(via scinerds)Source: expose-the-light
First Mars Express gravity results plot volcanic history
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 of between 275–330 km above the Tharsis volcanic ‘bulge’, which includes Olympus Mons 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 new data 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.
The data also describe the thickness of the lithosphere – the outermost shell of the planet, including the upper portion of the mantle – and find surprising lateral variations between Olympus Mons and the Tharsis Montes, with the three smaller volcanoes having a much higher density underground ‘root’ than Olympus Mons.
Animation: 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.