Hand magnets erased billion-year-old data from meteorites, reveals study

Jérôme Gattacceca, a paleomagnetist, found that chunks of the famed Black Beauty meteorite no longer contained secret clues to the Martian magnetic field.
Deena Theresa
Representational image.
Representational image.


Meteorites contain secret messages. That is, a host of information revealing mysteries older than our Solar System. They contain clues on how newborn stars fuse elements, the dense molecular clouds that form between stars, and much more. And so, accidentally erasing the history or memory of a meteorite is incredibly disappointing, to say the least. 

When Jérôme Gattacceca, a paleomagnetist at the European Centre for Research and Teaching in Environmental Geosciences, came across chunks of the Black Beauty (a meteorite that contained crystals that formed on Mars more than 4.4 billion years ago), he was hopeful that it would contain a message from the now-defunct Martian magnetic field, which might have helped the planet sustain life.

But, to his utter disappointment, he found that the meteorite's memory had been wiped and replaced by a strong signal.

Gattacceca instantly knew that strong hand magnets, a popular technique to identify meteorites, could be the culprit, and reported his experience in Science.

Reiterating Gattacceca's views, a new study by Foteini Vervelidou, a planetary scientist at the Massachusetts Institute of Technology (MIT), and her colleagues have documented the "destructive" power of the hand magnets, often made from rare-earth metals such as neodymium. They are typically about 10,000 times stronger than Earth’s magnetic field. 

Iron-rich rocks easily attract hand magnets, but meteorites from Mars and Moon often don't

Vervelidou hopes the study will lead to awareness, convincing hunters, collectors, and researchers to forgo a method promoted by the U.S. Geological Survey and universities. "It’s like having this unique piece [of information] destroyed," Vervelidou told Science. "Why would you buy an amazing painting and then throw some sauce on it?"

But collectors have 'valid' reasons to employ hand magnets. For starters, hand magnets can easily distinguish ordinary chondrites, which is a typical kind of meteorite. But they have limits.

For example, iron-rich rocks attract magnets easily, but Mars and Moon meteorites often don't. "The irony is that the [meteorites] that don’t stick to magnets are actually the most valuable of all," said Ben Weiss, a study co-author, and MIT planetary scientist.

After finding that their Black Beauty samples were contaminated, the researchers dove into the issue right away. They calculated how magnets of different strengths could disrupt a meteorite's magnetic records as they neared the rock. The results revealed how the magnet resets fields from the outside in.

This helped researchers figure out how deep they would have to cut to find a pure sample. "Now we’re 100 percent sure, if we weren’t already, that this is what’s going on," Weiss said.

So, what are our alternatives?

Vervelidou and her colleagues recommend susceptibility meters. 

These devices do apply a magnetic field, but they are rather weak and cannot erase records. However, these devices are expensive and challenging to handle. Gattacceca and colleagues are building handheld susceptibility meters with only one button and cost a few hundred dollars.

The study is published in the Journal of Geophysical Research: Planets.

Study Abstract:

Meteorites provide invaluable records of planetary formation and evolution. Studies of their paleomagnetism have constrained accretion in the protoplanetary disk, the thermal evolution and differentiation of planetesimals, and the history of planetary dynamos. Yet, the potential of these magnetic records in advancing the field of planetary science is severely hindered by a widely used technique: application of hand magnets to assist in meteorite classification. Touching a meteorite with a magnet results in near-instantaneous destruction of its magnetic record. Here, we showcase the destructive effects of exposing meteorites to magnets through numerical modeling, a controlled remagnetization experiment on a terrestrial basalt, and a paleomagnetic study of the oldest known Martian meteorite, the Northwest Africa (NWA) 7034 pairing group. NWA 7034 is a polymict regolith breccia containing zircon crystals with crystallization ages older than 4.4 billion years. As such it contains materials that are sufficiently old to have formed during the time Mars is most likely to have had a core dynamo. Unfortunately, we found that all 9 paired stones of NWA 7034 that we investigated were remagnetized by hand magnets, as has been observed for many other hot desert meteorites. We recommend that magnets not be applied to meteorites during collection and curation. Instead, a low-field susceptibility meter is a far more sensitive and completely nondestructive tool for meteorite classification.

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