地球的形成速度比之前認為的還快
哥本哈根大學的研究人員利用鐵同位素的測量結果，證實地球最初的形成速度比之前認為的還快。對於行星的形成過程以及宇宙其他地方是否有水和生命的可能性來說，這項發現都給出了新的見解。
雖然看起來毫不起眼，但是照片中這些微小的隕石碎片，稱為碳質球粒隕石，卻被認為是和太陽系總體成分最為接近的物質。圖片來源：哥本哈根大學
哥本哈根大學寰宇研究所(Globe
Institute)恆星與行星形成研究中心(StarPlan)的新研究證明了我們星球的前身――原始地球――大概用了五百萬年的時間形成。
研究人員解釋以宇宙的時間尺度來看，這是極為迅速的。
太陽系估計已經存在了46億年，如果把這段時間化為24小時的長度，那麼新的研究結果中的原始地球形成速度對應來說大概是一分半。
StarPlan的這項結果推翻了原始地球如何形成的傳統理論。該理論認為原始地球是由越來越大的行星體彼此隨機撞擊而成，整個過程用了數千萬年的時間。這在上述假想的24小時中大約等同於5到15分鐘。
反之，這項新的結果支持了另一種最近提出的行星形成理論，認為行星是由宇宙塵埃聚積而成。研究主要作者Martin Schiller副教授對此的解釋如下：
「另外一種想法認為我們基本上是從塵埃中誕生：數毫米大小的物體聚集起來之後墜落到成長中的星體之上，一口氣把行星製造出來。」他接著說：
「地球以高速形成不僅對於我們自身的太陽系來說具有十分有趣的意涵，在評估銀河系其他地方形成行星的可能性有多高時也是如此。」
太陽系的總體成分
這項新發現的關鍵在於他們得到了目前在科學期刊上發表過最精密的鐵同位素測量結果。
研究人員探討了不同類型的隕石裡這種金屬元素的同位素如何混和之後，發現跟地球的組成類似的只有一種隕石：碳質球粒隕石(CI chondrites)。
進行此研究的科學家敘述這種易碎的隕石裡面的塵埃，是我們所能取得的物質中和太陽系整體組成最相似的。這類塵埃和氣體結合在一起後，透過恆星周遭的吸積盤匯流到成長中的太陽表面。
這項過程持續了大約五百萬年，期間吸積盤裡的物質也形成了我們的地球。研究人員推測原始地球的鐵質核心在這段期間也已經形成，並把早期聚積的鐵從地函中帶走。
兩種不同的鐵組成
其他種類的隕石(像是從火星來的)告訴我們最初供給地球成長的物質具有不同的鐵同位素組成。StarPlan的研究人員解釋說很可能是因為靠近年幼太陽的塵埃受到加熱作用的關係。
經過最初的數十萬年之後，太陽系的溫度才下降到未受影響的碳質球粒隕石塵埃可以從太陽系較遠的地方進入到原始地球的聚積區域。
Martin
Schiller解釋：「這些新來的碳質球粒隕石塵埃蓋掉了地函原本的鐵同位素組成。如果原先的鐵大部分都沒有移到地核當中就不會有這種現象，因此地核的形成必定要較早發生才行。」
他繼續解釋：「如果地球的形成過程是一堆天體撞在一起的隨機過程，那麼地球的鐵組成絕對不會只和一種隕石相符，應該會是許多東西混雜之後的結果。」
更多行星，更多水分，或許還有更多生命
根據行星是由宇宙塵埃聚積而成的這些證據，研究人員相信宇宙其他地方可能也會發生相同作用。
這意味著如果其他行星的形成過程不是只靠太空中天體的隨機撞擊作用，或許它們也同樣地能以更快的速度形成。
天文學家自90年代中期以來發現了數以千計的系外行星――即太陽系之外的行星――也證實了這項推測。研究共同作者，StarPlan的主任Martin Bizzarro教授如此解釋：
「我們現在知道宇宙中到處都有行星，而且行星系統的誕生過程和運作方式有通用規則。若我們可以從自身所處的太陽系中瞭解這些規則，或許也能將類似規則推演至銀河系的其他行星系統，包括水是在什麼時候加入行星以及有多常見。」他繼續說道：
「如果行星聚積作用很早就完成的理論正確無誤，那麼類似地球的行星在形成過程中的副產物之一可能就是水，因此我們所知的這種生命原料在宇宙其他地方可能更為常見。」
The Earth formed much faster than
previously thought
By measuring iron isotopes, researchers
from the University of Copenhagen have shown that our planet originally formed
much faster than previously thought. This finding provides new insights on both
planetary formation and the likelihood of water and life elsewhere in the
universe.
The precursor of our planet, the proto-Earth, formed
within a time span of approximately five million years, shows a new study from
the Centre for Star and Planet Formation (StarPlan) at the Globe Institute at
the University of Copenhagen.
On an astronomical scale, this is extremely fast, the
researchers explain.
If you
compare the solar system's estimated 4.6 billion years of existence with a
24-hour period, the new results indicate that the proto-Earth formed in what
corresponds to about a minute and a half.
Thus,
the results from StarPlan break with the traditional theory that the
proto-Earth formed by random collisions between larger and larger planetary
bodies throughout several tens of millions of years – equivalent to about 5-15
minutes out of the above-mentioned fictional 24 hours of formation.
Instead,
the new results support a more recent, alternative theory about the formation
of planets through the accretion of cosmic dust. The study's lead author,
Associate Professor Martin Schiller, explains it as follows:
‘The other idea is that we start from dust, essentially. Millimetre-sized
objects, all coming together, raining down on the growing body and making the
planet in one go,’ he says, adding:
‘Not only is this implication of the rapid formation of the Earth
interesting for our solar system. It is also interesting to assess how likely
it is for planets to form somewhere else in the galaxy.’
The bulk composition of the solar system
The key to the new finding came in the form of the most precise
measurements of iron isotopes that have so far been published scientifically.
By studying the isotopic mixture of the metallic element in different
meteorites, the researchers found only one type of meteoritic material with a
composition similar to Earth: The so-called CI chondrites.
The researchers behind the study describe
the dust in this fragile type of meteorite as our best equivalent to the bulk
composition of the solar system itself. It was dust like this combined with gas
that was funnelled via a circumstellar accretion disk onto the growing Sun.
This
process lasted about five million years and our planets were made from material
in this disk. Now, the researchers estimate that the proto-Earth’s ferrous core
also formed already during this period, removing early accreted iron from the
mantle.
Two
different iron compositions
Other
meteorites, for example from Mars, tell us that at the beginning the iron
isotopic composition of material contributing to the growing Earth was
different. Most likely due to thermal processing of dust close to the young
sun, the researchers from StarPlan explain.
After
our solar system’s first few hundred thousands of years it became cold enough
for unprocessed CI dust from further out in the system to enter the accretion
region of the proto-Earth.
‘This
added CI dust overprinted the iron composition in the Earth’s mantle, which is
only possible if most of the previous iron was already removed into the core.
That is why the core formation must have happened early,’ Martin Schiller
explains.
‘If
the Earth’s formation was a random process where you just smashed bodies together,
you would never be able to compare the iron composition of the Earth to only
one type of meteorite. You would get a mixture of everything,’ he adds.
More
planets, more water, perhaps more life
Based
on the evidence for the theory that planets form through the accretion of
cosmic dust, the researchers believe that the same process may occur elsewhere
in the universe.
This
means that also other planets may likely form much faster than if they grow
solely from random collisions between objects in space.
This
assumption is corroborated by the thousands of exoplanets – planets in other
solar systems – that astronomers have discovered since the mid-nineties,
explains Centre Leader and co-author of the study, Professor Martin Bizzarro:
‘Now
we know that planet formation happens everywhere. That we have generic
mechanisms that work and make planetary systems. When we understand these
mechanisms in our own solar system, we might make similar inferences about
other planetary systems in the galaxy. Including at which point and how often
water is accreted’, he says, adding:
‘If
the theory of early planetary accretion really is correct, water is likely just
a by-product of the formation of a planet like the Earth – making the
ingredients of life, as we know it, more likely to be found elsewhere in the
universe’.
原始論文：Martin Schiller, Martin Bizzarro, Julien
Siebert. Iron
isotope evidence for very rapid accretion and differentiation of the proto-Earth. Science Advances,
2020; 6 (7): eaay7604 DOI: 10.1126/sciadv.aay7604
引用至：University of Copenhagen. "Earth formed
much faster than previously thought, new study shows."
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