The above image was generated by “hortonheardawho” for unmannedspaceflight.com and represents some of the earlier days of the Martian dust storm currently enveloping the planet. This is Mars as seen from the Opportunity Rover who seems to be suffering under the storm worse than Spirit on the opposite side of the globe. While these skies are somewhat dark take a look at what it might have looked like around July 15 (or Martian day #1235 of the mission since arriving).
Using an image developed by Jim Bell to show the relative brightness of the dust storm at Opportnity’s position, the above image was artistically re-interpreted to show what that same scene might have looked like a few weeks later. The rovers have since been shut-down almost completely to try and conserve power and word has it that at present it is even darker than suggested here. The loss of light reaching the surface of Mars at Opportunity’s position is currently more than 99% and should these storms go on for too long the rovers will run out of energy with no light reaching their solar panels and that would finally spell the end of these long-running missions. Even though all operations that use any kind of power have been shut-down, the rovers do need to constantly use enough power to stay warm. As of this time, there is so little energy being replenished that just running the heaters is using up more energy than can be regenerated from the solar collectors.
While the situation does seem dire, most specialists expect the rovers to make it through this situation and expect to carry on after the storm dissipates. Coincidentally, Opportunity was just about to enter the Victoria crater and thankfully it had not as that would have surely spelled doom for that mission. The solar energy collectors would not only have had to deal with the low-light situation, but would have suffered under crater wall shadows as well.