Martian Seasonal CO2 Frost Indicating Decameter-Scale Variability in Buried Water Ice

Details Martian Seasonal CO2 Frost Indicating Decameter-Scale Variability in Buried Water Ice Martian Seasonal CO2 Frost Indicating Decameter-Scale Variability in Buried Water Ice

Dec 2011

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011agufm.p23a1691m&link_type=abstract

American Geophysical Union, Fall Meeting 2011, abstract #P23A-1691

Physics

[0702] Cryosphere / Permafrost, [5422] Planetary Sciences: Solid Surface Planets / Ices, [6225] Planetary Sciences: Solar System Objects / Mars

Scientific paper

Several new lines of evidence indicate that subsurface water ice (ground ice) on Mars is more complexly distributed, and in variable concentrations, than had been previously envisioned. Understanding the current distribution of ground ice is a fundamental part of understanding how this ice was emplaced and the recent past climate conditions under which icy deposits formed and subsequently evolved. In this work we examine the seasonal defrosting of CO2 observed by HiRISE as an indicator of decameter-scale ground-ice heterogeneity. It is well known that CO2 dry ice accumulates on the martian surface in winter. The amount of dry ice and the time it spends on the ground depends strongly on surface properties. A readily observable attribute is the "crocus date", the season (Ls) when CO2 completely sublimates, exposing the soil surface. Many factors can affect the crocus date, but perhaps most important are the properties of CO2 frost and of the surface soil. We examine HiRISE observations, spanning more than a martian year, for decameter-scale patterns of CO2 frost and the crocus date. Year-to-year repeatability of CO2 ice patterns, both in polygon troughs and decameter-size patches, along with a lack of topography nor aeolian redistribution, suggests that differences in the surface substrate is the root cause for these patterns. In addition, only CO2 slab ice (solid, non-porous dry ice) is indicated throughout the observed seasons and at all spatial scales (down to meter scale), as evidenced by albedo (HiRISE and TES) and IR spectra (CRISM). In addition, the low emissivity and high albedo of fine-grained particulate CO2 frost would result in a crocus date much earlier than even the earliest observed. We present two scenarios of substrate differences which explain the observations: (i) the ice-table depth varies away from atmospheric equilibrium, such that a thicker "dry-soil" layer occurs in disequilibrium where the CO2 ice lingers longest; and (ii) the H2O concentration at the ice table and below is less than pore filling (or that subsurface pure-water-ice deposits contain substantial open porosity) in these same lingering CO2-ice locations. In the later case these CO2 ice patches may be outlining deposits of the light-toned, friable, pure water ice as was discovered by Phoenix. Neither of these two ground-ice scenarios is predicted by the current models of ground ice stability and dynamics. These results indicate substantial decameter-scale heterogeneity in the ground ice distribution and local disequilibrium with the current martian climate.

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