It cant be seen in the thumb but if you zoom in you can see that the rover is sitting on and partially capturing an outer ring as well.
Concentric rings really lead to only one possible answer.
(And the rover should hurry on its eay because if the Martians took the time to build a bullseye on that range then they intend to use it.)
Good obsrvation - thanks for pointing that out.
On a second look I can see that the outer rocks that are visible in the photo do take the form of a large ring around the inner circle.