The amount of time that a geologic deposit is exposed at the surface can be measured by counting the number of impact craters that it contains. The longer a deposit is exposed at the surface the more impact events that it endures.

In this image, there are at least two distinct geologic units, a light–toned bedrock and a surface veneer of dark–toned material that contains sand dunes. The light–toned bedrock must be older that the dark–toned veneer of sand; the bedrock must have been present first in order to be covered by the sand. The dark–toned sand however, contains many more impact craters than the light–toned bedrock. This suggests that the surface of the bedrock is younger than the veneer of sand.

This can be explained by the bedrock being more easily eroded by the wind than the veneer of sand. The surface of the bedrock is rapidly refreshed (craters smoothed away), while the sand veneer retains impact craters for a longer period of time. This indicates that the bedrock is very friable (weak and easily eroded, in this case by the wind) and the sand veneer is less friable.

Written by: Chris Okubo   (20 October 2010)

This is a stereo pair with PSP_002550_1725 .

More info and image formats at http://hirise.lpl.arizona.edu/PSP_003618_1725

Image: NASA/JPL/University of Arizona