The geology of Mars

Excerpts from an interesting article at Wired:

Why, for instance, does Earth have a magnetic field, but Mars’ seems to have disappeared? Why are so many volcanoes spread all over Earth, while volcanoes are more localized—and bigger—on Mars? (At 374 miles in diameter and 16 miles high, Olympus Mons is the biggest known volcano in the solar system.)..

The researchers found the core density to be surprisingly low, at only about 6 grams per cubic centimeter, which is much lower than what they’d expected of an iron-rich center... In Stähler’s team’s paper, they report a core radius of 1,830 kilometers. Another team, led by ETH Zürich geophysicist Amir Khan, found that this size is so large it leaves little room for an Earth-like lower mantle, a layer that acts as a heat-trapping blanket around the core... Without this layer, the Martian core likely cooled much more readily than Earth’s. This is key to understanding the evolution of the Red Planet, and in particular why it lost its magnetic field, a barrier that would have protected the atmosphere—and potential life—from harsh solar winds. Creating a magnetic field requires a temperature gradient between the outer and inner core, high enough to create circulating currents that churn the core’s liquid and give rise to a magnetic field. But the core cooled so fast that these convection currents died out...

Khan’s analysis also shows that Mars has a thick lithosphere, the rigid and cold part of the mantle. This might be a clue as to why the Red Planet doesn’t have the plate tectonics that drive the frenzy of volcanism on Earth. “If you have a very thick lithosphere, it's going to be very difficult to break this thing up and create the exact equivalent of plate tectonics on Earth,” says Khan. “Maybe Mars had it very early on, but it's certainly shut down now.”