These pits formed through collapse above an underground void. The movement of rock along faults may have created this void deep underground.

Faults are commonly thought of as planar cracks in the ground. In reality, faults have very rough surfaces that can create voids as the rocks slide along the fault. Alternatively, the movement of magma (molten rock) underground may have also created such underground voids. As magma moves underground, it pushes aside the bedrock, making an underground tunnel. This tunnel remains behind as the magma drains away and subsequent collapse can occur into it. A combination of processes may also result in pit crater formation, as faults are pre-made passageways for the magma to move underground.

There is much evidence of faulting in this scene. The series of stair-stepped cliffs are actually faults, with each cliff representing the approximate location of a fault. Thus it seems likely that faults played an important role in the formation of these pit craters.

The role of magma flow in the formation of these pit craters remains unknown. The presence of eruptive vents near these pits would be a clue to the past presence of magma. Such vents are not observed in this image, although the absence of these vents does not rule out magma. Magma does not always erupt at the surface and can remain entirely underground.
Written by: Chris Okubo

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.