Ironing Out Mercury's Makeup

The surface of Mercury, believed to be curiously deficient in iron even though the planet has an enormous iron core, might instead be rich in iron and titanium, according to new data collected by NASA’s Messenger spacecraft. The finding could force scientists to retool models of the planet’s formation and evolution.

Scientists reported the first direct elemental abundance measurements of Mercury’s surface at a press conference on Nov. 3. Messenger captured the data with its neutron spectrometer during the craft’s third and final flyby of Mercury. In the coming months, Messenger will settle into orbit around the planet to begin mapping it in detail in 2011.

The neutron spectrometer’s detection of an iron-rich surface contradicts other, less definitive data from Messenger’s first flyby two years ago (C&EN, July 28, 2008, page 65). Understanding how the new results affect theories about Mercury’s formation “will keep people busy for awhile,” said Messenger mission scientist David Lawrence of Johns Hopkins University Applied Physics Laboratory at the conference.

The new iron-rich picture of Mercury’s surface that’s emerging resembles that of Earth’s moon, Lawrence said.

An error during the flyby caused Messenger’s power to turn off, and as a result, the team acquired only about 50% of the data they’d hoped for.

But in addition to the neutron spectrometer measurements, Messenger imaged most of the planet, including 6% of the surface that has never been explored before. In one striking image, a double-ring basin 290 km in diameter resembles a feature discovered during the first flyby, known as the Raditladi basin. Both features, which are believed to be volcanic in origin, are only about a billion years old, a fact that bolsters evidence for recent volcanism on the planet, said Brett W. Denevi, a Messenger mission member and postdoc at Arizona State University, Tempe, at the conference.