ESA’s Mars Express orbiter has captured an image of what appears to be the remains of an ancient supervolcano. The image was taken on 26 November 2014, and focuses on the Siloe Patera feature in the Arabia Terra region of Mars.
Siloe Patera comprises two large nested craters, close to the center of the image. It is around 40 km (25 miles) long and 30 km (19 miles) wide. The structure is named after the Castilian sculptor and architect Diego de Siloé (1495 – 1563).
Some researchers believe that this type of geological structure and several similar structures in this region of Mars are calderas of very flat volcano complexes – volcanoes that resemble the few supervolcanoes on Earth in shape.
One example of such a supervolcano is the Yellowstone volcano, with a caldera 60 km (37 miles) long and 40 km (25 miles) wide.
Supervolcanoes occur when magma is trapped below the surface, leading to a huge built up in pressure. They erupt suddenly in violent explosions and thus do not ‘grow’ sloping mountains like Olympus Mons, the largest volcano in the Solar System.
But a number of irregularly shaped craters have been detected in the Arabia Terra region that could represent a family of ancient supervolcano calderas.
Siloe Patera is one such example. It is characterized by two depressions with steep-sided walls, collapse features and low topographic relief.
The deepest depression reaches 1,750 m (5,700 feet) below the surrounding plains, and the somewhat shallower depression sinks another 700 m (2,300 feet) below this.
The depressions could even represent two different eruptive episodes due to collapse as the underlying magma pressure was released, or as the magma chamber migrated below the surface.
By comparison, impact craters include features such as a central peak, uplifted crater rims and ejecta blankets surrounding them. Indeed, impact craters are widespread in this scene: textbook examples can be found in the two side-by-side craters just above Siloe Patera, and in the large crater at the far right of the scene. These craters each exhibit a central peak, terraced crater walls and a surrounding ejecta blanket.
An impact crater with depth to diameter ratio comparable to Siloe Patera would be expected to show these features – unless perhaps the crater had undergone extensive erosion or modification – but it does not.
The structure is surrounded by numerous small channels and gullies partly flowing into the depression.
At the southern tip of the depression, a prominent valley-like feature is visible. These occur when groundwater escapes from beneath the edge of the terrain, the caverns created by this collapse and the eroded material is transported away along the course of the valley by the flowing water. A sharp rim that is somewhat uplifted in various places also appears here.
Directly above this feature is a region some 20 by 20 km (12.4 x 12.4 miles) in size that contains numerous smaller, dendritic outflow channels on a material that looks like ejecta.
This deposit could be a cooled lava sheet, or the ejecta blanket from an impact crater. In specialist literature, it is described as the remnant of a possible gas-rich, extremely hot pyroclastic lava flow. On the other hand, the lobate edges are also similar to ejecta blankets caused by small impact craters. Asymmetrical ejecta blankets occur at oblique impact angles of 15 degrees or less.