board, usually once per orbit. The tri-axial searchcoils implemented on the five THEMIS spacecraft are working nominally.
Calculations of the space charge, ion density, and conductivity in the Venus atmosphere were made. The presence of the cloud particles on Venus causes a profound reduction in the calculated values of the ion density and conductivity compared to the values that are obtained without consideration of the cloud particles. When the cloud particles are included in the calculations, the results for the ion density and conductivity are approximately the same as those of the terrestrial atmosphere at the same pressure-altitude. Because the particles span such a large range of sizes and are abundant over a substantial range of pressure, the space charge varies strongly with altitude and particle size. Differential settling of the particles is expected to produce weak electric fields in the clouds.
The implications for mantle conductivity of data collected at the Tucson geomagnetic observatory on very long period magnetotelluric (MT) impedences, is discussed. Forward modeling, a linearized resolution analysis, and constrained one-dimensional inversion are used to delineate the range of models which are consistent with the estimated impedances. Although the MT data have limited resolution, large-scale vertical averages of mantle conductivity are well constrained. The following conclusions are reached concerning mantle conductivity beneath Tucson: (1) the upper 200 km has a conductance of order 10⁴ Siemens (S); (2) typical conductivities in the transition zone (400-700 km) are ~ 0.1-0.3 S m⁻¹; (3) resolvable large-scale averages of conductivity increase from ~ 0.2 Sm⁻¹ to ~ 1.0 Sm⁻¹ between 600 and 900 km depth; (4) between 900-1500 km, conductivity increases slowly. The implications of these results are discussed. -from Authors
The Venus STDT has defined the goals, objectives, mission architecture, science investigations and payload for a Flagship-class mission to Venus. The mission puts advanced exploration capabilities in orbit, in the atmosphere, and on the surface.
Recent laboratory measurements of electrical conductivity of mantle minerals are used in forward calculations for mantle conditions of temperature and pressure. The electrical conductivity of the Earth's mantle is influenced by many factors, which include temperature, pressure, the coexistence of multiple mineral phases, and oxygen fugacity. Ln order to treat these factors and to estimate the resulting uncertainties, we have used a variety of spatial averaging schemes for mixtures of the mantle minerals and have incorporated effects of oxygen fugacity. In addition, to better calculate lower mantle conductivities, we report new measurements for electrical conductivity of magnesiowustite (Mg0.89Fe0.11)O. Because the effective medium theory averages lie between the Hashin-Shtrikman bounds for the whole mantle, a laboratory-based conductivity-depth profile was constructed using this averaging scheme. Comparison of apparent resistivities calculated from the laboratory-based conductivity profile with those from field geophysical models shows that the two approaches agree well.
The nearly global rader imaging and altimetry measurements of the surface of Venus obtained by the Magellan spacecraft have revealed that deformational features of a wide variety of styles and spatial scales are nearly ubiquitous on the planet. Many areas of Venus record a superposition of different episodes of deformation and volcanism. The common coherence of strain patterns over hundreds of kilometers implies that even many local features reflect a crustal response to mantle dynamic processes. The styles and scales of tectonic deformation on Venus may be consequences of three differences from the Earth: 1) The absence of a hydrological cycle and significant erosion dictates that multiple episodes of deformation are typically well-preserved. 2) A high surface temperature and thus a significantly shallower onset of ductile behavior in the middle to lower crust gives rise to a rich spectrum of smaller-scale deformational features. 3) A strong coupling of mantle convection to the upper mantle portion of the lithosphere leads to crustal stress fields that are coherent over large distances. -from Authors
In the magnetotelluric (MT) method, natural electromagnetic fields are used to investigate the electrical conductivity structure of the earth. Natural sources of MT fields above about 1 Hz are thunderstorms worldwide, from which lightning radiates fields which propagate to great distances. At frequencies below 1 Hz, the bulk of the signal is due to current systems in the magnetosphere set up by solar activity.
In both cases the electromagnetic (EM) fields at the surface of the earth behave almost like plane waves, with most of their energy reflected but with a small amount propagating vertically downward into the earth. The amplitude, phase, and directional relationships between electric (E) and magnetic (H or B) fields on the surface depend on the distribution of electrical conductivity in the subsurface. By use of computed models, field measurement programs can be designed to study regions of interest within the earth from depths of a few tens of meters to the upper mantle.
Equipment to carry out the measurements consists of magnetometers for the frequency range of interest; pairs of electrodes separated by suitable spacings to sense the electric field variations; plus amplifiers, filters, and suitable digital recording and processing systems to permit the signals to be captured and analyzed. The magnetometers in particular must have very low noise and great stability because those signals are so weak.
Scitation is the online home of leading journals and conference proceedings from AIP Publishing and AIP Member Societies
Water (hydrogen) likely enhances electrical conductivity in olivine
either directly through charge transport by proton or indirectly through
enhancement of ionic conductivity by M-site vacancies (Karato, 1990).
Although this hypothesis has been used to interpret results of
geophysical measurements of electrical conductivity, there has been no
experimental data to test it. In fact, the recent study on wadsleyite
and ringwoodite (Huang et al. 2005) did not entirely validate Karato's
hypothesis: electrical conduction in these minerals (with water) occurs
through the motion of free proton and not by all protons (protons at
M-site do not contribute to conductivity directly). Here we report the
results of laboratory measurements of electrical conductivity in
olivine as a function of water content (and other factors).
Polycrystalline samples of olivine (with ~5% of opx) were hot-pressed
with or without the addition of water. We control water content,
grain-size and oxygen fugacity in addition to temperature (and
pressure). We have evidence that a large amount of water is present in
polycrystalline olivine at grain-boundaries and therefore conductivity
is measured for samples with different grain-sizes. Grain-size of
samples range is ~ 2 to ~ 20 microns (except for a dunite for which
grain-size is ~2mm). The electrical conductivity was measured under
high-pressure (2~3GPa) and temperature (900~1600K) using an impedance
spectroscopy within a frequency range of 102~106Hz. Electrode or
"capsule (shield)" materials are Mo, or Fe or Ni that define oxygen
fugacity. Water content and grain-size of all samples were measured both
before and after conductivity measurements. We minimize the water loss
by choosing low voltage and high frequency, but in all cases, there is
some water loss during a measurement. Our current results show clear
evidence of enhanced conductivity by water, and weak dependence of
conductivity on grain-size. The results will be compared with various
models to identify the microscopic mechanisms of conduction and with
geophysical measurements to infer the water contents in the upper mantle
Mantle Convection in the Earth and Planets is a comprehensive synthesis
of all aspects of mantle convection within the Earth, the terrestrial
planets, the Moon, and the Galilean satellites of Jupiter. The authors
include up-to-date discussions of the latest research developments that
have revolutionized our understanding of the Earth and the planets. The
book features a comprehensive index, an extensive reference list,
numerous illustrations (many in color) and major questions that focus
the discussion and suggest avenues of future research. It is suitable
as a text for graduate courses in geophysics and planetary physics, and
as a supplementary reference for use at the undergraduate level. It is
also an invaluable review for researchers in the broad fields of the
Earth and planetary sciences.
It is suggested that episodic plate tectonics occurs on Venus; episodes of rapid plate tectonics are separated by periods of surface quiescence. For the last 500+/-200 m.y. it is postulated that the surface of Venus has been a single rigid plate that has been thickening due to conductive cooling. A near-uniform surface age is consistent with observed crater densities and the relatively small number of craters modified by surface tectonics or embayed by lava flows. A lithosphere that has conductively thickened for some 500 m.y. has a thickness of about 300 km, nearly an order of magnitude greater than the thickness associated with steady state conductive heat loss. Such a thick lithosphere can support the high topography and associated gravity anomalies on Venus as well as the unrelaxed craters; studies of lithospheric flexure at coronae are also consistent with a thick elastic lithosphere. Incipient subduction associated with large coronae may represent the onset of a new episode of rapid plate tectonics. On the Earth, 75-90% of mantle heat transport is attributed to the creation of new oceanic lithosphere at ocean ridges. This process is not operative on Venus. This paper suggests that episodic plate tectonics on Venus constitutes the primary mechanism for mantle heat transport on that planet.
The tectonics and volcanism of the terrestrial planets are controlled by the loss of heat from the planetary interior. On the Earth, about 70% of the heat flow through the mantle is attributed to the subduction of cold lithosphere. In order to understand the tectonic and volcanic processes on Venus it is necessary to understand how heat is transported through its mantle. In this paper, three alternative end-member hypotheses are considered. The first is the steady state loss of heat through the mantle to the surface in analogy to the Earth. However, without plate tectonics and subduction on Venus, a steady state requires either a very high plume flux or very rapid rates of lithospheric delamination. The required plume flux would be equivalent to about 80 plumes with the strength of the Hawaiian plume. The required delamination flux implies a 50% delamination of the entire Venus lithosphere every 10 m.y. Neither appears possible, so that it is concluded that Venus cannot transport heat through its mantle to its surface on a steady state basis. The second hypothesis is that there has been a strong upward concentration of the heat-producing elements into the crust of Venus; the heat generated is then lost by conduction. Surface measurements of the concentrations of the heat-producing elements place constraints on this model. If everything is favorable this hypothesis might be marginally acceptable, but it is considered to be highly unlikely. The third hypothesis is that heat is lost by episodic global subduction events followed by long periods of surface quiescence. The near-random distribution of craters suggests that the last subduction event occurred about 500 Ma. This model implies a thick thermal lithosphere (~300 km) at the present time, which is consistent with a variety of surface observations. Lava lakes on the Earth are considered as analogies to plate tectonics; they also exhibit episodic subduction events.
A mountainous area in northern Maine of predominantly slate, but containing an igneous stock, was surveyed at 150 m mean flight altitude. The 150-m surveyed was repeated at 300 m, and two of the 150-m flight lines were repeated at a total of three other altitudes. A comparison of the 150-m survey with the topography and with the 300-m survey revealed that although most of the resistivity information of the 150-m survey was retained at 300 m, serious difference arose due to topographic influences. Profiles of the individual electric field components at the various altitudes, then revealed that topography was distorting resistivity values through its effect upon only the vertical component of the electric field. The separate influences of phase and amplitude were analyzed using the results of a ground survey of the total, complex surface impedance. The phase of the tilt proved to be important in the airborne differentiation of the rock types. The entire 150-m survey was reevaluated with topographic effects removed from the vertical electric field. The resolution of the igneous geology improved and several of these improvements were verified by the ground measurements. In addition, it is concluded from a comparison of the 300-m survey with both the topographically corrected and uncorrected 150-m surveys that wavetilt is not preserved with altitude over ground resistivity anomalies.
In order to extend the useful temperature range of interpretation of olivine electrical conductivity sigma, the nonlinear iterative Marquardt technique is used to fit experimental data over the range 720-1500 C to a parametric form. The model describes conduction by migration of two different thermally activated defect populations with activation energies A1 and A2, and preexponential terms sigma (1) and sigma (2) that depend on number of charge carries and their mobility and that may be different for each crystallographic direction. A combined interpretation of recent high (San Carlos olivine) and low (Jackson County dunite) temperature measurements has been made that demonstrates that a single activation energy A1 for all three crystallographic directions adequately fits the data. The parametric fits show that the high-temperature conduction mechanism has far greater anisotropy than the low-temperature mechanism, consistent with previous assignments to ionic and electronic conduction, respectively.
Previous experimental studies of convection in fluids with temperature-dependent viscosity reached viscosity contrasts of the order of 105. Although this value seems large, it still might not be large enough for understanding convection in the interiors of Earth and other planets whose viscosity is a much stronger function of temperature. The reason is that, according to theory, above 104-105 viscosity contrasts, convection must undergo a major transition-to stagnant lid convection. This is an asymptotic regime in which a stagnant lid is formed on the top of the layer and convection is driven by the intrinsic, rheological, temperature scale, rather than by the entire temperature drop in the layer. A finite element multigrid scheme appropriate for large viscosity variations is employed and convection with up to 1014 viscosity contrasts has been systematically investigated in a 2D square cell with free-slip boundaries. We reached the asymptotic regime in the limit of large viscosity contrasts and obtained scaling relations which are found to be in good agreement with theoretical predictions.
A global geomagnetic response function, sensitive to the average radial electrical conductivity structure of Earth's mantle to depths of at least 1800km, is obtained by averaging published, single-site response functions estimated at periods between 105-107 seconds from magnetic observatory records. Geotherms inferred from these conductivities using a laboratory model for the temperature dependence of dry subsolidus olivine yield temperatures of 1750°C at a depth of 410km. Inclusion of a sharp jump in conductivity at the 660km seismic discontinuity lowers the electrogeotherm to 1600°C at 410km, while an explicit penalty on the conductivity at this depth demonstrates that a temperature of 1400° is compatible with the global response function if 1000 S of additional conductance is included above 200km. The electrical conductivity below the jump at 660km is 1 S/m increasing to 2 S/m at 200km, in excellent agreement with recent diamond anvil measurements of lower mantle materials. -from Author
Recent laboratory measurements of electrical conductivity of mantle
minerals are used in forward calculations for mantle conditions of
temperature and pressure. The electrical conductivity of the Earth's
mantle is influenced by many factors, which include temperature,
pressure, the coexistence of multiple mineral phases, and oxygen
fugacity. In order to treat these factors and to estimate the resulting
uncertainties, we have used a variety of spatial averaging schemes for
mixtures of the mantle minerals and have incorporated effects of oxygen
fugacity. In addition, to better calculate lower mantle conductivities,
we report new measurements for electrical conductivity of
magnesiowüstite (Mg0.89Fe0.11)O. Because the
effective medium theory averages lie between the Hashin-Shtrikman bounds
for the whole mantle, a laboratory-based conductivity-depth profile was
constructed using this averaging scheme. Comparison of apparent
resistivities calculated from the laboratory-based conductivity profile
with those from field geophysical models shows that the two approaches
agree well.
Water plays an important role in most processes within the Earth's mantle, e.g. transport phenomena, differentiation and seismic properties. This paper reviews the various aspects of water in the mantle, with a special emphasis on the water content in nominally anhydrous minerals, especially lower-mantle minerals. The saturation of the upper mantle with respect to water is calculated as a function of pressure, based on available water-solubility data obtained for upper-mantle minerals. The result indicates that the upper mantle is saturated at pressures between 2 and 4 GPa for bulk water contents of 250 to 700 ppm wt., as retrieved from measurements on mid-ocean ridge basalts (MORBs) and ocean island basalts (OIBs). Whereas up to 4000 ppm wt. of H2O could be dissolved in the upper mantle at pressures corresponding to 410 km depth, such a value is less than the 1.5—2.5 wt.% solubility stored in the hydrous phases of the transition zone. Water solubility in mantle perovskite is still controversial, because of the difficulty of synthesizing samples free of impurities. Reported data indicate that water solubility in perovskite decreases with increasing temperature and Al content. Water partitions preferentially into ferropericlase rather than into perovskite, and its water solubility increases with the incorporation of trivalent cations.
The effects of dissolved H2O on the electrical conductivity and its anisotropy in olivine (Fo90) at 8GPa were investigated by complex impedance spectroscopy. At nominally anhydrous conditions, conduction along [100] and [001] is slightly higher than along [010] in contrast to observations made at lower pressures in earlier studies. Increasing H2O content increases conductivities but activation energies are lower and H2O concentration dependent. The use of polarized FTIR spectroscopy to determine H2O concentrations reveals a weaker than expected effect that water has on olivine conductivity and distinguishes our results from earlier studies based on analyses using non-polarized infrared spectroscopy. We show that at H2O concentrations of a few hundred wt ppm or less, that the dominant conduction mechanism at mantle temperatures continues via small polarons, such as that observed for anhydrous olivine. Our results also suggest that at depths greater than 200km, the presence of H2O may not be necessary to explain regions in the upper mantle where both electrical and seismic anisotropy are observed. This can be explained by differences in the pressure dependence of the activation energy for conduction along each of the three crystallographic axes. However, while electrical anisotropy of anhydrous olivine remains weak at 8GPa, it is nevertheless enhanced by elevated concentrations (>several hundred wt ppm) of dissolved H2O. At these conditions dominated by proton hopping, conductivity along [010] is highest, approximately an order of magnitude greater than along [100]. Additionally, at 1000wt ppm and 1500°C, an isotropic conductivity derived from the data is about 1 order of magnitude higher than that for nominally anhydrous olivine. Thus, in regions of the mantle characterized by anomalously high conductivities and both electrical and seismic anisotropy, significant amounts of dissolved hydrogen can be expected.
1] A partially uniform knee (PUK) model is a combination of two two-dimensional telegraph equation (TDTE)-based techniques: the ''knee'' model, which addresses the problem of approximating the knee-like conductivity profile (on a semilogarithmic scale) of the Earth's ionosphere, and the global partially uniform day-night model, which allows a convenient treatment of the day-night asymmetry. Incorporation of the ''knee'' conductivity profile allows to overcome the shortcoming of the two-exponential technique, widely used in extremely low frequency (ELF) work, too flat frequency dependence of quality factor in the Schumann resonance (SR) range (5–40 Hz). The PUK model predictions for Schumann resonance parameters reasonably represent observations in the SR frequency range. Propagation parameters for other planets were calculated on the basis of existing ionospheric models of the planets. To allow the approximation of structured conductivity profiles of Venus and Mars, the ''knee'' model was upgraded to a double-''knee'' by inserting an additional ''knee'' to the profile. In general, this technique allows to approximate very structured profiles by adding as many ''knees'' as necessary. Calculations show that the detection of Schumann resonances on Venus, Mars, and Titan is possible, though low-quality factors on Mars and Titan imply that pronounced peaks are not to be presumed on these planets.
Applied electromagnetic research in recent years has been influenced by the growing importance of geothermal energy, coal, and permafrost, in addition to the traditional area of minerals. The interest in near-insulators such as coal and ice encouraged development of radars and other VHF-UHF techniques. Interpreting such measurements required reliable physical properties data for those materials over a frequency range of 6–10 decades. The utility of the high frequency field data has been improved through the use of transient techniques for data acquisition, and data processing schemes similar to those used in reflection seismology. The major developments in the more usual frequency range of applied geophysics (30 Hz—3 kHz) have also dealt with transients. In certain circumstances they appear to have a fundamental sensitivity not readily obtained by discrete frequency methods.Computer modelling of 3-D problems is progressing slowly. Improved 2-D inversion programs are in use, but their capabilities are very limited.Superconductivity plays a role in several new instrument developments. SQUIDs, and SQUID gradiometers have improved considerably since the last Workshop. Robust SQUID magnetometers having noise levels of
10 - 5 - 10 - 6 nT/ÖHz10^{ - 5} - 10^{ - 6} nT/\surd Hz
can now be obtained commercially. Gradiometer sensitivities have improved accordingly. A superconducting loop 3 metres in diameter, to be test flown early in 1979, is the prototype of a new low frequency system to map conductivity from a helicopter. It is expected to have greater depth penetration in conductive terranes than the best existing systems, because of the low frequency and anticipated low system noise.A new magnetotelluric procedure, using a remote field reference, reduced the scatter in apparent resistivities and other response functions to a few percent. Further improvements must now be made in modelling and interpreting MT results if we are to benefit from this development.
Our current knowledge of the spatial structure of the Venus ionosphere and its temporal behavior is reviewed, with emphasis on the more recent Pioneer Venus measurements and analysis not covered in earlier reviews. We will stress the ionosphere structure, since other papers in this issue deal with its dynamics, and its magnetic properties. We also discuss some of the limitations that the orbit has placed on the spatial and temporal coverage of the ionosphere. For the benefit of future users of the data some of the factors which affect the measurement accuracies are discussed in an Appendix.
The theory of three-dimensional and finite-amplitude convection in a viscous spherical shell with temperature and pressure dependent physical parameters is developed on the basis of a modified Boussinesq fluid assumption. The lateral dependences of the variables are resolved through their spherical harmonic representations, whereas their radial and time dependences are determined by numerical procedures. The theory is then applied to produce thermal evolution models for Venus. The emphasis is on illustrating the effects of certain physical parameters on the thermal evolution rather than proposing a specific thermal history for the planet. The main conclusions achieved in this paper are (1) a significant portion of the present temperature in the mantle and heat flux at the surface of Venus is probably owing to the decay of a high temperature established in the planet at the completion of its core formation, (2) the effective Rayleigh number of the mantle is so high that even the lower order modes of convection cool the planet sufficiently and maintain an almost adiabatic temperature gradient in the convecting region and high temperature gradients in the thermal boundary layers, (3) the convection is oscillatory with avalanche type properties which induces oscillatory features to the surface heat flux and the thickness of the crustal layer, and (4) a planetary model with a recycling crust cools much faster than those with a permanently buoyant crust.The models presented in this paper suggest that Venus has been highly convective during its history until ∼ 0.5 Ga ago. The vigorous convection was bringing hot and fresh material from the deep interior to the surface and dragging down the crustal slags, floating on the surface, in to the mantle. The rate of cooling of the planet was so high that its core has solidified. In the last 0.5 Ga the vigour of convection diminished considerably and the crustal slags developed into a global and permanently buoyant crustal layer. The tectonic style on Venus has, consequently, changed from the recycling of crustal plates to hot spot volcanics. At the present time the planet is completely solid, except in the upper part of its mantle where partial melting may occur.
The electrical conductivity of San Carlos olivine aggregate of various water content was measured at a pressure of 10 GPa in a Kawai-type multi-anvil apparatus. Conductivity measurements were performed on two sets of samples to determine the effect on conductivity of water in olivine: 1) a hydrogen-doped sample and 2) a hydrogen-undoped sample. To minimize water escape from the hydrogen-doped samples, the conductivity measurement was carried out below 1000 K. Three conduction mechanisms were identified from the Arrhenian behavior of the undoped samples, which include a small amount of water. A change in the activation enthalpy indicated that the dominant conduction mechanism changed from proton conduction to small polaron conduction with increasing temperature. At temperatures above 1700 K, the activation enthalpy exceeds 2 eV suggesting that the dominant mechanism of charge transport would be ionic conduction. The conductivity increased with increasing water content. The activation enthalpy for proton conduction tends to decrease slightly with increasing water content. The activation enthalpy determined for each run had similar values (~ 0.9 eV). Taking the water concentration dependence of activation enthalpy into account for proton conduction, all data were fitted to the electrical conductivity formula σ=σ0Iexp[−EI/kT]+σ0Hexp[−EH/kT]+σ0PCWexp[−(E0−αCW1/3)/kT], where σ0 represents a pre-exponential term, CW is the water content in weight percent, E is the activation enthalpy, E0 is the activation enthalpy for proton conduction at very low water concentration, α is the geometrical factor, k is the Boltzmann constant, T is absolute temperature and subscripts I, H and P denote ionic, hopping (small polaron) and proton conductions, respectively. The conductivity jump at the 410 km discontinuity (olivine–wadsleyite transition) is much smaller than that previously predicted. Since the contribution of proton conduction to the bulk electrical conductivity decreases with increasing temperature the high conductivity anomaly at the top of the asthenosphere cannot be explained by olivine hydration.
In the past decade, both inductive electromagnetic survey instrumentation and associated interpretive techniques have become refined to the point that electromagnetic techniques are widely used for geological mapping as well for the direct detection of conductive ore bodies. Electromagnetic survey techniques have been particularly successful in exploration for potable groundwater, for measuring salinity levels in aquifers and monitoring coastal saline intrusion, and for mapping soil salinity in connection with crop growth.Regardless of the techniques employed, it is the terrain conductivity that is measured, and it is a particular advantage of electromagnetic techniques that small variations in the bulk conductivity of the terrain can often be detected. A further advantage is that most electromagnetic techniques allow measurements to be made rapidly, and survey costs are generally less than those associated with conventional DC resistivity surveys or, conversely, larger areas can be surveyed in greater detaul for comparable cost. A disadvantage of electromagnetic instrumentation is that although the shallower units cost about the same as resistivity equipment, the deeper penetration systems are relatively expensive. In general, electromagnetic systems are most effective in looking for the better conductors and are ineffective in searching for resistive material. In all cases some knowledge of electromagnetic theory is desirable for a successful interpretation.In this paper we present several case history selected from the literature in which a variety of electromagnetic systems (horizontal loop EM, ground conductivity meters and VLF) are used either alone or in conjunction with conventional resistivity to explore for groundwater.
Geoid and topography observations are used to constrain the density structure of the venusian lithosphere under the assumptions that convective stresses are small and local isostasy prevails. Results are presented for 13 venusian highlands, including Ishtar Terra. In general, Venus has a thick (200–400 km) thermal lithosphere which is thinned beneath volcanic highlands by as much as 80%, inducing slopes of 0.05 to 0.18 in the base of the lithosphere. From scalings of geoid/topography ratio and lithospheric basal slope as a function of internal Rayleigh number and viscosity contrast, we estimate lower bounds to these parameters of 107and 105, respectively. Several lines of evidence point to values near these lower limits, which puts Venus in the sluggish-lid regime of convection. There also exist several accumulations of crustal material, most notably Ovda Regio, where there may be as much as 90 km of crust.