Geezer Gullies at Tempe Terra
This image shows gullies in the walls of a large impact crater in Tempe Terra. These gullies look quite older than other gullies imaged by HiRISE elsewhere on Mars.
We cannot tell what is the absolute age of these “geezer gullies” (that is, how long ago they formed), because actual rock samples would have to be analyzed with radiometric dating techniques to get that information. However, this image indicates that they have been inactive for a long time.
The walls of these venerable gullies have gentle, smooth-looking slopes which contrast with the tall and steep walls of younger gullies. Their floor is even and lacks sharply incised channels that would indicate recent activity. The weathering processes required to produce such a smooth landscape are very slow and thus must have acted during a long period of time.
Image is less than 5 km (3 mi) across and is 298 km (185 mi) above the surface. For full images including scale bars, visit the source link.