Chemical composition of the first rocks sampled by the Perseverance rover in Jezero crater, Mars

Abstract

International audienceThe Perseverance rover will collect its first sample in August 2021 by coring low-standing, polygonally fractured rocks associated with the crater-retaining terrain to the southeast of Jezero delta. Future laboratory analyses of this first sample could determine the origin and depositional/emplacement age of the fractured unit in Jezero crater. Depending on the lithology, further analyses can address: 1. the potential utility of this unit for the constraining the age of the delta and possibly calibration of the crater chronology of Mars; 2. the evolution of martian interior, surface and climate. The dusty polygons are flat on a m-to-cm scale, largely lack visible layering and occasionally grade into darker, higher-standing nubs. WATSON and SuperCam Remote Micro-Imager (RMI) images of these rocks reveal mm-scale light and dark patches and specular reflection in a matrix with otherwise difficult-to-see grains. Analyses by SuperCam laser-induced breakdown spectroscopy (LIBS) and visible and infrared spectroscopy (VISIR) do not support a compositional distinction between the low areas and nubs, suggesting two different weathering expressions of the same starting material. The same analyses reveal hydration signals on the surfaces of all these rocks and suggest alteration by fluids, whereas the dark mm-to-cm scale pits, holes and reflective patches on the rock surfaces indicate wind abrasion. The low-standing rocks are compositionally heterogeneous at the scale of LIBS spot raster spacings (2-5 mm), with a bulk composition consistent with basaltic andesite. The major element compositions of some individual ~0.3 mm-diameter LIBS spots are consistent with iron oxides, ilmenite, various pyroxene minerals, feldspars and glass-like phases enriched in SiO2. This supports the presence of pure mineral grains at this scale. VISIR data point to a potential mixture of iron-rich phyllosilicate and iron oxyhydroxide on rock surfaces. These observations are consistent with the interpretations of the polygonally fractured rocks as either fine-grained sandstone or tuff. New constraints on their origin are expected in August 2021, when WATSON, SuperCam, PIXL and SHERLOC will be able to examine the texture and composition of abraded rocks millimeters below the dusty rock surfaces and any rock surface coatings