科學家發現地核會下起「鐵之雪」
地球內核的溫度相當熾熱且壓力十分巨大――而且還有雪堆覆蓋。這項新的研究成果有助於科學家更加了解影響整個地球的作用力。
上圖為新研究提出的地球內部簡圖。黑色和白色的地方代表含有鐵晶體，像是泥漿一般的層位。這道黏稠的層位在外核的部分(白色)會形成鐵晶體，他們會像「雪花」一樣飄落到內核頂部，堆積在此並且壓密(黑色)。這道已經壓密的層位在西半球(W)的內核頂端較厚，在東半球(E)的內核頂端則較薄。圖片來源：德州大學奧斯汀分校傑克遜地球科學院
這些雪花是由顆粒相當微小的鐵組成，比地表任何一片雪花都要重上許多。它們從融化的外核往下飄落，堆積在內核最上方，把內核蓋上一層厚度可達200英里的鐵堆。
雖然這幅景象聽起來像是外星世界的冬日奇景，但是主持研究的科學家表示這就像是火山裡面的岩石形成方式。
德州大學奧斯汀分校傑克遜地球科學院的Jung-Fu
Lin是研究共同作者，他說：「地球金屬核心的運作方式就像我們較為熟知的地殼裡的岩漿庫。」
這項研究已經發表在網路上，並於12月23日刊登在紙本版的《地球物理研究期刊：固體地球》(JGR
Solid Earth)。
主持這項研究的為中國四川大學的副教授Youjun
Zhang。其他共同作者包括德州大學奧斯汀分校傑克遜地球科學院的研究生Peter
Nelson；以及田納西大學的助理教授Nick
Dygert，他在傑克遜地球科學院進行博士後研究時進行了這項研究。
由於科學家無法直接採集地核的樣品，因此他們把地震波(一種能量波)通過地球內部時產生的訊號記錄下來並進行分析，藉此來研究地核。
不過，最近得到的地震波數據跟現行的地核模型預測值有所差異而引起了一些問題。地震波經過外核底部時速度比預期的要慢；通過東半球的內核頂部時則比預期的要快。
這項研究提出內核頂部被鐵之雪覆蓋住的現象可以解釋這些差異。1960年代早期S.I.
Braginkskii提出內核和外核之間存在著一層像是泥漿的物質，但是對於地核環境中溫度壓力條件的主流看法駁斥了這項理論。然而，Zhang利用類似地核的材料實驗之後得到了新數據，並從最近的科學文獻中尋找新的資料，他發現結晶作用確實可以發生，而且外核最底部的15%可能是由以鐵為主的晶體組成，這些晶體會從液態的外核落下，最終堆積在固態的內核頂端。
「這是一種光想就很怪異的現象。」Dygert表示。「外核裡面會有晶體飄落數百公里之後，堆在內核頂端。」
研究人員指出這些雪堆便是地震波訊號異常的來源。組成像是泥漿一般會讓地震波的速度減慢。而雪堆在東半球較薄，西半球較厚的大小差異則解釋了速度變化。
Zhang表示：「內核邊界並不是一道單純的平滑表面，這可能會影響地核熱能如何傳導與對流。」
論文把鐵之雪的顆粒和近地表岩漿庫內部發生的作用相比，其中包含了礦物從岩漿結晶出來，然後結合在一起的過程。這種礦物在岩漿庫內部壓密的作用會形成所謂的「堆積岩」。而在地核內部，鐵經過壓密之後會讓內核成長，外核則會縮小。
由於地核內部發生的現象，從產生磁場以至於散發熱量而驅使板塊運動都會影響整個地球。因此更加了解地核的成分和行為，有助於我們了解這些大尺度的作用是如何運作。
加州大學柏克萊分校的地球科學教授，專長為研究行星內部的Bruce
Buffet並未參與此研究。他表示這項研究正視了長久以來有關地球內部的一些謎題，結果甚至能幫助解開地核的形成過程。
「找出模型預測值和觀測異常值之間的關聯可以讓我們推論液態外核的可能成分，而這項資訊或許還能讓我們推出地球形成當時的整體環境條件。」他說，「要了解地球如何變成我們現在所見的樣貌，起始條件是相當重要的因子。」
研究經費來自於中國國家自然科學基金會、中央高校基本科研專項資金、傑克森地球科學院、美國國家科學基金會、斯隆基金。
Scientists find
iron ‘snow’ in Earth’s core
The Earth’s inner core is hot, under
immense pressure and snow-capped, according to new research that could help
scientists better understand forces that affect the entire planet.
The snow is made of tiny particles of iron – much
heavier than any snowflake on Earth’s surface – that fall from the molten outer
core and pile on top of the inner core, creating piles up to 200 miles thick
that cover the inner core.
The image may sound like an alien winter wonderland.
But the scientists who led the research said it is akin to how rocks form
inside volcanoes.
“The Earth’s metallic core works like a magma chamber
that we know better of in the crust,” said Jung-Fu Lin, a professor in the
Jackson School of Geosciences at The University of Texas at Austin and a
co-author of the study.
The study is available online and will be published
in the print edition of the journal JGR
Solid Earth on December 23.
Youjun Zhang, an associate professor at Sichuan
University in China, led the study. The other co-authors include Jackson School
graduate student Peter Nelson; and Nick Dygert, an assistant professor at the
University of Tennessee who conducted the research during a postdoctoral
fellowship at the Jackson School.
The Earth’s core can’t be sampled, so scientists
study it by recording and analyzing signals from seismic waves (a type of
energy wave) as they pass through the Earth.
However, aberrations between recent seismic wave data
and the values that would be expected based on the current model of the Earth’s
core have raised questions. The waves move more slowly than expected as they
passed through the base of the outer core, and they move faster than expected
when moving through the eastern hemisphere of the top inner core.
The study proposes the iron snow-capped core as an
explanation for these aberrations. The scientist S.I. Braginkskii proposed in
the early 1960s that a slurry layer exists between the inner and outer core,
but prevailing knowledge about heat and pressure conditions in the core
environment quashed that theory. However, new data from experiments on
core-like materials conducted by Zhang and pulled from more recent scientific
literature found that crystallization was possible and that about 15% of the
lowermost outer core could be made of iron-based crystals that eventually fall
down the liquid outer core and settle on top of the solid inner core.
“It’s sort of a bizarre thing to think about,” Dygert
said. “You have crystals within the outer core snowing down onto the inner core
over a distance of several hundred kilometers.”
The researchers point to the accumulated snow pack as
the cause of the seismic aberrations. The slurry-like composition slows the
seismic waves. The variation in snow pile size – thinner in the eastern
hemisphere and thicker in the western – explains the change in speed.
“The inner-core boundary is not a simple and smooth
surface, which may affect the thermal conduction and the convections of the
core,” Zhang said.
The paper compares the snowing of iron particles with
a process that happens inside magma chambers closer to the Earth’s surface,
which involves minerals crystalizing out of the melt and glomming together. In
magma chambers, the compaction of the minerals creates what’s known as
“cumulate rock.” In the Earth’s core, the compaction of the iron contributes to
the growth of the inner core and shrinking of the outer core.
And given the core’s influence over phenomena that
affects the entire planet, from generating its magnetic field to radiating the
heat that drives the movement of tectonic plates, understanding more about its
composition and behavior could help in understanding how these larger processes
work.
Bruce Buffet, a geosciences professor at the
University of California, Berkley who studies planet interiors and who was not
involved in the study, said that the research confronts longstanding questions
about the Earth’s interior and could even help reveal more about how the
Earth’s core came to be.
“Relating the model predictions to the anomalous
observations allows us to draw inferences about the possible compositions of
the liquid core and maybe connect this information to the conditions that
prevailed at the time the planet was formed,” he said. “The starting condition
is an important factor in Earth becoming the planet we know.”
The research was funded by the National Natural
Science Foundation of China, Fundamental Research Funds for the Central
Universities, the Jackson School of Geosciences, the National Science Foundation
and the Sloan Foundation.
原始論文：Youjun Zhang, Peter Nelson, Nick Dygert, Jung‐Fu Lin. Fe
Alloy Slurry and a Compacting Cumulate Pile Across Earth's Inner‐Core Boundary. Journal
of Geophysical Research: Solid Earth, 2019; DOI: 10.1029/2019JB017792
引用自：University of Texas at Austin. "Scientists
find iron 'snow' in Earth's core."
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