Martian 'Spiders' in Sharper Look, Thanks to Volunteers

This image shows spidery channels eroded into Martian ground. It is an example from high-resolution observation of more than 20 places that were chosen in 2016 on the basis of about 10,000 volunteers' examination of lower-resolution images of larger areas near Mars' south pole. These sharper looks use the High Resolution Imaging Science Experiment (HiRISE) camera on NASA's Mars Reconnaissance Orbiter. The volunteers, through the Planet Four: Terrains website, categorize surface features in images from the same orbiter's Context Camera (CTX). This image is a portion of HiRISE observation ESP_047487_1005, taken on Sept. 12, 2016, of a site at 79.4 degrees south latitude, 18.8 degrees east longitude. The ground area shown is about half a mile (0.8 kilometer) wide. This terrain type, called spiders or "araneiform" (from the Latin word for spiders), appears in some areas of far-southern Mars that are covered by sheets of frozen carbon dioxide ("dry ice") during the winter. When the slab ice thaws from the underneath side in the spring, carbon dioxide gas trapped beneath the ice builds pressure until it rushes toward a fissure or vent where it bursts out. The venting gas carries dust and sand that it picks up as it carves these channels. At this location, the spiders are surrounded by ground called "basketball terrain" because of its texture. http://photojournal.jpl.nasa.gov/catalog/PIA21126