岩漿是組成月球的關鍵成分
By Jim Shelton
一百多年來，科學家對於月球如何形成一直沒有定論。但最近耶魯大學和日本的科學家表示他們或許找到了答案。
模擬巨型撞擊事件形成月球過程中的兩張截圖。圖片中間是原始地球；紅點是原始地球岩漿海飛濺出來的物質；藍點則是撞擊體的組成物質。圖片來源：耶魯大學
許多理論學家相信一顆跟火星差不多大的星體撞上了早期地球，飛濺出來的物質成為了月球的基本材料。運用電腦模擬該理論的結果，顯示月球的主要組成應該來自於該顆撞上地球的星體。但實際情形卻恰好相反。科學家分析阿波羅登月任務帶回來的岩石，得知月球的主要組成其實來自於地球。
4月29日發表於《自然―地球科學》(Nature Geoscience)的新研究提出了解釋。其中一位共同作者為耶魯大學的地球物理學家唐戶俊一郎。
唐戶表示關鍵是許久以前(大概是在太陽形成後的5000萬年)的原始地球表面覆蓋了一層熾熱的岩漿海，而撞擊體則可能全由固體組成。因此唐戶和同僚測試的新模型，基本假設便是原始地球表面覆蓋了一層岩漿海，而撞擊體則是由固體組成。
研究人員說模型顯示兩者碰撞之後，岩漿的加熱幅度遠遠超出從撞擊體飛散開來的固體。這讓岩漿的體積膨脹，並飛濺到地球的運行軌道而形成月球。結果可以解釋為什麼月球的組成中大部分都是地球來的物質。相較之下，過往的模型並未考量到原始地球的矽酸鹽和撞擊體會被加熱到不同的溫度。
唐戶曾對原始地球的岩漿化學性質進行了諸多研究。他說：「我們的模型顯示月球的組成大概80%來自於原始地球。而之前大部分的模型卻顯示月球的組成大概80%來自於撞擊體。這是十分巨大的差異。」
唐互表示新模型證實了之前有關月球形成的理論，而不用增加特殊的碰撞條件――某些理論學家之前必須這麼做才能給出合理的解釋。
在此研究中，唐戶領導了矽酸鹽融化後的壓縮實驗。另一組東京工業大學和日本理化學研究所(RIKEN)計算科學研究中心組成的團隊，則建立出新的電腦模型來預測碰撞產生的物質如何形成月球。
研究第一作者為RIKEN的細野七月，其他共同作者為牧野淳一郎和斎藤貴之。
Magma is the key to the moon’s makeup
For more than a century, scientists have
squabbled over how the Earth’s moon formed. But researchers at Yale and in
Japan say they may have the answer.
Many theorists believe a Mars-sized object slammed
into the early Earth, and material dislodged from that collision formed the
basis of the moon. When this idea was tested in computer simulations, it turned
out that the moon would be made primarily from the impacting object. Yet the
opposite is true; we know from analyzing rocks brought back from Apollo
missions that the moon consists mainly of material from Earth.
A new study published April 29 in Nature Geoscience, co-authored by Yale
geophysicist Shun-ichiro Karato, offers an explanation.
The key, Karato says, is that the early, proto-Earth
— about 50 million years after the formation of the Sun — was covered by a sea
of hot magma, while the impacting object was likely made of solid material.
Karato and his collaborators set out to test a new model, based on the collision
of a proto-Earth covered with an ocean of magma and a solid impacting object.
The model showed that after the collision, the magma
is heated much more than solids from the impacting object. The magma then
expands in volume and goes into orbit to form the moon, the researchers say.
This explains why there is much more Earth material in the moon’s makeup.
Previous models did not account for the different degree of heating between the
proto-Earth silicate and the impactor.
“In our model, about 80% of the moon is made of
proto-Earth materials,” said Karato, who has conducted extensive research on
the chemical properties of proto-Earth magma. “In most of the previous models,
about 80% of the moon is made of the impactor. This is a big difference.”
Karato said the new model confirms previous theories
about how the moon formed, without the need to propose unconventional collision
conditions — something theorists have had to do until now.
For the study, Karato led the research into the
compression of molten silicate. A group from the Tokyo Institute of Technology
and the RIKEN Center for Computational Science developed a computational model
to predict how material from the collision became the moon.
The first author of the study is Natsuki Hosono of
RIKEN. Additional co-authors are Junichiro Makino and Takayuki Saitoh.
原始論文：Natsuki
Hosono, Shun-ichiro Karato, Junichiro Makino, Takayuki R. Saitoh. Terrestrial
magma ocean origin of the Moon. Nature Geoscience, April 29,
2019; DOI: 10.1038/s41561-019-0354-2
引用自：Yale University. "Magma is the key to the
moon's makeup."
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