GEOLOGIC CONTEXT OF THE MARS RADAR STEALTH REGION IN SOUTHWESTERN THARSIS

''Stealth'' is a region on Mars that has no distinguishable radar retu rn. 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 wit h a minimum thickness of several meters, with the most likely explanat ion being that Stealth is a volcanic tephra deposit [Muhleman et al., 1991, 1995]. Stealth overlies some of the youngest geologic units on M ars, including lava flows and a giant, curvilinear-ridged lobate unit on the west flank of Arsia Mons. Stealth also overlies part of a regio n of smooth and wind-scoured materials called the Medusae Fossae Forma tion. Examination of high-resolution Viking orbiter images suggests th at Stealth is the youngest of several discontinuous, wind-worked mantl es of fine-grained (much less than 1 cm) sediment. Older deposits of f ine sediment appear to be interbedded with lava flows and show evidenc e of aeolian deposition (dunes and deposits in the lee of topographic obstacles) and erosion (yardangs and pedestal craters). The geologic c ontext supports a volcanic ash origin for Stealth. If Stealth is compo sed of tephra, then its extent and the nature of landforms in the regi on indicate that it resulted from multiple eruptions that occurred rel atively late in Martian history. Possible source vents include linear depressions (flanked by constructive volcanic landforms) that are loca ted on the lower western slopes of Arsia and Pavonis Montes.