METEORITE hunters are unwittingly ruining their finds by swiping them with bar magnets. This common field test for identifying genuine meteorites could magnetise them and wipe them clean of subtle fields that tell us about the early Solar System.
Most terrestrial rocks aren’t magnetic because heavy metals like iron sank from the crust to the planet’s core while the Earth was still molten. In contrast, most meteorites are magnetised, and it was presumed this was because they had broken off iron-rich magnetic asteroids that formed in the primordial Solar System.
So when NASA’s NEAR space probe landed on the surface of the asteroid Eros in 2001, scientists were surprised to find no sign of a magnetic field. They eventually decided this was because of Eros’s large size: the bigger the body, the harder it is to magnetise. Small pieces that have broken off such large asteroids and wound up on Earth must have become magnetised by passing through the Earth’s magnetic field.
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But Peter Wasilewski of NASA Goddard Space Flight Center reckoned something more earthly was afoot. In a review to be published in June’s The Meteoritical Bulletin he says he became suspicious when he found that some meteorites were hundreds of times more magnetic than similar specimens. And meteorites carefully collected by specialist teams in the Antarctic were less magnetic than those turned in elsewhere, even though they had presumably undergone similar conditions on entry. “Some of this has to be contamination,” he says.
Tim Jull who studies meteorites at the University of Arizona in Tucson says Wasilewski is right. “It makes a lot of sense,” he says. “People in the field put bar magnets on meteorites all the time because that’s how you tell if they’re magnetic.”
Meteorites may in fact have very weak magnetic signatures that reflect the conditions in which their parent asteroids were formed. But contact with magnets on Earth would wipe or distort that information just as exposure to a magnet can ruin a credit card’s magnetic strip. “You destroy everything that was there and you introduce new magnetic signatures,” says Wasilewski.
Despite increasing interest in collecting meteorites, there’s no protocol for avoiding magnetic contamination, and no one knows if it’s possible to detect weak primordial signals in contaminated rocks. Yet such data would be extremely valuable to planetary scientists. A meteorite’s magnetic fields and chemical composition would help pin down when and where in the Solar System its parent asteroid formed. With enough samples, it might be possible to map out the primordial magnetic fields that helped to shape planetary formation 5 billion years ago.