When Opportunity got caught in a sand trap four years ago, the wheels had to spin for the equivalent of 200 meters (yards) before the rover popped out of the trench it had dug for itself, Callas said.

• 这种变化主要是由于地球表面的海洋和陆地对太阳光线反射率的不同而引起的，而与地球上空的云层关系不大。科学家们认为，关键是要看地球长期的亮度变化情形。