Dark, rippling dunes of volcanic ash 鈥� similar to Hawaii鈥檚 black sand beaches 鈥� cast a teardrop shape in an ancient Martian crater, reveal the latest images from Mars Express.
Wind is likely to have carved the 12-kilometre-long tear shape a million or more years ago when the Martian atmosphere was thicker, says Gerhard Neukum, principal investigator of the High Resolution Stereo Camera on Europe鈥檚 Mars orbiter.
Now, the atmosphere has thinned so much that the dune鈥檚 shape is likely to remain fixed for at least hundreds of thousands of years, he says.
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The sand鈥檚 dark colour suggests it is basaltic, or volcanic in origin. Neukum believes an impact created the 45-kilometre-wide crater perhaps a billion years ago, ejecting the basalt from a subsurface layer of volcanic ash.
The fact that the sand remains dark 鈥� as opposed to reddening with time 鈥� suggests the material has not been altered by water or other chemical processes.
Elliptical path
鈥淭his blackish, bluish material was not so much known before,鈥� Neukum told New 杏吧原创. Unlike Mars Express, previous orbiters have not been able to image the planet in full colour and 3-D. 鈥淧reviously, one could not distinguish it very well 鈥� it appeared darker but it could have been any colour.鈥�
The camera, which has been taking data since January 2004, captured the latest images in May at a resolution of 16 metres. The two-kilometre deep depression lies in a basin of craters in Mars鈥檚 southern hemisphere called Argyre Planitia.
鈥淭he HRSC is doing very, very well,鈥� says Neukum. But he adds that right now the camera is taking images for just 10 minutes during each of Mars Express鈥檚 seven-hour orbits. That is because the closest part of the spacecraft鈥檚 orbit, which takes an elliptical path around the planet鈥檚 poles, changes with time.
Now the closest approach is relatively far from the planet and occurs over the planet鈥檚 night side, limiting photo opportunities. But the geometry will begin to improve again in September.