Dramatic underground explosions, perhaps involving ice, are responsible for the pits inside the two large martian impact craters, that has been captured by ESA's Mars Express on January 4.
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.
The northernmost large crater was officially given the name Arima in early 2012, but the southernmost crater remains unnamed. Both are just over 50 km wide and display intricate interior features.
Multiple terraces slump from the southernmost 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.
Central pit craters are common on Mars, as well as on the icy moons orbiting the giant planets in our Solar System. But how did they form?
When an asteroid hits the rocky surface of a planet, 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.
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. (ANI)