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Salts are important secondary components of the icy shells of outer-planet satellites. Salt hydrates formed from freezing brines may influence reflectivity and absorption of surface-penetrating radars. We compare new measurements of frozen mixtures of water and magnesium sulfate with those made by Pettinelli et al. (2016) and Grimm et al. (2008) . We find a high-frequency real dielectric constant of the hydrate endmember (meridianiite) of 4.9 ± 0.3 at −85 °C, within the error bounds derived by Pettinelli and colleagues. We confirm the existence of a dielectric relaxation in meridianiite, similar to the well-known response of ice but at higher frequency and with lower amplitude. However, the meridianiite relaxation is evident only at supereutectic concentrations and for a freezing history that maximizes annealing and partitioning of soluble impurities into the lattice. Dielectric relaxation is nonetheless important for radar sounding of Europa because it controls absorptive loss. Chloride is the most abundant ice-soluble impurity on Europa and will have the greatest impact on radar absorption, whether in ice or hydrates. Salt hydrates may generate reflectivity contrasts in surface-penetrating radar but are unlikely to be uniquely identified.

The surface of Mars has been well mapped and characterized, yet the subsurface — the most likely place to find signs of extant or extinct life and a repository of useful resources for human exploration — remains unexplored. In the near future this is set to change.

The Surface Electrical Properties (SEP) experiment deployed at the Apollo 17 landing site in the Taurus-Littrow valley was a continuous-wave, radiofrequency (1–32 MHz) interferometer designed to probe the subsurface from depths of meters to kilometers. Signals were transmitted by orthogonal electric dipoles laid out near the lunar module and three components of the magnetic field were measured on the lunar rover at useful distances up to 1.6 km. The gross range decay of the signals is well fit by a simple equation combining spherical spreading, first-order interference, and attenuation. The derived loss tangents ∼0.01 are consistent with laboratory-measured radiofrequency absorption in Apollo 17 basalts. Thus the attenuation in the SEP data can be explained by absorption with negligible small-scale scattering; the latter can be quantified as a dearth of 10-m scale lateral heterogeneity and a mean-free path of kilometers. Numerical modeling was used to compare predicted vs. observed waveforms and to invert for vertically varying dielectric structure. The dielectric constant is readily converted to density and porosity for lunar rocks. The preferred model shows a sharp decrease in porosity in the top 20 − 30 m, with little change below 300 m. This likely tracks the transition from fully gardened regolith, to impact-fractured rock, to largely intact 3.7-Ga basalt. The bottom of the basalt was not detected by the SEP to a depth of ∼2 km or more, which is consistent with reanalysis of Apollo 17 seismic and gravity data.

Bulletin of the AAS, 53 (4)

Venus Corona and Tessera Explorer (VeCaTEx) would use an aerobot to descend repeatedly beneath the dense clouds for imaging targeted area of the surface in the near infrared to address six of the prime investigations prioritized by VEXAG. The technologies needed could be matured during the next decade.

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.

International audience; More than 85% of the 23 investigations developed by VEXAG are largely accomplished via a NF mission centered on a variable-altitude balloon (aerobot) supported by a science/comm orbiter. Circling Venus >15 times over ~90 days, the aerobot repeatedly visits 52-62 km alts as it semi-continuously samples a host of environmental & surface parameters.

Charles D. Edwards (Jet Propulsion Laboratory, California Institute of Technology). Co-Authors: 1. Vlada Stamenkovic Jet Propulsion Laboratory, California Institute of Technology; 2. Penelope Boston NASA Ames; 3. Kennda Lynch LPI/USRA … et al.

Intense bombardment is considered characteristic of the Hadean and early Archean eons, yet some detrital zircons indicate that near-surface water was present and thus at least intervals of clement conditions may have existed. We investigate the habitability of the top few kilometers of the subsurface by updating a prior approach to thermal evolution of the crust due to impact heating, using a revised bombardment history, a more accurate thermal model, and treatment of melt sheets from large projectiles (>100 km diameter). We find that subsurface habitable volume grows nearly continuously throughout the Hadean and early Archean (4.5–3.5 Ga) because impact heat is dissipated rapidly compared to the total duration and waning strength of the bombardment. Global sterilization was only achieved using an order of magnitude more projectiles in 1/10 the time. Melt sheets from large projectiles can completely resurface the Earth several times prior to ∼4.2 Ga but at most once since then. Even in the Hadean, melt sheets have little effect on habitability because cooling times are short compared to resurfacing intervals, allowing subsurface biospheres to be locally re-established by groundwater infiltration between major impacts. Therefore the subsurface is always habitable somewhere, and production of global steam or silicate-vapor atmospheres are the only remaining avenues to early surface sterilization by bombardment.

Recurring slope lineae (RSL) are narrow dark features that incrementally lengthen down steep low-albedo slopes when temperatures are warm, subsequently fade, and reoccur annually. RSL could be caused by wet, wet-triggered debris, or dry granular flow mechanisms. We have classified 190 candidate and confirmed southern mid-latitude (SML) RSL sites using six Mars years of high-resolution imagery to provide new constraints on RSL flow mechanisms. We demonstrate that at least three confirmed SML RSL sites have at least two pulses of active lengthening. We have also confirmed that RSL start much earlier than previously thought. The first SML RSL pulse occurs from solar longitude (Ls) 187° ± 6° to 226° ± 14° or 62 ± 32 sols, while the second pulse occurs from Ls 260° ± 26° to 329° ± 29° or 113 ± 89 sols. Even with the newly observed early pulse, the total SML RSL active duration is still shorter than RSL found elsewhere on the planet. No significant seasonal variations exist between the dominant (N, NW, W) slope-face orientations. The central peak of Hale crater appears to be anomalous in that it has a third pulse that rejuvenates RSL late in the southern summer (Ls 330° ± 1° to 348° ± 19° or 48 ± 26 sols). These additional RSL pulses of activity add increased complexity to RSL behavior. Follow-up missions may be needed to unambiguously discriminate between RSL mechanisms.

Recurring slope lineae (RSL) are dark narrow features that incrementally lengthen and fade at least once each Mars year. Their origin has remained enigmatic. We report quantitative modeling of three hypotheses related to triggering of over-steepened sand caches that result in dry-grain flow. First, sand could be supplied by either wind blowing upslope from within the crater or by wind blowing into the crater from the outside. Without cementation, grain flows should be directly correlated with temporal variations in potential sediment transport. Second, deliquesced perchlorate salts could increase soil cohesion; with the loss of water this soil cohesion may be reduced. The timing of accumulation versus release can be distinct from each other. Third, seasonal water frost could act in a similar cementation capacity. We assess these hypotheses using atmospheric modeling to determine wind speed, wind direction, temperature, and relative humidity at three craters (Rauna, Krupac, and Palikir) that host confirmed RSL and that cover a large latitudinal range. Overall, we find no convincing support for any of these hypotheses. Deliquescence is predicted to occur only at Rauna crater and its formation does not correlate with RSL activity. The occurrence of frost is inconsistently correlated with RSL activity among the three craters. Upslope winds at Palikir crater transport a significant amount of sediment only when RSL are active. However, the largest sediment flux into the crater is also during periods of RSL activity, thus supporting both internal and external contributions. Sediment transport into Krupac crater to S- and SW-facing slopes is also maximized when RSL are active, but there is no correlated upslope transport. This supports the hypothesis that RSL formation is dominated by external sediment supply alone. On the other hand, W- and NW-facing Krupac and all RSL at Rauna show no correlations with any directional sediment transport. Nonetheless, we suggest that the mixed success of the external sediment transport model is still quantitatively better than any competitor (including water), and that we simply lack the model and data resolution to treat RSL at the required meter scales. In all of the aeolian models, favorably-directed potential sediment flux greatly exceeds the volume of sand that must be displaced to form RSL. We conclude that RSL may require a particular combination of favorable strong winds, local sediment traps, and a supply of sand grains that easily saltate on Mars (~100 μm in diameter).

The ice-rich dwarf planet Ceres is the largest object in the main asteroid belt and is thought to have a brine or mud layer at a depth of tens of kilometers. Furthermore, recent surface deposits of brine-sourced material imply shallow feeder structures such as sills or dikes. Inductive sounding of Ceres can be performed using the solar wind as a source, as was done for the Moon during Apollo. However, the magnetotelluric method -- measuring both electric and magnetic fields at the surface -- is not sensitive to plasma effects that were experienced for Apollo, which used an orbit-to-surface magnetic transfer function. The highly conductive brine targets are readily separable from the resistive ice and rock interior, such that the depth to deep and shallow brines can be assessed simultaneously. The instrumentation will be tested on the Moon in 2023 and is ready for implementation on a Ceres landed mission.

A prototype ground-penetrating radar (GPR) for planetary exploration was tested in the Western Desert of Egypt by Ciarletti et al. (Planet. Space Sci., 117, 177, 2015), and penetration depths of 200–300 m were reported. However, geoelectrical and electromagnetic soundings performed during the same campaign indicate very low resistivities (

Recurring slope lineae (RSL) are narrow (0.5–5 m) dark features on Mars that incrementally lengthen down steep slopes, fade in colder seasons, and recur annually. These traits suggest that liquid water is flowing in the shallow subsurface of Mars today. Here we describe High Resolution Imaging Science Experiment (HiRISE) observations of RSL within Valles Marineris (VM). We have identified 239 candidate and confirmed RSL sites within all the major canyons of VM, with the exception of Echus Chasma. About half of all the globally known RSL locations occur within VM and the areal density of RSL on Coprates Montes appears to be the greatest on the planet. VM RSL are heterogeneously distributed, as they are primarily clustered in certain areas while being conspicuously absent in other locations that appear otherwise favorable. RSL have been found on many of the interior layered deposits (ILDs) within VM. Such ILD RSL appear to traverse bedrock, instead of regolith like all other RSL. Forty-six of the VM RSL sites show incremental lengthening and exhibit similar behavior in most of the canyons of VM, but the RSL duration at one site in Juventae Chasma is significantly reduced. Furthermore, the lengthening seasonality depends solely on slope orientation, with typical VM RSL on a given slope lengthening for ∼42–74% of a Mars year. There are always RSL lengthening within VM, regardless of the season. If RSL are caused by water, such a long active season at hundreds of VM RSL sites suggests that an appreciable source of water must be recharging these RSL. Thermophysical modeling indicates that a melting temperature range of ∼246 − 264 K is needed to reproduce the seasonal phenomenology of the VM RSL, suggesting the involvement of a brine consisting of tens of wt% salt. The mechanism(s) by which RSL are recharged annually remain uncertain. Overall, gaining a better understanding of how RSL form and recur can benefit the search for extant life on Mars and could provide details about an in situ water resource.

Recurring slope lineae (RSL) are narrow (0.5–5 m) dark features on Mars that incrementally lengthen down steep slopes, fade in colder seasons, and recur annually. These features have been identified from the northern to southern mid-latitudes. Here, we describe how observations of northern mid-latitude RSL in northern Chryse Planitia and southwestern Acidalia Planitia (CAP) suggest that brines start flowing before northern spring equinox and continue for more than half a Mars-year (490 ± 40 sols, spanning solar longitude 337° ± 11°–224° ± 20°). All CAP RSL are found on the steep slopes of craters and their source zones are at or below the elevation of the surrounding plains. Spacecraft-derived surface temperature observations cannot resolve individual RSL, so thermal modeling was used to determine that CAP RSL have a freezing temperature of 238–252 K, freeze and melt diurnally, and flow only occurs within the top ∼8 cm of the regolith. Furthermore, we calculate that a typical CAP RSL has a water budget of 1.5–5.6 m3/m of headwall. Therefore, such a large water budget makes annual recharge via atmospheric or subsurface diffusion sources unlikely. Alternatively, we hypothesize that the most plausible RSL source is a briny aquifer with a freezing temperature less than or equal to the mean annual CAP surface temperature (220–225 K). The annual cycle is as follows: in late autumn, the shallowest part of the brine feeding the source zone freezes, forming an ice dam. As spring approaches, temperatures rise and the dam is breached. Brine is discharged and the RSL initially lengthens rapidly (>1.86 m/sol), the lengthening rate then slows considerably, to ∼0.25 m/sol. Eventually, the losses equal the discharge rate and the RSL reaches its equilibrium phase. As brine flows in the RSL some of the water is lost to the atmosphere, therefore the freezing temperature of the brine within the RSL is higher (238–252 K) as the brine transitions to a super-eutectic salt concentration. In the late autumn, falling temperatures restore the ice dam and the H2O in the RSL slowly sublimates away. Overall, CAP RSL possess a significantly different seasonality and much longer duration than typical southern mid-latitude RSL, suggesting that RSL at different latitude bands have different source types. Lastly, CAP RSL are the best evidence that shallow groundwater may still exist on Mars.

Recurring slope lineae (RSL) are narrow (0.5–5 m) low-albedo features that incrementally lengthen down steep slopes during warm seasons, fade in colder seasons, and recur each Mars year. To reduce the effort involved in manually mapping and analyzing each RSL in Garni crater, we developed Mapping and Automated Analysis of RSL (MAARSL) to analyze a set of orthorectified High Resolution Imaging Science Experiment (HiRISE) images and a digital elevation map. MAARSL along with manual mapping allowed us to detect RSL, compute descriptive statistics, and characterize changes over time. We mapped 2910 RSL in 22 orthoimages, from Mars Year (MY) 31 solar longitude (Ls) 133.0° to MY32 Ls 323.7°. The MAARSL results confirmed that RSL lengthening and fading occur concurrently on slopes with the same orientation and many times within some individual RSL. Slope angles of RSL and a slope slump show that some RSL start, stop, and have mean slope angles that are below the angle of repose. Our analysis shows that RSL are actively lengthening on at least one slope-facing direction in all HiRISE observations of the crater. We also found that NE-, N-, and NW-facing RSL in Garni crater lengthened during times of increasing shortwave insolation, while S- and SW-facing RSL lengthened during increasing and decreasing shortwave insolation. A (non-orthorectified) HiRISE image acquired shortly after the MY34 dust storm and shows RSL on every slope-facing direction, which is anomalous with respect to observations prior to the dust storm. We find that dust removal and deposition could explain the darkening and fading (respectively) of RSL, and could also explain the apparent lack of material being transported. Observations of RSL lengthening and fading occur concurrently on slopes could suggest overprinting of dry granular flows. Dry flows could also explain the significant lengthening activity of every slope-facing direction after a fresh layer of dust was deposited via the MY34 dust storm. Alternatively, briny shallow subsurface flows are consistent with observations of RSL on slopes below the angle of repose and those that exhibit concurrent lengthening and fading. However, the most significant problem with briny RSL flows is accessing a source of briny water and removing excess salt from the regolith.

Imaging the interior structure of comets and asteroids can provide insight into their formation in the early Solar System, and can aid in their exploration and hazard mitigation. Accurate imaging can be accomplished using broadband wavefield data penetrating deep inside the object under investigation. This can be done in principle using seismic systems (which is difficult since it requires contact with the studied object), or using radar systems (which is easier since it can be conducted from orbit). We advocate the use of radar systems based on instruments similar to the ones currently deployed in space, e.g. the CONSERT experiment of the Rosetta mission, but perform imaging using data reflected from internal interfaces, instead of data transmitted through the imaging object. Our core methodology is wavefield extrapolation using time-domain finite differences, a technique often referred to as reverse-time migration and proven to be effective in high-quality imaging of complex geologic structures. The novelty of our approach consists in the use of dual orbiters around the studied object, instead of an orbiter and a lander. Dual orbiter systems can provide multi-offset data that illuminate the target object from many different illumination angles. Multi-offset data improve image quality (a) by avoiding illumination shadows, (b) by attenuating coherent noise (image artifacts) caused by wavefield multi-pathing, and (c) by providing information necessary to infer the model parameters needed to simulate wavefields inside the imaging target. The images obtained using multi-offset are robust with respect to instrument noise comparable in strength with the reflected signal. Dual-orbiter acquisition leads to improved image quality which is directly dependent on the aperture between the transmitter and receiver antennas. We illustrate the proposed methodology using a complex model based on a scaled version of asteroid 433 Eros.

We modeled orbital surface-penetrating radar of an asteroid and comet using two-dimensional finite-difference wavefield migration, in order to assess key target properties and experiment parameters required to fully image internal structure. Migration places radar echoes in their correct positions in a complex subsurface and is a complementary tool to travel-time tomography. The target shape was scaled from 433 Eros to 0.5-km mean diameter for an asteroid and 10 km for a comet. The interiors were populated with a power-law distribution of spherical blocks. We used an image structural similarity index to compare the internal surfaces reconstructed under different assumptions to a “best” image using optimum parameters. We found that successful internal imaging of the asteroid was not sensitive to whether the block interstices were regolith or void. Frequency dependence between 5 and 15 MHz was also minor. Internal interfaces could also be imaged if the attenuation was higher than that inferred within volcanic plains on Mars, but not as high as measured in a strongly fractured volcanic tuff on Earth. The overall imaging quality for the comet was statistically similar to the asteroid, but there was less variability due to smaller internal contrasts. A key finding is that imaging was vastly improved by using a second spacecraft as a radar receiver. A subsatellite with a different orbit will eventually provide a range of different illumination geometries over each part of the target. Finally, the results depend strongly on the specified internal velocity distribution, representing partial progress in complementary tomographic velocity estimation. The modeled impedance contrasts within the asteroid are larger than those typically encountered in exploration seismology and very much larger than in medical imaging, and so the velocity used to migrate the reflections must be close to the actual distribution. This again emphasizes the need for joint traveltime tomography and wavefield migration for asteroid imaging, which is optimized using two orbiters.

We analyzed the dielectric spectra (0.1 Hz–1 MHz) of 49 firn and ice samples from ice sheets and glaciers to better understand how differing ice formation and evolution affect electrical properties. The dielectric relaxation of ice is well known and its characteristic frequency increases with the concentration of soluble impurities in the ice lattice. We found that meteoric ice and firn generally possess two such relaxations, indicating distinct crystal populations or zonation. Typically, one population is consistent with that of relatively pure ice, and the other is significantly more impure. However, high temperatures (e.g. temperate ice), long residence times (e.g. ancient ice from Mullins Glacier, Antarctica) or anomalously high impurity concentrations favor the development of a single relaxation. These relationships suggest that annealing causes two dielectrically distinct populations to merge into one population. The dielectric response of temperate ice samples indicates increasing purity with increasing depth, suggesting ongoing rejection of impurities from the lattice. Separately, subglacially (lake) frozen samples from the Vostok (Antarctica) 5G ice core possess a single relaxation whose variable characteristic frequency likely reflects the composition of the source water. We conclude that multi-frequency methods are essential to dielectric discrimination between different types of glacier ice.

Low-frequency (LF, > 1 kHz). From laboratory measurements of samples collected at the US Army Permafrost Tunnel (Fox, Alaska), we find temperature-dependent relationships between ice volume fraction and the resistivity frequency effect (RFE, defined as the LF-normalised difference in LF and HF resistivities). We report the first field detection of H2O polarisability in permafrost, using a broadband spectral-induced polarisation system at the permafrost tunnel. By comparing laboratory and field spectra, we found a best-fitting ice temperature of -3 ± 0.5 °C. Laboratory RFE at the selected temperature was then used to map the RFE in the tunnel wall to 45 − 95 per cent ice by volume. Both of these results agreed quantitatively with the bulk properties of the tunnel, and the ice content image correlated qualitatively with major permafrost features. The RFE approach may be expedient using simpler instrumentation, but the close agreement of laboratory and field spectra indicates that the ice and interfacial water signatures can be individually quantified by broadband fitting of both amplitude and phase. This will provide more accurate constitutive relations, but more importantly will yield better remote temperature measurement of the subsurface using known dependencies of the dielectric relaxation frequencies. Copyright © 2015 John Wiley & Sons, Ltd.

Flowing water, possibly brine, has been suggested to cause seasonally reappearing, incrementally growing, dark streaks on steep, warm slopes on Mars. We modeled these Recurring Slope Lineae (RSL) as isothermal water flows in thin surficial layers driven by gravity and capillary suction, with input from sources in the headwall and loss to evaporation. The principal observables are flow duration and length. At 40% porosity, we find that flow thicknesses reaching saturation can be just 50 mm or so and freshwater RSL seasonally require 2–10 m3 of H2O per m of source headwall. Modeled water budgets are larger for brines because they are active for a longer part of each day, but this could be partly offset by lower evaporation rates. Most of the discharged water is lost to evaporation even while RSL are actively lengthening. The derived water volumes, while small, exceed those that can be supplied by annual melting of near-surface ice (0.2–2 m3/m for a 200-mm melt depth over 1–10 m height). RSL either tap a liquid reservoir startlingly close to the surface, or the actual water budget is several times smaller. The latter is possible if water never fully saturates RSL along their length. Instead, they would advance like raindrops on a window, as intermittent slugs of water that overrun prior parts of the flow at residual saturation. Annual recharge by vapor cold trapping might then be supplied from the atmosphere or subsurface.

Southern mid-latitude (SML) recurring slope lineae (RSL) are narrow (0.5–5 m) dark albedo features that emanate from bedrock and incrementally lengthen down steep slopes that preferentially face the equator. We observe that SML RSL begin lengthening prior to southern summer at a solar longitude (Ls) of 245° ± 11° when Mars Global Surveyor Thermal Emission Spectrometer (TES)-derived near-maximum surface temperatures are 296 ± 5 K and Mars Reconnaissance Orbiter Mars Climate Sounder (MCS) – and Mars Odyssey Thermal Emission Imaging System (THEMIS)-derived mid-afternoon surface temperatures are >273 K. SML RSL continue to lengthen for 104 ± 38 sols with an average near-maximum surface temperature of 298 ± 5 K. The SML RSL then stop lengthening at Ls = 314° ± 12° when mid-afternoon surface temperatures drop below 273 K. They remain dark for another 116 ± 41 sols (until Ls = 16° ± 14°) as surface temperatures continue to fall. Although the RSL recharge mechanism remains unknown, our observation that the vast majority of RSL lengthen only when mid-afternoon surface temperatures are >273 K supports the hypothesis that they are formed by shallow subsurface liquid water flows without significant freezing-point depression. The number and length of RSL at multiple sites increased dramatically following the Mars Year 28 globe-encircling dust storm. We interpret this increase to be due to warmer subsurface temperatures created by a dust-laden greenhouse effect that may be unique to the southern mid-latitudes near Ls = 270°. Therefore SML RSL flow is quite sensitive to ground temperature and may only occur under favorable orbital parameters when mean insolation during the RSL lengthening season is above that of the current southern mid-latitude mean insolation value. This value is currently at a peak that has not been attained for the last ∼100 ka. Meanwhile, the RSL-poor northern mid-latitude mean insolation is near a minimum and has a value 27% lower than the southern value. If SML RSL are indeed formed by shallow subsurface freshwater flows, then they may be some of the best locations on Mars to explore for extant life.

[1] The Moon's Procellarum KREEP Terrane (PKT) is distinguished by unique geochemistry and extended volcanic history. Previous thermal-conduction models using enhanced radionuclide abundances in subcrustal potassium, rare earth elements, and phosphorus (KREEP) predicted the existence of a contemporary upper-mantle melt zone as well as heat flow consistent with Apollo measurements. Here I show that such models also predict large gravity or topography anomalies that are not observed. If the topography is suppressed by a rigid lithosphere, it is possible to eliminate the gravity anomaly and still match heat flow by completely fractionating the excess radionuclides into a thin crust. This implies that upper-mantle heat sources for mare volcanism were spatially discontinuous or transient and that radionuclides defining the PKT are not necessarily directly related to mare volcanic sources. However, the mantle temperature of a crustally fractionated PKT is insufficient to match the observed electrical conductivity: globally enhanced mantle heating or a thick megaregolith may be required. Alternatively, upper-mantle enrichment in iron, hydrogen, or aluminum can provide the requisite conductivity. Iron is the most plausible: the derived lower limit to the upper-mantle magnesium number 75–80% is consistent with seismic modeling. Regardless of the specific mechanism for electrical-conductivity enhancement, the overall excellent match to simple thermal-conduction models indicates that the lunar upper mantle is not convecting at present.

[1] Electrical conduction through meteoric polar ice is controlled by soluble impurities that originate mostly from sea salt, biomass burning, and volcanic eruptions. The strongest conductivity response is to acids, yet the mechanism causing this response has been unclear. Here we elucidate conduction mechanisms in ice using broadband dielectric spectroscopy of meteoric polar ice cores. We find that conduction through polycrystalline polar ice is consistent with Jaccard theory for migration of charged protonic point defects through single ice crystals, except that bulk DC conduction is impeded by grain boundaries. Neither our observations nor modeling using Archie's Law support the hypothesis that grain-boundary networks of unfrozen acids cause significant electrolytic conduction. Common electrical logs of ice cores (by electrical conductivity measurement [ECM] or dielectric profiling [DEP]) and the attenuation of radio waves in ice sheets thus respond to protonic point defects only. This response implies that joint interpretation of electrical and chemical logs can determine impurity partitioning between the lattice and grain boundaries or inclusions. For example, in the Greenland Ice Core Project (GRIP) ice core from central Greenland, on average more than half of the available lattice-soluble impurities (H+, Cl–, NH4+) create defects. Understanding this partitioning could help further resolve the nature of past changes in atmospheric chemistry.

The electrical properties of water ice impact the study of diverse frozen environments, in particular the radar sounding of ice masses. The high-frequency response of meteoric polar ice depends partly on the bulk concentration of ammonium (NH4+), but the nature of this response has been unclear. Here we use broadband dielectric spectroscopy to investigate the electrical response of laboratory-frozen solutions. By analyzing the relaxation frequency of these samples and its temperature dependence, we show that the mobility of Bjerrum D-defects formed in the ice lattice by ammonium is 1.4 ±0.8 x 10–9m2 V–1 s–1 at -20°C, or about an order of magnitude smaller than that of Bjerrum L-defects formed by chloride. However, co-substitution of both ions increases the ice-lattice solubility of chloride by a factor of ∼7, causing an enhanced conductivity response due to greater concentrations of Bjerrum L-defects. Thus, despite its low mobility, ammonium can also affect the high-frequency electrical response of polar ice, but its covariance with chloride must be considered.

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.

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 � 10–40 Hz may provide a global source of electromagnetic energy that penetrates � 10–100 km. 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– 500 km, 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

Reconstructions of the early thermal history of the H-chondrite parent body have focused on two competing hypotheses. The first posits an undisturbed thermal evolution in which the degree of metamorphism increases with depth, yielding an “onion-shell” structure. The second posits an early fragmentation–reassembly event that interrupted this orderly cooling process. Here, we test these hypotheses by collecting a large number of previously published closure age and cooling rate data and comparing them to a suite of numerical models of thermal evolution in an idealized parent body. We find that the onion-shell hypothesis, when applied to a parent body of radius 75–130 km with a thermally insulating regolith, is able to explain 20 of the 21 closure age data and 62 of the 71 cooling rates. Furthermore, six of the eight meteorites for which multiple data (at different temperatures) are available, can be accounted for by onion-shell thermal histories. We therefore conclude that no catastrophic disruption of the H-chondrite parent body occurred during its early thermal history. The relatively small number of data not explained by the onion-shell hypothesis may indicate the formation of impact craters on the parent body which, while large enough to excavate all petrologic types, were small enough to leave the parent body largely intact. Impact events fulfilling these requirements would likely have produced transient crater diameters at least 30% of the parent body diameter.

We describe an innovative, dual-mode, fluxgate-induction sensor (FIS) that combines a DC magnetometer and an electromagnetic (EM) sensor. This sensor will enable one-pass unexploded ordnance (UXO) detection and discrimination using the joint measurements. The FIS is based on a [Formula: see text] long, high-permeability magnetic core and a sensing coil that are shared by both EM and magnetic (MAG) modes. This integration makes the sensor very compact and removes the potential crosstalk problem of the core material of one sensor dominating the response of the other. A prototype receiver was developed and true serial, dual-mode operation demonstrated. The FIS achieved sensitivities of [Formula: see text] rms in MAG mode and [Formula: see text] at [Formula: see text] in EM mode, which compare favorably to existing commercial instruments. Triaxial dipole modeling confirms that three-component EM data are better for shape characterization than one (vertical) component. Assuming axisymmetry, inversions of three-component EM measurements of 22 cylindrical and disk-shaped targets yielded 100% correct classification of UXO-like objects (cylinders) and 38% misclassification of disks as cylinders. Degraded classification performance with increasing vertical distance may be caused by the small cross-sectional area of the FIS; an array will provide better field sampling. Triaxial dipole modeling of the MAG data confirmed its utility for detecting deeper targets using only the vertical component. Based on these results, the FIS is ready for field demonstration and validation.

A prototype time–domain electromagnetic (TDEM) sounder was developed to technical readiness level (TRL) 5 to detect and characterize deep groundwater on Mars. The TDEM method induces eddy currents in the subsurface by abrupt extinction of a steady current in a large, flat-lying loop antenna, and the subsurface response is measured using the same loop or a separate receiver. TDEM has been widely used in terrestrial groundwater exploration and is ideally suited to sense the high electrical conductivity associated with saline groundwater expected on Mars. The inductive regime of TDEM is distinct from ground-penetrating radar: the latter has higher resolution but smaller depth of investigation. Our Mars-prototype TDEM was tested in the laboratory and at a local field site before the principal test was performed on Maui, Hawaii. This location was chosen because of its analogy to Mars in electrical properties: dry, resistive basalt over saline pore water. Results compared favorably to soundings made with a commercial TDEM, clearly detecting the seawater interface at depths of 250 m. We subsequently developed a ballistic deployment system for the loop antenna suitable for robotic missions. Compressed gas launches two projectiles; each consists of two spools on a guide stick. Payout on one spool is back towards the launcher and on the other toward its twin on the other projectile. In this way a triangular loop antenna is formed. The full system was tested twice, successfully achieving a distance of ∼70 m in both. A system capable of deploying a 200 m loop antenna on Mars would have mass

We developed a numerical model to assess the lithoautotrophic habitability of Mars based on metabolic energy, nutrients, water availability, and temperature. Available metabolic energy and nutrient sources were based on a laboratory-produced Mars-analog inorganic chemistry. For this specific reference chemistry, the most efficient lithoautotrophic microorganisms would use Fe(2+) as a primary metabolic electron donor and NO(3)(-) or gaseous O(2) as a terminal electron acceptor. In a closed model system, biomass production was limited by the electron donor Fe(2+) and metabolically required P, and typically amounted to approximately 800 pg of dry biomass/ml ( approximately 8,500 cells/ml). Continued growth requires propagation of microbes to new fecund environments, delivery of fresh pore fluid, or continued reaction with the host material. Within the shallow cryosphere--where oxygen can be accessed by microbes and microbes can be accessed by exploration-lithoautotrophs can function within as little as three monolayers of interfacial water formed either by adsorption from the atmosphere or in regions of ice stability where temperatures are within some tens of degrees of the ice melting point. For the selected reference host material (shergottite analog) and associated inorganic fluid chemistry, complete local reaction of the host material potentially yields a time-integrated biomass of approximately 0.1 mg of dry biomass/g of host material ( approximately 10(9) cells/g). Biomass could also be sustained where solutes can be delivered by advection (cryosuction) or diffusion in interfacial water; however, both of these processes are relatively inefficient. Lithoautotrophs in near-surface thin films of water, therefore, would optimize their metabolism by deriving energy and nutrients locally. Although the selected chemistry and associated model output indicate that lithoautotrophic microbial biomass could accrue within shallow interfacial water on Mars, it is likely that these organisms would spend long periods in maintenance or survival modes, with instantaneous biomass comparable to or less than that observed in extreme environments on Earth.

Nonlinear complex-resistivity (NLCR) cross-hole imaging of the vadose zone was performed at the A-014 Outfall at the Savannah River Site, Aiken, SC. The purpose of this experiment was to field-test the ability of this method to detect dense nonaqueous phase liquids (DNAPLs), specifically tetrachloroethene (PCE), known to contaminate the area. Five vertical electrode arrays (VEAs) were installed with [Formula: see text] [Formula: see text] separations in and around the suspected source zone to depths of [Formula: see text] [Formula: see text], and measurements were carried out at seven nearest-neighbor panels. Amplitude and phase data were edited for quality and then inverted to form three-dimensional (3D) images. The comparatively small magnitude of the nonlinear resistivity Hilbert distortion allowed approximate linearized imaging of the 3D distribution of this quantity as well. Laboratory analysis of nearby soil contaminated in situ indicated that the NLCR response to the PCE-clay reaction is maximized near [Formula: see text], leading to the development of a metric involving the phase and resistivity Hilbert distortion to infer the 3D distribution of PCE. Variations in PCE content were independently detailed at three drilling locations within the NLCR survey area using direct penetration-based soil-collection tools. Approximately 400 soil samples were collected and analyzed for chlorinated solvent mass composition at [Formula: see text] [Formula: see text] vertical intervals to compare with the NLCR-predicted distribution of DNAPL. The optimum performance for [Formula: see text] PCE was [Formula: see text] detection (true positives) with [Formula: see text] false alarms (false positives) at an effective resolution of [Formula: see text] [Formula: see text], or [Formula: see text] of the interwell separation. When smoothed to [Formula: see text] [Formula: see text] resolution (comparable to well spacing), detection was 100% with just 12% false alarms. NLCR successfully predicted the general distribution of PCE at parts-per-thousand soil-mass fractions, specifically widespread near-surface contamination and a zone of discontinuous stringers and pods below the source.

Time-domain electromagnetic data collected for discrimination of unexploded ordnance (UXO) span one spatial component in one time gate to three spatial components and multiple time channels. The relative merits of additional temporal and spatial-component information were assessed by analyzing decimated data from the Geonics EM61-3D. The responses of 25 unique ordnance- and scrap-like objects buried in 45 depth-orientation states were modeled as triaxial time-dependent dipoles and were classified using the properties of the inferred model parameters. Performance was measured by the fraction of false positives for ordnance-like objects at 91% true positives. The best false-positive rate for the full multicomponent, multichannel data was 5%, which was achieved using the power-law slope of the time decay of the secondary-field EMF: ordnance-like items remain in “intermediate” time longer and display a characteristic [Formula: see text] slope, whereas scrap-like items (even those made of steel) transition to “late” time and the recovered power-law indices vary widely. However, this excellent performance is largely due to the fact that the ordnance-like objects are often larger than the scrap-like objects in this data set. Better generalization may be obtained with discriminants based solely on shape—ordnance-like objects are usually axisymmetric about a long axis—which yielded 32% false positives, again in the power-law slope. In contrast, the best false-positive rate for single-component, single-channel data was 68%. Other instrument configurations (single-component, multichannel or multicomponent, single channel) were generally ranked intermediately between these extremes. Additional experiments with intermediate numbers and positions of time gates revealed that the best discrimination is achieved when recording a broad time range for the secondary-field decay [Formula: see text]; the number or density of time gates has a smaller impact. These results demonstrate in an internally consistent way the significant performance improvement in UXO discrimination that can be achieved with multicomponent, multichannel electromagnetic sensors.

We use a dynamic finite-difference model to simulate martian landslides in the Valles Marineris canyon system and Olympus Mons aureole using three different modal rheologies: frictional, Bingham, and power law. The frictional and Bingham modes are applied individually. Fluidized rheology is treated as a combination of frictional and power-law modes; general fluidization can include pore pressure contributions, whereas acoustic fluidization does not. We find that general fluidization most often produces slides that best match landslide geometry in the Valles Marineris. This implies that some amount of supporting liquid or gas was present in the material during failure. The profile of the Olympus Mons aureole is not well matched by any landslide model, suggesting an alternative genesis. In contrast, acoustic fluidization produces the best match for a lunar slide, a result anticipated for dry crust with no overlying atmosphere. The presence of pressurized fluid during Valles Marineris landsliding may be due to liquid water beneath a thin cryosphere (

Multiazimuth binning of 3-D P-wave reflection data is a relatively simple but robust way of characterizing the spatial distribution of gas‐producing natural fractures. In our survey, data were divided into two volumes by ray azimuth (approximately perpendicular and parallel (±45° to the dominant fracture strike) and separately processed. Azimuthal differences or ratios of attributes provided a rough measure of anisotropy. Improved imaging was also attained in the more coherent fracture‐parallel volume. A neural network using azimuthally dependent velocity, reflectivity, and frequency attributes identified commercial gas wells with greater than 85% success. Furthermore, we were able to interpret the physical mechanisms of most of these correlations and so better generalize the approach. The apparent velocity anisotropy was compared to that derived from other P- and S-wave methods in an inset three‐component survey. Prestack determination of the azimuthal moveout ellipse will best quantify velocity anisotropy, but simple two‐ or four‐azimuth poststack analysis can adequately identify regions of high fracture density and gas yield.

Venus, a planet similar in size, mass, and substance to Earth, has clearly undergone a vastly different thermal evolution because it does not currently lose heat by means of plate tectonics. Other manifestations of this difference are the thickness of the mechanical lithosphere and the geothermal gradient, which are intrinsic to the outer thermal boundary layer of the mantle convective system and are regulated by the interior heat loss. Observed tectonic styles and the results of geodynamic modeling indicate that the mean lithospheric thickness on Venus has increased with time. Pervasive deformation of the plateau highlands tessera, the oldest preserved terrain, requires a weak, thin lithosphere. Later features, such as ridge belts, coronae, chasmata, and shield volcanoes, reflect a lithosphere of intermediate thickness that causes deformation to be distributed. A broad arc of focused, coherent underthrusting at Artemis Chasma and the inferred partial regional support of volcanic highlands suggest that at present the average lithosphere is considerably stronger and thicker, and the heat flow much lower, than expected by comparison to Earth. The relatively thin lithosphere at young volcanic structures is consistent with localized reheating, while the few features representative of the thermally stable plains are suggestive of a ∼200-km thermal lithosphere. Heat loss on Venus today cannot be in steady state with mantle radiogenic heat production. Collectively, the deformation styles and modeled lithospheric thicknesses imply a passive, monotonic cooling of the thermal boundary layer over the interval of Venus's history recorded on the surface, approximately the past billion years.

Readers of TLE were introduced to the bulk similarities and bizarre differences between Venus and earth in “Geophysical interpretation of heat‐flow provinces of Venus utilizing Magellan SAR imaging” by Jeff Kelley (March 1998). In this short follow‐up, I would like to correct some inaccuracies in the earlier article, and share with the exploration geophysics community some of the ongoing consensus and controversy about Venus. Several compilations of papers on the post‐Magellan view of Venus are cited below. I will also point out other basic, related results for the moon and Mars.

Recent spherical harmonic representations of the gravity field of Mars have sufficient resolution to examine the regional crustal and lithospheric structure of the Valles Marineris and are used as constraints for rift modeling. While gravity signatures of individual troughs are not evident, a broad 190–260 mgal low indicates that the central chasmata as a whole are compensated at a depth of 30–80 km, representing the thickness of crust surrounding the troughs. Furthermore, at the time large-scale relief was established, the effective thickness of the elastic lithosphere was 20 m W m−2. The calculated range of crustal thickness and heat flow can be used to test two semianalytic models of rift formation. In the first model, the distinction between single (“narrow rift”) and multiple (“wide rift”) troughs is determined by the wavelength of necking instabilities. In the second model, narrow-versus-wide morphology is controlled by the evolution of lithospheric strength during rifting. The large-scale, parallel, multiple troughs of the central Valles Marineris are morphologically similar to terrestrial wide rifts, but a lack of distinct faulting in some of the chasmata could imply that the faulted troughs may have formed as isolated narrow rifts. Our results are only marginally consistent with necking leading to a wide rift; allowable combinations of crustal thickness and heat flow lead to decoupling within the lithosphere resulting in a second principal necking wavelength that is smaller than the main trough spacing, for which evidence is equivocal. At high heat flow (>40 m W m−2), however, necking of the strong upper crust alone can yield a shorter wavelength characteristic of a single trough, allowing narrow-rift origins of faulted chasmata. In contrast, the inferred range of crustal thickness and heat flow poorly match the narrow-rift regime of the strength-evolution model but are in good agreement with its predictions for wide rifting. Furthermore, the distinction between wide rift and core complex in this model places an upper bound on heat flow of 70 m W m−2.

Geodynamic modeling of tessera deformation at the ancient Venusian plateau highlands implies lithospheric thermal gradients exceeding 17 K km−1. Regularly spaced contractional ridges in the highlands manifest the unstable growth of perturbations at a dominant wavelength of ≤ 20 km controlled by a mechanical layer of relatively uniform thickness. We model the deformation as the shortening of a plastic layer over a viscous substrate; a layer thickness of less than 6 km is consistent with the observed ridge spacings. Assuming an anhydrous diabase crustal rheology and a minimum strain rate of 10−16 S−1, the geotherm was at least 17 K km−1. Higher strain rates — consequently higher thermal gradients — are more probable. We rule out the possibility that the plateau highlands were locally anomalously hot at the time ridges formed in the tessera, that the highlands are dominantly composed of rock more felsic than diabase, or that the crust was weak because it was hydrated. Therefore, the average global lithospheric heat flux when the highlands tesserae were deforming was considerably higher than it has been since.

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.

The topography and geology of large craters on Venus reveal no evidence for floor rebound or relaxation; rather, the floors have subsided. Depressions of the floor centers relative to the margins are evident in topography, and floor faulting is interpreted as contractional failure. Of the likely processes responsible, only subsolidus thermal contraction applies to craters that both have and have not been infilled by lavas, assuming volcanism occurred within a few tens of millions of years after the impact. Thermal subsidence satisfies the measured floor depressions for reasonable scaling of impact energies and temperature distributions in the lithosphere. Further, the predicted stresses are generally consistent with observed floor fracturing. We constrain the impact heat deposited in the lithosphere to be less than roughly 5 × 1023 J for diameters of ∼100 km. The absence of perceptible floor subsidence at craters this size on the Moon and icy satellites is readily explained by the scaling dependence of impact energy not only on transient crater diameter, but also on gravity and target density. The unfractured melt sheets of three large, young, bright-floored craters imply sufficient lithospheric rigidity to support the crater cavities. An elastic flexural rebound model restricts the elastic plate thickness to at least 10–15 km for the three craters, corresponding to maximum geotherms of ∼20–30 K km−1. Structural evidence for early rebound in older, dark-floored craters may have been buried by subsequent volcanism.