Coarse (pebble to cobble sized) clasts on the intercrater plains of the Mars Exploration Rover Spirit landing site exhibit a nonrandom (i.e., uniformly spaced) distribution. This pattern has been attributed to the entrainment and redistribution of coarse clasts during extreme wind events. Here we propose an alternative mechanism readily observable in wind tunnels and numerical models at modest wind speeds. In this process, coarse clasts modify the air flow around them, causing erosion of the underlying substrate on the windward side and deposition on the leeward side until a threshold bed-slope condition is reached, after which the clast rolls into the windward trough. Clasts can migrate across an erodible substrate in repeated cycles of trough formation and clast rolling, “attracting” or “repelling” one another through feedbacks between the local clast density, substrate erosion and/or deposition rate, and substrate elevation. The substrate beneath areas of locally high clast densities aggrades, building up a topographic high that can cause clasts to repel one another to form a more uniform distribution of clasts through time. This self-organized process likely plays a significant role in the evolution of mixed grain size eolian surfaces on Earth and Mars.
January 13th, 2009