Massive underground explosions, perhaps involving ice, have created the pits inside two large impact craters in the Thaumasia Planum region on Mars, say researchers from the European Space Agency.
Arima twin craters in the Thaumasia Planum region on Mars (ESA / DLR / FU Berlin / G. Neukum)
Thaumasia Planum is a large plateau that lies immediately to the south of Valles Marineris, the largest canyon in the Solar System.
The twin craters, imaged by ESA’s Mars Express on January 4, 2013, are over 30 miles (50 km). The northernmost crater (at the right on the image) was officially given the name Arima, but the southernmost (left) crater remains unnamed.
Multiple terraces slump from the crater walls onto a flat floor. But, according to ESA scientists, 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 depressurize, 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.
A perspective view of a 30-mile crater in Thaumasia Planum, Mars (ESA / DLR / FU Berlin / G. Neukum)
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, vapourizing in an explosion. As a result, the rocky surface is excavated forming an explosive pit surrounded by rocky debris. The pit is in the center of the main crater, where most of the impact energy was deposited.
Though the twin craters have similar diameters, their central pits are rather 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 vaporized in the southern crater, punching through slightly thinner crust to leave a larger pit.
Many neighboring small impact craters also show evidence for subsurface water or ice at the time of impact as evidenced by their ejecta blankets.
Impact craters like these can thus 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.
