re: What is it....
Obviously not evey vaguely circular feature is neccessarily of impact origin. However, apart from obvious collapse features and volcanic vents, most features I have seen on Mars described as impact craters clearly appear to be that, variably modified by erosion. The criteria for recongising impact origin are fairly straight forward and well understood. If you have problems with particular features then they need to be discussed on a case by case basis.
Cheers
Jon
The problem