There are other ways to make circular depressions and formations on planetary surfaces. Of the craters visited so far by Opportunity, only Fram seems certainly of impact origin. The others might be fossil hot springs or something of the sort.

Of particular interest to me in the case of Endurance is the raised far rim, compared to the essentially rimless near side of the crater. That would tend to suggest impact formation by a low-angle impactor, yet overhead views show a strongly circular formation. Impact origin is possible, but alternatives should be explored as well.

The absence of ejecta is possibly due to the dissolution of the rocks -- rocks that may be mostly salts -- over time leaving nothing but the blue berries and a tiny amount of light colored fragmentary material (as seen in those fancy dunes on the bottom of Endurance). Or there may never have been any ejecta to speak of.

Fram shows all the appropriate signs of impact origin, including well-developed rays, a heavily broken and overturned interior, even what appear to be some erratics. None of these features are seen (at least not yet) at Endurance, and the Opportunity crater itself actually shows what could be interpreted as remnant ledges from previous "wet" episodes. Although the rocks are broken they don't seem to be overturned, and their condition suggests collapse from below rather than impact from above.

Given Moomaw's article that references the sulfuric acid -- concentrated as well as dilute -- in any standing liquid on the surface at any time now or in the past, the dissoution of native rock over time is entirely possible, but I still like the idea that Meridiani is full of fossil springs -- some of which may still show occasional activity...

Wouldn't that be fun?

Cheers, BRFelix