地球早期的磁場可能並非由地核產生，而是來自地函
一系列新研究支持了斯克里普斯海洋研究所的科學家主張的理論
一名加州大學聖地牙哥分校斯克里普斯海洋研究所的地球物理學家，以不同於傳統的情節重述了早期地球的歷史。最新研究支持了這段最早由他提出的故事。
圖片來源：Naeblys/istockphoto
3月15日發表在期刊《地球與行星科學通訊》(Earth
and Planetary Science Letters)的研究中，斯克里普斯海洋研究所的科學家Dave
Stegman、Leah
Ziegler、Nicolas
Blanc重新評估了早期地函的液態部分在熱動力學上是否能產生磁場，以及這種磁場可以維持多久。
這篇論文打開了一扇大門，使我們有機會解決描述地球幼年日子的故事中有所出入的地方。值得注意的是，另外兩篇同樣最近發表的研究中，加州大學洛杉磯分校和亞利桑那州立大學的地球物理學家把Stegman提出的概念加以拓展，並且應用到新的層面。
「目前還沒有一個大一統理論可以解釋地球熱力學的演變過程。」Stegman表示。「我們尚未擁有這類理論框架來理解地球的演變過程。在此我們提出了一種可行的假說。」
這三篇研究代表了一種典範轉移過程中的最新發展，或許將改寫我們解讀地球歷史的方式。
一直以來地球物理學最基本的教條之一，便是地球的磁場從古至今都是由地球的液態外核作為發電機而產生。地球和其他行星可以形成磁場，是因為它們擁有快速旋轉的液態金屬核心，並且具有發生熱對流的條件。
科學家長久以來假設地函從地球相當初期就維持在整個都是固體的狀態。但在2007年，法國的研究人員提出了一種截然不同的理論。他們主張地球45億年歷史的前半段，地函底部三分之一都處於融化狀態，稱為「basal
magma ocean(基底岩漿海)」。六年之後，Stegman和Ziegler進一步拓展此理論，發表了第一篇研究指出地球當時可以跨過產生磁場所需門檻的部分，其實是下部地函曾為液態的地方而非地核。
矽酸鹽是組成地函的物質，正常來說它們的導電度非常差。因此，即使最底層的地函處於液態數十億年，其中快速流動的液體也無法跟目前地核裡的發電機一樣，產生夠大的電流來形成磁場。不過Stegman的團隊主張液態矽酸鹽的導電度事實上比一般認為的還要更強。
加州大學洛杉磯分校的地球物理學家Lars
Stixrude表示：「Ziegler和Stegman最先提出早期地球的地磁發電機是矽酸鹽的理論。」該理論招致了某些懷疑，因為先前的結果顯示「矽酸鹽要能發電的情況只有當矽酸鹽液體的導電度高得出奇，比在低溫低壓下的矽酸鹽液體測量到的導電度還要高出許多的情況下才能辦到。」
Stixrude領導的團隊首度利用量子力學的計算方式來預測基底岩漿海裡矽酸鹽液體的導電度。
Stixrude表示：「我們發現它的導電度相當高，足以讓矽酸鹽持續發電。」加州大學洛杉磯分校的這篇研究刊登在2月25日發行的《自然通訊》(Nature Communications)。
另一篇論文中，亞利桑那州立大學的地球物理學家Joseph
O’Rourke應用了Stegman的概念，考量金星的液態地函是否可能曾在某個時候有磁場產生。
這些新研究顯示出此假說已經開始扎根，但是離廣為接受還有很長一段路要走。
Stegman說：「只有當人們親自測試我的理論才會相信它，而現在已經有另外兩位備受尊崇的科學家這麼做了。」
「Dave
Stegman和他的同事進行的研究極具開創性，這直接啟發了我對金星的研究工作。」O’Rourke表示。「他們最近的研究有助於回答困擾科學家許多年的問題：地球的磁場如何持續數十億年之久？」
如果Stegman的假說正確無誤，意味著地函為幼年地球建立了第一個磁場屏障來抵擋宇宙輻射。不僅如此，這也能為地球歷史稍後的板塊運動是如何演化出來的研究提供基礎。
Stegman表示：「如果磁場是由地核之上，熔化的下部地函產生的，代表說地球從相當初期就開始具有這層保護，因此生命可能很快便能出現在地球上。」
「總結來說我們的論文相輔相成，它們呈現出基底岩漿海對於類地行星的演化來說十分重要。」O’Rourke表示。「雖然地球的基底岩漿海已經凝固，但它是我們的地磁可以維持這麼久的關鍵。」
由斯克里普斯海洋研究所進行的研究，經費來自美國國家科學基金會、美國能源部，以及加州大學聖地牙哥分校的SEED獎學金。
Earth’s mantle, not its core, may have
generated planet’s early magnetic field
Scripps Oceanography researcher’s
assertion bolstered by series of new studies
New research lends credence to an
unorthodox retelling of the story of early Earth first proposed by a
geophysicist at Scripps Institution of Oceanography at UC San Diego.
In a study appearing March 15 in the journal Earth and Planetary Science Letters,
Scripps Oceanography researchers Dave Stegman, Leah Ziegler, and Nicolas Blanc
provide new estimates for the thermodynamics of magnetic field generation
within the liquid portion of the early Earth’s mantle and show how long that
field was available.
The paper provides a “door-opening opportunity” to
resolve inconsistencies in the narrative of the planet’s early days.
Significantly, it coincides with two new studies from UCLA and Arizona State
University geophysicists that expand on Stegman’s concept and apply it in new
ways.
“Currently we have no grand unifying theory for how
Earth has evolved thermally,” Stegman said. “We don’t have this conceptual
framework for understanding the planet’s evolution. This is one viable
hypothesis.”
The trio of studies are the latest developments in a
paradigm shift that could change how Earth history is understood.
It has been a bedrock tenet of geophysics that
Earth’s liquid outer core has always been the source of the dynamo that
generates its magnetic field. Magnetic fields form on Earth and other planets
that have liquid, metallic cores, rotate rapidly, and experience conditions
that make the convection of heat possible.
In 2007, researchers in France proposed a radical
departure from the long-held assumption that the Earth’s mantle has remained
entirely solid since the very beginnings of the planet. They argued that during
the first half of the planet’s 4.5-billion-year history, the bottom third of
Earth’s mantle would have had to have been molten, which they call “the basal
magma ocean.” Six years later, Stegman and Ziegler expanded upon that idea,
publishing the first work showing how this once-liquid portion of the lower
mantle, rather than the core, could have exceeded the thresholds needed to
create Earth’s magnetic field during that time.
The Earth’s mantle is made of silicate material that
is normally a very poor electrical conductor. Therefore, even if the lowermost
mantle were liquid for billions of years, rapid fluid motions inside it
wouldn’t produce large electrical currents needed for magnetic field
generation, similar to how Earth’s dynamo currently works in the core.
Stegman’s team asserted the liquid silicate might actually be more electrically
conductive than what was generally believed.
“Ziegler
and Stegman first proposed the idea of a silicate dynamo for the early Earth,”
said UCLA geophysicist Lars Stixrude. The idea was met with skepticism because
their early results “showed that a silicate dynamo was only possible if the
electrical conductivity of silicate liquid was remarkably high, much higher
than had been measured in silicate liquids at low pressure and temperature.”
A team led by Stixrude used quantum-mechanical
computations to predict the conductivity of silicate liquid at basal magma
ocean conditions for the first time.
According to Stixrude, “we found very large values of
the electrical conductivity, large enough to sustain a silicate dynamo.“ The
UCLA study appeared in the Feb. 25 issue of Nature
Communications.
In another paper, Arizona State geophysicist Joseph
O’Rourke applied Stegman’s concept to consider whether it’s possible that Venus
might have at one point generated a magnetic field within a molten mantle.
These new studies are signs that the premise is
starting to take hold, but is still far from being widely accepted.
“No
one is going to believe it until they do it themselves and now two other highly
esteemed scientists have done it themselves,” said Stegman.
"The pioneering studies of Dave Stegman and his
collaborators directly inspired my work on Venus,” said O’Rourke. “Their recent
paper helps answer a question that vexed scientists for many years: How has
Earth's magnetic field survived for billions of years?”
If Stegman’s premise is correct, it would mean the
mantle could have provided the young planet’s first magnetic shield against
cosmic radiation. It could also underpin
studies of how tectonics evolved on the planet later in history.
“If
the magnetic field was generated in the molten lower mantle above the core,
then Earth had protection from the very beginning and that might have made life
on Earth possible sooner,” Stegman said.
“Ultimately,
our papers are complementary because they demonstrate that basal magma oceans
are important to the evolution of terrestrial planets,” said O’Rourke. “Earth's
basal magma ocean has solidified but was key to the longevity of our magnetic
field.”
The Scripps Oceanography study was funded by the
National Science Foundation, the U.S. Department of Energy, and a UC San Diego
SEED Fellowship.
原始論文：
1.
Nicolas A. Blanc, Dave R. Stegman,
Leah B. Ziegler. Thermal and magnetic evolution of a crystallizing
basal magma ocean in Earth's mantle. Earth and Planetary Science
Letters, 2020; 534: 116085 DOI: 10.1016/j.epsl.2020.116085
2.
J. G. O'Rourke. Venus: A Thick
Basal Magma Ocean May Exist Today. Geophysical Research Letters,
2020; 47 (4) DOI: 10.1029/2019GL086126
3.
Lars Stixrude, Roberto Scipioni,
Michael P. Desjarlais. A silicate dynamo in the early Earth. Nature
Communications, 2020; 11 (1) DOI: 10.1038/s41467-020-14773-4
引用自：University of California - San Diego.
"Earth's mantle, not its core, may have generated planet's early magnetic
field."
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