Astronomy Picture of the Day

Explanation: What caused this unusual white rock formation on Mars? Intrigued by the possibility that they could be salt deposits left over as an ancient lakebed dried-up, detailed studies of these fingers now indicate a more mundane origin: volcanic ash. Studying the exact colour of the formation indicated the volcanic origin. The light material appears to have eroded away from surrounding area, indicating a very low-density substance consistent with the ash hypothesis. The stark contrast between the rocks and the surrounding sand is compounded by the unusual darkness of the sand. The above picture was taken with the Thermal Emission Imaging System on the Mars Odyssey spacecraft currently orbiting Mars. The image spans about 10 kilometres inside a much larger crater.