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Astronomy in the News

Mercury Summer 2012 Table of Contents


Olympus Mons

Olympus Mons colour-coded according to height from white (highest) to blue (lowest). The new data find that Olympus Mons is built on a rigid lithosphere whereas the nearby Tharsis Montes partially sank into a less rigid lithosphere. Courtesy ESA / DLR / FU Berlin
(G. Neukum).

First Mars Express Gravity Results Plot Volcanic History

European Space Agency

Five years of Mars Express gravity mapping data are providing unique insights into what lies beneath the Red Planet’s largest volcanoes. The results show that the lava grew denser over time and that the thickness of the planet’s rigid outer layers varies across the Tharsis region.

The measurements were made while Mars Express was at altitudes between 275 and 330 km above the Tharsis volcanic ‘bulge’ during the closest points of its eccentric orbit, and were combined with data from NASA’s Mars Reconnaissance Orbiter.

The Tharsis bulge includes Olympus Mons, the tallest volcano in the solar system, at 21 km, and the three smaller Tharsis Montes that are evenly spaced in a row.

The region is thought to have been volcanically active until 100-250 million years ago, relatively recent on a geological timescale.

The large mass of the volcanoes caused tiny ‘wobbles’ in the trajectory of Mars Express as it flew overhead; these were measured from Earth via radio tracking and translated into measurements of density variations below the surface.

The new data also reveal how the lava density changed during the construction of the three Tharsis Montes volcanoes. They started with a lighter andesitic lava that can form in the presence of water, and were then overlaid with heavier basaltic lava that makes up the visible surface of the Martian crust.

“Combined with the varying height of the volcanoes, we can say that Arsia Mons is the oldest, then Pavonis Mons formed and finally Ascraeus Mons,” says Mikael Beuthe of the Royal Observatory of Belgium.


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