隕石揭示了地球的揮發性化學物的可能來源

原文網址：https://www.imperial.ac.uk/news/242771/meteorites-reveal-likely-origin-earths-volatile/

by Caroline Brogan

倫敦帝國學院的研究人員透過隕石得知了地球的揮發性化學物可能來自遠方，而這些物質當中有部分構成了生命的基石。

鋅是一種揮發性的元素，研究人員發現地球整體的鋅大約有一半是來自外太陽系的小行星——也就是小行星帶之外，木星、土星、天王星等行星所在區域的小行星。他們預估這些天體也供應了其他重要的揮發性物質，像是水。

揮發性物質(volatile)是指在相對較低的溫度下就會從固態或液態轉變成氣態的元素或化合物，包括了生物體內最常見的六種元素與水。因此，這些物質的加入對於地球生命的出現來說至關重要。

先前研究人員認為地球大部分的揮發性物質是來自於地球附近形成的小行星。因此這項發現對於地球如何保有維持生命所需的特別條件來說，帶來了重要的線索。

倫敦帝國學院地球科學與工程學系的教授Mark Rehkämper是此研究的資深作者，他說：「我們的數據顯示地球整體的鋅大約一半是從外太陽系，也就是木星軌道之外的物質而來。以太陽系早期發育過程的現行模型來說，這完完全全是在預料之外。」

前人研究認為地球幾乎是由內太陽系的物質組成，因此研究人員推論它們也是地球揮發性化學物質的主要來源。相較之下，這項新發現提出外太陽系占有的地位比之前認為的還高。

Rehkämper教授進一步說明：「外太陽系的物質在構成地球整體的揮發性化學物中具有相當重要的貢獻。如果說沒有來自外太陽系的物質所帶來的貢獻，地球的揮發性物質就會遠遠少於我們今日所見，使其變得乾燥許多，而可能無法滋養並維繫生命。」

此發現發表在《科學》。

為了進行這項研究，研究人員探討了18顆不同來源的隕石：11顆來自內太陽系，稱為非碳質隕石；另外7顆來自外太陽系，稱為碳質隕石。

他們一一測量每顆隕石中鋅的五種不同形式(即「同位素」)之間的相對含量。接著他們將每顆隕石中的同位素指紋跟代表地球的樣本對比，藉此估計地球整體的鋅來自每種材料的比例各有多少。結果指出雖然地球的質量大約只有10%是來自碳質隕石，但這種材料卻供應了地球大約一半的鋅。

研究人員表示含有高濃度的鋅以及其他揮發性成分的材料也可能有較高的含水量，因此結果也暗示了地球水份的來源。

地球科學與工程學系的博士候選人Rayssa Martins是論文第一作者，她說：「雖然我們一直都知道地球的成分有部分來自於碳質隕石，但我們的發現指出這種材料在地球儲備揮發性元素的過程中是相當重要的來源，而這些元素當中有一些對於生物滋長來說是不可或缺的。」

研究人員下一步準備要分析火星(在徹底乾枯之前，火星在距今41億到30億年是有水的)與月球的岩石。Rehkämper教授說：「廣為接受的理論認為大約在45億年前，有顆巨大的小行星跟新生地球相撞而產生了月球。分析月岩的鋅同位素有助有我們驗證這項假說，並且判定水分等揮發性物質遞送到地球的過程中，這顆撞上來的小行星是否扮演了重要的腳色。」

 

Meteorites reveal likely origin of Earth’s volatile chemicals

Meteorites have told Imperial researchers the likely far-flung origin of Earth’s volatile chemicals, some of which form the building blocks of life.

They found that around half the Earth’s inventory of the volatile element zinc came from asteroids originating in the outer Solar System – the part beyond the asteroid belt that includes the planets Jupiter, Saturn, and Uranus. This material is also expected to have supplied other important volatiles such as water.

Volatiles are elements or compounds that change from solid or liquid state into vapour at relatively low temperatures. They include the six most common elements found in living organisms, as well as water. As such, the addition of this material will have been important for the emergence of life on Earth.

Prior to this, researchers thought that most of Earth’s volatiles came from asteroids that formed closer to the Earth. The findings reveal important clues about how Earth came to harbour the special conditions needed to sustain life.

Senior author Professor Mark Rehkämper, of Imperial College London’s Department of Earth Science and Engineering, said: “Our data show that about half of Earth’s zinc inventory was delivered by material from the outer Solar System, beyond the orbit of Jupiter. Based on current models of early Solar System development, this was completely unexpected.”

Previous research suggested that the Earth formed almost exclusively from inner Solar System material, which researchers inferred was the predominant source of Earth’s volatile chemicals. In contrast, the new findings suggest the outer Solar System played a bigger role than previously thought.

Professor Rehkämper added: “This contribution of outer Solar System material played a vital role in establishing the Earth’s inventory of volatile chemicals. It looks as though without the contribution of outer Solar System material, the Earth would have a much lower amount of volatiles than we know it today – making it drier and potentially unable to nourish and sustain life.”

The findings are published in Science.

To carry out the study, the researchers examined 18 meteorites of varying origins – eleven from the inner Solar System, known as non-carbonaceous meteorites, and seven from the outer Solar System, known as carbonaceous meteorites. 

For each meteorite they measured the relative abundances of the five different forms – or isotopes – of zinc. They then compared each isotopic fingerprint with Earth samples to estimate how much each of these materials contributed to the Earth’s zinc inventory. The results suggest that while the Earth only incorporated about ten per cent of its mass from carbonaceous bodies, this material supplied about half of Earth’s zinc.

The researchers say that material with a high concentration of zinc and other volatile constituents is also likely to be relatively abundant in water, giving clues about the origin of Earth’s water.

First author on the paper Rayssa Martins, PhD candidate at the Department of Earth Science and Engineering, said: “We’ve long known that some carbonaceous material was added to the Earth, but our findings suggest that this material played a key role in establishing our budget of volatile elements, some of which are essential for life to flourish.”

Next the researchers will analyse rocks from Mars, which harboured water 4.1 to 3 billion years ago before drying up, and the Moon. Professor Rehkämper said: “The widely held theory is that the Moon formed when a huge asteroid smashed into an embryonic Earth about 4.5 billion years ago. Analysing zinc isotopes in moon rocks will help us to test this hypothesis and determine whether the colliding asteroid played an important part in delivering volatiles, including water, to the Earth.”

原始論文：Rayssa Martins, Sven Kuthning, Barry J. Coles, Katharina Kreissig, Mark Rehkämper. Nucleosynthetic isotope anomalies of zinc in meteorites constrain the origin of Earth’s volatiles. Science, 2023; 379 (6630): 369 DOI: 10.1126/science.abn1021

引用自：Imperial College London. "Meteorites reveal likely origin of Earth's volatile chemicals."