Mystery of Mars rover's 'carwash' rolls on

New Scientist | December 23 2004

NASA's Mars rover Opportunity seems to have stumbled into something akin to a carwash that has left its solar panels much cleaner than those of its twin rover, Spirit. A Martian carwash would account for a series of unexpected boosts in the electrical power produced by Opportunity's solar panels.

The rovers landed on Mars in January 2004 with solar cells capable of providing more than 900 watt-hours of electricity per day. Spirit's output has dropped to about 400 watt-hours, partly because Martian dust has caked its solar panels.

Opportunity's output also declined at first - to around 500 watt-hours - but over the past six months it has regained power (New Scientist print edition, 30 October). Lately, its solar cells have been delivering just over 900 watt-hours.

Rover team leader Jim Erickson at NASA's Jet Propulsion Laboratory in Pasadena, California, told New Scientist that a process still not understood has repeatedly removed dust from the solar panels. "These exciting and unexplained cleaning events have kept Opportunity in really great shape," he says.

Whatever the process, it has taken place while Opportunity was parked during the Martian night. On at least four occasions over a six-month period, the rover's power output increased by up to 5% overnight. At the time, the team speculated that wind may have swept the dust off the panels or frost may have caused it to clump, exposing more of the panels.

Self inspection
Now an inspection of the rover's surface using its own camera has confirmed that dust has been removed from the vehicle. Erickson estimates that the cleaning accounts for about 15% of the difference between Spirit's and Opportunity's power output. Most of the remaining disparity is due to the difference in sunshine in their two locations.

But the mystery of why only Opportunity has been cleaned remains. The answer might lie in the nature of the two rovers' missions. Spirit has been prospecting in an area called Columbia Hills, while Opportunity has been exploring the wall of Endurance crater.

While climbing, Opportunity spent a lot of time with its solar panels tilted, which could have caused any dust to tumble off. And the researchers suspect the shape of the crater may encourage the development of dust devils or other wind patterns that could help scrub the panels.

If the crater does provide a natural, wind-driven car wash then Opportunity's days as a clean machine could be at an end. On 12 December, it drove out of the crater to explore the terrain beyond. "If in three or four months Opportunity is still operating and hasn't had another power boost that would suggest the crater was the key," Erickson says.