Physics
Scientific paper
Dec 2007
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2007agufm.p14a..03h&link_type=abstract
American Geophysical Union, Fall Meeting 2007, abstract #P14A-03
Physics
0702 Permafrost (0475), 1823 Frozen Ground, 5422 Ices, 5462 Polar Regions, 5470 Surface Materials And Properties
Scientific paper
Conditions during previous climate epochs on Mars may have allowed subsurface ice to form via diffusion from a moist atmosphere. The deposition and recharge of such reservoirs is driven by subsurface humidity gradients; an atmospheric frostpoint greater than that of the subsurface results in a net influx of vapor which deposits in pore space as ice. Observations of the hydrogen distribution by Mars Odyssey indicate that the ice content of some high-latitude regions (e.g. Olympia Undae) exceeds 70% by volume. Reconciliation of this concentration with typically lower porosities of soils demands a process of ice segregation (lensing) and mechanical expansion, or direct precipitation. We investigate the possibility and consequences of volumetrically significant subsurface ice derived from the Mars atmosphere by vapor diffusion, at present and in the past. Experiments conducted at the Mars Simulation and Ice Laboratory at Caltech demonstrate that diffusion processes produce significant pore-filling ice under controlled lab conditions. Atmospherically derived water vapor is deposited within an initially dry porous medium subject to a strong (~15~K/cm) temperature gradient forcing a humidity gradient. This mimics the humidity gradient caused by time varying temperatures in the shallow subsurface of Mars with a static experimental setup. The vertical profile of water content is determined at the end of the experiment by gravimetric analysis and the thermal conductivity of the ice-bearing sample is calculated. Pore filling fractions up to 100% have been measured. Profiles with a marked transition in ice content at the frostpoint depth are observed corresponding to a subsurface ice table. The data enable calculation of time-varying diffusion coefficients which exhibit a reduction of up to an order of magnitude with respect to ice-free regolith. These are compared to numerical models of vapor diffusion incorporating ice deposition and pore constriction. Formation theories of high-volume subsurface ice deposits are informed by these results. Implications and predictions for the state of the regolith at the Phoenix landing site are discussed.
Aharonson Oded
Hudson Troy L.
Oslund K.
Schorghofer Norbert
Siegler Matthew
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