EDIT: Original thread title was "small, round structures on Mars" - now it is solved. ... on CTX imagery strip firstly I have spotted three, 100 meters diameter, perfectly round, ringed outcrops (as if were parked UFOs), then swarms of smaller ones, ending a cluster of both type.
later, on another CTX slice I have found similar objects again and image title seemed to solve the mistery: "Ring/cone structures" - however these studied objects are much bigger.
in attached kmz folder, placemarks for 4 sites can be found. (keep GMars / Primary Database / Featured Satellite Imagery layer enabled) small round structures on Mars.kmz (1.16 KB)
meteorites create non-round impact craters, as the gas pressure is created by a non-round object, giving the gas pressure a shape, overlayed on the shapes produced by the varying mechanical properties rock strata it hit, and then its gone.
circular impacts are from comets. The comet fragment creates gas pressure for a while, a lot longer than from an asteroid rock thing, and that flow of gas wipes out any feature.. (erosion occurs faster at corners.) It doesnt much matter as to the angle of impact with comet fragments.
On Mars there are 600,000 craters larger than 600 metres diamater. Imagine how many smaller craters there are...
These ones appear to be a an erosion zone.. a process that may also change the crater ? Or at least remove the streaks from high speed impacts. Meteors and errant Keiper belt objects may well hit at such a high speed, a small bit of rock makes a large explosion.. a gas based explosion ,circular, and fresh ones have the streaks.
Straying asteroids make for a far slower impact speed. perhaps creating an asteroid shaped dent, showing the differences of mechanic properties of the Mars strata, and with debris acculation at the rim perhaps more to one side.
These are very numerous on the plains of Mars and are caused by lava moving over frozen groundwater, causing a massive steam explosion and an associated crater. They are not pingos as pingos lack central craters.