“…(S)omething — or someone — had regularly cleaned layers of dust from the solar panels of the Mars Opportunity vehicle while it was closed down during the Martian night.

The cleaning had boosted the panels’ power output close to their maximum 900 watt-hours per day after at one stage dropping to 500 watt-hours because of the heavy Martian dirt.

By contrast, the power output of the solar panels of Mars Spirit — on a different part of the Red Planet — had dropped to just 400 watt-hours a day, clogged by the heavy dust.” Yahoo! News