Eyes of Mars Watching? Twin Crater Pits Result of Huge Underground Explosions

Two large and very similar Martian impact craters also both reveal deep pits in their center, which most likely resulted from dramatic underground explosions. There are different theories about the cause of these explosions though. The 'twin' craters, imaged by the ESA's Mars Express orbiter, are located in the Thaumasia Planum region, a large plateau that lies immediately to the south of Valles Marineris, the largest canyon in the Solar System. Both are just over 50 km wide and display intricate interior features.

These kind of craters are labeled central pit craters and are common both on Mars and on the icy moons orbiting the giant planets in our Solar System. The possible options of how they formed is explained by the European astronomers who analyzed and released the new high detail topographic material recorded by the Mars orbiter.

The classic case is caused by an asteroid that hits the rocky surface of a planet, which compresses both itself and the surface 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. Central pit formation could be more pronounced when rock or ice melted during the impact drains away through fractures beneath the crater, it leaves a pit.

Another theory which is very interesting in the Martian context 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 will form in the center of the main crater, since most of the impact energy was deposited there.

Though the large craters in this scene 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 vapourised in the southern crater, punching through slightly thinner crust to leave a larger pit.

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.

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.