From K. Lewis, et al., Science, Vol. 322, pp. 1532–5, December 5, 2008. © 2008, AAAS.

Stereographic analysis of photos from the HiRISE camera on the Mars Reconnaissance Orbiter allowed the topography of Becquerel crater (8° W, 22° N) to be reconstructed. Ten “bundles” of layered beds can be seen here. The individual beds are 3.6 ± 1 meters thick, and the bundles are 36 ± 9 meters thick.

Ice Ages on Mars?

Some layered deposits on Mars may have been caused by regular variations in the planet’s tilt. On Earth, similar “astronomical forcing” drives ice-age cycles. Grad student Kevin Lewis and Oded Aharonson, associate professor of planetary science, along with John Grotzinger, the Jones Professor of Geology, examined outcrops in four craters in the Arabia Terra region and found that each set of layers have similar thicknesses and similar features.

The scientists propose that each layer was formed over a period of about 100,000 years, corresponding to a change in the tilt of Mars’s axis by tens of degrees analogous to the (smaller) Milankovitch cycles on Earth. When the axis is near vertical, the sun hovers over the equator and the poles stay cold. This would cause volatiles in the atmosphere, like water and carbon dioxide, to migrate poleward, where they’d be locked up as ice.

As the axis tilts, the poles get relatively more sunlight, and those materials would migrate away. “If you move carbon dioxide away from the poles, the atmospheric pressure would increase, which may cause a difference in the ability of winds to transport and deposit sand,” explaining the layering, Aharonson says.

Groups of 10 layers are bundled into larger units that were laid down over approximately million-year periods. This corresponds to a known modulation in the tilt cycle caused by solar-system dynamics.

Lewis is the lead author on the paper, which appeared in the December 5 issue of Science. Other authors include Randolph Kirk (MS ’84, PhD ’87), of the U.S. Geological Survey; Alfred McEwen of the University of Arizona, and Caltech staff member Terry-Ann Suer. —KS