Effects of soil heterogeneity on martian ground-ice stability and orbital estimates of ice table depth

Data from the Mars Odyssey Gamma-Ray Spectrometer (GRS) instrument suite and results from numerical simulations of subsurface ground-ice stability have been used to estimate the depth of martian ground-ice. Geographic correlation between these estimates is remarkable; the relative ice table depth distributions also agree well. However, GRS-based estimates of ice table depth are generally deeper than predictions based on ground-ice stability simulations. This discrepancy may be related to heterogeneities in the martian surface such as rocks, dust, and albedo variations. We develop a multi-dimensional numerical model of ground-ice stability in a heterogeneous martian subsurface and use it to place the first quantitative constraints on the response of the ice table to meter-scale heterogeneities. We find that heterogeneities produce significant undulations/topography in the ice table at horizontal length scales of a few meters. Decimeter scale rocks create localized areas of deep ice, producing a vertical depression of 10-60 cm in the ice table over a horizontal range of 1-2 rock radii. Decimeter scale dust lenses produce locally shallow ice; however the magnitude of the vertical deflection of the ice table is small (1-4 cm). The effects of decimeter scale albedo variations on the ice table are nearly negligible, although albedo very weakly enhances the effects of dark rocks and bright dust on the ice table. Additionally, we investigate the role played by rocks in estimates of ice table depth based on orbital data. Surface rocks can account for more than half of the discrepancy between ice table depths inferred from GRS data and those predicted by theoretical ice-stability simulations that utilize thermophysical observations. Our results have considerable relevance to the up-coming Mars Scout Mission, Phoenix, because they indicate that the uncertainty in the ice table depth of a given region is greater than differences between current depth estimates. Likewise, small-scale depth variability due to heterogeneities at the eventual landing site is potentially greater than differences between current depth estimates.     

The presence and stability of ground ice in the southern hemisphere of Mars

We calculate new estimates of ground-ice stability and the depth distribution of the ice table (the depth boundary between ice-free soil above and ice-cemented soil below) and compare these theoretical estimates of the distribution of ground ice with the observed distribution of leakage neutrons measured by the Neutron Spectrometer instrument of the Mars Odyssey spacecraft's Gamma Ray Spectrometer instrument suite. Our calculated ground-ice distribution contains improvements over previous work in that we include the effects of the high thermal conductivity of ice-cemented soil at and below the ice table, we include the surface elevation dependence of the near-surface atmospheric humidity, and we utilize new high resolution maps of thermal inertia, albedo, and elevation from Mars Global Surveyor observations. Results indicate that ground ice should be about 5 times shallower than in previous predictions. While results are dependent on the atmospheric humidity, depths are generally between a few millimeters and a few meters with typical values of a few centimeters. Results are also geographically similar to previous predictions with differences due to the higher resolution of thermal inertia and the inclusion of elevation effects on humidity. Comparison with the measured epithermal-neutron count rates in the southern hemisphere indicate that the geographic distribution of the count rate is best correlated with ground ice in equilibrium with 10 to 20 pr μm (precipitable micrometers) column abundance of atmospheric water, assuming a uniform distribution with CO₂; however, given the uncertainties, 5 to 30 pr μm also may be viable. This water abundance represents a longer-term average over 100 to 1000 yr. There is a high degree of correlation between the depth of the ice table and the epithermal count rate that agrees remarkably well with predicted count rates as a function of ice-table depth. These results indicate that ground ice in the upper meter of the martian soil is in diffusive equilibrium with the atmosphere. Since ground ice in this depth zone is expected to undergo saturation/desiccation cycles with orbital variations, this ice should be younger than about 500 kyr and was emplaced under similar cold and dry climate conditions of today. Remaining differences between the predicted depths of the ice table and those inferred from the neutron data are likely to be due to subpixel heterogeneity in the martian surface including the presence of rocks, slopes, and patches of soil with varying thermophysical properties.     

Mineralogy of the Phoenix landing site from OMEGA observations and how that relates to in situ Phoenix measurements

We present an analysis comparing observations acquired by the Mars Express Observatoire pour la Minéralogie l’Eau, les Glaces et l’Activité (OMEGA) and Phoenix lander measurements. Analysis of OMEGA data provides evidence for hydrous and ferric phases at the Phoenix landing site and the surrounding regions. The 3 μm hydration band deepens with increasing latitude, along with the appearance and deepening of a 1.9 μm H₂O band as latitude increases ∼60° polewards. A water content of 10-11% is derived from the OMEGA data for the optical surface at the Phoenix landing site compared to 1-2% derived for subsurface soil by Phoenix lander measurements. The hydration of these regions is best explained by surface adsorbed water onto soil grains. No evidence for carbonate or perchlorate-bearing phases is evident from OMEGA data, consistent with the relatively small abundances of these phases detected by Phoenix. The identification of spectral features consistent with hydrated phases (possibly zeolites) from OMEGA data covering regions outside the landing site and the ubiquitous ferric absorption edge suggest that chemical weathering may play a role in the arctic soils.     

A historical search for habitable ice at the Phoenix landing site

A time-resolved energy balance model in the latitude range targeted by Phoenix, and extending back in time over the past 10 Ma, has been developed and used to predict the time-varying temperature field in ground ice over scales ranging from minutes to millions of years. The temperature history is compared to the population doubling times of terrestrial psychrophiles as a function of temperature, and the lifetime of analog microbe spores against de-activation by galactic cosmic rays (GCR), in order to assess the habitability of ground ice and surrounding materials that may be sampled by Phoenix. Metrics are derived to quantify “habitability” and compare different model configurations, including total and maximum continuous time, per year, that ground ice temperatures exceed various thresholds, maximum and average dormancy periods, and maximum and average consecutive growing seasons. The key unknowns in assessing the position, and hence the temperature, of the ground ice table at high northern latitude is the fate of the perennial north polar cap at high obliquity. If enough H₂O ice can persist at polar latitudes to buffer at least the high-latitude atmosphere at all orbital configurations, ground ice is found to be relatively shallow over much of the past 10 Ma, and regularly achieves temperatures in excess of those required for the growth of terrestrial psychrophiles. The dry overburden expected at the landing site can easily be sampled by Phoenix, and includes the “sweet spot” that is characterized by the optimal habitability metrics over the past 10 Ma. If the atmosphere is buffered only by low-latitude ice deposits at obliquities greater than about 30°, the frequency and duration of habitable ice is considerably diminished, and the intervening dormancy periods, during which cosmic ray damage accumulates, are correspondingly longer. In all cases, the maximum dormancy period that must be survived by putative martian psychrophiles is at least an order of magnitude greater than the amount of time required to reduce terrestrial psychrophile spore viability by 10⁻⁶ (∼7×¹⁰⁴ years). Depending on the fate of high-obliquity polar ice, the maximum dormancy period can exceed 4×¹⁰⁶ years, a factor of 60 longer than terrestrial psychrophile spore lifetimes. Habitability of martian ground ice is therefore dependent on putative martian psychrophiles developing robustness against GCR deactivation at least an order of magnitude greater than their terrestrial counterparts. Simulations of ground ice throughout the 65° N-72° N latitude range accessible to Phoenix suggest that higher-latitude ground ice has better habitability metrics, although the discrepancy is less than an order of magnitude for all metrics and across the entire latitude range.     

Modeling sublimation of ice exposed by new impacts in the martian mid-latitudes

New impacts in the martian mid-latitudes have exposed near-surface ice. This ice is observed to slowly fade over timescales of months. In the present martian climate, exposed surface ice is unstable during summer months in the mid-latitudes and will sublimate. We model the sublimation of ice at five new impact sites and examine the implications of its persistence. Even with generally conservative assumptions, for most reasonable choices of parameters it is likely that over a millimeter of sublimation occurred in the period during which the ice was observed to fade. The persistence of visible ice through such sublimation suggests that the ice is relatively pure rather than pore-filling. Such ice could be analogous to the nearly pure ice observed by the Phoenix Lander in the “Dodo-Goldilocks” trench and suggests that the high ice contents reported by the Mars Odyssey Gamma Ray Spectrometer at high latitudes extend to the mid-latitudes. Our observations are consistent with a model of the martian ice table in which a layer with high volumetric ice content overlies pore-filling ice, although other structures are possible.     

Small-scale trench in the north polar region of Mars: Evolution of surface frost and ground ice concentration

In this paper we attempt to answer the question, how formation of a small-scale trench in the martian regolith affects local distribution of the subsurface ice. We are especially interested in the consequences of digging a trench to search for buried ice, as has been done during the Phoenix Mars Lander mission. However, the results may be also applicable for natural troughs, or cracks. We present results of simulations of diurnal exchange of water between the regolith and the atmosphere. Our model includes the heat and vapor migration in the regolith surrounding the trench, as well as formation of diurnal frost. We take into account scattering of light in the atmosphere and on the trench facets, as well as changes of atmospheric humidity on diurnal and seasonal time scales. Our calculations show, that the measurements of ice content in a sample obtained within one, or two days from the beginning of digging should not be affected. However, on somewhat longer time scale at the south facing site of the trench the regolith can be significantly depleted from ice. This effect should be taken into account if the excavation and taking samples from different depths will be performed in stages separated in time by a month, or more.     

The geology of the Viking Lander 2 site revisited

Reevaluating the geologic history of the prior Mars landing sites provides important ground truth for recent and ongoing orbital missions. At the Viking 2 Lander (VL2) site, topographic measurements of relict landforms indicate that at least 100 m of sedimentary mantle material has been stripped away. The observed paucity of impact craters <100 m in diameter suggests that resurfacing processes (likely in the form of the recent deposition and removal of thin 1-10 m mantle layers) continue up to the present. A dearth of craters in the 100-500 m diameter range, however, also necessitates erosion of a thicker mantle layer. Partially inverted chains of secondary craters from nearby Mie Crater indicate that the mantle was already in place when the impact occurred. The density of craters superposed on Mie ejecta is consistent with a Late Hesperian age and provides a minimum age constraint for the mantle's emplacement. The thermophysical properties of the surface around VL2 as observed with Thermal Emission Imaging System (THEMIS) data indicate that the landing site occurs in an intracrater region that may typify mid to high northern latitude sites. Elevated thermal inertias of a pedestal crater superposed atop a larger pedestal crater suggest that rocky or indurated material can be created by impacts into sedimentary targets. Rock abundances at VL2 are consistent with the addition of impact-emplaced material from the missing small impact crater population documented in this study. Thus, the VL2 site may be a reasonable proxy for the landscape expected at the upcoming Phoenix Lander site.     

Latitudinal distribution of temporary liquid cryobrines on Mars

Simulations of the surface temperature and atmospheric humidity with a modern Mars climate model (MCD) and with Phoenix data are used to study the conditions for a liquefaction of brines as a function of latitude and season. The results show that, in the presence of appropriate salts, liquid cryobrines can in course of the diurnal cycle temporarily evolve at high latitudes on Mars’ current climate. The conditions for the liquefaction of “Mars-relevant” cryobrines and time and duration of their stability during the diurnal cycle are calculated for northern spring and for the Phoenix landing site.     

Lunar heat flow and regolith structure inferred from interferometric observations at a wavelength of 49.3 cm

We discuss observations of the Moon at a wavelength of 49.3 cm made with the Owens Valley Radio Observatory Interferometer. These observations have been fit to models in order to estimate the lunar dielectric constant, the equatorial subsurface temperature, the latitude dependence of the subsurface temperature, and the subsurface temperature gradient. The models are most consistent with a dielectric constant of 2.52 ± 0.01 (formal errors), an equatorial subsurface temperature of 249_?5⁺⁸K, and a change in the subsurface temperature with latitude (ψ), which is proportional to cos^0.38ψ. Since the temperature of the Moon has been measured by the Apollo Lunar Heat Flow Experiment, we have been able to use our determination of the equatorial temperature to estimate the error in the flux density calibration scale at 49.3cm (608 MHz). This results in a correction factor of 1.03 ± 0.04, which must be applied to the flux density scale. This factor is much different from 1.21 ± 0.09 estimated by Muhleman et al. (1973) from the brightness temperature of Venus and apparently indicates that the observed decrease in the brightness temperature of Venus at long wavelengths is a real effect.The estimates of the temperature gradient, which are based on the measurement of limb darkening, are small and negative (temperature decreases with depth) and may be insignificantly different from zero since they are only as large as their formal errors. We estimate that a temperature gradient in excess of 0.6K/m at 10m depth would have been observed. Thus, a temperature gradient like that measured in situ at the Apollo 15 and 17 landing sites in the upper 2m of the regolith is not typical of the entire lunar frontside at the 10m depths where the 49.3 cm wavelength emission originates. This result may indicate that the mean lunar heat flow is lower than that measured at the Apollo landing sites, that the thermal conductivity is greater at 10m depth than it is at 2m depth, or that the radio opacity is greater at 10m depth than at 2m depth. The negative estimates of the temperature gradient indicate that the Moon appeared limb bright and might be explained by scattering of the emission from boulders or an interface with solid rock. The presence of solid rock at 10m depths will probably cause heat flows like those measured by Apollo to be unobservable by our interferometric method at long wavelengths, since it will cause both the thermal conductivity and radio opacity of the regolith to increase. Thus, our data may be most consistent with a change in the physical properties of the regolith to those of solid rock or a mixture of rock and soil at depths of 7 to 16m. Our results show that future radio measurements for heat flow determinations must utilize wavelengths considerably shorter than 50 cm (25 cm or less) to avoid the rock regions below the regolith.     

Development and testing of subsurface sampling devices for the Beagle 2 lander

For planetary landing missions, the capability to acquire samples of soil and rock is of high importance whenever complex analyses (e.g. isotopic studies) on these materials are to be carried out, or when samples are to be returned to Earth. Not only surface samples are of relevance, but in recent concepts at least for Mars landing missions also subsurface samples are required. Subsurface material on Mars is believed to have been protected from the inferred oxidants at the immediate surface while also being protected from the UV influx. Therefore, there is considerable hope that in subsurface soil samples on Mars, at least organic matter delivered by meteorites may be detected, and possibly also relics of earlier simple microbial life on the planet. Likewise, samples from the inside of Martian surface rocks promise to have been protected from weathering and for the same reason they are important for organic chemistry studies. In this paper, an overview is given of the development and science of two different subsurface sampling devices for the Beagle 2 lander of ESA's Mars Express mission, being a “Mole” subsurface soil sampler and a small rock coring and sampling mechanism. Besides their sampling function, both the Mole and the Corer/Grinder will provide data on physical properties of Martian soils and rock, respectively, through the way they interact with the sampled materials. Details of the Mole and Corer/Grinder design are presented, along with results of recent tests with prototypes in the laboratory on physically analogous sample materials.     

On the sublimation of ice particles on the surface of Mars; with applications to the 2007/8 Phoenix Scout mission

Experimental studies related to the sublimation of ice, in bulk or as small particles, alone or mixed with dust similar to that expected on the surface of Mars, are reported. The experiments, a cloud physics particle sublimation model, and a convection model presented by Ingersoll, all indicate a strong dependence of sublimation rate on temperature, and this appears to be the dominant factor, assuming that the relative humidity of the air is fairly low. In addition the rate of loss of water vapour appears to depend primarily on exposed surface area and less on particle size and the total mass of the sample, or the mass of ice in the sample. The 2007/8 Phoenix Scout mission plans to obtain and analyse samples of sub-surface ice from about 70° N on Mars. A concern is that these samples, in the form of ice chips of size about 1 mm diameter, could be prone to sublimation when exposed for prolonged periods (many hours) to a relatively warm and dry atmosphere. Our laboratory simulations confirm that this could be a problem if particles are simply left lying on the surface, but also indicate that samples kept suitably cold and collected together in confined piles will survive long enough for the collection and delivery (to the analysis instruments) procedure to be completed.     

Digging deep for ice in Isidis Planitia—New constraints on subsurface volatile concentrations from thermal modeling

The Isidis Planitia region on Mars usually is regarded as a comparably attractive site for landing missions based on engineering constraints such as elevation and smooth regional topography. The Mars Express landed element Beagle 2 was deployed to this area, and the southern margin of the basin was selected as one of the backup landing sites for the NASA Mars Exploration Rovers.Especially in the context of the Beagle 2 mission, Isidis Planitia has been discussed as a place which might have experienced a volatile-rich history with associated potential for biological activity [e.g. Bridges et al., 2003. Selection of the landing site in Isidis Planitia of Mars Probe Beagle 2. J. Geophys. Res. 108(E1), 5001, doi: 10.1029/2001JE001820]. However the measurements of by the GRS instrument on Mars Odyssey indicate a maximum inferred water abundance of only 3 wt% in the upper few meters of the surface [Feldman et al., 2004. Global distribution of near-surface hydrogen on Mars. J. Geophys. Res. 109, E09006, doi: 10.1029/2003JE002160]. Based on these measurements this area seems to be one of the driest spots in the equatorial region of Mars.To support future landing site selections we took a more detailed look at the minimum burial depth of stable ice deposits in this area, focusing as an example on the planned Beagle 2 landing site. We are especially interested in the likelihood of ground ice deposits within the range of proposed subsurface sampling tools as drills or ‘mole’-like devices [Richter et al., 2002. Development and testing of subsurface sampling devices for the Beagle 2 Lander. Planet. Space Sci. 50, 903-913] given reasonable physical constraints for the surface and near surface material.For a mission like ExoMars [Kminek, G., Vago, J.L., 2005. The Aurora Exploration Program—The ExoMars Mission. In: Proceedings of the 35th Lunar and Planetary Science Conference, abstract no. 1111, 15-19 March 2004, League City, TX] with a focus on finding traces of fossil life the area might be of potential interest, because these traces would be better conserved in the dry soil. Modeling and measurement indicate that Isidis Planitia is indeed a dry place and any hypothetical ground ice deposits in this region are out of range of currently proposed sampling devices.     

Apparent thermal inertia and the surface heterogeneity of Mars

Thermal inertia derivation techniques generally assume that surface properties are uniform at horizontal scales below the footprint of the observing instrument and to depths of several decimeters. Consequently, surfaces with horizontal or vertical heterogeneity may yield apparent thermal inertia which varies with time of day and season. To investigate these temporal variations, we processed three Mars years of Mars Global Surveyor Thermal Emission Spectrometer observations and produced global nightside and dayside seasonal maps of apparent thermal inertia. These maps show broad regions with diurnal and seasonal differences up to 200 J m⁻² K⁻¹s^−1/2 at mid-latitudes (60° S to 60° N) and 600 J m⁻² K⁻¹s^−1/2 or greater in the polar regions. We compared the seasonal mapping results with modeled apparent thermal inertia and created new maps of surface heterogeneity at 5° resolution, delineating regions that have thermal characteristics consistent with horizontal mixtures or layers of two materials. The thermal behavior of most regions on Mars appears to be dominated by layering, with upper layers of higher thermal inertia (e.g., duricrusts or desert pavements over fines) prevailing in mid-latitudes and upper layers of lower thermal inertia (e.g., dust-covered rock, soils with an ice table at shallow depths) prevailing in polar regions. Less common are regions dominated by horizontal mixtures, such as those containing differing proportions of rocks, sand, dust, and duricrust or surfaces with divergent local slopes. Other regions show thermal behavior that is more complex and not well-represented by two-component surface models. These results have important implications for Mars surface geology, climate modeling, landing-site selection, and other endeavors that employ thermal inertia as a tool for characterizing surface properties.     

Explosive erosion during the Phoenix landing exposes subsurface water on Mars

While steady thruster jets caused only modest surface erosion during previous spacecraft landings on the Moon and Mars, the pulsed jets from the Phoenix spacecraft led to extensive alteration of its landing site on the martian arctic, exposed a large fraction of the subsurface water ice under the lander, and led to the discovery of evidence for liquid saline water on Mars. Here we report the discovery of the ‘explosive erosion’ process that led to this extensive erosion. We show that the impingement of supersonic pulsed jets fluidizes porous soils and forms cyclic shock waves which propagate through the soil and produce erosion rates more than an order of magnitude larger than that of other jet-induced processes. The understanding of ‘explosive erosion’ allows the calculation of bulk physical properties of the soils altered by it, provides insight into a new behavior of granular flow at extreme conditions and explains the rapid alteration of the Phoenix landing site’s ground morphology at the northern arctic plains of Mars.     

Slope morphology of hills at the Mars Pathfinder landing site

The study of hillslopes is a primary element of geomorphology and has successfully been used in many terrestrial arenas. In this study we take advantage of High Resolution Imaging Science Experiment (HiRISE) imagery as well as Mars Orbiter Camera (MOC) derived DEMs of the Pathfinder landing site to study regional hillslopes at resolutions many times greater than previously available and compare them with Mars Pathfinder lander images. This site was thought to be modified by massive flooding 1.8-3.5 byr ago and although evidence of flood activity was not obvious at the finer scale of this study, possible lee deposits and terracing were seen in some of the features. Evidence of post flood processes of ice related creep, aeolian and dry mass wasting were observed at the site and have likely obscured flood related morphology present in these features. Regional slopes were found to vary with aspect and suggest processes intensities operating at different orientations, possibly related to the prevailing wind direction, as well as the origin of the ancient flood event.     

H layering in the top meter of Mars

We explore the capability of a method of mapping the depth distribution of a hydrogen-rich layer in the top meter of Mars from the neutron currents measured by the Mars Odyssey Neutron Spectrometer. Assuming the soil can be modeled by two layers of known composition having different hydrogen contents, simulations allow an inversion of the neutron data into knowledge of depth and hydrogen content of the lower layer. The determination of these variables is sensitive to the hypothesis of chemical composition of the soil. We quantify this contribution to the uncertainty in the method first in terms of individual chemical elements and then in terms of macroscopic absorption cross sections. To minimize this source of error, an average composition was inferred from Mars Exploration Rover data. Possible compositions having a wide range of macroscopic absorption cross sections were used to evaluate the uncertainty associated with our calculations. We finally compare our results to ice table depth estimates predicted by two published theoretical models at locations where the composition is relatively well known. The fit is excellent in the southern high latitudes but questionable in the northern high latitudes. Possible explanations of these differences include the high geographical variations of the neutron currents relative to the spatial width of the response function of the instrument and the overly simple model we, of necessity, used for surface layering.     

High latitude patterned grounds on Mars: Classification, distribution and climatic control

Patterned grounds such as polygonal features and slope stripes are the signature of the presence of ground ice and of temperature variations in cold regions on Earth. Identifying similar features on Mars is important to understand its past climate as well as the ground ice distribution. In this study, young patterned grounds are classed and mapped from the systematical analysis of Mars Observer Camera high resolution images. These features are located poleward of 55° latitude which fits the distribution of ground ice found by the Neutron Spectrometer onboard Mars Odyssey. Thermal contraction due to seasonal temperature variations is the predominant process of formation of polygons formed by cracks which sizes vary from 15 to 300 m. The small (<40 m) widespread polygons are very recent and degraded by the desiccation of ground ice from the cracks which enhances the effect of ice sublimation. The large polygons (50 to 300 m) located only around the south CO₂ polar cap indicate the presence of ground ice and thus outline the limit of the CO₂ ice cap. They could be due to the blanketing of water ice deposits by the advances and retreats of the residual CO₂ ice cap during the last thousand years. Large (50-250 m) and homogeneous polygons similar to ice wedge polygons, hillslope stripes and solifluction lobes may indicate that specific environments such as crater floors and hillslopes could have been submitted to freeze-thaw cycles, possibly related to higher summer temperatures in periods of obliquity higher than 35°. These interpretations must be strengthened by higher resolution images such as those of the HiRise mission of the Mars Reconnaissance Orbiter because locations with past seasonal thaw could be of major interest as potential landing sites for the Phoenix mission.     

Viscous liquid film flow on dune slopes of Mars

It is shown that viscous liquid film flow (VLF-flow) on the surfaces of slopes of martian dunes can be a low-temperature rheological phenomenon active today on high latitudes. A quantitative model indicates that the VLF-flows are consistent with the flow of liquid brines similar to that observed by imaging at the Phoenix landing site. VLF-flows depend on the viscosity, dynamics, and energetics of temporary darkened liquid brines. The darkening of the flowing brine is possibly, at least partially, attributed to non-volatile ingredients of the liquid brines. Evidence of previous VLF-flows can also be seen on the dunes, suggesting that it is an ongoing process that also occurred in the recent past.     

Sorted clastic stripes, lobes and associated gullies in high-latitude craters on Mars: Landforms indicative of very recent, polycyclic ground-ice thaw and liquid flows

Self-organised patterns of stone stripes, polygons, circles and clastic solifluction lobes form by the sorting of clasts from fine-grained sediments in freeze-thaw cycles. We present new High Resolution Imaging Science Experiment (HiRISE) images of Mars which demonstrate that the slopes of high-latitude craters, including Heimdal crater - just 25 km east of the Phoenix Landing Site - are patterned by all of these landforms. The order of magnitude improvement in imaging data resolution afforded by HiRISE over previous datasets allows not only the reliable identification of these periglacial landforms but also shows that high-latitude fluviatile gullies both pre- and post-date periglacial patterned ground in several high-latitude settings on Mars. Because thaw is inherent to the sorting processes that create these periglacial landforms, and from the association of this landform assemblage with fluviatile gullies, we infer the action of liquid water in a fluvio-periglacial context. We conclude that these observations are evidence of the protracted, widespread action of thaw liquids on and within the martian regolith. Moreover, the size frequency statistics of superposed impact craters demonstrate that this freeze-thaw environment is, at least in Heimdal crater, less than a few million years old. Although the current martian climate does not favour prolonged thaw of water ice, observations of possible liquid droplets on the strut of the Phoenix Lander may imply significant freezing point depression of liquids sourced in the regolith, probably driven by the presence of perchlorates in the soil. Because perchlorates have eutectic temperatures below 240 K and can remain liquid at temperatures far below the freezing point of water we speculate that freeze-thaw involving perchlorate brines provides an alternative “low-temperature” hypothesis to the freeze-thaw of more pure water ice and might drive significant geomorphological work in some areas of Mars. Considering the proximity of Heimdal crater to the Phoenix Landing Site, the presence of such hydrated minerals might therefore explain the landforms described here. If this is the case then the geographical distribution of martian freeze-thaw landforms might reflect relatively high temperatures (but still below 273 K) and the locally elevated concentration of salts in the regolith.     

Fast numerical method for growth and retreat of subsurface ice on Mars

Subsurface water ice on Mars evolves due to exchange of vapor with the atmosphere, in the form of loss of ice to the atmosphere or in the form of the growth of interstitial ice. Described here is an accelerated numerical method for the long-term evolution of subsurface ice. This accelerated method is five orders of magnitude faster than explicit vapor transport calculations, enabling fundamentally new types of climate models. Its speed matches that of purely thermal models. The speedup is achieved primarily by solving time-averaged equations for vapor transport and ice volume change. Processes incorporated are growth of interstitial pore ice, retreat of pore ice, retreat of an ice sheet, and retreat of pore ice due to geothermal heating from below. Two example applications illustrate this numerical method’s capabilities. Near the permafrost margin at 55° latitude, ice is periodically depleted and slowly recharged, leading to a pore ice layer estimated to be currently no more than a few meters thick. At the Phoenix Landing Site, it shows the formation of a three layered structure, whereby the layer of pore ice can be very thin.