Pit Craters of Tractus Catena
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.
Image PSP_002420_2040 was taken by the High Resolution Imaging Science Experiment (HiRISE) camera onboard the Mars Reconnaissance Orbiter spacecraft on 01-Feb-2007. The complete image is centered at 23.8 degrees latitude, 256.2 degrees East longitude. The range to the target site was 280.6 km (175.3 miles). At this distance the image scale is 56.1 cm/pixel (with 2 x 2 binning) so objects ~168 cm across are resolved. The image shown here has been map-projected to 50 cm/pixel and north is up. The image was taken at a local Mars time of 03:37 PM and the scene is illuminated from the west with a solar incidence angle of 57 degrees, thus the sun was about 33 degrees above the horizon. At a solar longitude of 176.2 degrees, the season on Mars is Northern Summer.