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Officials last week revealed that the U.S. contribution to ITER could cost $3.9 billion by 2034—roughly four times the...
An experimental hepatitis B drug that looked safe in animal trials tragically killed five of 15 patients in 1993. Now,...
Using the two high-quality genomes that exist for Neandertals and Denisovans, researchers find clues to gene activity...
A new report from the Intergovernmental Panel on Climate Change (IPCC) concludes that humanity has done little to slow...
Astronomers have discovered an Earth-sized planet in the habitable zone of a red dwarf—a star cooler than the sun—500...
Three years ago, Jennifer Francis of Rutgers University proposed that a warming Arctic was altering the behavior of the...
- 17 April 2014 12:48 pm , Vol. 344 , #6181
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Mercury's Lively Core
14 January 2005 (All day)
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.