Common handheld magnets can erase crucial data in ancient meteorites.
Planetary scientists say meteorite hunters must re-think their technique in the field. In a recent study, Foteini Vervelidou (MIT) and colleagues found that using standard magnets on terrestrial basalt — a common analog for stony chondrite/achondrite meteorites — destroyed the material's magnetic record. They published the study, titled Hand Magnets and the Destruction of Ancient Meteorite Magnetism, in the Journal of Geophysical Research: Planets.
Magnets are a common tool in the modern meteorite hunter’s kit. Cheap, powerful neodymium-iron-boron magnets are now widely available online, and amateurs can use them to quickly identify iron-nickel meteorites. However, these magnets can erase the original magnetic field of any meteorite, damaging crucial fragments of asteroids, the Moon, and Mars.
“Over the last few decades, we have discovered that meteorites contain unique magnetic records that tell us how planets formed and evolved,” says team member Benjamin Weiss (also at MIT). “However, even just briefly touching a fist-sized meteorite with a magnet essentially obliterates this record forever.”
The meteorite dubbed “Black Beauty” is a good case in point. Searchers in Morocco found the black fragments in 2011 strewn across the Saharan Desert. (Like Antarctica, the Sahara is prime terrain to search for meteorites, as they stand out against the background.)
Scientists found that Black Beauty (Northwest Africa 7034) originated on Mars 4.4 billion years ago, right around the time the Red Planet was thought to have a magnetic field of its own. Unfortunately, exposure to hand magnets erased the samples’ primordial magnetic fields, taking away precious records of early Mars in the process.
Vervelidou and colleagues advocate using magnetic “susceptibility meters,” which won’t disturb the original field embedded in the sample. This technique has the added benefit of being able to discern which subgroup a meteorite belongs to while still in the field.
“Magnetic susceptibility meters are far more useful than magnets for meteorite identification because they provide a quantitative measure of the meteorite’s magnetism,” Weiss explains. “As a result, magnets are largely only useful for distinguishing meteorites from Earth rocks, while susceptibility meters can be used to distinguish between meteorites of different groups.”
One problem with susceptibility meters, however, are that they’re expensive — in the range of hundreds to thousands of dollars per unit. Procuring one is difficult, especially in places where local meteorite hunters work for meager subsistence pay.
Visual inspection can also go a long ways toward identifying whether a sample is worth preserving. Meteorites often display a fusion crust, a black glass layer created from ablation during their passage through Earth’s atmosphere. Iron-nickel meteorites also display a jagged Widmanstätten pattern, although this is often only apparent after a cross-section is cut.
Safety is paramount during sample returns. In NASA’s annual Antarctic Search for Meteorites program (ANSMET), scientists look for meteorites embedded in the ice. Other samples come from farther afield: The Artemis initiative will bring fresh samples back from the Moon in the coming decade, and Perseverance is already caching samples for the Mars sample return. The latter could be back on Earth for analysis by 2031. Vervelidou and his colleagues recommend that all of these samples be protected from magnetic contamination.
Mars’s loss of its global magnetic field may have led to the Red Planet's transition from a wet world to the cold, dry one we see today. “To test this idea,” Weiss says, “we would like to measure the strength of Mars’s magnetic field through time by analyzing ancient Martian rocks.” Even if Perseverance’s samples make it back to Earth, Martian meteorites landing on Earth provide a good point of comparison.
“Amateur use of magnets is the number one problem limiting meteorite paleomagnetic studies,” Weiss says. But techniques for finding meteorites, and even for finding incoming asteroids before they become meteorites, are improving. Perhaps handheld magnets will soon become a thing of the past.