No doubt the conspiracy theorists are abuzz over this. BTW, happy 10th birthday to Opportunity. It's amazing that this rover has lasted this long.

MSN
edited to add:
Ha, that didn't take long. Here is a popular UFO conspiracy site speculating on what this might be:
UFO sightings daily.com

MSN
CAPE CANAVERAL, Fl. — Scientists are stumped as to how a rock mysteriously appeared in images taken two weeks apart by NASA's Mars rover Opportunity.
The rover, which landed in an area known as Meridiani Planum a decade ago, is exploring the rim of a crater for signs of past water.
Another rover, Curiosity, touched down on the opposite side of the planet in 2012 for a more ambitious mission to look for past habitable environments.
For the moment, however, scientists are pondering a more immediate question. On January 8, while preparing to use its robotic arm for science investigation, Opportunity sent back a picture of its work area.
edited to add:
Ha, that didn't take long. Here is a popular UFO conspiracy site speculating on what this might be:
UFO sightings daily.com
I enlarged the object and it seems to be an odd shaped creature of some sort. In the enlargement you can also see that its sort of a square shape, but its center is pushed downward. It would be cool if this was some kind of crab-like creature. Very cool and please visit the source for the full story about it. SCW
Last edited: