A Darkened Mars Sky

This series of images, taken by the Opportunity rover in July 2007, shows a darkening sky as a huge regional dust storm covered both Opportunity and her twin rover, Spirit. In July 2007, a series of severe Martian summer dust storms swept over both rovers.. At its worst, the dust in the Martian atmosphere over Opportunity blocked 99 percent of direct sunlight to the rover, leaving only the limited diffuse sky light to power it. Scientists feared the storms might continue for several days, if not weeks. The numbers across the top of the image report a measurement of atmospheric opacity, called by the Greek letter tau. The lower the number, the clearer the sky. Both Opportunity and Spirit have been recording higher tau measurements in July 2007 than they had seen any time previously in their three and a half years on Mars. The five sol numbers across the bottom correspond (left to right) to June 14, June 30, July 5, July 13 and July 15, 2007.