Something (or somebody??) has been regularly cleaning the Mars rover Opportunity's solar panels during the night, enabling the device to continue to work much better and longer.
NASA rover team leader Jim Erickson has said that the unexplained cleaning events have kept Opportunity's solar panels operating close to their maximum daily output of 900 watt-hours. Due to a previous accumulation of Martian dust, which is heavy and dense, output had dropped to 500 watt-hours per day.
The output of Mars Spirit's solar panels, which are not getting cleaned, have dropped to 400 watt-hours per day.
NASA has no explanation for the cleaning till this day ...