They are always circular. The impactor plows into the crust, vaporizing to a plasma bolus, and then explodes deep below ground level like an artillery shell, so the scar is always circular. The material is thrown out, and then mostly slumps back into the hole. The shock waves in the fluidized stone interfere so that a central peak is thrown up, like the center spike of a milk droplet falling into a liquid..jpg)
If the thin crust overlays an ice mantle, perhaps this mechanism could extrude an upwelling of liquid ice into a dome shape, which can then reflect sunlight straight back at the spacecraft. But if that's the case, these structures should be common.
Then again, it might be a polished titanium dome in an alien base placed there millennia ago to surveille the inner solar system. Seriously, the asteroid belt is a logical place to erect a monitoring station, and the biggest rock out there is a logical place to leave the Sentinel; a place where we would be sure to find it once we developed the technology.
Either way, I look forward to further data.
Space/Science » in reply to No away team, but,
Craters are never oval, regardless of the angle of impact of the projectile.