科學家經由研究月球，發現地球在某個時期之後更常受到小行星撞擊
小行星的撞擊塑造了太陽系的樣貌，而月球是人類最容易取得、也最為完整的年鑑記載了這些撞擊事件的發生時間。最近一組國際科學團隊在觀察月球之後，挑戰了我們對地球部分歷史的理解。
這幅月球地圖標示出這篇研究中所有撞擊坑的位置，旁邊的小圖顯示出各隕石坑的相對大小，最上方的為哥白尼環形山。資料來自：NASA
GSFC / LRO / USGS，圖片來源： Alex
Parker)
今日發表於《科學》(Science)的研究中，研究團隊表示大約從2.9億年前開始，撞擊地球和月亮的小行星數量增加了兩到三倍左右。
主要作者Sara
Mazrouei說：「我們研究中呈現的證據顯示古生代結束之際，地球和月亮受到小行星撞擊的頻率發生了劇烈變化。」Mazrouei剛從多倫多大學文理學院的地球科學系獲得博士學位。
「這項結果指出我們從2.9億年前開始，就處在更常被小行星撞擊的時代當中，頻率是在此之前的2.6倍。」
之前科學家推測年代老於2.9億年、由小行星造成的撞擊坑，大部分都被侵蝕作用和其他地質作用給抹平了，但新的研究結果顯示並非如此。
「地球上年代介於2.9億年至6.5億年的大型撞擊坑數量較少，不是因為它們消失了，而是那段時間小行星撞擊地球的頻率低於現在。」研究共同作者之一，多倫多大學地球科學系的副教授Rebecca
Ghent表示，「如果有人想要了解地球和月亮受小行星撞擊的歷史，以及撞擊事件在地球生命的歷史發展中扮演了什麼樣的腳色，我們認為這項結果會讓他們很有興趣。」
科學家數十年來試著瞭解小行星撞擊地球的頻率變化，方法是運用放射性定年來定出撞擊坑附近的岩石年代。但科學家之前認為侵蝕作用會讓一部分的撞擊坑消失，因此要精確測出撞擊頻率並判定是否隨著時間而變化，就變得非常困難。
要規避侵蝕造成的問題，有個方法是研究月球。因為月球和地球被小行星撞擊的頻率，不論何時兩者之間的比例都是一樣的。然而以往並沒有任何方法可以測量月球撞擊坑的年代，直到10年前NASA的月球勘測軌道飛行器(Lunar
Reconnaissance Orbiter, LRO)開始繞行月球並研究月球表面，才使這個問題有了解決方法。
NASA戈達德太空飛行中心LRO計畫的科學家Noah
Petro說：「LRO搭載的儀器讓科學家可以看見過去塑造月球表面的力量。」
LRO搭載的儀器之一「探勘師」(Diviner)是一個輻射計，它可以測量月球表面散發出來的熱輻射，進而追蹤年代較近的撞擊坑的崩解速率。團隊運用「探勘師」的數據整理出一份清單，列出月球上所有年代小於10億年的撞擊坑的形成時間。
月球年代較近的撞擊坑附近，會有小行星撞擊時噴濺至月球表面的岩塊。在夜晚岩石散發出來的熱量會高於稱作風化層的細粒土壤，使科學家可以在熱影像中分辨出兩者。Ghent先前利用熱輻射給予的資訊，計算出撞擊坑附近的岩石數千萬年來崩解成土壤的速率。團隊加以應用這個想法，使他們可以計算之前未經定年的月球撞擊坑年代。
他們比較了大約同一時間範圍內，月球和地球撞擊坑的形成年代表，發現兩者對於小行星轟炸的歷史紀錄如出一轍。
「研究結果清楚指出2.9億年以前小行星撞擊的頻率較低，使得地球最為穩定的陸塊內部，年代較老的撞擊坑數量較少。」另一位研究共同作者，美國科羅拉多州博爾德市西南研究所的小行星專家William
Bottke表示，「小行星撞擊地球的頻率到底有何變化？答案其實一直都在你我眼前。」
研究人員不確定撞擊頻率突然上升的原因，但他們猜測跟3億多年前的一起事件有關。當時火星和木星軌道之間的主小行星帶發生了大型對撞事件，這類事件形成了許多碎屑進入到內太陽系。
在英國南安普敦大學的地球科學家Thomas
Gernon的合作下，Ghent及同事的這項發現得到了強而有力的證據支持。Gernon的研究對象是一種特殊的陸上構造，稱為金伯利岩管(kimberlite
pipe)。這種埋藏於地下、許久以前就已經停止噴發的火山，有著類似胡蘿蔔的管狀外型，可以延伸到地下數公里。在地球某些侵蝕最慢的地區中可以同時發現金伯利岩管和撞擊坑。
Mazrouei說：「加拿大地盾在這類侵蝕極慢的地體中，是世界上保存情況最佳，也是研究最徹底的其中之一；不僅如此，加拿大地盾上也有幾個經過詳盡研究的撞擊坑。」
Gernon證實早於6.5億年前形成的金伯利岩管受到的侵蝕極少，意謂在穩定陸塊上6.5億年之後才形成的大型撞擊坑，必定也保存得十分完整。
Ghent說：「因此我們確定這些撞擊坑幾乎就是小行星撞擊的完整紀錄。」
雖然研究作者不是第一個提出這種理論，認為過去數億年來小行星撞擊地球的頻率其實有所變動，但他們是第一個研究團隊可以拿出統計數據並把撞擊頻率定量化。
「這項發現對於地球生命的歷史來說也別具意義。在生命的歷史紀錄中，曾穿插過數次滅絕事件以及新物種的快速演化。」Ghent表示，「在生命這篇仍在書寫的史詩中，推動上述事件的力量相當複雜，可能包括了其他地質作用力，像是大型火山爆發，另外還有生物因子，但小行星撞擊必然具有十分重要的地位。」
「現在的問題是：雖然我們推估出來小行星撞擊的頻率有所變化，但跟地球許久之前發生的事件是否擁有直接關聯？」
詳述這項發現的論文發表於《科學》，題名為「Earth
and Moon impact flux increased at the end of the Paleozoic」。研究資金來自加拿大自然科學暨工程研究委員會、NASA的探索太陽系虛擬技術研究所、以及英國自然環境研究委員會。
Scientists identify period of
increased asteroid impacts on ancient Earth by studying the moon
An international team of scientists is
challenging our understanding of a part of Earth’s history by looking at the
moon, the most complete and accessible chronicle of the asteroid collisions
that carved our solar system.
In a study published today in Science, the team shows the number of asteroid impacts on the moon
and Earth increased by two to three times starting around 290 million years
ago.
“Our research provides evidence for a dramatic change
in the rate of asteroid impacts on both Earth and the moon that occurred around
the end of the Paleozoic era,” said lead author Sara Mazrouei, who recently
earned her PhD in the department of Earth sciences in the University of
Toronto’s Faculty of Arts & Science.
“The implication is that since that time we have been
in a period of relatively high rate of asteroid impacts that is 2.6 times
higher than it was prior to 290 million years ago.”
It had previously been assumed that most
asteroid-produced craters on the Earth older than 290 million years had been
erased by erosion and other geologic processes. But the new research shows
otherwise.
“The relative rarity of large craters on Earth older
than 290 million years and younger than 650 million years is not because we
lost the craters, but because the impact rate during that time was lower than
it is now,” said Rebecca Ghent, an associate professor in U of T’s department
of Earth sciences and one of the paper’s co-authors. “We expect this to be of
interest to anyone interested in the impact history of both Earth and the moon,
and the role that it might have played in the history of life on Earth.”
Scientists have for decades tried to understand the
rate that asteroids hit Earth by using radiometric dating of the rocks around
craters to determine their ages. But because it was believed erosion caused
some craters to disappear, it was difficult to find an accurate impact rate and
determine whether it had changed over time.
A way to sidestep this problem is to examine the
moon, which is hit by asteroids in the same proportions over time as Earth. But
there was no way to determine the ages of lunar craters until NASA’s Lunar
Reconnaissance Orbiter (LRO) started circling the moon a decade ago and
studying its surface.
“The LRO’s instruments have allowed scientists to
peer back in time at the forces that shaped the moon,” said Noah Petro, an LRO
project scientist based at NASA Goddard Space Flight Center.
Using LRO data, the team was able to assemble a list
of the ages of all lunar craters younger than about a billion years. They did this
by using data from LRO’s Diviner instrument, a radiometer that measures the
heat radiating from the moon’s surface, to monitor the rate of degradation of
young craters.
During the lunar night, rocks radiate much more heat
than fine-grained soil called regolith. This allows scientists to distinguish
rocks from fine particles in thermal images. Ghent had previously used this
information to calculate the rate at which large rocks around the moon’s young
craters – ejected onto the surface during asteroid impact – break down into
soil over tens of millions of years. By applying this idea, the team was able
to calculate ages for previously undated lunar craters.
When compared to a similar timeline of Earth’s
craters, they found the two bodies had recorded the same history of asteroid
bombardment.
“It became clear that the reason why Earth has fewer
older craters on its most stable regions is because the impact rate was lower
up until about 290 million years ago,” said William Bottke, an asteroid expert
at the Southwest Research Institute in Boulder, Colo., and another of the
paper’s co-authors. “The answer to Earth’s impact rate was staring everyone
right in the face.”
The reason for the jump in the impact rate is
unknown, though the researchers speculate it might be related to large
collisions taking place more than 300 million years ago in the main asteroid
belt between the orbits of Mars and Jupiter. Such events can create debris that
can reach the inner solar system.
Ghent and her colleagues found strong supporting
evidence for their findings through a collaboration with Thomas Gernon, an
Earth scientist based at the University of Southampton in England who works on
a terrestrial feature called kimberlite pipes. These underground pipes are
long-extinct volcanoes that stretch, in a carrot shape, a couple of kilometres
below the surface, and are found on some of the least eroded regions of Earth
in the same places that preserved impact craters are found.
“The Canadian Shield hosts some of the best-preserved
and best-studied of this terrain – and also some of the best-studied large
impact craters,” said Mazrouei.
Gernon showed that kimberlite pipes formed over the
past 650 million years had not experienced much erosion, indicating that large
impact craters younger than this on stable terrains must also be intact.
“This is how we know those craters represent a
near-complete record,” Ghent said.
While the researchers weren’t the first to propose
that the rate of asteroid strikes to Earth has fluctuated over the past billion
years, they are the first to show it statistically and to quantify the rate.
“The findings may also have implications for the
history of life on Earth, which is punctuated by extinction events and rapid
evolution of new species,” said Ghent. “Though the forces driving these events
are complicated and may include other geologic causes, such as large volcanic
eruptions, combined with biological factors, asteroid impacts have surely
played a role in this ongoing saga.
“The question is whether the predicted change in
asteroid impacts can be directly linked to events that occurred long ago on
Earth.”
The findings are described in the study “Earth and
Moon impact flux increased at the end of the Paleozoic,” published in Science. Support for the research was
provided by the National Science and Engineering Research Council of Canada,
NASA’s Solar System Exploration Research Virtual Institute, and the Natural
Environment Research Council of the United Kingdom.
原始論文：Sara Mazrouei,
Rebecca R. Ghent, William F. Bottke, Alex H. Parker, Thomas M. Gernon. Earth
and Moon impact flux increased at the end of the Paleozoic. Science,
2019 DOI: 10.1126/science.aar4058
引用自：University of Toronto. "Team of scientists
led by U of T identify period of increased asteroid impacts on ancient Earth by
studying the moon."
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