Geologic context of the Mars radar ''Stealth'' region in southwestern Tharsis

''Stealth'' is a region on Mars that has no distinguishable radar return. Stealth was discovered in 1988 by Muhleman et al. [1991] in 3.5 cm Earth-based radar images, and it was found to extend similar to 2000 km along the Martian equator west of Arsia and Pavonis Montes. Stealth was proposed to be a deposit of unconsolidated, granular material with a minimum thickness of several meters, with the most likely explanation being that Stealth is a volcanic tephra deposit [Muhleman et al., 1991, 1995]. Stealth overlies some of the youngest geologic units on Mars, including lava flows and a giant, curvilinear-ridged lobate unit on the west flank of Arsia Mons. Stealth also overlies part of a region of smooth and wind-scoured materials called the Medusae Fossae Formation. Examination of high-resolution Viking orbiter images suggests that Stealth is the youngest of several discontinuous, wind-worked mantles of fine-grained (much less than 1 cm) sediment. Older deposits of fine sediment appear to be interbedded with lava flows and show evidence of aeolian deposition (dunes and deposits in the lee of topographic obstacles) and erosion (yardangs and pedestal craters). The geologic context supports a volcanic ash origin for Stealth. If Stealth is composed of tephra, then its extent and the nature of landforms in the region indicate that it resulted from multiple eruptions that occurred relatively late in Martian history. Possible source vents include linear depressions (flanked by constructive volcanic landforms) that are located on the lower western slopes of Arsia and Pavonis Montes.