Post by Dr. Lori Fenton, Carl Sagan Center SETI Institute, 189 Bernardo Ave., Suite 100, CA 94043
Fields of dunes and sand sheets are common on Mars. The largest type of dune is made of dark sand, which is thought to be similar in composition to the mafic sands often found near volcanoes on Earth (such as the “black beaches” of Hawaii). As on Earth, aeolian dune fields accumulate where sand is deposited by the wind, typically in low-lying areas such as basins, canyons, and craters.
In a survey of high southern latitude dune fields on Mars, Fenton and Hayward (2010) found several intra-crater dune fields that are surrounded by concentric bands of dark sand. Because of this striking and unusual shape, they have been named “bullseye” dune fields. Image 1 shows an example of one of these intriguing features, with several of these sand rings appearing prominently on the northern and western side. Individual dunes are most common on the eastern margin, where they have formed from the remnants of concentric bands that were once more pristine and extensive. The center of the dune field is a flat sand sheet, which is unusual for dune fields both on Earth and Mars.
It is not yet known what causes the sand to form into a bullseye shape. Dunes form in response to the wind regime to which they are exposed, taking a shape as transverse as possible to all winds strong enough to move sand. The concentric bands are typically most pronounced where they are adjacent to topography. This is the case in the image, where the western and northern edges of the dune field are adjacent to the steep walls of the crater. It is possible that strong slope winds dominate at the edge of the dune field and create large transverse waves in the sand. It is not clear, however, why this would be the case only at high southern latitudes, and is not observed in other intra-crater dune fields across Mars. Until this is better understood, the bullseye dune fields are one more mystery that hides geological secrets of Mars.
Fenton, L.K. and Hayward, R.K. (2010) Southern high latitude dune fields on Mars: Morphology, aeolian inactivity, and climate change, Geomorphology, 121, 98-121, doi:10.1016/j.geomorph.2009.11.006.