Megaripples and Their Sedimentary Deposits on Earth and Mars

Introduction: Aeolian sediments commonly are well sorted, but aeolian megaripples are exceptions, having bimodal grain size-frequencies. Distinguishing aeolian megaripple deposits from mixed grain size fluvial deposits is important for martian sedimentary rocks, because indicators of flowing water in the martian past (if revealed by legitimate fluvial deposits) are mission drivers for rovers and landers. Megaripples (aka granule ripples or coarse-grained ripples [1-3]) develop when saltating sand has sufficient energy to drive coarser grains in creep. Megaripples typically develop thin surface layers of coarse material overlying interiors dominated by finer, saltation-capable sand [1, 4]. Although crest heights can exceed 1 meter [5], megaripples are relatively minor components of terrestrial aeolian settings (e.g., as interdune features), so have attracted little attention for their potential to contribute to the sedimentary rock record [e.g., 4]. On Mars, however, megaripples have been encountered in many locations by landers and rovers as the most common aeolian bedform class [e.g., 3, 6-9]. Megaripples can be very long-lived on the martian surface [10] due to self-armoring surface layers of coarse materials that become indurated. For reasons of abundance and durability, aeolian megaripple deposits might be expected in the martian sedimentary rock record. Distinguishing aeolian megaripple deposits in martian sedimentary rocks is complicated by (1) most outcrops are never imaged at hand lens resolution; (2) outcrops typically are mantled in dust; (3) stand-off images of outcrops typically are obtained from only a single point of view, and usually are returned to Earth only after the rover has already moved on. Therefore we have undertaken wind tunnel experiments, fieldwork, and image analysis to assist interpretations distinguishing aeolian megaripple deposits from mixed grain fluvial materials. Field Investigations of Active Bedforms. Active megaripples were examined for grain size distribution and internal structures at White Sands, NM, Coachella Valley, CA, and Great Sand Dunes, CO. Coarse fractions were 1-2 mm very coarse sand, with the saltating fraction ~250 μm sand. The presence of internal structures, particularly foresets of coarser grains, varied widely with setting. Medano Creek borders the main dune field at Great Sand Dunes, CO, and when the creek bed is dry, grain sizes from dune sand to cobbles are exposed to the wind. Megaripples develop from this mixture and rapidly achieve good sorting of the coarse material (1-2 mm) over relatively short migration distances. Wind Tunnel Experiments. All experiments were conducted at the 1-atmosphere wind tunnel at the Arizona State University Planetary Aeolian Laboratory. Sorting during megaripple development and migration was investigated using 250 μm sand, impacting coarser grains 600-2800 μm divided into nine equal (by mass) fractions. At 10-12 m/s, megaripples with sorted crests 850-1180 μm developed spontaneously. Other experiments indicate that creep rates of individual saltationdriven coarse grains decline exponentially with increasing grain size (Fig. 1), leading to very effective sorting within a meter of migration downwind.