Ever since NASA's Mariner 10 spacecraft zipped past Mercury in 1974, scientists have wondered why the planet's magnetic field is so much wimpier than expected. Now, a new study led by researchers at Braunschweig University of Technology in Germany suggests that the solar wind—the incessant flow of charged particles boiling off the sun's surface—suppresses the field generated by the flow of molten iron in the planet's outer core. On the sunward side of Mercury, the magnetopause—the protective shield created by the planet's magnetic field—sits just 1200 kilometers above the planet's surface. That's so close, the team's computer models indicate, that magnetic fields created by particles flowing along the magnetopause reach deep into Mercury itself, counteracting the internally-generated field. Without the external fields generated by the solar wind, Mercury's magnetic field might be about 30 times stronger than it actually is, the researchers report today in Science. NASA's MESSENGER probe (artist's concept above) has been orbiting Mercury since mid-March and will provide unprecedented measurements of the strength and direction of the planet's magnetic field, revealing more about how such fields are generated in the first place.
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