Meet the Martian twins: The ¿double craters¿ discovered on the red planet formed by massive underground ice explosions Each giant crater is 50km wide and contains a central 'pit' formed during the explosions Craters are to the south of Valles Marineris, the largest canyon in the Solar System Massive underground ice explosions on Mars have created a unique set of twin craters, it has been revealed. A passing European Space Agency spacecraft captured the 50km wide craters, named the Arima twins, in January. They contain a unique central 'pit' formed by underground explosions, the space agency says. Double take: The 50km wide craters, contain a unique central 'pit' formed by underground explosions, the European Space Agency says The "twin" craters are in the Thaumasia Planum region, a large plateau that lies immediately to the south of Valles Marineris, the largest canyon in the Solar System. 'Multiple terraces slump from the crater walls onto a flat floor, but perhaps the most striking feature is the central pit, a feature it shares with Arima crater to its north,' said the agency. Dramatic underground explosions, perhaps involving ice, are responsible for the pits inside these two large martian impact craters, which was imaged by ESA's Mars Express spacecraft on 4 January. Researchers say impact craters like these can provide windows into the past of a planet's surface. 'In this case, they provide evidence for the Thaumasia Planum region having once hosted plentiful subsurface water or ice that was liberated during impact events both small and large,' it is claimed. A perspective view of one of the two 50 km diameter craters in Thaumasia Planum. The image was made by combining data from the High-Resolution Stereo Camera on ESA's Mars Express with digital terrain models. This colour-coded overhead view is based on an ESA Mars Express High-Resolution Stereo Camera digital terrain model of the crater, and reveals the relative depth of the craters The craters were formed when an asteroid hits the rocky surface of a planet, and both it and the surface are compressed to high densities. Immediately after the impact, the compressed regions rapidly depressurise, exploding violently. In low-energy impacts, a simple bowl-shaped crater results. In more dramatic events, larger craters are produced with more complex features, such as uplifted central peaks or sunken pits. 'One idea for central pit formation is that when rock or ice melted during the impact drains away through fractures beneath the crater, it leaves a pit,' said ESA. Another theory is that subsurface ice is rapidly heated, vapourising in an explosion. As a result, the rocky surface is excavated forming an explosive pit surrounded by rocky debris. The pit is in the centre of the main crater, where most of the impact energy was deposited. Where is it? The area imaged on 4 January 2013 lies just south of the Coprates Catena region of Vallis Marineris, in the Thaumasia region of Mars Though the large craters in this scene have similar diameters, their central pits are different in size and depth, as is clearly evident in the topographical map. Compared to the Arima crater, perhaps more subsurface ice was present and more readily vapourised in the southern crater, punching through slightly thinner crust to leave a larger pit, ESA says. Many neighbouring small impact craters also show evidence for subsurface water or ice at the time of impact as evidenced by their "rampart" ejecta blankets. Ejecta blankets are debris deposits surrounding the crater, excavated from inside the crater during its formation. They have petal-like lobes around their edges: these result from liquid water bound up in the ejected material, allowing it to flow along the surface and giving it a fluid appearance.