火星子午线平原火山灰图片发布

Deposits of volcanic¹ ash colour this view of the Meridiani Planum, as seen by the Mars Express High Resolution Stereo Camera. They also give clues to the prevailing³ wind（主风，恒风） direction in this region of Mars. Meridiani Planum, a plain at the northern edge of the southern highlands of Mars, is half way between the volcanic Tharsis Region to the west and the low-lying Hellas Planitia impact basin to the south-east. Through a telescope, Meridiani Planum is a striking, dark feature, close to the martian（火星的） equator（赤道） .

Meridiani Planum extends 127 km by 63 km and covers an area of roughly 8000 sq km, which is about the size of Cyprus（塞浦路斯） . It was chosen as a central reference point（控制点，参考点） for Mars' geographical⁴ coordinate⁵ system. So the martian prime meridian²（本初子午线） , the equivalent of the Greenwich, UK, prime meridian on Earth, has been set to run through this region.

The High-Resolution Stereo Camera (HRSC) took this image. It was obtained on 1 September 2005, during orbit 2097, at a resolution of about 13 m per pixel.

In the centre of the image, the floor of an impact crater⁶（陨石坑，撞击坑） almost 50 km wide is covered in dark material. This resembles volcanic ash, which is predominantly（显著地，主要地） composed of minerals such as pyroxene（辉石） and olivine（橄榄石） . Poking⁷ through the dark covering are small mounds⁸（堆，高地） , probably made of more resistant⁹ material. The softer material around them has been eroded¹⁰（侵蚀，损坏） and blown out of the crater by north-easterly winds and now forms dark streaks¹¹（条痕） on the surroundings.

An impact crater just 15 km wide, to the upper left, exhibits the same dark material on its south-western edge. It is likely that this material was blown into the smaller crater from the larger one. The nearly black structures are almost certainly dunes¹²（沙丘） made of volcanic ash-rich sediments¹³. In contrast, the 34-km-wide impact crater to the lower right of the image is largely filled with light material.

The southern area, to the left of the image below the smaller crater, exhibits dark features. Located on the lee（向下风，在背风处） side of ridges¹⁴, these are probably similar deposits of ash-rich material, again blown out of the crater.