Deposits of volcanic ash color this view of the Meridiani Planum, as seen by the Mars Express High-Resolution Stereo Camera (HRSC). They also give clues to the prevailing wind direction in this region of Mars.
Meridiani Planum, a plain at the northern edge of the southern highlands of Mars, is halfway between the volcanic Tharsis Region to the west and the low-lying Hellas Planitia impact basin to the southeast. Through a telescope, Meridiani Planum is a striking dark feature close to the martian equator.
Meridiani Planum extends 79 miles (127 kilometers) by 39 miles (63 kilometers) and covers an area of roughly 3,100 square miles (8,000 square km), which is about the size of Cyprus. It was chosen as a central reference point for Mars’ geographical coordinate system. So the martian prime meridian, the equivalent of the Greenwich, United Kingdom, prime meridian on Earth, has been set to run through this region.
In the center of the image, taken by HRSC, the floor of an impact crater almost 31 miles (50 km) wide is covered in dark material. This resembles volcanic ash, which is predominantly composed of minerals such as pyroxene and olivine. Poking through the dark covering are small mounds, probably made of more resistant material. The softer material around them has been eroded and blown out of the crater by northeasterly winds and now forms dark streaks on the surroundings.
An impact crater just 9 miles (15 km) wide, in the upper left, exhibits the same dark material on its southwestern edge. It is likely that this material was blown into the smaller crater from the larger one. The nearly black structures are almost certainly dunes made of volcanic ash-rich sediments. In contrast, the 21-mile-wide (34 km) impact crater in the lower right of the image is largely filled with light material.
The southern area, to the left in the image below the smaller crater, exhibits dark features. Located on the lee side of ridges, these are probably similar deposits of ash-rich material, again blown out of the crater.
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