This image shows a pair of pit craters on Alba Patera, a massive volcano on Mars.

The pit craters are the prominent deep features near the center of the image. Most of the other circular holes are impact craters. Pit craters form when material collapses into a subsurface void (commonly on volcanoes, such as Kilauea in Hawai'i). Voids may form on volcanoes when parts of the "plumbing" system transporting lava are drained, or when extension and stretching of the volcano causes gaps below the surface.

Pit craters may resemble impact craters. However, there is evidence that the two large depressions in this image are pit craters. Impact craters as large and deep as these would likely have a raised rim of material ejected during its formation. Erosion of the rim would partially fill the crater, but these are relatively deep despite having no raised rim. Additionally, these pits are roughly aligned with other depressions outside the image, a likely arrangement for pit craters.

Layers exposed around the edges of these craters are likely old lava flows which covered the surface of the region, breaking up into boulders. The uppermost material has no boulders. This is likely a layer of dust (or possibly volcanic ash) which has draped the entire area, obscuring much detail of the volcanic landscape.
Written by: Colin Dundas

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.