A cleaning event is a phenomenon whereby dust is removed from solar panels, in the context of exploration and science rovers on Mars, supposedly by the action of wind. The term is used on several NASA webpages; generally supposing that Martian winds have blown dust clear off the probes on Mars, including the solar panels which power them, increasing their energy output. The nature of the cleaning events is not known, but the assumption is that wind blows the dust off the panels. Cleaning events were discovered in 2004 when, several times, power levels increased by 5% overnight, in contrast to the expectation they would decrease—the added power allowed the missions to continue beyond their expected lifetime. One theory is that wind currents in craters aid in cleaning events, and also if the rover is sloped, this is speculated to help remove dust.