These images from [the High Resolution Stereo Camera on] ESA’s Mars Express show a crater named Ismenia Patera on the Red Planet. Its origin remains uncertain: did a meteorite smash into the surface or could it be the remnants of a supervolcano?
Ismenia Patera – patera meaning ‘flat bowl’ in Latin – sits in the Arabia Terra region on Mars. This a transition area between the planet’s northern and southern regions – an especially intriguing part of the surface. (…)
Ismenia Patera is some 75 km across. Its centre is surrounded by a ring of hills, blocks and lumps of rock thought to have been ejected and flung into the crater by nearby impacts. (…)
There are two leading ideas for its formation. One links it to a potential meteorite that collided with Mars. Sedimentary deposits and ice then flowed in to fill the crater until it collapsed to form the fissured, uneven landscape seen today.
The second idea suggests that, rather than a crater, Ismenia Patera was once home to a volcano that erupted catastrophically, throwing huge quantities of magma out into its surroundings and collapsing as a result… [More at link]