Multi-scale Variability in the Ice-table Depth at Potential Phoenix Landing

Introduction: A major objective of the Mars Scout mission, Phoenix, is to characterize the present state of water in the martian environment, at a location where water may play a significant role in the present and past habitability of Mars [1]. Given the generally dry and cold climate of Mars today, any substantial amount of water is expected to occur in the form of ground ice within the regolith. Indeed, the Mars Odyssey Gamma Ray Spectrometer (GRS) has indicated abundant subsurface hydrogen and inferred ground ice at high latitudes in both hemispheres [2]. Therefore, the Phoenix mission will be targeted to land in the northern high latitudes (approximately 65 N 72 N) where ground ice is expected to be abundantly available for analysis [1]. Numerous lines of evidence can be employed to provide an indication of the presence or absence of shallow ground ice at the potential landing sites. Geomorphology, GRS data, and ground ice stability theory (based on thermophysical data) each contribute clues to an overall understanding of the distribution of ice. Orbital observations provide information on regional scales (10s to 100s of km) and local scales (10s of m to km). Understanding the behavior of ground ice at these scales is important for the selection of a Phoenix landing site. Understanding ground ice behavior at the scale of the lander and its work area (less than a few m) is of particular interest for primary mission operations. However, data at this scale is not presently available. While ground ice may be stable on a regional scale, local-scale slopes and changes in the physical characteristics of soils can result in significant variations in the distribution of ice. Likewise, small surface heterogeneities at the landing site may cause undulations in the ice-table, presenting challenges (or opportunities) during excavation (Figure 1). In the present work we address questions of ice-table depth and variability on all scales relevant to Phoenix, with particular emphasis on scales relevant to excavation. Ground Ice Stability: In the current cold, dry martian environment, stable ground ice is in diffusive equilibrium with atmospheric water vapor. The icetable is a sharp boundary separating dry and densely ice-cemented soil, which occurs at the depth where net sublimation balances net condensation on annual time scales or longer. Temperatures in the martian subsurface (and thus water vapor density in pore spaces) are controlled primarily by the thermophysical properties of surface and subsurface materials, which are variable at regional, local and lander scales. At all scales, we simulate the thermal behavior of the martian surface and subsurface using a radiative-conductive model and determine the icetable depth assuming diffusive equilibrium. We present maps of predicted ice-table depth on global and regional scales. At local scales we examine the ice-table variability due to changes in bulk soil characteristics (thermal inertia and albedo) and other factors that influence the depth of ice (e.g., slope). Finally, at the scale of the lander and the area to be excavated, we explore the effects of meterand decimeter-size hetereogeneities (e.g. rocks, variations in soil grain size and albedo) on the ice-table.