研究30億年前的礦物，結果認為彗星是地球大氣的成因

原文網址：http://www.geologypage.com/2017/05/comets-contributed-earths-atmosphere-says-study-3-billion-year-old-minerals.html

研究30億年前的礦物，結果認為彗星是地球大氣的成因

科學家揭露地球大氣層的組成中，可能有部分是數十億年前由彗星帶來的。

地球大氣如何形成？這道謎題已經困惑科學家許久。某些研究人員認為地球剛形成時，部分的水、有機物和氣體分子可能是由彗星帶來的，這些物質構成了今日生命組成的原料。

由英國曼徹斯特大學、法國岩象暨地球化學研究中心(CRPG)和洛林大學(位於法國南錫)的研究人員刊登在《自然通訊》(Nature Communications)的新研究中，找到了支持該理論的證據。

科學家在年代可追溯至超過30億年前的石英礦物中找到了水泡。他們分析內部含有的微量古代空氣樣品，發現岩石中的空氣有一部分是由化學元素氙的極稀有形式，稱為鈾-氙(U-Xe)所組成。它之所以相當稀有的原因是在地球上通常很難找到它的存在。這種成分既不存在於地球地函，也不存在隕石當中。

因此，團隊認為鈾-氙必定是在初始大氣開始形成之後才加入地球。簡單來說，彗星是把鈾-氙帶到地球的最佳候選者。

共同作者，曼徹斯特大學地球與環境科學院的Ray Burgess教授解釋：「地球形成的位置跟太陽過於接近，使得像是鈾-氙之類的揮發性物質難以輕易凝結下來，因此它們會從表面迅速汽化並逸散到太空中。」

「海洋和大氣能夠存在，全都是因為地球形成後揮發性物質仍持續進入地球系統中。問題在於找到揮發性成分的來源，以及是什麼物質把它們帶到早期地球當中。」

「困難點在於最初加入地球的發揮性物質，許多不同的成分都已經在地球漫長的地質歷史中，因為各種地質作用的影響而徹底混和在一起。」

為了對抗此「混和」問題，團隊在非洲大陸的南非巴伯頓地區採集年代非常古老，但保存情況相當良好的岩石。他們先從此處的岩芯取出石英，然後分析石英內部的水泡含有的微量古代空氣樣品。團隊發現在巴伯頓石英中年代為33億年的鈾-氙，其成分跟現今地球大氣中的氙氣組成有很大的差異。

主要作者，CRPG的Guillaume Avice博士表示：「我們以前所未見的精準度來測量33億年前的空氣中，含有多少氙以及其同位素豐度。」

「氙是一種惰性氣體，由於它的化學活性很低且擁有九種同位素，使得岩石中的氙成為一種理想元素來找出地球原始大氣的氙同位素組成，也讓其成為一種理想工具來追查大氣的起源。」

最先開始進行此研究，且同樣任職於CRPG的Bernard Marty教授表示：「我們的研究顯示在30億年前，地球大氣的氙氣組成已經跟太陽系氣體與小行星的氙氣組成不同。其中一個可能原因為地球的氙氣是來自於彗星。」

這項發現同樣也顯示出，研究地球深部的空氣組成是有其研究價值的。Avice博士補充：「地球的揮發性元素是地球之所以適居的關鍵因素。研究遠古岩石含有的氣體，可以讓我們在瞭解地球揮發性元素的起源及演化過程時有新的見解。」

Comets contributed to Earth’s atmosphere, says study of 3 billion-year-old minerals

Scientists have revealed that some of Earth’s atmosphere may have been brought to the planet by comets billions of years ago.

The mystery of how the Earth’s atmosphere was formed has long baffled scientists. Some researchers think comets might have originally brought some of the water, organic and atmospheric molecules to Earth that now make up its life.

Now a new study, published in Nature Communications, by researchers from The University of Manchester, UK, Centre de Recherches Pétrographiques et Géochimiques (CRPG) and Université de Lorraine (Nancy, France), has found evidence to back up the theory.

The scientists have been analysing tiny samples of ancient air trapped in water bubbles found in the mineral, quartz, which dates back more than three billion years. The team found that the air in the rocks is partly made up of an extremely rare form of the chemical element, xenon. It is known as U-Xe and what makes it so rare is that it isn’t usually found on Earth. The component is not present in the Earth’s mantle, nor is it found in meteorites.

Therefore, the team believe that the U-Xe must have been added to the Earth after a primordial atmosphere had developed. Simply put, comets are the best candidates for carrying the U-Xe to the planet.

Co-author, Prof Ray Burgess, from Manchester’s School of Earth and Environmental Sciences explains: “The Earth formed too close to the Sun for volatile elements, such as U-Xe, to easily condense and they would have rapidly boiled off the surface and been lost to space.

“The reason that oceans and an atmosphere exist at all is because volatiles were still being added after the Earth formed. The puzzle is in identifying where the volatiles came from and what objects carried them to the early Earth.

“The difficulty is that many of the different volatile ingredients that were originally added have been thoroughly mixed together by geological processes during Earth’s long geological history.”

To combat this ‘mixing’ issue the team used tiny samples of ancient air trapped in water bubbles found in quartz in drill cores from the Barberton area of South Africa. The rocks from this region of the continent are extremely old and very well preserved. The team found that, in the Barberton quartz, 3.3 billion year old U-Xe has a composition very different from the xenon found in the Earth’s atmosphere today.

Lead author Dr Guillaume Avice from CRPG, said: “We measured the amount and isotopic abundance of xenon in the 3.3 billion year old air with unparalleled precision.

“Xenon is a noble gas which, being chemically inert and having nine isotopes, is an ideal element to reveal the xenon isotopic composition in the Earth’s primary atmosphere. This also makes it an ideal way of finding out where the atmosphere came from.”

Prof Bernard Marty, who initiated the study and who is also based as CRPG, said: “Our study reveals that 3 billion years ago there was already a xenon component in the Earth’s atmosphere different from solar gases and in asteroids. One possibility is that this xenon was from comets.”

But the discovery also shows the research possibilities of studying gases found trapped deep in the earth. Dr Avice added: “The study of gases trapped in ancient rocks opens new perspectives in our understanding of the origin and evolution of Earth’s volatile elements which are key factors for our planet’s habitability.”

引用自University of Manchester. “Comets contributed to Earth’s atmosphere, says study of 3 billion-year-old minerals.” Geology Page. May 18, 2017.