Deposition and erosion of a Light-Toned Yardang-forming unit of Mt Sharp, Gale crater, Mars

Abstract

Gale crater is a large impact crater with a ca 5 km thick sequence of stratified rocks in it, expressed today as a central eroded mound (i.e., Aeolis Mons informally named Mt. Sharp). A goal of the current Mars Science Laboratory mission in Gale crater is to investigate the processes that deposited, lithified, and eroded this fill. The Light-Toned Yardang Unit (LTYu) unit, the subject of the present research, is one of the geological units of Mt Sharp. Our specific purpose here is to refine and interpret the imaging documentation of the morphologic and stratal components of the LTYu, at large outcrop scale. In combination with established orbital images, we use the Remote Micro-Imager (RMI) of ChemCam, a remote sensing instrument currently operated onboard Curiosity rover, which provides several types of context imaging. RMI capabilities now include “Long Distance” acquisitions of targets several kilometers away. In these new acquisitions, substantial differences are visible in LTYu yardang attitudes from lowest to uppermost elevations allowing tentative subdivision of the LTYu into subunits. Bedding geometries in the lower LTYu are consistent with eolian dune foresets which collectively prograde towards an average N134° direction. Based on stratal architectures, the LTYu is viewed as an amalgamated stack of at least two, and possibly four, ancient erg systems bounded by large deflationary “supersurfaces”. Observations point to a multistory generation of yardangs interpreted to have been successively buried during the stratigraphic building of Mt Sharp. We conclude that the successive sequences of eolian deposition-erosion recorded by the LTYu have been generated by cyclic changes from semi-arid to arid conditions, coupled in climatic cycles, including wind regime change. The regional unconformity that tops the Lower mound formation of Mt Sharp, and the subsequent emplacement of the LTYu, collectively express a clear tendency toward sustained arid environments for this region of Mars around the Early – Late Hesperian transition. Given the large time scale involved (i.e., a few tens of million years as a minimum), we consider it likely that the local evidence for increased aridity in the Mt Sharp stratigraphy is a manifestation of climate change affecting the whole planet.