This image shows pit craters on Mars, running north-south and east-west, possibly tracking an unmilled lava tube. Credit: NASA/JPL-Caltech/UArizona On Mars, pit craters are typically bowl-shaped and tend to occur in otherwise flat and featureless terrain, and planetary scientists can tell a pit crater from an impact crater because pit craters typically do not have a raised rim or ejecta; as impact craters do. Most of them can be easily identified by the lack of raised rims or ejecta, which would be present if an impact created the crater. But the Mars Reconnaissance Orbiter and other previous missions orbiting the Red Planet have spotted more than 100 pit craters around the Tharsis region of Mars that exhibit unusual features compared to other pit craters. Remove all ads on Universe Today Join our Patreon for just $3! Get the ad-free experience of a lifetime Called Atypical Pit Craters (APCs) they generally have sharp and distinct rims, vertical or projecting walls that extend to their floor. They are usually cylindrical or bell-shaped, and their surface diameter can be one-third larger than ordinary pit craters. They can range from 50 to 350 meters in diameter. The Tharsis region is the large volcanic plateau near the equator in the western hemisphere of Mars, which is home to the largest volcanoes in the Solar System, and scientists believe that the abundance of APC in this region comes from underground pipes that may intersect between the giant volcanoes of Mars. An atypical pit crater near Elysium Mons. Credit: NASA/JPL-Caltech/UArizona But now MRO has found several APCs in another volcanic area that is older than Tharsis. The atypical craters seen here are near the Elysium Mons volcano, which is the third largest mountain on Mars and is located in the eastern hemisphere of Mars. Scientists from MRO are excited to discover these older APCs, as they believe they “should reveal weathering features and possibly whether they are ancient or formed more recently. HiRISE has targeted several APCs on Tharsis, but scientists say these are the first to be suggested on older land. The area around Elysium Mons has many craters on its surface, and while some were likely formed by meteorite impacts, there are many that show no ejecta pattern. Also, many are aligned in linear patterns that are radial to the summit caldera, and so planetary scientists believe they were likely formed by collapse as lava retreated from beneath the surface, rather than by a meteorite impact. As we discussed in a recent article, these pit craters create entrances to underground lava tubes. On the Moon, they could provide thermally stable habitats as well as safe shielding from cosmic rays, solar radiation and micrometeorites for future human explorers. Further reading: HiRISE Image of the Day: APC near Elysium MonsPaper: Atypical pit craters on Mars: New insights from NASA’s THEMIS, CTX and HiRISE observations: Pit Craters and Giant Volcanoes
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