CRACKS AND FINS: PIECES OF MARS AT WHITE SANDS NATIONAL MONUMENT, NEW MEXICO

In 2004 the Mars Exploration Rover Opportunity returned evidence from Meridiani Planum for sulfate-rich sedimentary rocks that formed in both aqueous and eolian environments. Polygonal cracks crosscut bedding in outcrops in Endurance Crater and on the plains of Meridiani, suggesting contraction after exposure of the outcrops. Some cracks in Endurance Crater are associated with fins, which are cemented features that protrude a few centimeters above outcrops. Fin geometry is consistent with differential cementation along cracks. Frost observed on Opportunity demonstrates modern atmospheric water cycling. We use observations from an analog site at White Sands National Monument, NM, to provide insights into processes forming cracks and fins.

Dunes and playas at White Sands National Monument are excellent analogs to Meridiani Planum because of the ubiquitous sulfate (gypsum) sand. Fieldwork at White Sands in January, March, and June 2005 demonstrated that cracks and fins form seasonally in gypsum sand. In January, sand was moist from abundant frost, and polygonal cracks formed in sand along interdune-dune boundaries, on dune slopes, and in pedestals of cemented dune sand. Cracks on dune slopes crosscut bedding as do Meridiani cracks. Slightly cemented fins were also present in January and protruded a few centimeters out of the dune sand along crack edges and along differentially eroded ripple cross lamination and bedding. In March and June, cracks on dune slopes were dry and not actively forming; no fins were present. Cracks were produced in gypsum sand in the laboratory when temperature was repeatedly cycled between -5° and 20°C in the presence of water to mimic frost; cracks did not form when sand was dry. These results, combined with field observations, suggest that atmospheric water cycling promotes crack formation in sulfate sand. Field observations suggest that fins form from 1) preferential cementation along cracks and 2) adhesion of fine-grained particles to structures above the sediment surface. Near-surface water cycling may also play a role in fin formation since these features were only observed in January when nighttime temperatures were sufficiently low to form a heavy frost. These or similar processes may be important for crack and fin formation in Meridiani Planum outcrops.