Here's a French website dealing with the phenomenon.

It's interesting that this paper (pre-Mars Global Surveyor) asserts there are no maars on the surface of Mars, based on Viking imagery, even though they are expected in large numbers due to presumed ice/magma interactions.

Something tells me the formations were seen in Viking imagery but were unrecognized, indeed misinterpreted as impact or constructional volcanic features. A fair review of MOC images would show thousands of candidates, including possibly the ring formation referenced in the THEMIS image.

The French website shows that these maars can be explosive steam vents, something like geysers but on a much larger scale than we are used to.

Such explosive releases of steam on Mars, given its colder temperatures and thinner atmosphere, should be capable of producing far larger formations than on earth.

Throughout Acidalia and Utopia, among other areas, there are large ring formations that have traditionally been interpreted as impact craters. Some of them actually look like they are still releasing steam or some other volatile. Maybe they are Martian maars.

Need some open minds here.

Cheers, BFelix