恐竜絶滅させた天体、木星軌道外側由来の小惑星

【ワシントンＡＦＰ＝時事】地球に衝突し、恐竜を絶滅させた天体として、科学界で長年論争を引き起こしてきた「チチュルブ衝突体」の起源について、驚くべき重要データを示す新研究が１５日、米科学誌「サイエンス」に発表された。（写真は、デンマークのスティーヴンス・クリントにある６６００万年前の白亜紀－古第三紀〈Ｋ－Ｐｇ〉境界層。ユーレカ・アラート提供）
　この研究チームは革新的な技術を用い、約６６００万年前、現在のメキシコ・ユカタン半島沖に衝突し、地球史上最も直近の大量絶滅を引き起こした破滅的な天体が、木星軌道の外側に由来することを示した。
　チームはまた、チチュルブ衝突体が彗星（すいせい）だったという見解を否定している。
　論文の主著者で独ケルン大学の地球化学者マリオ・フィッシャーゴッデ氏はＡＦＰに対し、「今やこれらの知識に基づき、この小惑星が形成されたのは木星（軌道）の外側だということができる」と語った。
　この種の小惑星が地球に衝突することは非常にまれなため、新研究の結論は特に注目される。
　フィッシャーゴッデ氏はこの結論について、将来の天体衝突の脅威予測や、地球の水の由来解明に役立つかもしれないと述べた。
　■サンプル
　新しい発見は、中生代白亜紀と新生代古第三紀の間の時期に形成された堆積物サンプルの分析に基づいている。この時期は、チチュルブ衝突の影響で生物が大量絶滅したＫ－Ｐｇ境界に相当する。
　研究チームは、ルテニウムという元素の同位体を測定した。ルテニウムは小惑星では珍しくないが、地球では非常にまれな元素だ。
　チチュルブ衝突によるがれきが堆積したいくつもの地層を調査したところ、検出されたルテニウムは「１００％、この小惑星に由来する」ことが確認できた。
　小惑星の主要なグループには、太陽系の外側で形成され、炭素を多く含む「Ｃ型小惑星」と、太陽系の内側で形成され、ケイ酸塩を多く含む「Ｓ型小惑星」があるが、ルテニウム同位体はこの二つを区別するために使用することができる。
　チチュルブ衝突は巨大地震を引き起こし、地球規模の冬をもたらし、恐竜や他の多くの生命体を絶滅させたが、今回の研究はその原因となった天体が、木星の外側で形成されたＣ型小惑星だったことを確認する結果となった。
　２０年前の研究でもそうした仮説は立てられていたが、当時は現在よりもはるかに確実性が低かった。
　フィッシャーゴッデ氏によると、地球に落下する小惑星の破片たる隕石（いんせき）のほとんどはＳ型小惑星ものであるため、この結論は非常に驚くべきものだと述べた。【翻訳編集ＡＦＰＢＢＮｅｗｓ】
〔ＡＦＰ＝時事〕（2024/08/19-16:54）

An intense debate surrounding the cosmic rock that killed the dinosaurs has stirred scientists for decades, but a new study has revealed some important -- and far-out -- data about the impactor's origin story.
Researchers, whose findings were published Thursday in the journal Science, used an innovative technique to demonstrate that the apocalyptic culprit which slammed into the Earth's surface 66 million years ago, causing the most recent mass extinction, had formed beyond Jupiter's orbit.
They also refute the idea that it was a comet.
The new insights into the apparent asteroid that cratered into Chicxulub, in what is present-day Mexico's Yucatan Peninsula, could improve the understanding of celestial objects that have struck our planet.
Now we can, with all this knowledge... say that this asteroid initially formed beyond Jupiter, Mario Fischer-Godde, lead author of the study and a geochemist at the University of Cologne, told AFP.
The conclusions are particularly notable, given how rarely this type of asteroid collides with Earth.
Such information may well prove useful in assessing future threats, or determining how water arrived on this planet, Fischer-Godde said.
- Samples -
The new findings are based on analysis of sediment samples formed at the period between the Cretaceous and Paleogene eras, the time of the asteroid's cataclysmic impact.
Researchers measured the isotopes of the element ruthenium, not uncommon on asteroids but extremely rare on Earth. So by inspecting the deposits in multiple geological layers that mark the debris from the impact at Chicxulub, they could be sure that the ruthenium studied came 100 percent from this asteroid.
Our lab in Cologne is one of the rare labs that can do these measurements, and it was the first time such study techniques were used on impact debris layers, Fischer-Godde said.
Ruthenium isotopes can be used to distinguish between the two main groups of asteroids: C-type, or carbonaceous, asteroids that formed in the outer solar system, and S-type silicate asteroids from the inner solar system, nearer the sun.
The study affirms that the asteroid that triggered a mega-earthquake, precipitated a global winter and wiped out the dinosaurs and most other life, was a C-type asteroid that formed beyond Jupiter.
Studies from two decades ago had already made such an assumption, but with far less certainty.
The conclusions are striking, because most meteorites -- pieces of asteroids that fall to Earth -- are S-types, Fischer-Godde pointed out.
Does that mean the Chicxulub impactor formed beyond Jupiter and made a beeline for our planet? Not necessarily.
We cannot be really sure where the asteroid was kind of hiding just before it impacted on Earth, Fischer-Godde said, adding that after its formation, it may have made a stopover in the asteroid belt, located between Mars and Jupiter and where most meteorites originate.
- Not a comet -
The study also dismisses the idea that the destructive impactor was a comet, an amalgam of icy rock from the very edge of the solar system. Such a hypothesis was put forward in a much-publicized study in 2021, based on statistical simulations.
Sample analyses now show that the celestial object was far different in composition from a subset of meteorites which are believed to have been comets in the past. It is therefore unlikely the impactor in question was a comet, Fischer-Godde said.
As to the wider usefulness of his findings, the geochemist offered two suggestions.
He believes that more accurately defining the nature of asteroids that have struck Earth since its beginnings some 4.5 billion years ago could help solve the enigma of the origin of our planet's water.
Scientists believe water may have been brought to Earth by asteroids, likely of the C-type like the one that struck 66 million years ago, even though they are less frequent.
Studying past asteroids also allows humanity to prepare for the future, Fischer-Godde said.
If we find that earlier mass extinction events could also be related to C-type asteroid impacts, then... if there's ever going to be C-type asteroid on an Earth-crossing orbit, we have to be very careful, he said, because it might be the last one we witness.