MERCURY鈥檚 magnetic field has been a puzzle for more than 30 years. Now radio astronomers have found the innermost planet has a partly molten core, which may be generating the field.
Earth鈥檚 magnetic field is created by molten material in its core, but Mercury is so small that its centre should have cooled and solidified long ago, making the source of its field a mystery.
Fluctuations in a planet鈥檚 rate of spin can reveal the nature of its core. Mercury rotates on its axis once every 59 days, but the sun鈥檚 uneven gravitational pull makes the planet鈥檚 spin rate fluctuate slightly over the course of its orbit. The size of these fluctuations hinge on whether Mercury鈥檚 core is solid or liquid.
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Jean-Luc Margot of Cornell University in Ithaca, New York, and colleagues used radio telescopes to bounce radar signals off Mercury and monitor the reflections. The signals reflect from irregularities in Mercury鈥檚 surface, turning it into something of a 鈥渄isco ball鈥�, Margot says. The team studied the reflected patterns to determine variations in the speed of Mercury鈥檚 rotation.
The fluctuations turned out to be about 1 part in 10,000 鈥� larger than expected for a completely solid planet. It seems Mercury鈥檚 heart has yet to harden (Science, vol 316, p 710).
