Mars is a dusty place. Without any precipitation, dust just kinda gets on everything and stays there. "Everything" includes, at the moment, two robotic rovers, one of which (Opportunity) relies on solar panels to stay alive. This presents a bit of a problem, because when you cover a solar panel with dust, its efficiency tanks. The newer and bigger Curiosity has a nuclear thermal generator that has no problem with dust, but Opportunity has had to be very careful about power over the past decade. Thirty seconds with a vacuum would solve the problem, but that's not an option. Instead, the rover just has to count on getting lucky with the Martian winds.
The Martian atmosphere isn't especially thick, but it's there, and where you've got atmosphere, you've got wind. Opportunity has witnessed some dust devils. Last month, the rover may have experienced a near miss: the above picture shows, on the left, the state of Opportunity's solar panels in January, and on the right, how they looked in March. This isn't just about looking better: the rover is now able to suck down more power from the Sun than it has since 2005. More power means longer life, more science, and more sending pictures back to us on Earth.
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