Local and regional Mars Orbiter Laser Altimeter (MOLA) topographic data support the presence of an extensive Hesperian-aged volatile-rich south polar deposit (the Dorsa Argentea Formation, Hd, and related units) underlying the present Amazonian-aged polar cap (Api, residual ice, and Apl, layered terrain) and covering a surface area that could be as large as 2.94×106 km2 (about 2% of the surface of Mars), over twice the area of the present Amazonian-aged south polar deposits. The deposit characteristics indicate that it contained significant quantities of water ice in amounts comparable to present-day polar deposits. Several lines of evidence for melting indicate that the ice sheet deposits underwent melt back and liquid water drainage into surrounding lows, including a large valley near the crater Schmidt and the Argyre basin. Narrow sinuous ridges lie in a broad linear depression extending from a high near the present polar cap continuously downslope to near the distal portion of Hd. The new topographic data support the interpretation of these ridges as eskers, representing meltwater distribution networks at the base of the receding deposit. Extensive development of large pits and depressions (cavi) have previously been interpreted as eolian etching or basal melting of ice-rich deposits. Analysis of MOLA topography supports the interpretation that they represent basal melting of ice-rich deposits and shows that they have links to the esker systems. Volumetric considerations and topographic lineations suggest that some of the basal melting occurred beneath regions presently occupied by Apl, and that some of the liquid water formed ponds and lakes in the distal parts of Hd. The presence of pedestal craters is further evidence of the removal of extensive volatile-rich deposits and contributes to the quantitative measure of the former deposit thickness. Where did the melt products go? Inspection of the margins of the Dorsa Argentea Formation reveals several large channels that begin there and drain downslope for distances between 900 and 1600 km onto the floor of the Argyre basin, some 3.5–4.0 km below their origin. These channels do not exhibit tributaries. Their broad lateral distribution supports other evidence that deposit melting was areally very widespread and volumetrically significant, and that a large part of the meltwater entered the surface distribution system and was deposited on the floor of the Argyre basin over 1000 km away. Estimates of the present deposit thicknesses together with amounts of the deposit removed by meltback suggest that the original volume could have been as much as 5.9×106 km3, equivalent to a global layer of water ∼20 m deep if the deposit consisted of ∼50% volatiles. A portion of these volatiles migrated across the surface to pond in adjacent valleys and basins, and into the groundwater system. A significant portion of the volatiles remain in the deposit, representing a net removal from the atmosphere and from the active hydrologic system in early to middle Mars history, and forming an accessible record of aqueous conditions and possible biological environments dating from that time.
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