MARS AEOLIAN DUST, SAND, AND CHALLENGES TO THE DISTINCTION OF IGNEOUS FROM SEDIMENTARY ROCKS - AN OVERVIEW OF SELECTED CURIOSITY ROVER MARS HAND LENS IMAGER (MAHLI) OBSERVATIONS

MAHLI is a color camera with a macro lens mounted on the Curiosity rover’s robotic arm. It obtains images of geological targets at scales of 16 to 100 microns per pixel. Since August 2012, the rover has been ascending the lower north slope of Aeolis Mons, a 5-km-high mountain of stratified rock inside Gale crater. MAHLI images have contributed to investigations of modern sediments and fluvial, alluvial, lacustrine, and aeolian sedimentary rocks. Here we present a few key observations that intersect with the career-spanning interests of this year’s GSA Planetary Geology Division G.K. Gilbert awardee, James R. Zimbelman (congratulations!).

We begin with unconsolidated aeolian dust and sand. MAHLI images show that modern aeolian sand in Gale contains sedimentary, igneous, and impact melt rock fragments alongside monomineralic grains and spherules formed by impact events or volcanism. The images also show that dust deposits in Gale are thin (usually << 1 mm) and coat skyward-facing surfaces, indicating they settled from suspension. Additionally, dust can coat the undersides of rock overhangs; a result attributed to horizontal storm winds. MAHLI images have documented aeolian sand grain motion across bedrock surfaces and within active aeolian bedforms. On Sol 1671, MAHLI captured a time-series of aeolian sand motion within a 5.4 by 4.0 cm rock surface over about a 10 second interval. MAHLI results indicate that sand impinging on rock surfaces prevents dust accumulation and abrades soft rock. Abrasion of lacustrine mudstones demonstrates that one source of modern aeolian dust is the erosion of these ancient rocks.

Next, we consider the challenge of distinguishing igneous from sedimentary rocks on Mars. MAHLI images are used to investigate rock texture and structure. Some dark, gray, mafic rocks are challenging to confidently classify as igneous (e.g., aphanitic lava) or sedimentary. Within the context of all other rover and orbiter observations of Gale, these rocks are likely to be sedimentary, but MAHLI images demonstrate a need for thin sections and petrographic microscopy. From orbit, dark gray rock exposures peppered with many sub-kilometer-diameter impact craters inside Gale might have been interpreted a decade ago as probable lavas, but MAHLI and other instruments aboard Curiosity show that these are sandstones.