科學家對於地球水分的來源有了新的見解

原文網址：https://www.global.hokudai.ac.jp/blog/new-insight-into-the-origin-of-water-on-the-earth/

科學家對於地球水分的來源有了新的見解

科學家發現星際間的有機物加熱之後可以產生充足的水分，意味著地球的水分來源或許就是這些有機物。

星雲中的有機物質可能是地球水份的來源。圖片來源：NASA, ESA, and the Hubble Heritage Team (STScI/AURA)

關於地球還有許多未解之謎，其中之一便是地球水分的來源仍然難以解釋。對此主動進行的研究提出水是由「雪線」(溫度低到能讓冰塊凝固出來的界線)之外、帶有含水矽酸鹽的冰凍彗星或隕石所帶來的。然而，近期某些研究得出的觀察結果卻和彗星起源理論相悖，但還是沒有辦法提出其他合理的解釋來回答地球水份的來源。北海道大學的行星科學家Akira Kouchi表示：「即便在雪線以內有許多有機物質，但它們受到的關注一直以來都遠遠不如冰和矽酸鹽。」

在目前發表於《科學報告》(Scientific Reports)的研究當中，Akira Kouchi領導的科學團隊證實了把星際間的有機物加熱到高溫可以產生許多水和油。這代表水可能是在雪線以內產生，完全不需要雪線之外的彗星或隕石攜帶過來。

研究人員首先用化學反應物製造出類似於星際空間的分子雲裡的有機物。他們為此參照的分析數據來自於前人利用紫外線照射含有水、一氧化碳和氨的混合物，藉此模擬星際間有機物的自然合成過程。接下來，他們把這團星際間有機物的類比放進鑽石鉆，在加壓環境下從24℃逐漸加熱到400℃。樣品在100℃之前仍然十分均勻，但到了200℃時則開始分離成兩相。大約350℃時水珠的形成愈加明顯，並且隨著溫度上升而變大。到了400℃，除了水珠之外還產生了黑色的油滴。

加熱到400℃，星際間有機物的類比明顯分成了兩層油(黑色)和水(透明的黃色)(a：加熱前，b：加熱至400℃過後)。圖片來源：Hideyuki Nakano et al., Scientific Reports, May 8, 2020

團隊也把這些有機物的份量增加來進行類似的實驗，結果同樣產生了水和油。他們分析吸收光譜得出液態物質的主要成分是純水；油狀產物的化學分析結果則顯示它的特性類似於一般地底下採到的原油。

「我們的結果顯示雪線以內的星際間有機物可能是地球水份的來源。此外，我們還觀察到了非生物來源的油，代表形成自遠古地球的石油可能比過往認為的還多，」Akira Kouchi表示。「日本的小行星探測器隼鳥二號明年將會把小行星龍宮的樣品帶回地球，屆時對其中有機物的後續分析結果，應該能讓我們更進一步的瞭解地球水份的起源。」

 

New insight into the origin of water on the earth

Scientists have found the interstellar organic matter could produce an abundant supply of water by heating, suggesting that organic matter could be the source of terrestrial water.

There remains a number of mysteries on our planet including the elusive origin of water on the earth. Active studies suggested that terrestrial water had been delivered by icy comets or meteorites containing hydrous silicates that came from outside the “snow line” — the boundary beyond which ice can condense due the low temperatures. More recent studies, however, have provided observations opposing to cometary origin theory, yet still failing to suggest plausible substitutions for the source of terrestrial water. “Until now, much less attention has been paid to organic matter, comparing to ices and silicates, even though there is an abundance inside the snow line” says planetary scientist Akira Kouchi at Hokkaido University.

In the current study published in Scientific Reports, a group of scientists led by Akira Kouchi demonstrates that heating of the interstellar organic matter at high temperature could yield abundant water and oil. This suggests that water could be produced inside the snow line, without any contribution of comets or meteorites delivered from outside the snow line.

As a first step, the researchers made an analog of organic matter in interstellar molecular clouds using chemical reagents. To make the analog, they referred to analytical data of interstellar organics made by irradiating UV on a mixture containing H₂O, CO, and NH₃, which mimicked its natural synthetic process. Then, they gradually heated the organic matter analog from 24 to 400 ℃ under a pressured condition in a diamond anvil cell. The sample was uniform until 100 ℃, but was separated into two phases at 200 ℃. At approximately 350 ℃, the formation of water droplets became evident and the sizes of the droplets increased as the temperature rose. At 400 ℃, in addition to water droplets, black oil was produced.

The group conducted similar experiments with larger amounts of organic matter, which also yielded water and oil. Their analysis of absorption spectra revealed that the main component of the aqueous product was pure water. Additionally, chemical analysis of produced oil showed similar characteristics to the typical crude oil found beneath the earth.

“Our results show that the interstellar organic matter inside the snow line is a potential source of water on the earth. Moreover, the abiotic oil formation we observed suggests more extensive sources of petroleum for the ancient Earth than previously thought,” says Akira Kouchi. “Future analyses of organic matter in samples from the asteroid Ryugu, which the Japan’s asteroid explorer Hayabusa2 will bring back later this year, should advance our understanding of the origin of terrestrial water.”

原始論文：Hideyuki Nakano, Naoki Hirakawa, Yasuhiro Matsubara, Shigeru Yamashita, Takuo Okuchi, Kenta Asahina, Ryo Tanaka, Noriyuki Suzuki, Hiroshi Naraoka, Yoshinori Takano, Shogo Tachibana, Tetsuya Hama, Yasuhiro Oba, Yuki Kimura, Naoki Watanabe, Akira Kouchi. Precometary organic matter: A hidden reservoir of water inside the snow line. Scientific Reports, 2020; 10 (1) DOI: 10.1038/s41598-020-64815-6

引用自：Hokkaido University. "New insight into the origin of water on Earth."