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Images from the High Resolution Stereo Camera (HRSC) operated by the German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt; DLR) on board ESA's Mars Express spacecraft show the collapsed depression Ismenia Patera. There are two hypotheses regarding the formation of this geological structure – it is either an impact crater or the caldera of a Martian supervolcano.

Ismenia Patera (Latin: patera – shallow bowl) is located in the Arabia Terra region, at the transition between the Martian southern highlands and northern lowlands. Inside the 75-kilometre-wide crater, hills and several hundred-metre-high mesas are arranged in a circle around the centre (Images 1, 4 and 5). On closer inspection, small gullies running into the crater are visible in some places on the rim of Ismenia Patera, which have been formed by water or ice flowing into the crater (Image 2). The floor of the crater is covered by deposits, the surfaces of which display traces of previous ice flow. These patterns were probably produced by rock glaciers – ice flows whose surfaces transport large quantities of boulders and debris. Similar phenomena are present on Earth in the Alpine and Polar regions, where glacial ice is completely covered by debris that has slid onto the ice from the surrounding mountain slopes. This ice-debris mass moves slowly downslope, leaving behind such flow structures.

Conspicuous groups of small impact craters, for example in the centre of Ismenia Patera, were probably caused by another meteorite impact in the immediate vicinity, which ejected large boulders towards the centre of the crater, producing small craters.

There are various opinions regarding the formation of the crater. According to one hypothesis, it was formed by a meteorite impact and subsequently filled with material deposits and, at times, with ice. Eventually, the crater surface collapsed as the underground ice melted and the water evaporated, leaving behind a rugged, hilly landscape. Another hypothesis suggests that Ismenia Patera is a volcanic caldera – a volcanic crater created by the emptying of the magma chamber following a large eruption. Volcanoes that produce a large amount of material during a single eruption are called supervolcanoes. Apart from Ismenia Patera, there are two other supervolcano candidates in Arabia Terra, namely Siloe Patera and Eden Patera. On Earth, volcanic structures such as the Phlegraean Fields near Naples, the Yellowstone area and Lake Taupo in New Zealand are considered to be supervolcanoes. On Mars, however, volcanic eruptions are no longer expected.

The theory of supervolcanoes in the ancient volcanic province in Arabia Terra is still a controversial topic for Mars researchers. In favour of the volcanic crater theory is the fact that the 'paterae' are irregularly-shaped collapse features with low topographic relief – partly without ejecta deposits elevated crater rim – which would be typical of impact craters. On the other hand, such an irregular shape may also be created by superimposed and subsequently altered impact craters. Future measurement methods for examining the Martian subsoil, to provide evidence for the existence of magma chambers below the paterae, for example, could shed light on this matter. These include seismic experiments, such as those on board NASA's InSight lander scheduled for launch on 5 May 2018, as well as orbiting radar experiments.