Documenting the morphology and distribution of polygonally patterned ground on Mars is critical for understanding the age and origin of the Martian latitude‐dependent mantle. Polygonally patterned ground on Mars is analyzed using High Resolution Imaging Science Experiment image data in order to document the variation of polygon morphology within latitude bands 30–80° in both northern and southern hemispheres. Small‐scale (<∼25 m diameter) polygons are classified on the basis of morphological characteristics into seven groups, which are present in both northern and southern hemispheres. Polygon morphology is shown to be consistent with thermal contraction cracking of an ice‐rich mantling unit, consistent with observations of sediment wedge thermal contraction crack polygons forming in ice‐cemented sediment at the Phoenix landing site. Polygon groups are distributed symmetrically in both northern and southern hemispheres, suggesting strong climate controls on polygon morphology. Northern hemisphere polygonally patterned surfaces are found to decrease in age from low to high latitude, spanning surface ages from ∼1 to <0.1 Ma, suggesting more recent deposition of ice‐rich material at high latitudes than at low latitudes. Six of the seven classes of polygons are interpreted to be capable of forming because of the combined effects of thermal contraction cracking and differential sublimation, suggesting that sublimation and sand wedge polygons dominate Martian high latitudes. Gully polygon systems present at midlatitudes suggest that small amounts of liquid water may have been involved in thermal contraction crack polygon processes, producing composite wedge polygons. No evidence is found for the presence of pervasive small‐scale ice wedge polygons.

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 signifi cant component of this cold desert land system and constitute the major fl ow path in a cryptic hydrological system. Geological, geochemical, and hydrological analyses show that the water tracks are generated by a combination of infi ltration 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 icecovered lakes, such as Lake Hoare, resulting in the interfi ngering of shallow groundwater solutions and glacier-derived stream water, adding complexity to the geochemical profi le. 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 fl ow; however, the solute delivery to Lake Hoare by water tracks equals or may exceed the mass of solutes delivered from stream fl ow, making water tracks signifi cant 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.

Glacial landforms on Mars, including concentric crater fill (CCF), lineated valley fill (LVF), and lobate debris aprons (LDA), were mapped between~±30 and 50°latitude, and geometric constraints were placed on the volume of these deposits based on their topography and relationship with host topography (e.g., massifs, valleys, and craters). LDA deposits were found to have the largest volume (2.63 × 10 5 km . The actual ice sequestered in these deposits is calculated depending on the estimated fraction of ice currently remaining in the deposits. We adopt end-member values of 30% (pore ice only) and 90% (relatively pure, debris-covered glacial ice). These volumes, 1.25 × 10 5 km 3 and 3.74 × 10 5 km 3 respectively, represent an equivalent global ice layer 0.9-2.6 m thick. On the basis of extensive recent local and regional analysis of these deposits indicating a debris-covered glacial origin, we favor the larger estimate. Taken together, these glacial landforms have an average thickness of~450 m. The glacial deposition responsible for these features occurred or recurred during an extended period of the middle to late Amazonian, which implies that long-term climate conditions were sufficient to produce and preserve debris-covered glacial landforms on Mars.

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A remnant of Taylor Glacier ice rests beneath a 40-80 cm thick layer of sublimation till in central Beacon Valley, Antarctica. A vapour diffusion model was developed to track summertime vapour flow within this till. As input, we used meteorological data from installed HOBO data loggers that captured changes in solar radiance, atmospheric temperature, relative humidity, soil temperature, and soil moisture from 18 November 2004-29 December 2004. Model results show that vapour flows into and out of the sublimation till at rates dependent on the non-linear variation of soil temperature with depth. Although measured meteorological conditions during the study interval favoured a net loss of buried glacier ice (~0.017 mm), we show that ice preservation is extremely sensitive to minor perturbations in temperature and relative humidity. Net loss of buried glacier ice is reduced to zero (during summer months) if air temperature (measured 2 cm above the till surface) decreases by 5.5ºC (from -7ºC to -12ºC); or average relative humidity increases by 22% (from ~36% to 58%); or infiltration of minor snowmelt equals ~0.002 mm day -1 . Our model results are consistent with the potential for long-term survival of buried glacier ice in the hyper-arid stable upland zone of the western Dry Valleys.

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