「６万５０００個に一つ」　太陽系誕生期の隕石を特定　仏研究

【パリＡＦＰ＝時事】太陽系の歴史のごく初期に形成された隕石（いんせき）を特定したとする研究結果が、このほど発表された。火山を起源とする隕石としては、これまで知られている中で最古のものになるという。（写真は資料写真）
　「Ｅｒｇ　Ｃｈｅｃｈ　００２」として知られるこの隕石は２０２０年５月、アルジェリアのサハラ砂漠で発見された。
　米科学アカデミー紀要（ＰＮＡＳ）に最近掲載された論文で、仏ブレスト大学のジャン・アリックス・バラ氏と研究チームは、今回の発見ならびに、隕石のいくつかの珍しい特徴について説明している。
　Ｅｒｇ　Ｃｈｅｃｈ　００２は外部が緑色、内部が茶色がかっている。一目見ただけではその特異性には気付くことができないが、実際には極めて希少なものだったのだ。
　地球上でこれまでに記録されている隕石約６万５０００個のうち、「（一度完全に溶けて）分化した物質」を含むものは約４０００個しかない。この含有物が意味するのは、地殻変動活動のある大型の母天体に隕石が由来するということだ。
　この４０００個のうちの９５％は、わずか二つの小惑星に由来するが、Ｅｒｇ　Ｃｈｅｃｈ　００２はこのどちらにも当てはまらず、残りの５％に属していた。
　「Ｅｒｇ　Ｃｈｅｃｈ　００２と同じスペクトルの特徴を持つ隕石は他に存在していない。このことは、この種の天体がほぼすべて消滅してしまったことを示している。その理由は、より大型の天体や惑星を形成する材料となったか、もしくは単に破壊されたからだ」と論文には記された。
　「このような隕石は６万５０００個に一つしかない」と、バラ氏は述べた。
　組成分析の結果、Ｅｒｇ　Ｃｈｅｃｈ　００２は約４６．５億年前に形成されたことが判明している。【翻訳編集ＡＦＰＢＢＮｅｗｓ】
〔ＡＦＰ＝時事〕（2021/03/18-16:18）

Scientists believe they have identified a meteorite formed in the first million years of our solar system, making it the oldest known meteor of volcanic origin.
The space rock, which began its journey some 4.5 billion years ago, has already proved an exceptional witness to the building blocks of the planets.
Known as Erg Chech 002, the meteorite was discovered in May 2020 by meteor hunters in the Algerian Sahara desert. It had rested undisturbed for at least 100 years, according to Jean-Alix Barrat, a geochemist at France's Brest University.
In a recent study published in the Proceedings of National Academy of Sciences journal, Barrat and his colleagues describe its discovery and several rare features.
There are 43 officially documented fragments, but probably about a hundred either still in the ground or unaccounted for, said the study. The largest ones are as big as a fist, Barrat told AFP.
With its greenish exterior and brownish interior, Erg Chech 002 might not appear extraordinary at first glance. But it is, in fact, extremely rare.
Of the roughly 65,000 meteorites so far documented on Earth, only around 4,000 contain what is known as differentiated matter. This means they came from celestial bodies large enough to have experienced tectonic activity.
Of those 4,000, 95 percent come from just two asteroids. But Erg Chech 002 is among the remaining five percent.
It's the only one out of 65,000 meteorites that is like it is, said Barrat.
Such rocks were quite common at the very beginning of the history of the solar system.
There are two possible explanations for Erg Chech 002's rarity.
The type of protoplanet from which it originated provided raw material for the growth of terrestrial planets such as Earth, said Barrat.
Others were pulverised in the great cosmic billiard game of the formation of the solar system.
The surface of the Moon, pockmarked with innumerable asteroid impacts, is a relatively recent witness to this second type of protoplanet.
No asteroid shares the spectral features of EC 002, indicating that almost all of these bodies have disappeared, either because they went on to form the building blocks of larger bodies or planets or were simply destroyed, the study said.
- 'Thrown into space' -
The so-called parent body of Erg Chech 002 could have measured around 100 kilometres across.
It was formed in the first million years of the solar system, according to the study's co-authors, March Chaussidon, from the Paris Globe Institute of Physics and Johan Villeneuve, a researcher from France's National Centre for Scientific Research at the University of Lorraine.
Metallic meteorites correspond to the nuclei of protoplanets, said Barrat.
But Erg Chech 002 is volcanic in origin, meaning that it was part of the crust of a protoplanet, rather than its core.
The experts believe that its unique composition was the result of a string of fortunate events.
On the protoplanet in question, lava must have accumulated on the surface, fuelled by the heat of its aluminium core.
The crust containing the meteorite solidified briefly but -- because it showed evidence of a sudden cooling -- instead of remaining on the parent body, some violent force cast it asunder.
The rock was thrown into space, said Barrat.
Further investigation into its composition found that Erg Chech 002 was formed around 4.65 billion years ago.
It travelled through the aeons, in a gravel shell, protected from solar radiation, said Barrat.
Then, around 26 million years ago, the rock was dislodged, continuing its journey until colliding with Earth.