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Mercury's magnetic field -- and why it has one

October 1, 2011 |  1:25 pm


Mercury is one oddball of a planet. For one thing, the tiny planet’s magnetic field is lopsided, shifted 300 miles north, according to data sent from an orbiting spacecraft.

But perhaps the more basic mystery is why Mercury has a magnetic field at all.

The Messenger spacecraft, launched in 2004, has been orbiting Mercury since March. Among the suite of spectrometers, cameras and other instruments, the spacecraft also sports a magnetometer to chart the planet’s magnetic field. 

“The fact that it still has a liquid shell in its core seems to imply the core is not really pure iron,” said Brian Anderson, a space physicist at Johns Hopkins University Applied Physics Laboratory who led one of seven studies published in Friday’s edition of the journal Science.

Here’s why Anderson thinks that: Magnetic fields are powered by liquid metal churning in a planet’s core. But at this point, Mercury should have cooled down and solidified -– unless its iron core has a good mix of, say, sulfur in it. Sulfur in iron can act like salt in water, keeping it liquid past its normal freezing point.

The idea that a little extra sulfur might be present in the core fits well with findings in the other studies, including one suggesting that sulfur is 10 times as abundant on Mercury’s surface as it is on the Earth or Moon.


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Photo: The Messenger spacecraft captured this image of Mercury when it was high above the planet's southern hemisphere on May 24. Data shows the planet has a magnetic field. Credit: Reuters / NASA /Johns Hopkins University Applied Physics Laboratory / Carnegie Institution of Washington