re: Not the RAT
This is an impression made by the Rover, and Donald's probably right about what they used.
If you go back through all the photos, there are a number of shots where they pressed an instrument into the dirt to make an impression. I think this is how they first tested the idea that pressure on the soil in Opportunity's crater caused the little stones to disappear under the dirt.
Bet the photos made a lot of other people jump, too. ;)