恐竜絶滅、小惑星の衝突角度は「最も致命的な」６０度

【パリＡＦＰ＝時事】約6600万年前、仏パリの約2倍の直径の小惑星が地球に衝突し、当時生息していた恐竜と、地球上の生命体の75％が全滅した。これはよく知られたことだが、その衝突は直撃だったのか、それともかすめた程度だったのか、どちらがより破壊的であり得たのかは謎のままだった。（写真は資料写真）
　だが、２６日の英科学誌ネイチャー・コミュニケーションズに掲載された研究によると、小惑星は「最も致命的な」角度である６０度で地球に衝突した。
　その衝撃は、地球の気候を根本的に変化させるほど大量のがれきやガスを高層大気中に巻き上げ、ティラノサウルス・レックス（Ｔ・レックス）からその餌まであらゆるものを絶滅させた。
　英インペリアル・カレッジ・ロンドン、独フライブルク大学、米テキサス大学オースティン校の科学者らによる研究チームは、この小惑星の衝突でできたメキシコ南部にある直径２００キロのクレーターの構造を分析した。
　研究チームは、可能性があると考えられる４パターンの衝突角度（９０度、６０度、４５度、３０度）と、２パターンの衝突速度（毎秒１２キロと同２０キロ）を用い、一連のシミュレーションを行った。その結果、クレーターから得られたデータと最も合致したのは６０度の衝突角度だった。
　論文の主著者でインペリアル・カレッジ・ロンドンのギャレス・コリンズ氏はＡＦＰに対し、「地球を覆いつくすほどの大量の物質をまたたく間に放出し、他よりも致命的な影響を与える衝突角度は、６０度だ」と述べた。
　「チチュルブの小惑星衝突が、大量のちりやガスをクレーターから巻き上げて地球全体に拡散させ、大量絶滅を引き起こした」とコリンズ氏は説明した。
　一方、小惑星が直角や６０度以外の角度で衝突していたならば、これほど大量のがれきが大気中に放出されることはなかっただろうとコリンズ氏は補足した。放出された大量の硫黄が微粒子となって空気中に浮遊し続け、太陽光を遮断したため、気温が大きく低下したと考えられている。【翻訳編集ＡＦＰＢＢＮｅｗｓ】
〔ＡＦＰ＝時事〕（2020/05/27-14:48）

This much we knew: some 66 million years ago an asteroid roughly twice the diameter of Paris crashed into Earth, wiping out all land-dwelling dinosaurs and 75 percent of life on the planet.
What remained a mystery was whether it was a direct hit or more of a glancing blow, and which would be more destructive.
As it turns out, according to a study published Tuesday in Nature Communications, the giant space rock struck at the deadliest possible angle -- 60 degrees.
The cataclysmic impact kicked up enough debris and gases into the upper atmosphere to radically change the climate, dooming T-Rex and everything it ever hunted to extinction.
Analysing the structure of the 200-kilometre-wide crater in southern Mexico where the asteroid hit, scientists ran a series of simulations.
Lead author Gareth Collins of Imperial College London and colleagues at the University of Freiburg and the University of Texas at Austin looked at four possible impact angles -- 90, 60, 45 and 30 degrees -- and two impact speeds, 12 and 20 kilometres per second.
The best fit with the data from the crater was a 60 degree strike.
Sixty degrees is a more lethal impact angle because it ejects a larger amount of material fast enough to engulf the planet, Collins told AFP.
The Chicxulub impact triggered a mass extinction because it ejected huge quantities of dust and gas out of the crater fast enough to disperse around the globe.
Had the asteroid hit head on or at a more oblique angle, not as much debris would have been thrown up into the atmosphere, he added.
Large amounts of sulphur in the form of tiny particles that remained suspended in the air blocked the Sun, cooling the climate by several degrees Celsius.
- Rocks 'rebound' -
Smoke, ash and debris engulfed the atmosphere, eventually destroying most plants and wiping out 75 percent of species on Earth.
Chicxulub is also thought to have triggered an earthquake whose seismic waves reached Tanis -? the fossil site 3,000 km away in North Dakota where definitive evidence of the asteroid's devastating impact was uncovered -? in just 13 minutes.
The seismic shock triggered a torrent of water and debris from an arm of an inland sea known as the Western Interior Seaway.
Thus far, scientists have only been able to study the early stages of the impact.
The researchers combed through geological data gathered during a recent dig to better understand how the cataclysm unfolded.
They soon realised that the asteroid did not, as long assumed, approach Earth from the southeast.
Our work overturns this hypothesis, Collins explained. The crater's central uplift is leaning slightly to the southwest, and numerical simulations of the impact reproduce this.
The findings could lead to a greater understanding about how craters are formed in general.
The 3-D simulations, for example, suggest that rocks rebound to fill in some of the impact layer during the final stage of crater formation, a process that takes only minutes, the researchers conjectured.
Scientists are still trying to figure out exactly how the asteroid triggered a mass extinction event and why some species survived while others didn't.
The Chicxulub impact was a very bad day for the dinosaurs, Collins said, adding that the new research showed it was even worse than had been previously thought.
It makes it even more remarkable that life survived and recovered as rapidly as it did.