Image PSP_004804_1105 shows the walls of a large pit inside Lyell Crater, near the south polar region.

This image was acquired during a period of elevated atmospheric dust, following a series of dust storms that encircled the planet; this is the cause of the longitudinal bands of contrasting gray levels in the image. We can still see through the thick atmosphere, though, gullies and other subtle features which could have been produced by water and ice present at or near the surface.

The gullies shown in the subimage (approximately 650 x 1000 meters or 710 x 1100 yards; 2618 x 4009 pixels, 11MB) have well developed alcoves, deeply incised channels, and large depositional fans, and are similar to terrestrial landscapes sculpted by surficial water.

The gullies shown in the subimage experienced several periods of activity, as evidenced by older channels cut by younger ones or by their deposits. The current and recent Martian temperatures and atmospheric conditions would not allow for water to be stable at the surface for extended periods of time: it is so cold that the water would freeze, and then it would sublime quickly, because the air is very thin and dry. These gullies could have formed under a different climate, or maybe by repeated bursts of transient fluids. Current leading hypotheses explaining the origin of gullies includes erosion from seepage or eruption of water from a subsurface aquifer, melting of ground ice, or dust-blanketed surface snow.

The otherwise subdued surface in this region is criss-crossed by numerous fissures, forming polygons some 10 meters (11 yards) across. Similar features in both shape and scale can be found in terrestrial periglacial regions such as Antarctica, where ice is present at or near the surface. Antarctica's features are the product of repeated expansion and contraction of the soil-ice mixture due to seasonal temperature oscillations. This results in polygonal networks of stress fractures.
Written by: Sara Martinez-Alonso

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.