ESA has published some images of a crater with more than 30 kilometers (about 20 miles) across north of the large Hellas Planitia basin on the planet Mars obtained thanks to the High Resolution Stereo Camera (HRSC) instrument of the Mars Express space probe. The appearance of the crater along with the river valleys to the south indicates that at the time of impact there was groundwater near the surface. This is another indication that there was a lake in that region.
Hellas Planitia is itself an impact crater, one of the largest existing on the planet Mars with a diameter of about 2,300 kilometers (about 1,400 miles) and a depth of about 7 kilometers (almost 23,500 feet). The winds constantly raise dust in that region, making it uniform with a high albedo that makes it very visible even from Earth. It dates back to the period of late heavy bombardment between 4.1 and 3.8 billion years ago, when the inner planets were hit by a particularly high number of asteroids and comets.
Over time, the Hellas Planitia basin but also the nearby areas suffered further impacts that created more craters. Clues about the presence of water in the basin were found in 2008 thanks to the findings of the NASA’s Mars Reconnaissance Orbiter’s SHARAD (Shallow Radar Sounder) instrument in three of those craters. That’s ice that might be covered by debris and dust that prevents its sublimation.
On the northern edge of the Hellas Planitia basin, traces of the ancient water presence in the Terby crater were found thanks to the discovery of various space probes including Mars Express. Now something similar has been found in this other crater, not far from Terby. The clues of the ancient presence of groundwater derive from the characteristics of the materials ejected, which can provide indications of the conditions on the surface and its environment at the time of the impact.
The energy transferred to the ground at the time of the impact of a meteorite causes the melting and vaporisation of its various parts and of a surface layer. At the same time, an amount of materials are excavated from the ground and are ejected all around as a “blanket” of debris. In the case of this crater north of Hellas Planitia, the “fluidified” nature of those debris suggests that there was a water-rich subsoil that allowed debris to flow more easily.
In many impact craters on Mars, debris blankets are lobe-shaped, with ridges at the edges and multiple layers of debris. In this specific crater, it’s possible to identify up to three layers of materials, some of which end up in ridges. Small channels can be seen south of the crater, which in the upper image are to its left, further evidence that in a remote past there was water in the region.
The geological situation in the area around the Hellas Planitia basin is rather complex between direct traces of the ancient water presence and other indirect ones such as those shown by this crater’s characteristics. There’s a suspicion that in that region for some time there was a lake so the research will continue to reconstruct the situation from a time when Mars was similar to Earth.