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Tropical ground ice on Mars undergoes long-term sublimation and likely exospheric escape. Without restriction of sublimation, the cryosphere would eventually breach, leading to massive loss of any underlying groundwater. We seek to understand the conditions under which the ground-ice seal, groundwater, and subsurface habitability are preserved. Using multi-reservoir models for the evolution of deuterium-to-hydrogen ratios (D/H) we derive a median estimate of the Hesperian-Amazonian H2O loss of 60 m (interquartile range 30 − 120 m) Global Equivalent Layer (GEL), neglecting magmatic degassing. Most of the loss may have been early, with consequent low loss in the “modern” cold and dry climate. We modeled global H2O transport within Mars following an assumed abrupt transition to modern conditions at 3 Ga. Sublimation is retarded (in order of decreasing priority) by higher obliquity, smaller porosity, higher tortuosity, lower heat flow, and smaller pore radius. Higher obliquity reduces H2O loss by decreasing tropical surface temperatures and raising global atmospheric water vapor. Time-variable obliquities do not overly influence outcomes as long as the total duration

[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.

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.

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.

Artemis Chasma is an arcuate, 2000-km-diameter zone of convergence and lithospheric underthrusting on Venus. Inelastic flexure modeling of the topography at Artemis, combined with observations of trench tectonics, allow us to document local temperature gradients below 4 K km−1 and an immense compressive in-plane force at the trench. Lithospheric rheology on Venus may be stronger than would be predicted from conventional extrapolations of rock mechanics experiments; in particular, the brittle surface strength must reach a few tens of megapascals to satisfy the observed lack of flexurally induced surface faulting. Elastic plate bending models provide an adequate estimate of the bending moment at Artemis, but they fail to constrain—or even recognize—the in-plane force. The inelastic analysis implies an in-plane force of the order of −1 × 1014 N m−1; a potential driving force is thermal thinning of regionally thick lithosphre in the highlands to the north. The low heat flux at Artemis, which is a comparatively young structure, is compatible with the notion that Venus has experienced a prolonged period of cooling in the last several hundred million years. The inference of such exceptionally low thermal gradients embraces three end-member possibilities: (1) the surface age is >600 Ma, (2) the lithospheric thermal age is greater than the surface age, and/or (3) the upper mantle temperature is anomalously low (∼1550 K).

Constraints on the recent geological evolution of Venus may be provided by quantitative estimates of the rates of the principal resurfacing processes, volcanism and tectonism. This paper focuses on the latter, using impact craters as strain indicators. The total postimpact tectonic strain lies in the range 0.5-6.5%, which defines a recent mean strain rate of 10(exp -18)-10(exp -17)/s when divided by the mean surface age. Interpretation of the cratering record as one of pure production requires a decline in resurfacing rates at about 500 Ma (catastrophic resurfacing model). If distributed tectonic resurfacing contributed strongly before that time, as suggested by the widespread occurrence of tessera as inliers, the mean global strain rate must have been at least approximately 10(exp -15)/s, which is also typical of terrestrial active margins. Numerical calculations of the response of the lithosphere to inferred mantle convective forces were performed to test the hypothesis that a decrease in surface strain rate by at least two orders of magnitude could be caused by a steady decline in heat flow over the last billion years. Parameterized convection models predict that the mean global thermal gradient decreases by only about 5 K/km over this time; even with the exponential dependence of viscosity upon temperature, the surface strain rate drops by little more than one order of magnitude. Strongly unsteady cooling and very low thermal gradients today are necessary to satisfy the catastrophic model. An alternative, uniformitarian resurfacing hypothesis holds that Venus is resurfaced in quasi-random 'patches' several hundred kilometers in size that occur in response to changing mantle convection patterns.

[1] The real part of the dielectric permittivity of the Martian regolith was measured by the Thermal and Electrical Conductivity Probe (TECP) on the Phoenix lander. We interpret these data using laboratory measurements of permittivity as a function of H2O and salt content, soil type, and temperature. Due to variability in sensor coupling, we focus on data taken at one locality (“Vestri”) three separate times, spanning multiple sols. A daytime increase in permittivity suggests progressive melting of a heterogeneous, disconnected, salty ice with a eutectic temperature of ∼239 K, which is close to the eutectic temperatures of NaClO4 or MgCl2. We found no evidence for Mg(ClO4)2. NaClO4 and MgCl2 are consistent with precipitation by freezing following a prior epoch of high obliquity. The evaporation of diurnal briny meltwater is inhibited by surface tension in small pores. An increase in permittivity occurred on the night of sol 70 that coincided with surface frost and measurement of a decrease in atmospheric water vapor. The permittivity jump can be matched by an increase in adsorbed H2O from ∼1 monolayer to 3 monolayers in an analog soil with a Viking-like specific surface area (17 m2/g). However, the amount of adsorbed H2O is an order of magnitude larger than that inferred to have precipitated during the night. We suggest that the electrical signature of adsorbed water on Mars is stronger than we measured in the laboratory, possibly due to radiation damage of the regolith.