This HiRISE image captures a small part of the giant scarp that marks the base of Olympus Mons, the largest volcano in the solar system. This scarp was most likely produced by massive landslides as the volcano falls apart under its own weight. Earlier images from the Mars Observer Camera on the Mars Global Surveyor spacecraft showed that there were layers in this area. HiRISE now reveals new details about them.

The layers consist of ledges of hard rock protruding out of the slope. This rock is fractured but not disrupted; such fractures are called “joints” by geologists. (If there were motion along them, then they would merit the name “faults.”) These joints are not as narrow or well-developed as in lava flows that we interpret to be water-cooled. It is not surprising that we do not find evidence for liquid water at this high elevation.

In most places the rock is covered by dust and mass wasting material. However, in the southern part of this image the stack of rocks (stratigraphy) is well-exposed. The rock between the hard ledges consists of broken up rock (breccia), with some blocks a few meters in diameter.

Overall, this pattern of hard rock encased in breccia is consistent with a type of lava called “aa” by Hawaiians and geologists. The solidifying surface of aa lavas are broken up as the flow moves, producing a carapace of debris surrounding the liquid lava. When the lava flow finally stops, the liquid interior freezes into a hard layer. The contraction during cooling produces irregular jointing like that seen in this HiRISE image.
Written by: Laszlo P. Keszthelyi

All of the images produced by HiRISE and accessible on this site are within the public domain: there are no restrictions on their usage by anyone in the public, including news or science organizations. We do ask for a credit line where possible: Image: NASA/JPL/University of Arizona

For information about NASA and agency programs on the Web, visit: http://www.nasa.gov. NASA's Jet Propulsion Laboratory, a division of the California Institute of Technology in Pasadena, Calif., manages the Mars Reconnaissance Orbiter for NASA's Science Mission Directorate, Washington. Lockheed Martin Space Systems is the prime contractor for the project and built the spacecraft. The HiRISE camera was built by Ball Aerospace and Technology Corporation and is operated by the University of Arizona. The image data were processed using the U.S. Geological Survey’s ISIS3 software.