隕石ハンターが科学的な金鉱を誤って破壊する方法

新しい研究は、単純な手の磁石が隕石の磁気記憶を消去することを示しています.

新しい[{” attribute=””>MIT study reveals that hand magnets, commonly used by meteorite hunters and collectors to verify meteorites’ identities, often erase the rocks’ magnetic memory, thereby destroying valuable scientific data. The researchers found that hand magnets reorient the microscopic grains within the rocks, eliminating traces of their magnetic origins. The team has analyzed multiple samples of a meteorite known as Northwest Africa (NWA) 7034 or “Black Beauty,” only to find that all samples have been remagnetized since landing on Earth. To avoid further destruction of magnetic records, the researchers recommend using susceptibility meters instead of hand magnets, despite the higher cost.

Each year, thousands of space rocks pierce through the Earth’s atmosphere and hit the ground as meteorites. These fragments of comets and asteroids can land anywhere but are most often spotted in open terrain, such as the deserts of Africa and the Antarctic blue ice, where a meteorite’s blackened exterior can stand out.

Still, these extraterrestrial remnants can resemble Earth rocks, and to tell the difference meteorite hunters often expose their “finds” to hand magnets, which can attract more strongly to metal-rich meteorites than to terrestrial rocks. Meteorite hunters, dealers, collectors, and curators often rely on hand magnets to verify a meteorite’s identity.

But a new MIT study finds that the same magnets used to identify a meteorite usually erase its magnetic memory. They show that exposure to a magnet can reorient a rock’s microscopic grains, undoing their original orientation and any trace of its magnetic origins.

The researchers make their case with Northwest Africa (NWA) 7034, a meteorite known in collectors’ circles as “Black Beauty” for its obsidian exterior. Multiple shards of the meteorite were first discovered in the deserts of northwest Africa, and scientists determined that the rock contained crystals that formed on Mars more than 4.4 billion years ago.

Black Beauty is thought to have formed at a time when the Red Planet harbored a magnetic field, much like the Earth does today. If the rock bears any trace of Mars’ ancient field, this could give scientists valuable clues to the planet’s past climate and composition.

Unfortunately, the MIT team found that multiple samples of Black Beauty have been remagnetized since landing on Earth, and that any hint of an ancient Martian field has been wiped clean.

“There was an incredible record there, and a unique opportunity to understand the early history of Mars’ magnetism,” says study author Benjamin Weiss, professor of planetary sciences at MIT. “But we found it’s all been obliterated by magnets.”

With their new study, published on April 6 in the Journal of Geophysical Research: Planets, the researchers hope to raise awareness in the planetary science community about the destructive effects of hand magnets. Weiss’ co-authors are MIT postdoc Foteini Vervelidou and France Lagroix of the Paris Institute of Planetary Physics.

Dead ends

Tens of thousands of meteorites have been discovered to date. Nearly every found meteorite has been traced to about 100 parent bodies across the solar system, including asteroids, the moon, and Mars. Scientists attempting to read the history of these rocks have only recently come to realize that some interpretations were way off the mark, due to the influence of hand magnets.

For instance, samples of Allende, the largest and most studied meteorite on Earth, bear traces of exposure to a strong magnetic field. Scientists assumed this field was evidence that the meteorite formed long ago in a solar nebula that hosted an extremely high magnetic field. Only later did they realize that hand magnets were to blame for the meteorite’s curiously strong pull.

Weiss has also been duped by artificially reset rocks. When he first joined the MIT faculty, he discovered signs of strong magnetism in fallen samples of an asteroid. The findings would have been the first evidence that asteroids can differentiate and form metallic cores like the Earth. But he later discovered, much to his frustration, that the meteorite had been reset by hand magnets.

“There’s a long history of dead ends and confusion over remagnetized rocks,” Weiss says.

For the MIT team, the tipping point came with NWA 7034. In 2014, fellow paleomagnetist Jérôme Gattacceca measured a sample of Black Beauty and found its original magnetism, which was set more than 4.4 billion years ago, had been entirely undone by much stronger hand magnets on Earth. Weiss and Vervelidou recently analyzed numerous other samples of Black Beauty, hoping to find at least one magnetically preserved sample.

“Our initial hope was that by testing as many [samples] この隕石を可能な限り分析すると、再磁化されていない隕石がいくつか見つかることになります」と Vervelidou 氏は言います。 「私たちが調査したすべてのサンプルが再磁化されたと結論付けた後、その動機はハンドマグネットの破壊的な影響についての言葉を広めることでした.」

フィールドのシフト

彼らの新しい研究では、チームは手の磁石が岩石の自然な磁気に影響を与える方法を説明しました. 彼らは最初に、磁気の物理学に基づいて数値モデルを開発し、典型的な手磁石を取り巻く磁場と、それがさまざまなサイズの岩石にどのように影響するかを計算しました。

次に、同じ地球の岩石のサンプルをさまざまな強度のさまざまな距離の磁場にさらして実験を行い、各サンプルの固有の磁気が応答でどのように変化するかを測定しました。 これらの測定値はモデルの予測と一致し、モデルを使用して岩石が再磁化されたかどうかを判断できることが示されました。 このモデルは、岩石の磁化に基づいて、岩石がまだ影響を受けていない深さを推定するためにも使用できます。

最後に、チームは9つのBlack Beautyサンプルの測定値を報告し、発見された隕石のすべての破片が実際にハンドマグネットにさらされたことをモデルで確認しました.

「この論文で得られたのは、最終的に、岩が磁石にぶつかったかどうかを確認するための明確で明確な作業計画です」とワイスは言います。

研究者たちは、隕石ハンター、収集家、博物館の学芸員に、磁石の代わりに感受率計を使用することを推奨しています。これは、磁気記憶を混乱させることなく、隕石を迅速かつ正確に識別することが示されているハンドヘルド機器です。

Weiss 氏は、磁化率計は売れ筋ではないことを認めています。商用モデルは数千ドルの価値がありますが、ほとんどの費用がかからない手磁石もあります。 隕石取引の中で、彼はまず博物館の学芸員や収集家など上流の人々を説得したいと考えています。 そこから、地上で発見をしている人々に言葉が流れ込むかもしれません。

「過去 20 年ほどの間に、隕石の多様性と数は信じられないほど爆発的に増加しました。私たちは隕石ハンターに、これらのものを見つけてくれたことに感謝しなければなりません」と Weiss 氏は言います。 「しかし、悪魔の取引であるトレードオフは、彼らがそれらを見つけるためにしばしば磁石を使用しており、その過程で磁気記録を即座に破壊していることです。」

参照: 「ハンド マグネットと古代隕石磁気の破壊」、Foteini Vervelidou、Benjamin P. Weiss、France Lagroix 著、2023 年 4 月 6 日、 Journal of Geophysical Research: 惑星.
DOI: 10.1029/2022JE007464

この研究は、一部、欧州連合の Horizo​​n 2020 研究およびイノベーション プログラムによって資金提供されました。