Volcanic Features in Valles Marineris Reexamined with Mgs Data

Introduction: Viking images showed possible young volcanic deposits inside the Valles Marineris [1]. Some of these features are now covered by Mars Observer Camera (MOC) narrow-angle images at resolutions of only a few meters per pixel, and a new look is warranted. The features discussed in the earlier paper [1] were dark spots or bands at the base of trough walls, rimless craters, and a late hum-mocky, light colored deposit locally associated with flow lobes. Dark spots and cones: A north-south-trending trough that connects Ophir Chasma with Candor Chasma is lined by dark materials, locally clustered in dunes. Dark, cone-shaped structures in parallel rows were barely recognized on top of a low mesa in this trough [1,2] (Fig. 1). A MOC image shows the area in detail and reveals a line of circular and oval depressions situated on dark ridges. The structures resemble volcanic fissure vents and support the notion that much of the abundant dark material in this region is locally derived. The circular depressions are younger than the light interior deposits and the low mesa, and perhaps even younger than a set of fresh-looking landslides that shed debris into this region [3]. Dark bands: Dark outcrops line the base of the south wall of west Candor Chasma and a spur separating Ophir Chasma from Candor Chasma. Many MOC images cover these outcrops. Unfortunately, most of them are so dark that no morphologic detail can be seen. The low albedo suggests little contamination with lighter material derived from the trough walls or nearby interior deposits [4]. A lack of associated dark dunes makes it unlikely that the material is a wind deposit funneled along the base of the trough walls. The dark material covers the lower wall-rock spurs, which suggests that the material could be a thin veneer, but in some places it appears to be massive and composing the spurs. A volcanic origin for the dark material is suggested by associations with some flow lobes (Fig. 2) and by an irregular depression that could be a caldera. Mars Observer Laser Al-timeter (MOLA) measurements show that the elevations of the dark bands at the base of the trough walls rise and fall in concert with the elevations of the adjacent trough floors. This configuration supports an alignment of the dark material along faults. It is less compatible with an interpretation of the dark material as a basal layer in …