ESA | Space Science | 2011 Apr 01
ESA’s Mars Express has returned images of mist-capped volcanoes located in the northern hemisphere of the red planet. Long after volcanic activity ceased, the area was transformed by meteor impacts that deposited ejected material over the lower flanks of the volcanoes.
Permanent and transient features are on display in this newly released image of Mars. The image is derived from data acquired during three orbits of ESA’s Mars Express between 25 November 2004 and 22 June 2006. In that time, these dead volcanoes were not expected to change but, during the middle orbit, Mars Express captured icy clouds drifting past the summit of Ceraunius Tholus.
By the time Mars Express crossed again and took the final strip of data needed for this image, the clouds had long since dispersed and so there is a sharp line across them in the finished mosaic.
The Latin word “tholus” means a conical dome and the base of Ceraunius Tholus is 130 km across, while the peak rises 5.5 km above the surrounding plains. At its summit is a large caldera 25 km across. With similar morphology to its neighbour and lying 60 km to the north, Uranius Tholus is a smaller volcano, with a base diameter of 62 km and a height of 4.5 km.
The flanks of Ceraunius Tholus are relatively steep, at about 8° inclination, and are etched with valleys. They are deeply cut in many places, suggesting that soft and easily eroded material, such as layers of ash, were deposited during the volcano’s eruptions.
The largest and deepest of these valleys is about 3.5 km wide and 300 m deep. It terminates inside an otherwise unrelated elongated impact crater that happens to lie between the two volcanoes, and has created a fascinating fan shape of deposits.
Although the source of the fan is still being debated in scientific circles, it may have been formed when material from a lava channel or tube was washed downwards by a melting ice cap on the volcano.
Certainly, the summit crater – the caldera – is flat and smooth, so it may have contained a lake early in Mars’ history when the atmosphere was denser. It is also possible that the water was produced when volcanic activity melted buried ice lenses. An ice lens forms when moisture seeps below the surface and forms a frozen layer between the top ‘soil’ and the rocky layer below.
The elongated crater between the two volcanoes is called Rahe. It measures 35 km by 18 km and is the result of an oblique impact by a meteorite.
A smaller impact crater 13 km across can be seen to the west of Uranius Tholus. This one also formed after all the volcanic activity ended and served to cover the lower flanks of the volcanoes with ejected material, with the result that only the upper regions of the original structures are now visible.
Twin Martian volcanoes rise above a cratered landscape
New Scientist | Short Sharp Science | 2011 Apr 05
Rising above a crater-studded landscape, two Martian volcanoes sit side by side in the red planet's northern hemisphere. This newly released image from the European Space Agency's Mars Express orbiter maps contrasting elevations in the region, from the elongated Rahe crater in violet up to the grey summit of the higher volcano Ceraunius Tholus, which rises 5.5 kilometres high and spans 130 kilometres across, dwarfing its neighbour Uranius Tholus.
The smooth floor of the caldera at the summit of Ceraunius Tholus suggests that it may once have held a lake when Mars had a denser atmosphere. Alternatively, the caldera could be the result of ice that formed beneath the topsoil and then melted, undermining the surface.
Mars is also home to the largest known volcano in our solar system, the 22 kilometre-high Olympus Mons. Mars Express provided detailed shots of the giant volcano in 2004.
