Martian twins

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.
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.
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 more dramatic events, larger craters are produced with more complex features, such as uplifted central peaks or sunken pits.
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.