MOUNTAIN GLACIERS ON MARS?: WESTERN ARSIA FLANK DEPOSITS INTERPRETED AS COLD-BASED GLACIAL MORAINES AND ROCK GLACIERS

An unusual Amazonian-aged, fan-shaped deposit covers ~180,000 km² of the western flank of Arsia Mons and consists of three components: 1) an outermost ~60-90 km wide distal zone of over 100 parallel raised ridges; 2) a medial ~80-300 km wide zone of rough, hummocky topography; and 3) a proximal zone up to ~150 km wide abutting the upper flanks of Arsia and consisting of arcuate and lobate flow-like deposits. Upflank are several sinuous outward-facing scarps. The distal ridged deposits are superposed on lava flows and a large impact crater and underlying lava flows can be traced back underneath the medial hummocky unit. Using new MGS data and Earth analogs appropriate for Mars, we explore the hypothesis that the deposit is the remnant of a mountain glacier formed on the western flank of Arsia Mons (e.g., Scott and Zimbelman, USGS MI-2480, 1995). Conditions during the recent geological history of Mars suggest that glacial ice should commonly be below the pressure melting point, and thus analogous to polar glaciers, which are frozen to underlying beds (cold-based), and move by internal deformation, producing little record of basal scour or extensive meltwater features. Glaciers in the Antarctic Dry Valleys may be most appropriate terrestrial analogs, and we find many similarities between them and the western Arsia fan-shaped deposits. We interpret the outer parallel ridge zone to be distal dump moraines formed from the lateral retreat of a cold-based glacier, and the hummocky facies to be proximal hummocky moraines resulting from the sublimation, decay and downwasting of the ice sheet. The arcuate structures in the proximal zone are interpreted to be rock glaciers, formed by lobate flow deformation of debris-covered ice surfaces; some rock glaciers may still be ice-cored, preserving a record of the Amazonian atmosphere and Arsia Mons tephrochronology. We find little evidence for melting features in association with the deposit and thus conclude that it was predominantly cold-based throughout its history. In summary, we find abundant evidence to support the interpretation that the fan-shaped western Arsia Mons deposit was formed by a cold-based mountain glacier. Contemporaneous volcanic eruptions suggest that tephra may be the main non-ice component of the glacial deposits.