Ice Table Research Articles

Thermal behavior and ice-table depth within the north polar erg of Mars

We fully resolve a long-standing thermal discrepancy concerning the north polar erg of Mars. Several recent studies have shown that the erg’s thermal properties are consistent with normal basaltic sand overlying shallow ground ice or ice-cemented sand. Our findings bolster that conclusion by thoroughly characterizing the thermal behavior of the erg, demonstrating that other likely forms of physical heterogeneity play only a minor role, and obviating the need to invoke exotic materials. Thermal inertia as calculated from orbital temperature observations of the dunes has previously been found to be more consistent with dust-sized materials than with sand. Since theory and laboratory data show that dunes will only form out of sand-sized particles, exotic sand-sized agglomerations of dust have been invoked to explain the low values of thermal inertia. However, the polar dunes exhibit the same darker appearance and color as that of dunes found elsewhere on the planet that have thermal inertia consistent with normal sand-sized basaltic grains, whereas Martian dust deposits are generally lighter and redder. The alternative explanation for the discrepancy as a thermal effect of a shallow ice table is supported by our analysis of observations from the Mars Global Surveyor Thermal Emission Spectrometer and the Mars Odyssey Thermal Emission Imaging System and by forward modeling of physical heterogeneity. In addition, our results exclude a uniform composition of dark dust-sized materials, and they show that the thermal effects of the dune slopes and bright interdune materials evident in high-resolution images cannot account for the erg’s thermal behavior.

History and anatomy of subsurface ice on Mars

Ice buried beneath a thin layer of soil has been revealed by neutron spectroscopy and explored by the Phoenix Mars Lander. It has also been exposed by recent impacts. This subsurface ice is thought to lose and gain volume in response to orbital variations (Milankovitch cycles). We use a powerful numerical model to follow the growth and retreat of near-surface ice as a result of regolith–atmosphere exchange continuously over millions of years. If a thick layer of almost pure ice has been deposited recently, it has not yet reached equilibrium with the atmospheric water vapor and may still remain as far equatorward as 43°N, where ice has been revealed by recent impacts. A potentially observable consequence is present-day humidity output from the still retreating ice. We also demonstrate that in a sublimation environment, subsurface pore ice can accumulate in two ways. The first mode, widely known, is the progressive filling of pores by ice over a range of depths. The second mode occurs on top of an already impermeable ice layer; subsequent ice accumulates in the form of pasted on horizontal layers such that beneath the ice table, the pores are completely full with ice. Most or all of the pore ice on Mars today may be of the second type. At the Phoenix landing site, where such a layer is also expected to exist above an underlying ice sheet, it may be extremely thin, due to exceptionally small variations in ice stability over time.

Effects of extreme cold and aridity on soils and habitability: McMurdo Dry Valleys as an analogue for the Mars Phoenix landing site

AbstractThe McMurdo Dry Valleys are among the driest, coldest environments on Earth and are excellent analogues for the Martian northern plains. In preparation for the 2008 Phoenix Mars mission, we conducted an interdisciplinary investigation comparing the biological, mineralogical, chemical, and physical properties of wetter lower Taylor Valley (TV) soils to colder, drier University Valley (UV) soils. Our analyses were performed for each horizon from the surface to the ice table. In TV, clay-sized particle distribution and less abundant soluble salts both suggested vertical and possible horizontal transport by water, and microbial biomass was higher. Alteration of mica to short-order phyllosilicates suggested aqueous weathering. In UV, salts, clay-sized materials, and biomass were more abundant near the surface, suggesting minimal downward translocation by water. The presence of microorganisms in each horizon was established for the first time in an ultraxerous zone. Higher biomass numbers were seen near the surface and ice table, perhaps representing locally more clement environments. Currently, water activity is too low to support metabolism at the Phoenix site, but obliquity changes may produce higher temperatures and sufficient water activity to permit microbial growth, if the populations could survive long dormancy periods (∼106years).

Dawn’s Gamma Ray and Neutron Detector

The NASA Dawn Mission will determine the surface composition of 4 Vesta and 1 Ceres, providing constraints on their formation and thermal evolution. The payload includes a Gamma Ray and Neutron Detector (GRaND), which will map the surface elemental composition at regional spatial scales. Target elements include the constituents of silicate and oxide minerals, ices, and the products of volcanic exhalation and aqueous alteration. At Vesta, GRaND will map the mixing ratio of end-members of the howardite, diogenite, and eucrite (HED) meteorites, determine relative proportions of plagioclase and mafic minerals, and search for compositions not well sampled by the meteorite collection. The large south polar impact basin may provide an opportunity to determine the composition of Vesta’s mantle and lower crust. At Ceres, GRaND will provide chemical information needed to test different models of Ceres’ origin and thermal and aqueous evolution. GRaND is also sensitive to hydrogen layering and can determine the equivalent H2O/OH content of near-surface hydrous minerals as well as the depth and water abundance of an ice table, which may provide information about the state of water in the interior of Ceres. Here, we document the design and performance of GRaND with sufficient detail to interpret flight data archived in the Planetary Data System, including two new sensor designs: an array of CdZnTe semiconductors for gamma ray spectroscopy, and a loaded-plastic phosphor sandwich for neutron spectroscopy. An overview of operations and a description of data acquired from launch up to Vesta approach is provided, including annealing of the CdZnTe sensors to remove radiation damage accrued during cruise. The instrument is calibrated using data acquired on the ground and in flight during a close flyby of Mars. Results of Mars flyby show that GRaND has ample sensitivity to meet science objectives at Vesta and Ceres. Strategies for data analysis are described and prospective results for Vesta are presented for different operational scenarios and compositional models.

Water tracks and permafrost in Taylor Valley, Antarctica: Extensive and shallow groundwater connectivity in a cold desert ecosystem

Water tracks are zones of high soil moisture that route water downslope over the ice table in polar environments. We present physical, hydrological, and geochemical evidence collected in Taylor Valley, McMurdo Dry Valleys, Antarctica, which suggests that previously unexplored water tracks are a significant component of this cold desert land system and constitute the major flow path in a cryptic hydrological system. Geological, geochemical, and hydrological analyses show that the water tracks are generated by a combination of infiltration from melting snowpacks, melting of pore ice at the ice table beneath the water tracks, and melting of buried segregation ice formed during winter freezing. The water tracks are enriched in solutes derived from chemical weathering of sediments as well as from dissolution of soil salts. The water tracks empty into ice-covered lakes, such as Lake Hoare, resulting in the interfingering of shallow groundwater solutions and glacier-derived stream water, adding complexity to the geochemical profile. Approximately four orders of magnitude less water is delivered to Lake Hoare by any given water track than is delivered by surface runoff from stream flow; however, the solute delivery to Lake Hoare by water tracks equals or may exceed the mass of solutes delivered from stream flow, making water tracks significant geochemical pathways. Additionally, solute transport is two orders of magnitude faster in water tracks than in adjacent dry or damp soil, making water tracks “salt superhighways” in the Antarctic cold desert. Accordingly, water tracks represent a new geological pathway that distributes water, energy, and nutrients in Antarctic Dry Valley, cold desert, soil ecosystems, providing hydrological and geochemical connectivity at the hillslope scale.

Adsorptive fractionation of HDO on JSC MARS-1 during sublimation with implications for the regolith of Mars

A chamber was constructed to simulate the boundary between the ice table, regolith and atmosphere of Mars and to examine fractionation between H 2O and HDO during sublimation under realistic martian conditions of temperature and pressure. Thirteen experimental runs were conducted with regolith overlying the ice. The thickness and characteristic grain size of the regolith layer as well as the temperature of the underlying ice was varied. From these runs, values for the effective diffusivity, taking into account the effects of adsorption, of the regolith were derived. These effective diffusivities ranged from 1.8 × 10 −4 m 2 s −1 to 2.2 × 10 −3 m 2 s −1 for bare ice and from 2.4 × 10 −11 m 2 s −1 to 2.0 × 10 −9 m 2 s −1 with an adsorptive layer present. From these, latent heats of adsorption of 8.6 ± 2.6 kJ mol −1 and 9.3 ± 2.8 kJ mol −1 were derived at ice-surface temperatures above 223 ± 8 K and 96 ± 28 kJ mol −1 and 104 ± 31 kJ mol −1 respectively for H 2O and HDO were derived at colder temperatures. For temperatures below 223 K, the effective diffusivity of HDO was found to be lower than the diffusivity of H 2O by 40% on average, suggesting that the regolith was adsorptively fractionating the sublimating gas with a fractionation factor of 1.96 ± 0.74. Applying these values to Mars predicts that adsorbed water on the regolith is enriched in HDO compared to the atmosphere, particularly where the regolith is colder. Based on current observations, the D/H ratio of the regolith may be as high as 21 ± 8 times VSMOW at 12°S and L S = 357° if the regolith is hydrated primarily by the atmosphere, neglecting any hydration from subsurface ice.

Seasonal defrosting of the Phoenix landing site

The retreat of the seasonal polar cap at the Phoenix landing site is analyzed through observations of high resolution images and thermal modeling of the CO2 frost mass. Numerical simulations indicate that the onset of CO2 frost formation occurs around Ls = 220° at 68°N; however, the polar hood prevented imaging of the surface during this time. In the late winter/early spring a continuous layer of frost coated the surface obscuring the underlying polygonal patterns; however, rocks are clearly evident indicating that the frost depth was relatively shallow. Rare dark fans originating from rocks are also observed in an image from early spring. Frost dissipated from the polygon centers first and lingered longer in polygon troughs. Polygon centers were first exposed on Ls = 13.9°, and the polygon troughs were frost free by Ls ∼57°. Frost distributions varied slightly between geologic units; however, in general, global observations at similar latitudes are consistent with what is seen in the Phoenix region. A series of hypotheses is also explored to explain the preference for the CO2 frost to linger in the troughs: lower thermal inertia material in the troughs, aeolian redistribution of frost, burial of the ice table via aeolian redistribution of dust, and shadowing. Each of these hypotheses has drawbacks to explaining this phenomenon as stand‐alone components. Shadows do play a role in keeping the CO2 frost on the surface; however, this effect is small and cannot alone account for the observations.

Explosive lava‐water interactions in Elysium Planitia, Mars: Geologic and thermodynamic constraints on the formation of the Tartarus Colles cone groups

Volcanic rootless constructs (VRCs) are the products of explosive lava‐water interactions. VRCs are significant because they imply the presence of active lava and an underlying aqueous phase (e.g., groundwater or ice) at the time of their formation. Combined mapping of VRC locations, age‐dating of their host lava surfaces, and thermodynamic modeling of lava‐substrate interactions can therefore constrain where and when water has been present in volcanic regions. This information is valuable for identifying fossil hydrothermal systems and determining relationships between climate, near‐surface water abundance, and the potential development of habitable niches on Mars. We examined the western Tartarus Colles region (25–27°N, 170–171°E) in northeastern Elysium Planitia, Mars, and identified 167 VRC groups with a total area of ∼2000 km2. These VRCs preferentially occur where lava is ∼60 m thick. Crater size‐frequency relationships suggest the VRCs formed during the late to middle Amazonian. Modeling results suggest that at the time of VRC formation, near‐surface substrate was partially desiccated, but that the depth to the midlatitude ice table was ≲42 m. This ground ice stability zone is consistent with climate models that predict intermediate obliquity (∼35°) between 75 and 250 Ma, with obliquity excursions descending to ∼25–32°. For lava thicknesses ranging from 30 to 60 m and ground ice fractions ranging from 0.1 to 0.3, an ice volume of ∼4–23 km3could have been melted and/or vaporized by the time the lava solidified, and the associated hydrothermal systems could have retained temperatures >273 K for up to ∼1300 years.

In situ analysis of ice table depth variations in the vicinity of small rocks at the Phoenix landing site

Prior to the Phoenix mission, ice stability simulations predicted that surface rocks at the landing site would produce depressions in the ice table (the boundary between dry and ice‐cemented soil) if ground ice at the site were in diffusive equilibrium with atmospheric water vapor. We use stereo images and digital elevation models to investigate variations in the depth of the ice table near two decimeter‐scale rocks at the Phoenix landing site and compare the observed depth variations to ice stability simulations. We find that ice table depth variations observed near rocks at the landing site are consistent with our theoretical predictions. Based on our results we conclude that: 1) Water transport in the near‐surface regolith occurs primarily via vapor diffusion; 2) Environmental conditions in which water transport is dominated by diffusion have been persistent at the landing site for 100s to 1000s of years or longer; that is, ground ice is in diffusive equilibrium with atmospheric water vapor; and 3) In diffusive equilibrium, thermal stability dictates the depth of the ice table on horizontal length scales as small as centimeters.

Wet Chemistry experiments on the 2007 Phoenix Mars Scout Lander mission: Data analysis and results

Chemical analyses of three Martian soil samples were performed using the Wet Chemistry Laboratories on the 2007 Phoenix Mars Scout Lander. One soil sample was obtained from the top ∼2 cm (Rosy Red) and two were obtained at ∼5 cm depth from the ice table interface (Sorceress 1 and Sorceress 2). When mixed with water in a ∼1:25 soil to solution ratio (by volume), a portion of the soil components solvated. Ion concentrations were measured using an array of ion selective electrodes and solution conductivity using a conductivity cell. The measured concentrations represent the minimum leachable ions in the soil and do not take into account species remaining in the soil. Described is the data processing and analysis for determining concentrations of seven ionic species directly measured in the soil/solution mixture. There were no significant differences in concentrations, pH, or conductivity, between the three samples. Using laboratory experiments, refinement of the surface calibrations, and modeling, we have determined a pH for the soil solution of 7.7(±0.3), under prevalent conditions, carbonate buffering, and PCO2 in the cell headspace. Perchlorate was the dominant anion in solution with a concentration for Rosy Red of 2.7(±1) mM. Equilibrium modeling indicates that measured [Ca2+] at 0.56(±0.5) mM and [Mg2+] at 2.9(±1.5) mM, are consistent with carbonate equilibrium for a saturated solution. The [Na+] and [K+] were 1.4(±0.6), and 0.36(±0.3) mM, respectively. Results indicate that the leached portion of soils at the Phoenix landing site are slightly alkaline and dominated by carbonate and perchlorate. However, it should be noted that there is a 5–15 mM discrepancy between measured ions and conductivity and another species may be present.

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.

Buffering of sublimation loss of subsurface ice by percolating snowmelt: a theoretical analysis

AbstractSubsurface ice in cold hyperarid conditions retreats by sublimation and diffusion through the overlying soil layer. Here, it is shown that percolating meltwater, if present, can counterbalance sublimation loss effectively and thus increase the persistence time of subsurface ice. Time averaging of transport equations is used to evaluate the significance of percolation in an otherwise complex dynamical system. The reduction in sublimation loss is approximately given by the amount of meltwater multiplied by the percolation depth and divided by the depth to the ice table. It is plausible that percolation is even more effective during a warmer, wetter climate. Copyright © 2009 John Wiley & Sons, Ltd.

Laboratory experiments and models of diffusive emplacement of ground ice on Mars

Experiments demonstrate for the first time the deposition of subsurface ice directly from atmospheric water vapor under Mars surface conditions. Deposition occurs at pressures below the triple point of water and therefore in the absence of a bulk liquid phase. Significant quantities of ice are observed to deposit in porous medium interstices; the maximum filling fraction observed in our experiments is ∼90%, but the evidence is consistent with ice density in pore spaces asymptotically approaching 100% filling. The micromorphology of the deposited ice reveals several noteworthy characteristics including preferential early deposition at grain contact points, complete pore filling between some grains, and captured atmospheric gas bubbles. The boundary between ice‐bearing and ice‐free soil, the “ice table,” is a sharp interface, consistent with theoretical investigations of subsurface ice dynamics. Changes of surface radiative properties are shown to affect ice table morphology through their modulation of the local temperature profile. Accumulation of ice is shown to reduce the diffusive flux and thus reduce the rate of further ice deposition. Numerical models of the experiments based on diffusion physics are able to reproduce experimental ice contents if the parameterization of growth rate reduction has expected contributions in addition to reduced porosity. Several phenomena related to the evolution of subsurface ice are discussed in light of these results, and interpretations are given for a range of potential observations being made by the Phoenix Scout Lander.

Phoenix soil physical properties investigation

The Phoenix Lander investigated the polygonal terrain and associated soil and icy soil deposits of a high northern latitude site on Mars. The soil physical properties component involved the analysis of force data determined from motor currents from the Robotic Arm (RA)'s trenching activity. Using this information and images of the landing site, soil cohesion and angle of internal friction were determined. Dump pile slopes were used to determine the angle of internal friction of the soil: 38° ± 5°. Additionally, an excavation model that treated walls and edges of the scoop as retaining walls was used to calculate mean soil cohesions for several trenches in the Phoenix landing site workspace. These cohesions were found to be consistent with the stability of steep trench slopes. Cohesions varied from 0.2 ± 0.4kPa to 1.2 ± 1.8 kPa, with the exception of a subsurface platy horizon unique to a shallow trough for which cohesion will have to be determined using other methods. Soil on polygon mounds had the greatest cohesion (1.2 ± 1.8 kPa). This was most likely due to the presence of adsorbed water or pore ice above the shallow icy soil surface. Further evidence for enhanced cohesion above the ice table includes lateral increase in excavation force, by over 30 N, as the RA approached ice.

Ice table depth variability near small rocks at the Phoenix landing site, Mars: A pre-landing assessment

We combine thermal simulations of ground ice stability near small rocks with extrapolations of the abundance of rocks at the Phoenix landing site based on HiRISE rock counts to estimate the degree of ice table depth variability within the 3.8 m 2 workspace that can be excavated during the mission. Detailed predictions of this kind are important both to test current ground-ice theory and to optimize soil investigations after landing. We find that Phoenix will very likely have access to at least one rock in the diameter range 5 cm to 1 m. Our simulations, which assume the ice to be in diffusive equilibrium with atmospheric water vapor, indicate that all rocks in this size range are associated with an annulus of deep ice-free soil. Ice table depth variability of 1–5 cm is very likely at the landing site due to the presence of small rocks. Further, there are scenarios in which Phoenix might exploit the presence of individual large rocks and/or the arrangement of small rocks to sample soils at depths >10 cm below the average depth predicted from orbit (∼4 cm). Scale analysis to constrain uncertainties in simulation results indicates that estimates of maximum depths may be somewhat conservative and that ice table depressions associated with individual rocks could be deeper and laterally more extended than indicated by formal predictions by mm to cm.

A prelanding assessment of the ice table depth and ground ice characteristics in Martian permafrost at the Phoenix landing site

We review multiple estimates of the ice table depth at potential Phoenix landing sites and consider the possible state and distribution of subsurface ice. A two‐layer model of ice‐rich material overlain by ice‐free material is consistent with both the observational and theoretical lines of evidence. Results indicate ground ice to be shallow and ubiquitous, 2–6 cm below the surface. Undulations in the ice table depth are expected because of the thermodynamic effects of rocks, slopes, and soil variations on the scale of the Phoenix Lander and within the digging area, which can be advantageous for analysis of both dry surficial soils and buried ice‐rich materials. The ground ice at the ice table to be sampled by the Phoenix Lander is expected to be geologically young because of recent climate oscillations. However, estimates of the ratio of soil to ice in the ice‐rich subsurface layer suggest that that the ice content exceeds the available pore space, which is difficult to reconcile with existing ground ice stability and dynamics models. These high concentrations of ice may be the result of either the burial of surface snow during times of higher obliquity, initially high‐porosity soils, or the migration of water along thin films. Measurement of the D/H ratio within the ice at the ice table and of the soil‐to‐ice ratio, as well as imaging ice‐soil textures, will help determine if the ice is indeed young and if the models of the effects of climate change on the ground ice are reasonable.

Periglacial landforms at the Phoenix landing site and the northern plains of Mars

We examine potentially periglacial landforms in Mars Orbiter Camera (MOC) and High Resolution Imaging Science Experiment (HiRISE) images at the Phoenix landing site and compare them with numerical models of permafrost processes to better understand the origin, nature, and history of the permafrost and the surface of the northern plains of Mars. Small‐scale (3–6 m) polygonal‐patterned ground is ubiquitous throughout the Phoenix landing site and northern plains. Larger‐scale (20–25 m) polygonal patterns and regularly spaced (20–35 m) rubble piles (localized collections of rocks and boulders) are also common. Rubble piles were previously identified as “basketball terrain” in MOC images. The small polygon networks exhibit well‐developed and relatively undegraded morphology, and they overlay all other landforms. Comparison of the small polygons with a numerical model shows that their size is consistent with a thermal contraction origin on current‐day Mars and are likely active. In addition, the observed polygon size is consistent with a subsurface rheology of ice‐cemented soil on depth scales of about 10 m. The size and morphology of the larger polygonal patterns and rubble piles indicate a past episode of polygon formation and rock sorting in thermal contraction polygons, while the ice table was about twice as deep as it is presently. The pervasive nature of small and large polygons, and the extensive sorting of surface rocks, indicates that widespread overturning of the surface layer to depths of many meters has occurred in the recent geologic past. This periglacial reworking has had a significant influence on the landscape at the Phoenix landing site and over the Martian northern plains.

Size‐frequency distributions of rocks on the northern plains of Mars with special reference to Phoenix landing surfaces

The size‐frequency distributions of rocks >1.5 m diameter fully resolvable in High Resolution Imaging Science Experiment (HiRISE) images of the northern plains follow exponential models developed from lander measurements of smaller rocks and are continuous with rock distributions measured at the landing sites. Dark pixels at the resolution limit of Mars Orbiter Camera thought to be boulders are shown to be mostly dark shadows of clustered smaller rocks in HiRISE images. An automated rock detector algorithm that fits ellipses to shadows and cylinders to the rocks, accurately measured (within 1–2 pixels) rock diameter and height (by comparison to spacecraft of known size) of ∼10 million rocks over >1500 km2 of the northern plains. Rock distributions in these counts parallel models for cumulative fractional area covered by 30–90% rocks in dense rock fields around craters, 10–30% rock coverage in less dense rock fields, and 0–10% rock coverage in background terrain away from craters. Above ∼1.5 m diameter, HiRISE resolves the same population of rocks seen in lander images, and thus size‐frequency distributions can be extrapolated along model curves to estimate the number of rocks at smaller diameters. Extrapolating sparse rock distributions in the Phoenix landing ellipse indicate <1% chance of encountering a potentially hazardous rock during landing or that could impede the opening of the solar arrays. Extrapolations further suggest rocks large enough to depress the ground ice table and small enough to be picked up or pushed by the robotic arm should be present within reach for study after landing.

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.

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 H 2O 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 × 10 4 years). Depending on the fate of high-obliquity polar ice, the maximum dormancy period can exceed 4 × 10 6 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.