Estimating wind speed from spatial grain size sorting in ripples on Earth and Mars

The landscape initially seen by the Mars Exploration Rover Opportunity at Meridiani Planum is dominated by aeolian (wind-blown) ripples with concentrated surface lags of hematitic spherules and fragments. These ripples exhibit profound spatial grain size sorting, with well-sorted coarse-grained crests and poorly sorted, generally finer-grained troughs - they were the most common bed form encountered by Opportunity in its traverse from Eagle Crater to Endurance Crater. Similar bed features have been sporadically studied in the terrestrial literature, but not in detail. We measured wind speed and sediment flux profiles in White Sands National Monument, New Mexico, during conditions under which such coarse-grained ripples were forming. Data show that these bed features formed by the different transport modes of coarse- and fine-grain fractions in an initially bi-modal sediment distribution. Fine grains were transported via saltation, while coarse grains moved only by creep due to ballistic impacts of finer grains, as originally envisioned by Bagnold. We use this observation to place tight constraints on formative wind conditions of coarse-grained ripples on Mars: wind speed must have exceeded the threshold for saltation of fine grains, but was less than the saltation threshold for coarse grains. Estimated wind speeds are only moderately greater than those associated with modern dust storms. When combined with the observation of sand grains on Opportunity's solar panel following a dust storm, results indicate that modern winds may occasionally be strong enough to cause significant sediment transport on the Martian surface.