in the lower part of this image

Standard interpretation is that all such circular formations are impact craters. I suspect these are not impact craters. Note they appear to have formed in an icy region of Russel Crater. Whether the material is ice or not, I can't say. These "craters" look like they may have once been -- *or perhaps at the time of the picture were* -- filled with an icy material, perhaps water ice. Perhaps, too, if the surface or subsurface temperature reached the melting point of water on Mars, these circular depressions were temporarily filled with liquid water.

I can think of no way at all that liquid CO2 could have ever filled them. On the other hand, might there be other volatiles available on Mars? What could they be?

Interesting.... ;-)

Cheers, B R Felix