(Sen) - Scientists have used gravity data collected by the European Space Agency’s Mars Express to take a peek beneath the Red Planet’s largest volcanoes and gain an insight into the processes that built them.
Mars Express’ low orbital height (between 275-330km) makes it quite sensitive to gravitational fluctuations in the Martian surface. When the probe passes over a particularly dense region, the increased gravitational attraction causes the craft’s trajectory to wobble slightly. These movements are tracked from Earth and translated into measurements of the region’s density.
Using data gathered with this technique, researchers at the Royal Observatory of Belgium have mapped the density of the 21km-high Olympus Mons volcano and the three smaller Tharsis Montes volcanoes that make up Mars’ Tharsis region.
Mars Express’ measurements (combined with data from NASA’s Mars Reconnaissance Orbiter) have revealed that the three smaller Tharsis Montes volcanoes have a much higher underground density than Olympus Mons.
The data shows that the Tharsis Montes volcanoes (Arsia Mons, Pavonis Mons and Ascraeus Mons) were created by different geological processes than Olympus Mons, which is on the edge of a region known as the Tharsis bulge.
Image of Tharsis Montes and Olympus Mons from NASA’s Mars Global Surveyor. Credit: NASA
They were created by an upwelling of abnormally hot rock from deeper within the Martian mantle in a process that can be likened to a gigantic lava lamp that slowly moved sideways to create each of the three Tharsis Montes volcanoes in turn – a reverse of the process that builds chains of volcanoes on Earth (when it is the crust that moves above a stationary lave plume).
The researchers suggest that the lava’s unusually high temperature was caused by the shape of the crust at the Tharsis bulge which, being thicker at the centre, acted like an insulating lid to increase the temperature.
The upwelling first created Arsia Mons and then moved on to form Pavonis Mons and finally Ascraeus Mons. The whole region then partially sank into the lithosphere (the crust and upper region of the mantle) – creating the higher density observed by Mars Express.
In comparison, because it sits on the edge of the Tharsis bulge and not on top (unlike the Tharsis Montes), Olympus Mons was formed on a more rigid part of the crust - and so didn’t sink like the Tharsis Montes volcanoes - which would explain the less dense nature of its roots.
The region is thought to have been volcanically active until 100-250 million years ago, which, geologically speaking, is relatively recent.
The results have been published in the Journal of Geophysical Research.