THE DISTRIBUTION AND BEHAVIOR OF MARTIAN GROUND ICE DURING PAST AND PRESENT EPOCHS

Mars undergoes significant oscillations in its orbit, which will have a pronounced effect on its climate and, in particular, on the behavior of subsurface water ice. We explore and map the behavior of ice in th e Martian near-surface regolith over the past 1 m.y. using a diffusion and condensation model presented in an earlier paper, with two primar y modifications to include orbitally induced variations in insolation and atmospheric water abundance. We find that the past behavior of gro und ice differs significantly from that at the present epoch, primaril y the result of high-amplitude oscillations in obliquity (presently 25 degrees). In midlatitude and equatorial regions, ground ice will cond ense from atmospheric water during times of higher obliquity, filling the top few meters of the regolith with significant amounts of ice. At an obliquity of 32 degrees, ground ice becomes stable globally. Durin g times of lower obliquity, ground ice will sublime and diffuse back i nto the atmosphere, dessicating the regolith to a depth of about 1 to 2 m equatorward of 60 degrees to 70 degrees latitude. In the high-lati tude regions these oscillations are considerably subdued. Below this d epth of cyclic saturation and dessication a long-term stability of ice exists in some geographic regions. We present a map of the distributi on of ice expected at the present epoch. Cyclic exchange of water betw een the global regolith and polar regions will have significant implic ations for surface geology and the polar layered deposits.