DROP MORAINES, SUBLIMATION TILL, AND DEBRIS-COVERED GLACIERS: COLD-BASED GLACIATION OF WESTERN ARSIA MONS

Basal-ice and surface-ice temperatures are key parameters that govern the style of glacial erosion and deposition. Temperate glaciers contain basal ice at the pressure-melting point (wet-based) and commonly exhibit extensive areas of surface melting. Such conditions foster basal plucking and abrasion, as well as deposition of matrix-supported drift sheets, moraines, and glacio-fluvial outwash. Polar glaciers include those in which the basal ice remains below the pressure melting point (cold-based) and, in extreme cases, lack surface meltwater. These conditions inhibit significant glacial erosion and deposition. An intermediate classification of subpolar glaciers includes those with wet-based interiors and cold-based margins.

Present surface temperatures on Mars indicate that any Amazonian-aged glaciers were likely cold based. Our studies of glaciers in the Dry Valleys region of Antarctica provide diagnostic criteria for cold-based ice: 1) drop moraines, 2) scattered erratics and thin drift sheets atop undisturbed bedrock and unconsolidated landforms; and 3) notable absence of outwash, pro-glacial lacustrine sediments, and associated meltwater channels. Drop moraines and scattered erratics are produced as englacial and supraglacial debris derived from rockfall and/or volcanic ejecta are transported above the glacier bed and dropped passively as ice sublimates.

An Amazonian-aged deposit covering ~180,000 km2 of the western flank of Arsia Mons displays several features diagnostic of deposition from cold-based ice. Foremost are 1) an outermost zone of over 100 arcuate and parallel raised ridges, each up to 10 meters high and 100 km long; 2) a medial zone of rough, hummocky topography superposed on intact lava flows; and, 3) a proximal zone abutting the upper flanks of Arsia consisting of several individual lobes and superposed parallel raised ridges. The latter closely resemble terrestrial debris-covered glaciers. None of the deposits are associated with geomorphic features indicative of meltwater. Collectively, these features indicate glaciation of western Arsia Mons and suggest that relict glacier ice may remain in extant debris-covered glaciers.