Blue Planet: Study Proposes New Origin Theory for Earth's Water
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August 27, 2020 08:35 PM (source)
Blue Planet: Study Proposes New Origin Theory for Earth's Water
藍色星球：研究提出地球之水的新起源理論
August 27, 2020 08:35 PM
By AFP
Water covers 70% of the Earth's surface and is crucial to life as we know it, but how it got here has been a longstanding scientific debate.
The puzzle was a step closer to being solved Thursday after a French team reported in the journal Science they had identified which space rocks were responsible, and suggested our planet has been wet ever since it formed.
Cosmochemist Laurette Piani, who led the research, told AFP the findings contradicted the prevalent theory that water was brought to an initially dry Earth by far-reaching comets or asteroids.
According to early models for how the Solar System came to be, the large disks of gas and dust that swirled around the Sun and eventually formed the inner planets were too hot to sustain ice.
This would explain the barren conditions on Mercury, Venus and Mars — but not our blue planet, with its vast oceans, humid atmosphere and well-hydrated geology.
Scientists therefore theorized that the water came along after, and the prime suspects were meteorites known as carbonaceous chondrites that are rich in hydrous minerals.
But the problem was that their chemical composition doesn't closely match our planet's rocks.
The carbonaceous chondrites also formed in the outer Solar System, making it less likely they could have pelted the early Earth.
Planetary building blocks
Another group of meteorites, called enstatite chondrites, are a much closer chemical match, containing similar isotopes (types) of oxygen, titanium and calcium.
This indicates they were Earth's and the other inner planets' building blocks.
However, because these rocks formed close to the Sun, they had been assumed to be too dry to account for Earth's rich reservoirs of water.
To test whether this was really true, Piani and her colleagues at Centre de Recherches Petrographiques et Geochimiques (CRPG, CNRS/Universite de Lorraine) used a technique called mass spectrometry to measure the hydrogen content in 13 enstatite chondrites.
The rocks are now quite rare, making up only about two percent of known meteorites in collections, and it is hard to find them in pristine, uncontaminated condition.
The team found that the rocks contained enough hydrogen in them to provide Earth with at least three times the water mass of its oceans — and possibly much more.
They also measured two isotopes of hydrogen, because the relative proportion of these is very different from one celestial object to another.
"We found the hydrogen isotopic composition of enstatite chondrites to be similar to the one of the water stored in the terrestrial mantle," said Piani, comparing it to a DNA match.
The isotopic composition of the oceans was found to be consistent with a mixture containing 95% of water from the enstatite chondrites — more proof these were responsible for the bulk of Earth's water.
The authors further found that the nitrogen isotopes from the enstatite chondrites are similar to Earth's — and proposed these rocks could also be the source of the most abundant component of our atmosphere.
Piani added that research doesn't exclude later addition of water by other sources like comets, but indicates that enstatite chondrites contributed significantly to Earth's water budget at the time it formed.
The work "brings a crucial and elegant element to this puzzle" wrote Anne Peslier, a planetary scientist for NASA, in an accompanying editorial.
如我們所知，水覆蓋了地球表面的 70%，也是生命的關鍵成因，但水是如何來到地球，則是一個長期受到爭辯的科學議題。
這謎題在週四（編按：2020 年 8 月 27 日） 因一個法國（France）團隊在《科學》（Science）期刊的報導而有了進展。該團隊表示，他們已經找到構成地球之水的太空岩石，並提議我們的星球一直以來都是濕潤的。
領導這項研究的宇宙化學家勞雷特．皮亞尼（Laurette Piani）告訴法新社 ( Agence France-Presse )，這項發現與普遍認為水是由遙遠的彗星或小行星(asteroid， 註 1)帶入原先乾燥的地球之盛行(prevalent)理論有所抵觸。
根據早期關於太陽系如何形成的模型，大量圍繞太陽旋轉並最終形成內行星的氣體和塵埃盤因為溫度太高而無法結冰。
這可以解釋水星（Mercury）、金星（Venus）和火星（Mars）的荒蕪(barren， 註 2)狀況，但不能解釋我們身處的藍色星球為何擁有廣闊的海洋、潮濕的大氣層和充足水分。
因此，科學家認為水是後來形成的，而被懷疑的主要對象是一種名為碳質球粒隕石(carbonaceous chondrites)的隕石，其具有豐富的含水(hydrous， 註 3)礦物。
不過，它們的化學成分與我們星球的岩石並不相配。
碳質球粒隕石也在外太陽系中形成，因此它們不太可能撞擊早期的地球。
行星基石
另一組名為頑輝隕石(enstatite chondrites)的隕石，從化學角度上看更為符合，因它包含相似的氧、鈦和鈣同位素(isotopes)（類型）。
這表示它們是地球和其他內行星的構建基塊。
但是，由於這些岩石是在靠近太陽的位置形成的，因此它們被認為過於乾燥而無法解釋地球上豐沛的蓄水量。
為了檢驗這是否真的成立，皮亞尼和她在法國岩象暨地球化學研究中心（CRPG，CNRS /洛林大學）的同事使用了一種稱為質譜法 (mass spectrometry)的技術來測量 13 種頑輝隕石中的氫含量。
這些岩石現在非常稀少，只佔集合中已知隕石的 2%，而且很難找到未受污染的原始(pristine， 註 4)岩石。
研究小組發現這些岩石中含有足夠的氫，足以提供地球至少 3 倍海水量的能量，甚至可能更多。
他們還測量了氫的兩種同位素，因為它們的相對比例從一個天(celestial)體到另外一個非常不同。
「我們發現頑輝岩球粒隕石的氫同位素與儲存於地幔(mantle)中的水在組成上十分相似。」皮亞尼將其與去氧核糖核酸配對(DNA match)做比較後說道。
地球海洋的同位素結構與一種 95% 的水來自頑輝隕石的混合物相符。這進一步顯示地球大部分的水來自它。
這項研究的作者們進一步發現，頑輝隕石中的氮同位素與地球的相似，並提出我們大氣中最豐富的成分可能源自於此。
皮亞尼補充說，此研究並沒有排除彗星隕石等其他來源帶來的水的可能性，但顯示出頑輝隕石對地球形成初期的水量有著重大貢獻。
該作品「為這一難題帶來了重要且巧妙的元素。」 美國國家航空暨太空總署（NASA）行星科學家安妮．佩斯利爾（Anne Peslier）在隨附的社論中寫道。
Language Notes
註 1： asteroid 源自希臘文的「星星」"aster" 和「形狀」"eidos"
註 2： barren 於本文指「荒蕪」的表面 ；本字也常指「不孕」; 注意重音在第 1 音節
註 3： hydrous 由名詞 "hydr" 「水」與用於構成形容詞的 "-ous" 組成，意指「含水」
註 4： pristine 於本文指「原始」的狀態 ；本字也常指「嶄新的」; 注意重音在第 2 音節
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Check your comprehension!
Choose the BEST answer to each of the questions below. After you finish, highlight the parentheses to reveal the hidden answers.
1. ( A ) What was the widely accepted theory of the origin of earth's water that preceded the newest finding?
(A) Earth's water was brought by traveling asteroids or comets.
(B) Earth was lacking in water until its gas and dust became water.;
(C) Earth's water was brought by a wet meteorite thousands of years ago.
(D) Earth has always had water and did not need any outside source.
2. ( D ) What is FALSE about enstatite chondrites according to the article?
(A) They are extremely rare and undamaged ones are hard to find.
(B) They contain similar types of elements as earth’s rocks.
(C) They are the foundation of Earth and some other planets.
(D) They are asteroids full of hydrous minerals.
3. ( B ) Which of the following statements best summarizes the article?
(A) Scientists are no longer puzzled about earth's water origins through the discovery of enstatite chondrites.
(B) Enstatite chondrites similar in composition to earth's water and rock disproves previous theory about
earth's water origins.
(C) Scientists have been proven wrong about earth's water origins through new enstatite chondrites found.
(D) Enstatite chondrites' chemical makeup is different from that of carbonaceous chondrites and shows new
water origins for earth.
編譯：外語教學暨資源中心 編輯小組
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