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Mercury Gets a Dose of Extra Iron

When Pluto was demoted to dwarf status, Mercury assumed the mantle of our solar system's smallest world. But a new study may confer a catchier moniker: the Iron Planet. Team members on the MESSENGER mission orbiting Mercury—with some help from Earth-based radar scientists—are reporting that Mercury's iron core is even larger than had been previously thought. And they are suggesting that a scum of iron-rich mineral is now frozen around the rest of the core, which is still liquid. All this without laying a finger on the planet.

The trick to finding the iron beneath hundreds of kilometers of rock is its effects not only on the motions of an orbiting satellite but also on the motions of the planet itself. By precisely measuring the orbital movements of MESSENGER using the subtle Doppler frequency shifts of its radio signal, geodesist David Smith of the Massachusetts Institute of Technology in Cambridge and 16 colleagues measured how the pull of gravity varies across Mercury, as the team reports online today in Science. Those gravity variations, in turn, depend on where inside Mercury its mass is concentrated.

The internal distribution of Mercury's mass also affects the planet's motions. From Earth, big-dish radar can precisely measure the changing tilt of Mercury's rotation axis as well as what one of the co-authors calls "the planet doing the twist": tiny changes in its rotation speed due to solar tides.

Combined to give the most likely picture of Mercury's interior, the gravity and orbital data point to plenty of iron for the planet. By the team's figuring, the core extends 2030 kilometers from the center of Mercury, or 83% of the planetary radius; Earth's core is only 55% the size of the whole planet.

And the team sees another, unexpected place iron may be stored. The data suggest a mass concentration in a layer surrounding the core. Considering the chemistry of iron and its minerals and the geochemistry of Mercury's surface as determined by MESSENGER, it is plausible that, when Mercury was forming, iron and sulfur combined and froze out of the molten core, floated to the core top, and formed a solid iron-sulfide layer tens of kilometers to 200 kilometers thick. That would leave the planet with a rocky rind as little as 200 kilometers thick.

"If they're right, it's quite a remarkable result," says planetary physicist David Stevenson of California Institute of Technology in Pasadena. "There are a number of interesting things that might be affected." For one, geophysicists have had a hard time explaining how Mercury generates such a weak magnetic field. A possible answer: The magnetic field is strong where the churning core generates it, but the layer of electrically conducting iron sulfide partially screens out and weakens the field before it reaches the surface.

Further support for a scum layer inside Mercury could come as MESSENGER gathers more gravity data and team members use the accumulating gravity data as an independent check on the radar orbital data.