They have no ejecta. They tend to be a similar depth. If they are craters, (which I don't doubt is also a strong possibility), then they must relate to a single event, since they all have the same MO. They could be ejecta backsplatter from the Endurance impact, now eroded, or more likely, remnants of a more recent meteor shower. However, I think the biggest question is why they do not fill in with the abundant spherical windblown lag deposit. For that reason, I strongly favor the idea that they are due to subsurface dissolution or chemical weathering that includes some type of liquid. In that scenario, the various fissures are also formed in the same way, but along fracture lines. The round dimples may indeed be the sites of small impacts that created fracturing or depressions where continued corrosion occurred. Once a depression formed on the plain surface, it would perpetuate itself as a trap for slowly draining corrosive liquids. Candidate liquids are sulfuric acid solutions and brines. These might result from interaction of the hydrated Sulfate minerals with the reducing (low O2 content) atmosphere and UV radiation, leaving behind Sulfides, and aqueous Sulfuric acid (highly speculative, I know). I would not rule out the possibility of biologically produced fluids, either. As Nick said, I bet Sulfate reactions get a lot of attention now. It would be nice if they could trench or investigate one of these small depressions, though sinking in seems like a possible danger. My post below "a Dimple in Time" has more images of the dimples. Perhaps this super fine dust they trap also leads to corrosion. This possible "pure iron" dust would be a strong reducing agent or possible catalyst for the Sulfate reduction. All that would be required is the thinnest of fluid films containing some water to get the dry phases in contact for this corrosive reaction to occur. In conclusion, I suspect there is a basic chemical incompatability between Iron and varoius Sulfates. There is sort of a poetic beauty in this last theory in that late arriving Iron meteorites (or exposed Iron deposits) could have provided the dust that corroded the subaqueous sediments of the Noachian period to such great depth, slowly releasing some of the water that was bound there ages ago.