RECOGNITION AND OBSERVATIONS OF THE MAFIC SANDSTONES OF GALE CRATER, MARS, USING CURIOSITY'S MARS HAND LENS IMAGER (MAHLI)

Rock identification is fundamental to the Curiosity rover investigation in Gale crater, Mars. About 200 m of ancient (3-4 Ga) strata have been explored. The rocks record fluvial-deltaic, lacustrine, and eolian environments. Remote robotic investigation of rocks is challenging; Curiosity carries no rock hammer, makes no thin sections, nor do we often return the rover to a given site to collect additional data. The Mars Hand Lens Imager (MAHLI) contributes to rock identification via images at scales of, typically, 16 to 100 microns per pixel; these allow distinction of silt from very fine sand. The images are usually of natural weathered surfaces, sometimes coated with dust, in sunlight or shadow or under white light LEDs at night. Some sandstones were identified, as such, only after hundreds of days passed, after we gained experience with similar rocks in clearer context. Among the sandstones investigated thus far, framework grains are sometimes readily observed. In many cases, however, grain boundaries are difficult to distinguish due to low porosity and poor color (and luster) contrast between the sand grains versus the matrix and cement. Where clasts are large enough to observe details in them, they include lithic fragments (some with phenocrysts or microbreccia fragments); most are of various shades of gray to white and some are monomineralic. Most of the sandstones are dark gray and mafic, in contrast to the sulfate-bearing, light-toned sandstones of Meridiani Planum examined via the Opportunity rover. The dark gray rocks are among the most erosion-resistant materials encountered during the mission; they cap buttes and mesas and retain sub-km-diameter impact craters. Some lighter-gray sandstones, less often encountered, are more porous and recessive relative to the dark gray, resistant sandstones. Since mid-2016, we have also encountered sandstones with very fine to fine sand in what is otherwise a mudstone section of more than 100 m thickness. Most importantly, several different sandstones encountered include mudstone lithic clasts, and a conglomerate at the “Bimbe” site contains sandstone clasts, one of which is cut by a vein. These latter observations indicate that sediment recycling, the transfer of detrital and diagenetic material from an earlier sedimentary rock to a later one, has occurred on Mars.