Magnets wipe memories from meteorites, erasing billion-year-old data | Science

In 2011, nomads roaming the western Sahara encountered precious time capsules from Mars: coal-black chunks of a meteorite, strewn across the dunes. “Black Beauty,” as the parent body came to be known, captivated scientists and collectors because it contained crystals that formed on Mars more than 4.4 billion years ago, making it older than any native rock on Earth. Jérôme Gattacceca, a paleomagnetist at the European Centre for Research and Teaching in Environmental Geosciences, hoped it might harbor a secret message, imprinted by the now-defunct martian magnetic field—which is thought to have helped the planet sustain an atmosphere, water, and possibly even life.

But when Gattacceca obtained a piece of Black Beauty and tried to decode its magnetic inscription, he found its memory had been wiped—Men in Black style—and replaced by a stronger signal. He instantly knew the culprit. Somewhere along its journey from Moroccan desert to street dealers to laboratory, the rock had been touched by strong hand magnets, a widely used technique for identifying meteorites. “It’s a pity that, just by using magnets, we’ve been destroying this scientific information that was stored there for 4 billion years,” Gattacceca says.

In a new study, Foteini Vervelidou, a planetary scientist at the Massachusetts Institute of Technology (MIT), and her colleagues have documented the destructive power of the hand magnets, which are often made from rare-earth metals such as neodymium and are typically about 10,000 times stronger than Earth’s magnetic field. When brought within a few centimeters of a rock, the researchers found, the magnets overwrite vestigial fields contained in iron-based minerals such as magnetite and reset them to the higher strength and orientation of the magnet. In an instant, a unique view into the heart of distant rocky bodies can be erased. Black Beauty, for example, is the only known meteorite old enough to “remember” Mars’s magnetic field before it started to disappear about 4 billion years ago.

Vervelidou hopes that raising awareness will help convince hunters, collectors, and researchers to give up a technique that’s still promoted by the U.S. Geological Survey, universities, and popular meteorite hunting TV shows. “It’s like having this unique piece [of information] destroyed,” Vervelidou says. “Why would you buy an amazing painting and then throw some sauce on it?”

Hand magnets are effective for distinguishing ordinary chondrites, an abundant kind of meteorite that is often loaded with metals, from many Earth rocks. But their diagnostic abilities have limits. Some common iron-rich Earth rocks, such as basalt, can also attract magnets, whereas the meteorites that come from Mars or the Moon often don’t. Most of the iron on those larger bodies is in their core, not their crust, where meteorites originate. “The irony is that the [meteorites] that don’t stick to magnets are actually the most valuable of all,” says Ben Weiss, a study co-author and MIT planetary scientist.

The problem is not new. Two decades ago, Weiss was studying rare meteorite samples to see whether some asteroids were ever big enough to have a dynamo—a churning molten core that generates magnetic fields. He saw spectacularly high magnetizations in each of the samples only to realize, later, that he’d been duped by magnets. Weiss says the meteoritic literature is rife with false magnetic findings—such as reports of extreme fields in one of the most studied meteorites of all time, Allende, which fell in Mexico in 1969. “I wasn’t the first one to be fooled.”

After the frustration of finding magnetic contamination in nine different Black Beauty samples, the researchers decided to confront the hand-magnet issue head-on. In their study, accepted this week by the Journal of Geophysical Research: Planets, they calculated how magnets of different strengths would alter a meteorite’s magnetic records as they approached the rock. Study co-author France Lagroix, at the Paris Institute of Planetary Physics, helped check the calculations by measuring the fields in 13 specimens of terrestrial basalt before and after placing a typical hand magnet at various distances from them. The results showed how the magnet progressively resets fields from the outside in, giving researchers a guide for how deep they would have to cut to find a pristine sample. “Now we’re 100% sure, if we weren’t already, that this is what’s going on,” Weiss says.

But for most people—even other meteorite researchers—the magnetic records aren’t as precious as they are to paleomagnetists. The “quick-and-dirty” method of meteorite identification persists because “not everybody cares about magnetic fields like Ben Weiss does,” says Carl Agee, the meteoriticist at the University of New Mexico who first determined Black Beauty hailed from Mars.

For the past 2 decades, Hasnaa Chennaoui Aoudjehane, a planetary scientist at the Hassan II University of Casablanca, has been trying to educate Saharan meteorite hunters about the dangers of hand magnets. But the message doesn’t always sink in. “We try explaining to hunters … ‘it’s the human heritage; it’s the history of the Solar System,” Chennaoui Aoudjehane says. “But when someone needs to buy bread and things to live every day … their priority is not the science that is conducted on those rocks. It’s their source of revenue.” Saïd Yousfi, a meteorite collector and dealer in Morocco, agrees magnets will remain a fixture of the Saharan meteorite-hunting culture—despite the fact that most local hunters are skilled enough to identify meteorites by eye.

In their study, Vervelidou and her colleagues recommend an alternative: susceptibility meters. These devices apply a weak magnetic field that doesn’t erase the records, and they are better at identifying different types of meteorites. The catch is that commercial devices often cost a few thousand dollars, and they’re not as intuitive to use as a simple magnet. Gattacceca and colleagues are now building handheld susceptibility meters that have only one button and cost a few hundred dollars.

Gattacceca hopes to make inroads with scientists. He points to the Antarctic Search for Meteorites (ANSMET), an annual expedition funded by the National Science Foundation. His collaborators there told him they weren’t using magnets on their samples, so he assumed the large magnetizations he was finding were relics of the early Solar System.

But a few years ago, a colleague participating in the expedition brought back a small hand magnet adorned with the ANSMET logo. It had been distributed in the researchers’ field kits. “That’s what you call a smoking gun,” Gattacceca says. “Once you have the gun, you’re likely to use it.” 

In 2011, nomads roaming the western Sahara encountered precious time capsules from Mars: coal-black chunks of a meteorite, strewn across the dunes. “Black Beauty,” as the parent body came to be known, captivated scientists and collectors because it contained crystals that formed on Mars more than 4.4 billion years ago, making it older than any native rock on Earth. Jérôme Gattacceca, a paleomagnetist at the European Centre for Research and Teaching in Environmental Geosciences, hoped it might harbor a secret message, imprinted by the now-defunct martian magnetic field—which is thought to have helped the planet sustain an atmosphere, water, and possibly even life.

But when Gattacceca obtained a piece of Black Beauty and tried to decode its magnetic inscription, he found its memory had been wiped—Men in Black style—and replaced by a stronger signal. He instantly knew the culprit. Somewhere along its journey from Moroccan desert to street dealers to laboratory, the rock had been touched by strong hand magnets, a widely used technique for identifying meteorites. “It’s a pity that, just by using magnets, we’ve been destroying this scientific information that was stored there for 4 billion years,” Gattacceca says.

In a new study, Foteini Vervelidou, a planetary scientist at the Massachusetts Institute of Technology (MIT), and her colleagues have documented the destructive power of the hand magnets, which are often made from rare-earth metals such as neodymium and are typically about 10,000 times stronger than Earth’s magnetic field. When brought within a few centimeters of a rock, the researchers found, the magnets overwrite vestigial fields contained in iron-based minerals such as magnetite and reset them to the higher strength and orientation of the magnet. In an instant, a unique view into the heart of distant rocky bodies can be erased. Black Beauty, for example, is the only known meteorite old enough to “remember” Mars’s magnetic field before it started to disappear about 4 billion years ago.

Vervelidou hopes that raising awareness will help convince hunters, collectors, and researchers to give up a technique that’s still promoted by the U.S. Geological Survey, universities, and popular meteorite hunting TV shows. “It’s like having this unique piece [of information] destroyed,” Vervelidou says. “Why would you buy an amazing painting and then throw some sauce on it?”

Hand magnets are effective for distinguishing ordinary chondrites, an abundant kind of meteorite that is often loaded with metals, from many Earth rocks. But their diagnostic abilities have limits. Some common iron-rich Earth rocks, such as basalt, can also attract magnets, whereas the meteorites that come from Mars or the Moon often don’t. Most of the iron on those larger bodies is in their core, not their crust, where meteorites originate. “The irony is that the [meteorites] that don’t stick to magnets are actually the most valuable of all,” says Ben Weiss, a study co-author and MIT planetary scientist.

The problem is not new. Two decades ago, Weiss was studying rare meteorite samples to see whether some asteroids were ever big enough to have a dynamo—a churning molten core that generates magnetic fields. He saw spectacularly high magnetizations in each of the samples only to realize, later, that he’d been duped by magnets. Weiss says the meteoritic literature is rife with false magnetic findings—such as reports of extreme fields in one of the most studied meteorites of all time, Allende, which fell in Mexico in 1969. “I wasn’t the first one to be fooled.”

After the frustration of finding magnetic contamination in nine different Black Beauty samples, the researchers decided to confront the hand-magnet issue head-on. In their study, accepted this week by the Journal of Geophysical Research: Planets, they calculated how magnets of different strengths would alter a meteorite’s magnetic records as they approached the rock. Study co-author France Lagroix, at the Paris Institute of Planetary Physics, helped check the calculations by measuring the fields in 13 specimens of terrestrial basalt before and after placing a typical hand magnet at various distances from them. The results showed how the magnet progressively resets fields from the outside in, giving researchers a guide for how deep they would have to cut to find a pristine sample. “Now we’re 100% sure, if we weren’t already, that this is what’s going on,” Weiss says.

But for most people—even other meteorite researchers—the magnetic records aren’t as precious as they are to paleomagnetists. The “quick-and-dirty” method of meteorite identification persists because “not everybody cares about magnetic fields like Ben Weiss does,” says Carl Agee, the meteoriticist at the University of New Mexico who first determined Black Beauty hailed from Mars.

For the past 2 decades, Hasnaa Chennaoui Aoudjehane, a planetary scientist at the Hassan II University of Casablanca, has been trying to educate Saharan meteorite hunters about the dangers of hand magnets. But the message doesn’t always sink in. “We try explaining to hunters … ‘it’s the human heritage; it’s the history of the Solar System,” Chennaoui Aoudjehane says. “But when someone needs to buy bread and things to live every day … their priority is not the science that is conducted on those rocks. It’s their source of revenue.” Saïd Yousfi, a meteorite collector and dealer in Morocco, agrees magnets will remain a fixture of the Saharan meteorite-hunting culture—despite the fact that most local hunters are skilled enough to identify meteorites by eye.

In their study, Vervelidou and her colleagues recommend an alternative: susceptibility meters. These devices apply a weak magnetic field that doesn’t erase the records, and they are better at identifying different types of meteorites. The catch is that commercial devices often cost a few thousand dollars, and they’re not as intuitive to use as a simple magnet. Gattacceca and colleagues are now building handheld susceptibility meters that have only one button and cost a few hundred dollars.

Gattacceca hopes to make inroads with scientists. He points to the Antarctic Search for Meteorites (ANSMET), an annual expedition funded by the National Science Foundation. His collaborators there told him they weren’t using magnets on their samples, so he assumed the large magnetizations he was finding were relics of the early Solar System.

But a few years ago, a colleague participating in the expedition brought back a small hand magnet adorned with the ANSMET logo. It had been distributed in the researchers’ field kits. “That’s what you call a smoking gun,” Gattacceca says. “Once you have the gun, you’re likely to use it.”