地球の内核、地表より遅く回転している可能性　英科学誌に論文

【パリＡＦＰ＝時事】地球の中心部に存在する高温の鉄などでできた「内核」が、地表より速く回転していたのが止まり、今度はその回転速度が地表よりも遅くなっている可能性があるとする論文が２３日、英科学誌ネイチャージオサイエンスに掲載された。（写真は地球。米航空宇宙局（ＮＡＳＡ）提供）
　地下約５０００キロに存在する内核は、液体金属層の「外核」に浮かんでいるため独自に回転できるが、内核がどのように回転しているのかについては研究者の間で議論されてきた。
　内核について分かっていることは少なく、解析は、地震や核爆発による地震波が地球の中心を通過する際の小さな差異を測定することで行われる。
　論文の研究チームは、内核の動きを追跡するため、過去６０年間に発生した地震波を分析した。
　筆者の中国・北京大学の宋暁東教授らはＡＦＰに、「内核は、地表に対してブランコが前後しているような位置関係で回転していると考えられる」と話し、「１回の周期は約７０年」だと説明した。
　論文によると、内核は１９７０年代初期に、地球の他の部分に比べて少し速く回転するようになった。しかし速度は徐々に鈍化した。内核は現在、地表よりも遅く回転しており、次の変化が訪れるのは２０４０年代半ばとの見方を示した。
　内核の動きが地上に与える大きな影響は今のところほとんどないが、宋氏らは、内核から地表まで、地球のすべての層の間には物理的な相互作用があると考えられると述べ、「われわれの研究に触発されて、地球全体を一つのダイナミックなシステムと見なすモデルを構築する研究者が出てくるのを期待している」と語った。
　専門家からは今回の研究結果に懐疑的な意見も出ている。
　南カリフォルニア大学の地震学者ジョン・ビデール氏は、「優秀な科学者が数多くのデータを基に非常に慎重に行った研究」と評価した上で、「どの（数学）モデルも、すべてのデータの説明としては不十分」だと述べた。同氏は昨年、内核の速度変化周期はずっと短く、約６年ごとに起きているとの研究結果を発表している。
　さらに、内核は２００１～１３年に大幅に動き、それ以降は静止しているという説もある。
　また、オーストラリア国立大学の地球物理学者、フルボイエ・トカルチッチ氏は、内核の速度変化周期は今回発表された７０年ではなく、２０～３０年だとする研究結果を発表している。
　同氏は「示されている数学モデルが全部間違っている可能性もある」として、「地球物理学の世界では、今回の研究結果は論争を呼ぶだろう」と述べた。【翻訳編集ＡＦＰＢＢＮｅｗｓ】
〔ＡＦＰ＝時事〕（2023/01/25-12:57）

Earth's inner core, a hot iron ball the size of Pluto, has stopped spinning faster than the planet's surface and might now be rotating slower than it, research suggested on Monday.
Roughly 5,000 kilometres (3,100 miles) below the surface we live on, this planet within the planet can spin independently because it floats in the liquid metal outer core.
Exactly how the inner core rotates has been a matter of debate between scientists -- and the latest research is expected to prove controversial.
What little is known about the inner core comes from measuring the tiny differences in seismic waves -- created by earthquakes or sometimes nuclear explosions -- as they pass through the middle of the Earth.
Seeking to track the inner core's movements, new research published in the journal Nature Geoscience analysed seismic waves from repeating earthquakes over the last six decades.
We believe the inner core rotates, relative to the Earth's surface, back and forth, like a swing, the study's authors, Xiaodong Song and Yi Yang of China's Peking University, told AFP.
One cycle of the swing is about seven decades, the authors said.
The inner core started rotating slightly faster than the rest of the planet in the early 1970s, the study said.
But it had been slowing down before coming in sync with Earth's rotation around 2009, it added.
There has been a negative trend since, the study said, meaning the inner core is now rotating slower than the surface.
The researchers predicted the next change would occur in the mid-2040s.
They said this rotation timeline roughly lines up with changes in what is called the length of day -- small variations in the exact time it takes Earth to rotate on its axis.
- Stuck in the middle -
So far there is little to indicate that what the inner core does has much effect on surface dwellers.
But the researchers said they believed there were physical links between all Earth's layers, from the inner core to the surface.
We hope our study can motivate some researchers to build and test models which treat the whole Earth as an integrated dynamic system, they said.
Experts not involved in the study expressed caution about its findings, pointing to several other theories and warning that many mysteries remain about the centre of the Earth.
This is a very careful study by excellent scientists putting in a lot of data, said John Vidale, a seismologist at the University of Southern California.
(But) none of the models explain all the data very well in my opinion, he added.
Vidale published research last year suggesting that the inner core oscillates far more quickly, swinging around every six years or so.
His work was based on seismic waves from two nuclear explosions in the late 1960s and early 1970s.
That timeframe is around the point when Monday's research says the inner core was last in sync with Earth's rotation -- which Vidale called kind of a coincidence.
- Geophysicists 'divided' -
Another theory -- which Vidale said has some good evidence supporting it -- is that the inner core only moved significantly between 2001 to 2013 and has stayed put since.
Hrvoje Tkalcic, a geophysicist at the Australian National University, has published research suggesting that the inner core's cycle is every 20 to 30 years, rather than the 70 proposed in the latest study.
These mathematical models are most likely all incorrect because they explain the observed data but are not required by the data, Tkalcic said.
Therefore, the geophysical community will be divided about this finding and the topic will remain controversial.
He compared seismologists to doctors who study the internal organs of patients' bodies using imperfect or limited equipment.
Lacking something like a CT scan, our image of the inner Earth is still blurry, he said, predicting more surprises ahead.
That could include more about a theory that the inner core might have yet another iron ball inside it -- like a Russian doll.
Something's happening and I think we're gonna figure it out, Vidale said. But it may take a decade.