Martian Dunes Take Their Time.

They need 1000 years to travel a few meters. This is the conclusion of a new study that tries to simulate the observed structures of dunes on the red planet and to determine whether present conditions could have been responsible. On Earth, a sand dune is shaped by wind and water. On Mars, there doesn't seem to be any surface water movement (at least not any that would shape dunes), and as for wind, there isn't much of that either.

With an atmosphere only 1/100 the density of Earth's the wind speed on Mars would have to be considerable to move sand around. Eric Parteli of the Universitaet Stuttgart in Germany and his colleague Hans Herrmann of the Universidade Federal do Ceara in Brazil calculate that on Mars (where the gravity is only 1/3 the Earth strength) a dune at a height of 1 meter would require a wind velocity of 35 m/s (roughly 75 mph) to be moved appreciably.

This speed occurs only a few times a decade, hence the glacial pace of dunes on Mars. Their most surprising finding, Parteli said, comes from their study of bimodal sand dunes, those that bear evidence of being shaped by winds from two perpendicular directions. They deduce a wind oscillation period on Mars of 50,000 years (the time it takes for winds to shift around by 90 degrees), roughly the same as the period for the precession of Mars's axis. (Physical Review E, October 2007; parteli@icp.uni-stuttgart.de)