By all rights, the Mariner 10 spacecraft should have found a geophysically dead planet when it flew by Mercury in the mid-1970s. But to everyone's surprise, Mariner detected a weak magnetic field emanating from the sun's closest companion, indicating that Mercury—like Earth—may have a liquid core.
Earth's magnetic field is churned out by a molten iron core, but astronomers assumed that Mercury's field was too weak to be generated this way. And besides, planetary scientists thought Mercury's big iron core must have frozen solid eons ago. Alternatively, if an early field-generating core had locked its field into Mercury's crust before freezing up, the field would be much stronger than Mariner's discovery. No spacecraft has revisited Mercury, but at last month's meeting of the American Geophysical Union in San Francisco, two groups of researchers built a strong case that Mercury generates its magnetic field in a lingering remnant of a molten core, much the way Earth's geodynamo operates.
The trick to diagnosing Mercury's interior without leaving Earth was measuring the planet's rotation rate to 1 part in 100,000. Planetary scientist Jean-Luc Margot of Cornell University and colleagues used a previously proposed ground-based radar technique to precisely measure variations in Mercury's rotation during the past 2 years. They repeatedly beamed a radar pulse at Mercury from the 70-meter antenna at Goldstone, California, and picked up the reflected signal at both Goldstone and the 100–meter antenna at Greenbank, West Virginia, 3200 kilometers to the east. Matching up the distinctively "speckled" pattern in the signal received at each station, they could gauge the time lag of reception between stations and thus calculated the rotation rate precisely. It varied with Mercury's 88-day libration three times as much as it would if the planet were solid throughout.
Given such a definitive result, "it looks as if [a molten core] is the only explanation," says planetary geophysicist David Smith of NASA's Goddard Space Flight Center in Greenbelt, Maryland. That still would leave the difficulty of why Mercury's magnetic field has only 1/100th the strength of Earth's geodynamo-generated field. The Messenger spacecraft, launched last August, should be able test the state of Mercury's core and the nature of its magnetic field after entering orbit in 2011.