Mars-orbiting probes have spied hundreds of mounds, some up to 500 meters across and dozens of meters tall, inside an ancient crater near the planet's equator. In the 15 April issue of Earth and Planetary Science Letters, researchers make the case that these enigmatic features (depicted in blue in the main image) are mud volcanoes. For one thing, the near-circular mounds weren't formed by molten-rock volcanoes because there are no deposits of volcanic ash or lava nearby. Instead, the mounds contain boulders and other chunks of material apparently stripped from underlying layers of sediments (depicted in yellow-green), which range from 200 to 500 meters thick. Also, most of the mounds inside the 90-kilometer-wide Firsoff crater (inset) are found on slopes inside the crater rim and were likely created when mud under high pressure—which likely formed during a warmer, wetter phase on the Red Planet—oozed to the surface through a network of cracks there. Other teams have claimed finding mud volcanoes elsewhere on Mars, but the researchers contend that the new finds are the first that definitively link material in the mounds to underlying sediments.
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