地球早期空氣的重量不到現在大氣的一半

原始網址：www.sciencedaily.com/releases/2016/05/160509115114.htm

Early Earth's air weighed less than half of today's atmosphere

地球早期空氣的重量不到現在大氣的一半

The idea that the young Earth had a thicker atmosphere turns out to be wrong. New research from the University of Washington uses bubbles trapped in 2.7 billion-year-old rocks to show that air at that time exerted at most half the pressure of today's atmosphere.

地球幼年時擁有厚重大氣層的想法現在看來似乎是錯的。華盛頓大學進行的新研究利用了27億年前困在岩石中的氣泡，顯示當時空氣造成的壓力頂多只有現今大氣的一半而已。

The results, published online May 9 in Nature Geoscience, reverse the commonly accepted idea that the early Earth had a thicker atmosphere to compensate for weaker sunlight. The finding also has implications for which gases were in that atmosphere, and how biology and climate worked on the early planet.

以往被廣為接受的想法中認為早期地球擁有厚重的大氣層並遮蔽了陽光，但刊登在5月9日《自然—地質科學》(Nature Geoscience)線上版的研究結果卻顛覆了這個說法。這項發現對於了解地球早期的空氣成分、氣候運作方式和生物的行為，也都具有相當的啟發。

"For the longest time, people have been thinking the atmospheric pressure might have been higher back then, because the sun was fainter," said lead author Sanjoy Som, who did the work as part of his UW doctorate in Earth and space sciences. "Our result is the opposite of what we were expecting."

「一直以來人們都因為當時的陽光較現今微弱，而認為那時的大氣壓力會比較高。」第一作者Sanjoy Som說。這是他於華盛頓大學攻讀地球與太空科學博士學位時進行的研究中的一部分。「然而我們的成果卻跟我們預設的恰好相反。」

The idea of using bubbles trapped in cooling lava as a "paleobarometer" to determine the weight of air in our planet's youth occurred decades ago to co-author Roger Buick, a UW professor of Earth and space sciences. Others had used the technique to measure the elevation of lavas a few million years old. To flip the idea and measure air pressure farther back in time, researchers needed a site where truly ancient lava had undisputedly formed at sea level.

華盛頓大學地球與太空科學教授Roger Buick，同時也是本文的共同作者，在幾十年前就已經想到說可以把岩漿冷卻後困在其中的氣泡作為「古氣壓計」，來測量地球年輕時大氣的重量。其他的研究通常用這項技術測定數百萬年前岩漿噴發時位於的海拔高度。但本篇研究的人員則反向利用這個概念來得知過去的大氣壓力，為了執行這個想法，他們需要一處年代十分久遠的熔岩，其生成位置必須要位於海平面的高度。

Their field site in Western Australia was discovered by co-author Tim Blake of the University of Western Australia. There, the Beasley River has exposed 2.7 billion-year-old basalt lava. The lowest lava flow has "lava toes" that burrow into glassy shards, proving that molten lava plunged into seawater. The team drilled into the overlying lava flows to examine the size of the bubbles.

他們的野外工作地點是由共同作者，西澳大學的Tim Blake 發現。稱作Beasley River的這個地方有27億年前的玄武岩熔岩出露。他們發現熔岩流在地勢最低處的「熔岩趾(lava toe)」具有玻璃質孔洞，證實了這片熔岩流曾經流進海水當中。研究團隊接著在上方的熔岩流上鑽孔，以檢視氣泡的大小。

A stream of molten rock quickly cools from top and bottom, and bubbles trapped at the bottom are smaller than those at the top. The size difference records the air pressure pushing down on the lava as it cooled, 2.7 billion years ago.

熔岩流動時會從上而下快速地冷卻，造成困於底部的氣泡會比頂端的要小。它們的尺寸差距即成為了27億年前，熔岩冷卻當下大氣對它施加壓力大小的紀錄。

Rough measurements in the field suggested a surprisingly lightweight atmosphere. More rigorous x-ray scans from several lava flows confirmed the result: The bubbles indicate that the atmospheric pressure at that time was less than half of today's.

在野外粗略的估計就顯示出當時大氣重量輕的令人驚訝。在以更加精確的X光掃描了數個熔岩流樣品後確立了以下結論：氣泡指出當時的大氣壓力比現在的一半還要低。

Earth 2.7 billion years ago was home only to single-celled microbes, sunlight was about one-fifth weaker, and the atmosphere contained no oxygen. But this finding points to conditions being even more otherworldly than previously thought. A lighter atmosphere could affect wind strength and other climate patterns, and would even alter the boiling point of liquids.

27億年前的地球僅有一些單細胞生物生存於此，而陽光比現在弱了將近8成，大氣也完全沒有氧氣。但這個發現指出當時的地球可能比之前認為的更像外星世界。較稀薄的大氣會影響風的強度以及其他氣候模式，甚至還會改變液體的沸點。

"We're still coming to grips with the magnitude of this," Buick said. "It's going to take us a while to digest all the possible consequences." Other geological evidence clearly shows liquid water on Earth at that time, so the early atmosphere must have contained more heat-trapping greenhouse gases, like methane and carbon dioxide, and less nitrogen.

「我們仍然在釐清這會帶來多大的影響。」Buick說。「要整理出所有可能發生的效應得花上我們一段時間。」其他的地質證據顯示當時地表已經擁有液態水，因此早期大氣勢必要含有更多可以關住熱量的溫室氣體，像是甲烷、二氧化碳以及少量的氮氣。

The new study is an advance on the UW team's previous work on "fossilized raindrops" that first cast doubt on the idea of a far thicker ancient atmosphere. The result also reinforces Buick's 2015 finding that microbes were pulling nitrogen out of Earth's atmosphere some 3 billion years ago.

華盛頓大學團隊繼先前以「雨滴化石」的研究而首度對遠古大氣相當濃厚的說法拋出質疑後，這篇研究更進一步地鞏固了他們的論述。結論也支持了Buick於2015年發現將近30億年前的微生物，會從地球大氣中吸收氮氣的研究。

"The levels of nitrogen gas have varied through Earth's history, at least in Earth's early history, in ways that people just haven't even thought of before," said co-author David Catling, a UW professor of Earth and space sciences. "People will need to rewrite the textbooks."

「直到不久前才有人去仔細思考在整個地球歷史，或者至少是在地球歷史的早期，氮氣在大氣中的濃度是如何變化的。」共同作者，華盛頓大學的地球與太空科學家David Catling說。「人們需要為此而重新編撰教科書。」

The researchers will next look for other suitable rocks to confirm the findings and learn how atmospheric pressure might have varied through time.

研究人員接下來的目標是放在尋找其他符合條件的岩石，以驗證他們的發現並且試著去了解大氣壓力隨著時間可能是如何變化的。

While clues to the early Earth are scarce, it is still easier to study than planets outside our solar system, so this will help understand possible conditions and life on other planets where atmospheres might be thin and oxygen-free, like that of the early Earth.

雖然關於早期地球樣貌的是如此稀少，但是跟研究太陽系之外的行星一比還是簡單許多，因此這或許有助於我們了解跟早期地球相似，大氣層稀薄且不含氧氣的其他行星上的環境條件以及生命形式可能的樣貌。

Som is CEO of Seattle-based Blue Marble Space, a nonprofit that focuses on interdisciplinary space science research, international awareness, science education and public outreach. He currently does astrobiology research at NASA's Ames Research Center in California.

Som是Blue Marble Space組織的執行長，此非營利組織位於西雅圖，主要業務著重於跨領域太空科學研究、國際關懷事務、科學教育及推廣ㄧ。他目前於加州NASA艾姆斯研究中心從事有關天體生物學的研究。

引用自：University of Washington. "Early Earth's air weighed less than half of today's atmosphere." ScienceDaily. ScienceDaily, 9 May 2016.