COASTAL GEOMORPHOLOGY AT ENDEAVOUR CRATER: OBSERVATIONS BY THE OPPORTUNITY ROVER

Opportunity is currently approaching Solander Point, the third exposure of the 22-km Endeavour Crater rim in Maridiani Planum. The rim and interior of Endeavour Crater is partly obscured by the familiar sulfate sands/mudstones of Meridiani Planum. The contact between the plains and the rim inselbergs is defined by an apron, typically about 15-20 meters in width and elevated slightly above the surrounding plains. In monoscopic orbiter images, this bench suggests a horizontal surface at the base of the inselbergs. However, digital elevation models made from stereo orbiter images show these aprons define approximate planar surfaces sloping downward into the crater interior by several degrees. The bench is best exposed along the west and east rim mountains. From orbit, it exhibits four principal elements. From the plains, upslope, they are: 1, a basal scarp up to a few tens of centimeters high; 2, a margin of bright outcrop that appears to either onlap or protrude from beneath 3, a smooth darker slope extending from the bright outcrop up to 4, a low escarpment or “berm” at the base of the rougher-looking surface of the Endeavour rim and a few meters upslope from the basal scarp.

Cape York was the first of the west rim mountains visited by Opportunity, followed by Sutherland Pt. and Nobbys Head. Opportunity has imaged the bench at several locations around these inselbergs with the navigation and panorama cameras (navcam and pancam). From the ground, the basal scarp does appear to be an erosional feature, as does the berm separating the bench or apron from the Endeavour rim surfaces. The bright outcrop margin (the “Grasberg unit”) is the most prominent of several successive onlaps of thinly-laminated sulfate sediments onto the crater rim inselbergs. Morphologically and stratigraphically, these onlaps are strikingly similar to terrestrial beachrock, but are sulfate rather than carbonate. If it is assumed that the bench, berms and onlaps were once horizontal surfaces, then a mechanism must be found to explain the tilt of both the plains margin and the inselbergs inward toward the crater interior. The crater seems to be too small a feature for a tectonic mechanism. Perhaps sediment dewatering and compaction over time could have caused the tilt at the rim if the sediment were thicker or deposited more rapidly inside the crater interior than outside.