SAND CHARACTERISTICS OF THE BAGNOLD DUNES, GALE CRATER, MARS: MAHLI OBSERVATIONS OF “BARBY” ON SOL 1184

The (active) Bagnold dune field lies at the NW base of Aeolis Mons (informally, Mt. Sharp) in Gale crater, Mars. On the Mars Science Laboratory’s 1184^th sol, the Mars Hand Lens Imager (MAHLI) acquired pictures of an undisturbed loose-sand target called “Barby” near the crest of a 1.5 cm-high sand ripple. The closest view covers 3.9x2.9 cm at 0.024 mm/pixel, permitting measurement and characterization of individual sand grains. The majority are 0.2-0.45 mm across (medium sand), and can be divided into 6 main populations based on color, shape, and texture: dark grey sub-rounded (53% of all grains); medium grey (15%); tan (9%); translucent (7%); dark grey spheres (6%); and red (5%). Grain populations vary spatially within the image.

Most dark grey sub-rounded grains exhibit rounded depressions that are ~0.1 mm across, as well as bumps and depressions ~0.05 mm across. Some of this roughness appears to be inherent to the grains and some is due to particles adhering to their surfaces. A few have green to yellow-green crystals embedded in them. Medium grey grains are slightly larger (mostly 0.3-0.5 mm) than the overall average, but otherwise are similar to dark grey grains, including rare instances of embedded green crystals. Tan grains and red grains have sizes similar to those of dark grey grains. Dark grey spheres are mostly 0.15-0.35 mm across. In addition to their smaller size, they differ from the other dark grey grains by being very shiny, mostly devoid of small bumps, and almost perfectly spherical. There is not a shape continuum between the dark grey sub-rounded grains and the dark grey spheres. Translucent grains are slightly smaller than the population as a whole, and are pink (30%), grey (30%), tan and tan-green (27%), or colorless (10%), with a few other colors comprising the remainder.

The grey, tan, and red grains are likely mafic lithic fragments with varying amounts of oxidation, either eroded directly from mafic igneous rocks (which Curiosity has not encountered) or from mafic sedimentary rocks (which Curiosity has encountered repeatedly). The spheres may be very well-rounded sand grains, impact spherules, or concretions; some nearby outcrops of the Stimson sandstone contain spherical grains. The entire MSL team made collection of these data possible.