Western Australia crater formed by the oldest asteroid impact -"Frozen Earth" melt?

Last month CNN.co.jp reported the following:

豪西部のクレーター、最古の小惑星衝突で形成　「凍った地球」解かした？

2020.01.24 Fri posted at 09:30 JST

（ＣＮＮ） オーストラリア西部の奥地にある巨大クレーターを出現させた小惑星の衝突について、確認できる中で地球の歴史上最古のものだとする研究結果が、２１日刊行の科学誌に掲載された。衝突の年代は２０億年以上前で、地球上の他の事例を２億年さかのぼる。当時の地球は全体が氷で覆われていたとされるが、この衝突の影響で凍結状態に変化が生じた可能性もあるという。

地球の表面では侵食や地殻プレートの活動などが起こるため、最も古いクレーターを特定するのは難しい。直径およそ７０キロに及ぶ上記の「ヤラババ・クレーター」の年代を測定するため、研究者らはクレーターに含まれる鉱物を対象に同位体分析を行った。その結果、ジルコンやモナザイトといった物質について「衝撃に伴う」再結晶化を確認。小惑星の衝突によって構造的変化が起きたと結論付けた。

研究者らはこれらの鉱物の年代に基づき、ヤラババ・クレーターの形成時期を推計。従来よりも正確な２２億２９００万年前という数字を割り出した。

豪パースにあるカーティン大学の教授で今回の論文の共著者を務めたクリス・カークランド氏は、ヤラババ・クレーターについて、「全球凍結」として知られる時期が終わろうとするまさにそのタイミングで形成されたと指摘。当時は大気と海洋の酸素濃度が増加し、大陸に堆積（たいせき）した岩石の多くが氷河の状態にあったという。

もう１人の論文共著者、ニコラス・ティムス氏は「衝突の後、氷河の堆積は岩石の記録から４億年の間姿を消す」「こうした展開は、巨大な小惑星の衝突が地球全体の気候に影響を及ぼした可能性を示唆する」と述べた。

論文中に示された計算によれば、小惑星が凍結した大地に激突したことで５０００億トンの水蒸気が大気中に放出されたとみられる。水蒸気は効率的な温室効果ガスで、地球から放射されるエネルギーを吸収し、それを再び地表へ送る働きにより温暖化に寄与する。ただこの時の衝突が地球規模での気候に影響を与えたのかどうかについては、さらなる調査が必要だと研究者らはみている。

恐竜の絶滅につながった６６００万年前の小惑星衝突の際には地球全体で海水温度が低下し、広範囲で酸性雨が降り注いだことが知られている。

Translation

2020.01.24 Fri posted at 09:30 JST

 (CNN) A study on the collision of an asteroid that caused a giant crater in the interior of western Australia was confirmed to be the oldest in Earth's history, the study was published in a scientific journal published on Wednesday. The age of this collision was more than two billion years ago while other cases on earth dated up to only 200 million years. It was said that there was a possibility that earth's freezing state was changed due to the impact of this collision.

Because of erosion and crustal plate activities on the Earth's surface, to identify the oldest crater was difficult. In order to date the above-mentioned "Yarrabubba Crater" which was about 70 kilometers in diameter, researchers conducted an isotope analysis on minerals contained in the crater. As a result, it was confirmed that there were substances recrystallized "with impact" such as zircon and monazite. It was concluded that the asteroid impact had caused structural changes.

Based on the age of these minerals, researchers estimated the formation time the Yarrabubba Crater. The number worked out was 2,229 million years ago, which was more accurate than ever before.

Chris Kirkland, a professor at Curtin University in Perth of Australia, as a co-author of the paper noted that the Yarrabubba crater was formed at the very end of what was known as the "Global freeze." At that time, the oxygen concentration in the atmosphere and ocean had increased, and many rocks that had been deposited on the continent were in the state of glacier.

 "After a collision, rock records of glacier deposits disappear for 400 million years", said Nicholas Timus, the other co-author of the paper, and that “these developments suggest that the giant asteroid impact may have affected the global climate.”

Calculations shown in the paper suggested that 500 billion tons of water vapor was released into the atmosphere as the asteroid hit the frozen ground. Water vapor was an efficient greenhouse gas that contributed to global warming by absorbing the energy radiated from the earth and sending it back to the earth's surface. However, researchers believed that further investigations would be needed to determine whether the collision had affected the climate in a global scale.

It was known that during the collision of an asteroid 66 million years ago, which led to the extinction of dinosaurs, the temperature of the seawater decreased globally and acid rain fell over a wide area.

          So, it is an interesting theory about earth's development more than two billion years ago.