原文網址：https://news.yale.edu/2022/03/02/confessions-former-fireball-how-earth-became-habitable
By Jim Shelton
耶魯大學與加州理工學院的研究人員提出了一個大膽的新理論，解釋地球自身如何從一顆熾熱、被碳籠罩的岩石球體轉變成可以供養生命的星球。
這項理論關乎地球最早的歲月，並且牽涉到一種奇怪的岩石，它和海水作用的方式正好可以促進生命有關的物質誕生。
「這是地球歷史上最神秘的時期，」Jun
Korenaga表示。他是耶魯大學地球與行星科學的教授，也是這篇發表於期刊《自然》(Nature)新研究的共同作者。「我們對於地球最初五億年的樣貌提供了迄今最完整的理論。」
研究第一作者Yoshinori
Miyazaki之前為耶魯大學的研究生，現在則是加州理工學院的博士後研究員。他的耶魯大學博士論文的最後一章即為本研究的基礎。
大部分的科學家相信地球最初的大氣和金星十分類似。當時的天空充滿了二氧化碳，比現在的大氣還要高出十萬倍以上；地表的氣溫可能超過華氏400度(攝氏200度左右)。
科學家一致認為在這種條件下，生命連誕生都不可能，更別說是生存了。
「勢必有某種方式可以大量移除空氣裡的二氧化碳，」Miyazaki表示。「由於地球初期沒有留下任何的岩石紀錄，因此我們等於是從零開始來建立極為早期的地球的理論模型。」
Miyazaki和Korenaga在建立模型的時候結合熱動力學、流體力學和大氣物理學的觀點。最後他們確立的假設相當大膽：覆蓋早期地球的岩石類型如今已經不復存在。
「這些岩石富含一種稱為輝石的礦物，它們可能因此呈現出墨綠色，」Miyazaki表示。「最重要的是它們含有極為大量的鎂，在現今的岩石中很少觀察到這麼高的濃度。」
Miyazaki說富含鎂的礦物可以和二氧化碳發生反應產生碳酸鹽，因此在封存大氣裡的碳的過程中扮演了關鍵角色。
研究人員提出隨著熔化的地球開始凝固，帶有水分的地函，也就是地球內部厚達3000公里的岩石也開始旺盛地對流。含水地函與鎂含量高的輝石岩組合起來，使得大氣二氧化碳被抽取的過程大幅加速。
研究人員表示實際上當時的大氣碳封存只用了1億6000萬年，這樣的速率比現今的地函岩石所能達到的還要高出10倍以上。
「早期地球上這些『奇異的』岩石額外帶來的一項好處是，它們會跟海水快速反應產生大量的氫氣，許多人相信這是創造生物分子必備的原料，」Korenaga表示。
這種效應就像是現代一種稀有的深海熱泉類型，位於大西洋一處稱為「失落的城市」的水熱田。此處透過非生物作用產生的氫氣和甲烷，使其成為探討地球生命起源時的重要地點。
「我們的理論不只有機會解決地球為什麼變得適合生命居住的問題，還有生命為什麼會出現在地球上，」Korenaga補充。
美國國家航空暨太空總署與美國國家科學基金會為這項研究提供了經費。
Confessions of a former fireball — how
Earth became habitable
Researchers at Yale and Caltech have a
bold new theory to explain how Earth transformed itself from a fiery,
carbon-clouded ball of rocks into a planet capable of sustaining life.
The theory covers Earth’s earliest years and involves
“weird” rocks that interacted with seawater in just the right way to nudge
biological matter into existence.
“This period is the most enigmatic time in Earth
history,” said Jun Korenaga, a professor of Earth and planetary sciences at
Yale and co-author of a new study in the journal Nature. “We’re presenting the most complete theory, by far, for
Earth’s first 500 million years.”
The study’s first author is Yoshinori Miyazaki, a
former Yale graduate student who is now a Stanback Postdoctoral Fellow at
Caltech. The study is based on the final chapter of Miyazaki’s Yale
dissertation.
Most scientists believe that Earth began with an
atmosphere much like that of the planet Venus. Its skies were filled with
carbon dioxide — more than 100,000 times the current level of atmospheric
carbon — and Earth’s surface temperature would have exceeded 400 degrees
Fahrenheit.
Biological life would have been unable to form, much
less survive, under such conditions, scientists agree.
“Somehow, a massive amount of atmospheric carbon had
to be removed,” Miyazaki said. “Because there is no rock record preserved from
the early Earth, we set out to build a theoretical model for the very early
Earth from scratch.”
Miyazaki and Korenaga combined aspects of
thermodynamics, fluid mechanics, and atmospheric physics to build their model.
Eventually, they settled on a pretty bold proposition: early Earth was covered
with rocks that do not currently exist on Earth.
“These rocks would have been enriched in a mineral
called pyroxene, and they likely had a dark greenish color,” Miyazaki said.
“More importantly, they were extremely enriched in magnesium, with a
concentration level seldom observed in present-day rocks.”
Miyazaki said magnesium-rich minerals react with
carbon dioxide to produce carbonates, thereby playing a key role in
sequestering atmospheric carbon.
The researchers suggest that as the molten Earth
started to solidify, its hydrated, wet mantle — the planet’s
3,000-kilometer-thick rocky layer — convected vigorously. The combination of a
wet mantle and high-magnesium pyroxenites dramatically sped up the process of
pulling CO2 out of the atmosphere.
In fact, the researchers said the rate of atmospheric
carbon sequestration would have been more than 10 times faster than would be
possible with a mantle of modern-day rocks, requiring a mere 160 million years.
“As an added bonus, these ‘weird’ rocks on the early
Earth would readily react with seawater to generate a large flux of hydrogen,
which is widely believed to be essential for the creation of biomolecules,”
Korenaga said.
The effect would be similar to a rare type of modern,
deep-sea thermal vent, called the Lost City hydrothermal field, located in the
Atlantic Ocean. The Lost City hydrothermal field’s abiotic production of
hydrogen and methane has made it a prime location for investigating the origin
of life on Earth.
“Our theory has the potential to address not just how
Earth became habitable, but also why life emerged on it,” Korenaga added.
Grants from the National Aeronautics and Space
Administration and the National Science Foundation helped to fund the research.
Yoshinori Miyazaki, Jun Korenaga. A
wet heterogeneous mantle creates a habitable world in the Hadean. Nature,
2022; 603 https://doi.org/10.1038/s41586-021-04371-9
沒有留言:
張貼留言