About a month ago an intriguing pair of images from NASA's Opportunity rover on Mars showed a curious rock that had seemingly appeared our of nowhere during the course of 12 days. This small, brightly hued rock clearly had a fresh surface, suggesting that it was either broken off from somewhere or previously buried.
So was it a game of alien 'now you see me'? Or was it a rock kicked up by a nearby meteor strike? Or was it the fault of the only known moving object on this part of Mars - the rover itself?
As is often the case, Occam's razor comes to the rescue. When the rover was commanded to move away from the small rock (dubbed 'Pinnacle Island') scientists were able to see that Opportunity had in fact run over another rock just 3 feet away, on the uphill side of Pinnacle. This rock had broken open, and the smaller piece had slid or jumped down the slope.
Here's what the rover saw. I've drawn ellipses around Pinnacle Rock (lower left) and its parent rock 'Stuart Island' (center), with the rover tracks clearly visible, as well as a red and white edge that matches.
Why didn't anyone spot this before? Well, you can thank the difficulties of remote exploration for that. While Opportunity was parked and studying Pinnacle it happened to be orientated so that its solar panels blocked the view uphill.
With the excitement over (the mineral content of Pinnacle does indicate that it formed in the presence of water) Opportunity has resumed its upwards course towards a north-facing ridge, a place that should also help it keep its solar panels tilted for maximum sunlight in the martian winter.