延誤送達！水、碳、氮並沒有在一開始就供應給地球
在發表於《自然》(Nature)的論文中，科隆大學的科學家對地球生命和海洋的起源有了重大的新發現。他們測量保存在格陵蘭最古老的地函岩石之後，顯示出跟過往的假設相反，生命演化必需的元素要在地球幾乎完全成形時才抵達地球。
出露在格陵蘭西南部地表，年代有38億年的地函岩石。圖片來源：Kristoffer
Szilas
由柯隆大學的地球科學家領軍的國際地質學者團隊，發現的證據顯示在海洋與生命形成過程中十分重要的元素，像是水、碳、氮，抵達地球的時間其實相當晚。之前許多科學家相信這些元素在地球開始形成的時候就已經出現在地球上了。然而，地質研究證實大部分的水其實是在地球幾乎完全成形的時候才送達地球。
這項新發現由德國、丹麥、威爾斯、澳洲、日本的科學家協力完成，成果於2020/3/11發表在期刊《自然》，論文標題為「Ruthenium
isotope vestige of Earth’s pre-late veneer mantle preserved in Archean rocks」。
一般認為像水之類的發揮性元素是來自於小行星，這些「組成行星的基石」則是形成於外太陽系。但是小行星確切抵達地球的時間為何，仍是專家持續討論的議題。「我們成功把時間範圍縮的更加精準。」第一作者，科隆大學地質和礦物學研究所的Mario
Fischer-Gödde博士表示。「我們的方法是取得大約38億年前形成於太古宙、年代最老的地函岩石，以及可能是其來源的小行星，還有現今的地函岩石，然後比較這三者的成分。」
為了定出揮發性元素送達地球的時間範圍，研究人員測量了一種非常稀有的鉑系金屬——釕的同位素豐度。釕在太古宙時才開始存在於地函當中，就像基因指紋一樣，這種稀有的鉑系金屬可以標記出地球形成過程的晚期階段。Fischer-Gödde表示：「像釕這類的鉑系金屬非常容易跟鐵結合。因此地球成形之後必定所有的釕都會被密封在地球的金屬核心當中。」
Carsten Münker博士接著說道：「如果我們還是在地函當中發現少量的釕，就可以推斷它們是在地核完全形成之後才加入地函。因此釕必定是後來地球跟小行星或是更小的原行星，也就是微行星碰撞時才加入地函。」
這些相當晚期才透過碰撞而加入地球的原料被科學家稱為「late
veneer」。如果釕是在此階段加入地球，到了現在它們應該已經散布到地函當中並充分混和。另一方面，殘存在格陵蘭的古老太古宙地函則把地球的初始成分給保留下來。
Fischer-Gödde表示：「格陵蘭這些年齡高達38億年的岩石是留存至今最古老的地函岩石。它們就像一道窗口，使我們能稍微窺見地球早年的歷史。」有趣的是，地球最古老的地函就赤裸裸地暴露在格陵蘭西南部地表的岩石露頭，地質學家可以輕而易舉地採集到這些岩石樣品。
這兩位科隆大學地質學家發表的論文中，他們參照了之前在地球上找到的隕石作為小行星的釕同位素值，認為古老地函岩石裡的原始釕同位素，最有可能來自太陽系的內側。科學家推測大部分的水星和金星也是由此處的物質形成。
Fischer-Gödde總結：「我們的發現顯示水和其他的揮發性物質，像是碳和氮，確實是在非常晚期的late
veneer階段才加入地球。」這項結果令人感到相當驚訝，因為科學界過往推測構成地球的材料中，含有水的部分在地球形成的早期階段就已經到達地球。
這些科學家正預計前往印度和格陵蘭進行更加深入的野外調查以探討更多岩石樣品。他們的研究經費來自德國研究基金會的優先計畫1833號，由科隆大學統籌的「建造適合居住的地球」；另外還有歐盟提供給Münker教授的ERC(歐洲研究委員會)經費，計畫名稱為「地球嬰兒期」。
Arrival delayed! Water, carbon and
nitrogen were not immediately supplied to Earth
Writing in
Nature, Cologne scientists present important new findings regarding the
origin of oceans and life on Earth. Measurements on the oldest preserved mantle
rocks from Greenland show that – contrary to previous assumptions – the
elements necessary for the evolution of life were not delivered to Earth until
very late in the planet’s formation.
Spearheaded by earth scientists of the University of
Cologne, an international team of geologists has found evidence that a large proportion
of the elements that are important for the formation of oceans and life, such
as water, carbon and nitrogen, were delivered to Earth very late in its
history. Previously, many scientists believed that these elements were already
present when the Earth began to form. However, geological investigations have
now shown that most of the water in fact was only delivered to Earth when its
formation was almost complete.
The new findings, which are a result of collaboration
among scientists from Germany, Denmark, Wales, Australia and Japan, will be
published in Nature under the title
‘Ruthenium isotope vestige of Earth’s pre-late veneer mantle preserved in
Archean rocks’ on 11 March 2020.
It is a generally accepted fact that volatile
elements such as water originate from asteroids, the ‘planetary building
blocks’ that formed in the outer solar system. However, there is ongoing
discussion among experts as to when precisely they came to Earth. ‘We have now
been able to narrow down the timeframe much more precisely’, said first author
Dr. Mario Fischer-Gödde from the Institute of Geology and Mineralogy at the
University of Cologne. ‘To do so, we compared the composition of the oldest,
approximately 3.8 billion-year-old mantle rocks from the Archean Eon with the
composition of the asteroids from which they may have formed, and with the
present-day composition of the Earth’s mantle.’
To constrain the delivery of the so-called ‘volatile’
elements to Earth, the researchers measured the isotope abundances of a very
rare platinum metal called ruthenium, which was already present in Earth’s
mantle by Archean time. Like a genetic fingerprint, this rare platinum metal is
an indicator for the late growth phase of the Earth. ‘Platinum group metals
like ruthenium have an extremely high tendency to combine with iron. Therefore,
when the Earth formed all ruthenium must have been completely sequestered into
the Earth’s metallic core’, said Fischer-Gödde.
Professor Dr. Carsten Münker added: ‘If we still find
traces of the rare platinum metals in the Earth’s mantle, we can assume that
they were only added after the formation of the core was completed. They were
certainly added during later collisions of the Earth with asteroids or smaller
protoplanets, so called planetesimals.’
Scientists refer to these very late building blocks
of the Earth, which were delivered by these collisions, as the ‘late veneer’.
If ruthenium was added during this stage, it is distributed and well mixed into
Earth’s mantle by now. The old Archean mantle relics in Greenland, on the other
hand, have still preserved Earth’s pristine composition.
‘The up to 3.8 billion-year-old rocks from Greenland
are the oldest preserved mantle rocks. They allow us a glimpse into the early
history of the Earth as if through a window’, Fischer-Gödde said.
Interestingly, Earth’s oldest mantle is openly accessible in surface outcrops
in southwest Greenland, allowing the geologists to easily collect rock samples.
The pristine ruthenium preserved in the old mantle
rocks most likely originates from the inner part of the solar system, the two
Cologne-based geologists report. It is presumably the same material that – for
the most part – also formed Mercury and Venus. The reference values for the
asteroidal ruthenium were previously obtained from meteorites found on Earth.
‘Our findings suggest that water and other volatile
elements such as carbon and nitrogen did indeed arrive on Earth very late,
during the “late veneer” phase’, Fischer-Gödde concluded. This result is
surprising because the scientific community had previously assumed that
water-bearing planetary building blocks were already delivered to Earth during
the early stages of its formation.
The scientists are planning further field trips to
India and Greenland to investigate more rock samples. Their work is being
supported by the German Research Foundation’s Priority Programme 1833 ‘Building
a Habitable Earth’, which is coordinated in Cologne, as well as Professor
Münker’s ERC grant ‘Infant Earth’ by the European Union.
原始論文：Mario Fischer-Gödde, Bo-Magnus Elfers, Carsten
Münker, Kristoffer Szilas, Wolfgang D. Maier, Nils Messling, Tomoaki Morishita,
Martin Van Kranendonk, Hugh Smithies. Ruthenium isotope vestige of Earth’s pre-late-veneer
mantle preserved in Archaean rocks. Nature, 2020; 579
(7798): 240 DOI: 10.1038/s41586-020-2069-3
引用自：University of Cologne. “Arrival delayed!
Water, carbon and nitrogen were not immediately supplied to Earth”
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