This crater in Meridiani Planum, on the martian prime meridian, is a striking dark feature on images of the Red Planet. Now ESA scientists working with Mars Express High Resolution Stereo Camera data have identified the black material as wind-blown volcanic ash. Volcanic ash could be dark in colour as a result of ferromagnesian minerals such as pyroxene and olivine.
The floor of an impact crater around 50 km wide is almost covered with the ash, although what appear to be more resistant mounds poke through the covering. A smaller crater (about 15 km across) to the left also has some dark ash, while the right-hand crater, which is the shallowest, is floored by some lighter material.
Dunes within the dark material suggest that the ash has been reworked by the wind, as do dark streaks extending out of the crater. In addition, there are some dark sedimentary features around these craters that could be dark ash deposits on the lee slopes of dunes.
North is to the right on this image, so the pattern of streaks and dunes suggest that winds from the northeast dominate in this part of Meridiani Planum.