Probable ice-rich deposits on north-facing slopes in Alba Patera, Mars

Abstract

We examine unconformable lobate deposits along the north-facing slopes of Alba Patera (40°N, 250°E with an elevation of ~6 km), Mars, using data from the Mars Reconnaissance Orbiter Context Camera, High-Resolution Imaging Science Experiment, and the Mars Orbiter Laser Altimeter. We interpret the lobate north-facing slope deposits (NSFDs) to be small (<3 km from source to toe) lobate debris aprons (LDAs), making them the highest-elevation LDAs yet identified on Mars. The total volume of the deposits along the caldera walls is ~5–11 km3. Deposits filling impact craters around the Alba Mons summit appear to be similar to the NFSDs. These NFSDs bear two distinct textures either singly or together: a hummocky surface of decameter-scale hills and a smooth surface. Smooth-textured NFSDs have lobate margins and convex-up topographic profiles, and show evidence of having flowed; hummocky NSFDs have concave or linear topographic profiles and are generally found on steep slopes. These deposits likely formed due to reduced insolation on north-facing slopes, which allowed for the preferential accumulation or preservation of water ice. The presence of small LDAs restricted to north-facing slopes could indicate that Alba Patera was only a marginal environment for glaciation, possibly because little water vapor was available at such a high elevation. The hummocky material is interpreted to be the eroded remnants of a mantle of ice-cemented dust that was superposed on the NFSDs. This mantle has been almost completely removed in smooth-textured areas. This difference in rates of ice removal was the result of either lower insolation on steep north-facing slopes than on shallow slopes or the greater mobility of the thicker, lobate portions of NFSDs producing a more densely fractured mantle.