GRAIN-SIZE ANALYSIS AND MICRO-SCALE FEATURES IN MARS HAND LENS IMAGER (MAHLI) IMAGES FROM THE HARTMANN’S VALLEY MEMBER, MURRAY FORMATION, GALE CRATER, MARS: LINKS TO DEPOSITIONAL ENVIRONMENT

Since 2012, the Mars Science Laboratory (MSL) Curiosity Rover has traversed through hundreds of meters of sedimentary rocks at Gale crater. Fluvial-deltaic and lacustrine deposits of the Bradbury group and the Murray formation are interpreted to reflect progradation in a lake basin with standing water. The ~25 m thick Hartmann’s Valley member (HVM) of the Murray formation (Sols 1157-1202; Sols 1355-1420) possesses apparent m-scale trough cross stratification that has been interpreted to represent aeolian dunes. However, aeolian facies more closely fit with a lake basin dominated by evaporation. Thus, a detailed study of the HVM characteristics is critical to document and interpret depositional environment. We present a comparative analysis of 4 Mars Hand Lens Imager (MAHLI) images of the HVM with image resolutions at ~ 17 µm/pixel to identify grain-size characteristics and micro-scale features. At least 400 spots per image were analyzed for grain-size analysis. The pervasive result from all the images is an absence of resolvable grains. Among the 4 targets, Inamagando contains the greatest amount of resolvable grains (17%), with 75% of the analyzed points below the resolution of the image and 8% of the points obscured by surface dust. Seventy-one percent of resolvable grains are coarse silt, 25% are very fine sand, and 4% are medium silt. Oudam contains 5.8% resolvable grains. Among the resolvable grains, 96% are coarse silt and 4% are very fine sand. Koes contains 7 resolvable grains, which range from very fine sand to medium sand. No grains in Aubures were resolvable, though this target does contain trace amounts of apparent light-toned sand-sized clasts ranging from fine to coarse sand. The dominant grain size resolved in these 4 targets is coarse silt, which lies below the expected threshold for saltation required for dune construction. The lack of resolvable grains implies that much of the sediment is even finer than silt, although grain boundaries may be obscured by the effects of compaction, diagenesis, and/or much younger aeolian polishing of the rock surface. Therefore, it is possible that the grains that are not resolvable are coarser than the image resolution. Additional work is needed to help determine a more complete grain-size assessment of the HVM.