Martian sand sheet characterization and implications for formation: A case study

Windblown sand and dust dominate surface geologic processes in Mars' current environment. Besides sand dune fields, areally extensive sand sheets are common across Mars, blanketing the underlying topography with several meters of rippled sand. Earth's sand sheets commonly form upwind or cross-wind to dunes and both partially trap and source sediment to downwind dunes. In contrast, Mars' sheets are frequently located downwind of active barchan and dome sand dunes, suggesting they cannot be a sediment source for the dunes as on Earth. Here, we characterize a Martian sand sheet and its geologic context, model the regional atmospheric circulation, and more broadly consider the implications for sand sheet formation on Mars. Our case study sand sheet in central Herschel Crater is < 5 m thick, with an upwind margin forming in the lee of topographic obstacles such as dunes, crater rims, and small hills. The sheet has actively migrating superposing ripples with estimated total sand fluxes comparable to total fluxes measured from slip faces on local, regional, and global dunes, some of which have eroded away. A smooth geologic unit interpreted as outcrops of paleo-sand sheets is adjacent to the active sheets. Our observations and atmospheric modeling-which predict wind shear stresses above the sand suspension threshold-indicate that the upwind dunes may be eroding and their sand deposited downwind in sheets in what may be a cyclical process, possibly related to Mars' axial obliquity cycles.