Martian dunes form in rare bursts
DOI: 10.1063/1.4796630
The surface of Mars is covered with sand dunes of different shapes and sizes (see figure). How did they form? The answers aren’t obvious. Compared with the climate that prevails in the Sahara and other terrestrial deserts, Mars has a rather unfavorable climate for building dunes. The density of Mars’s atmosphere is 1/1000 that of Earth’s. Rarely—about once a decade—does the Martian wind blow strongly enough to loft grains, and then only for 10 seconds or so. The only favorable condition is surface gravity, which, at 3.71 m/s2, makes transporting grains easier than on Earth. Eric Parteli of the Universidade Federal do Ceará in Brazil and his colleague Hans Herrmann of ETH Zürich in Switzerland investigated whether the present Martian climate could form the present Martian dunes. The researchers’ principal tool was a model that had been applied successfully to dunes on Earth. Their conclusion: Mars is indeed making its own dunes, and variations in local conditions can account for the different types of dune. Parteli and Herrmann found a surprise when they looked at bimodal sand dunes, those that bear evidence of being shaped by winds that oscillate between two perpendicular directions. They deduced a wind oscillation period on Mars of 50 000 years. That period is roughly the same as the precession period of Mars’s rotation axis. (
