地球水分何處來？

 原文網址：https://carnegiescience.edu/how-did-earth-get-its-water

根據卡內基科學中心的Anat Shahar、加州大學洛杉磯分校的Edward Young和 Hilke Schlichtin進行的新研究，地球的水可能是在地球的成長期由富含氫氣的大氣層和行星胚胎的岩漿海發生的交互作用所形成。他們發表在期刊《自然》(Nature)的這項發現，也許可以解釋地球的標誌性特徵源自何方。

數十年來，研究人員對於行星形成過程的理解主要來自於我們自身的太陽系。雖然木星與土星這類氣體巨行星的形成方式有一些熱烈的討論，但研究人員普遍同意地球與其他岩質行星，是太陽年幼時周圍的盤狀氣體與塵埃吸積之後所形成。

隨著越來越多較大的物體彼此碰撞，最後會成為地球的初生原行星體積和溫度都會持續增加，並且因為碰撞及放射性元素產生的熱能而融化成廣袤的岩漿海。之後行星逐漸冷卻下來的過程中，密度最高的物質往內部沉降，使得地球分離成三層不同的結構——由金屬構成的地核，以及由岩石、矽酸鹽構成的地函與地殼。

然而，過去十年暴增的系外行星研究給出了新方法來模擬地球的胚胎期。

「地外行星的發現讓我們深刻理解到甫形成的行星在最初數百萬年的成長期當中，外圍有層富含氫分子的大氣是很常見的情況，」Shahar解釋。「這層包裹在外的氫分子最後會消失無蹤，但會在幼年行星的成分中留下它們的記號。」

研究人員利用這些資訊建構新模型來模擬地球的形成與演變過程，觀察地球特殊的化學特徵能否複製出來。

利用新建構的模型，卡內基研究所與加州大學洛杉磯分校的研究人員成功證明了地球存在之初，岩漿海與含有氫分子的原始大氣發生的交互作用，可能產生了幾項地球的標誌性特徵，像是富含水分、整體處於氧化態……等。

研究人員運用數學模型來探討岩漿海與含有氫分子的大氣之間的物質交換作用，對象為25種不同的化合物以及18種不同的反應類型——其複雜程度足以產生有用的數據來探討地球形成時的歷史，但也簡單到可以完整解釋它們的含義。

在他們模擬的嬰兒期地球當中，岩漿海與大氣的交互作用造成非常多的氫進入金屬地核，並且讓地函氧化、產生大量的水分。

圖中顯示地球的某些標誌性特徵，像是富含水分、整體處於氧化態，可能是在地球的成長期由富含氫氣的大氣層和行星胚胎的岩漿海發生的交互作用所形成。此圖由加州大學洛杉磯分校的Edward Young以及卡內基科學研究所的Katherine Cain繪製。

研究人員得出就算彼此碰撞而讓行星成長的岩石材料都是完全乾燥的，含有氫分子的大氣與岩漿海之間的交互作用也能產生可觀的水分。他們說雖然水分可能還有其他來源，但不需要它們也能解釋地球目前的狀態。

「雖然這只是一種可以解釋地球演變過程的說法，但可以讓地球形成的歷史跟『超級地球』與『亞海王星』之間產生重大的連結。在迄今發現環繞遠方恆星的行星當中，『超級地球』與『亞海王星』是最常見的類型，」Shahar總結。

此計畫是一個跨領域、多個研究單位合作的「AEThER」(乙太)計畫中的一部份。由Shahar發起與主持的以太計畫旨在尋找並揭開銀河系最常見的行星類型——『超級地球』與『亞海王星』的化學組成，並建立一套準則來偵測遙遠星球的生命跡象。發展這項研究的目的是讓科學家理解這類星球的形成與演變過程如何塑造它們的大氣，進而讓他們可以把真正的生命印跡(biosignature)，也就是只有生命存在才能產生的物質，跟大氣中非生物起源的分子區別開來。

「性能越來越強的望遠鏡使得天文學家對系外行星的大氣成分有了前所未見的詳細瞭解，」Shahar表示。「透過實驗與模擬得出的數據，乙太計畫的成果能為天文學家的觀察結果提供參考，我們希望未來能藉此創造出萬無一失的方法來偵測其他星球上的生命跡象。」

 

How did Earth get its water?

Earth's water could have originated from interactions between the hydrogen-rich atmospheres and magma oceans of the planetary embryos that comprised Earth's formative years, according to new work from Carnegie Science's Anat Shahar and UCLA's Edward Young and Hilke Schlichting. Their findings, which could explain the origins of Earth's signature features, are published in Nature.

For decades, what researchers knew about planet formation was based primarily on our own solar system. Although there are some active debates about the formation of gas giants like Jupiter and Saturn, it is widely agreed upon that Earth and the other rocky planets accreted from the disk of dust and gas that surrounded our sun in its youth.

As increasingly larger objects crashed into each other, the baby planetesimals that eventually formed Earth grew both larger and hotter, melting into a vast magma ocean due to the heat of collisions and radioactive elements. Over time, as the planet cooled, the densest material sank inward, separating Earth into three distinct layers—the metallic core, and the rocky, silicate mantle and crust.

However, the explosion of exoplanet research over the past decade informed a new approach to modeling the Earth's embryonic state.

"Exoplanet discoveries have given us a much greater appreciation of how common it is for just-formed planets to be surrounded by atmospheres that are rich in molecular hydrogen, H₂, during their first several million years of growth," Shahar explained. "Eventually these hydrogen envelopes dissipate, but they leave their fingerprints on the young planet's composition."

Using this information, the researchers developed new models for Earth's formation and evolution to see if our home planet's distinct chemical traits could be replicated.

Using a newly developed model, the Carnegie and UCLA researchers were able to demonstrate that early in Earth's existence, interactions between the magma ocean and a molecular hydrogen proto-atmosphere could have given rise to some of Earth's signature features, such as its abundance of water and its overall oxidized state.

The researchers used mathematical modeling to explore the exchange of materials between molecular hydrogen atmospheres and magma oceans by looking at 25 different compounds and 18 different types of reactions—complex enough to yield valuable data about Earth's possible formative history, but simple enough to interpret fully.

Interactions between the magma ocean and the atmosphere in their simulated baby Earth resulted in the movement of large masses of hydrogen into the metallic core, the oxidation of the mantle, and the production of large quantities of water.

Even if all of the rocky material that collided to form the growing planet was completely dry, these interactions between the molecular hydrogen atmosphere and the magma ocean would generate copious amounts of water, the researchers revealed. Other water sources are possible, they say, but not necessary to explain Earth's current state.

"This is just one possible explanation for our planet's evolution, but one that would establish an important link between Earth's formation history and the most common exoplanets that have been discovered orbiting distant stars, which are called Super-Earths and sub-Neptunes," Shahar concluded.

This project was part of the interdisciplinary, multi-institution AEThER project, initiated and led by Shahar, which seeks to reveal the chemical makeup of the Milky Way galaxy's most common planets—Super-Earths and sub-Neptunes—and to develop a framework for detecting signatures of life on distant worlds. This effort was developed to understand how the formation and evolution of these planets shape their atmospheres. This could—in turn—enable scientists to differentiate true biosignatures, which could only be produced by the presence of life, from atmospheric molecules of non-biological origin.

"Increasingly powerful telescopes are enabling astronomers to understand the compositions of exoplanet atmospheres in never-before-seen detail," Shahar said. "AEThER's work will inform their observations with experimental and modeling data that, we hope, will lead to a foolproof method for detecting signs of life on other worlds."

原始論文：Edward D. Young, Anat Shahar, Hilke E. Schlichting. Earth shaped by primordial H₂ atmospheres. Nature, 2023; 616, 306-311. DOI: 10.1038/s41586-023-05823-0

引用自：Carnegie Science. “How did Earth get its water?”