Research press release


Nature Geoscience

Earth’s mantle enriched by iron rain



Richard Krausらは、アメリカ合衆国のサンディア国立研究所が保有する世界最強のX線発生装置であるZマシンを用いて、アルミニウムの板を鉄試料に非常に高速で衝突させ、鉄試料を非常に高圧な衝撃圧にさらした。彼らは、鉄を蒸発させるために必要な衝撃圧はこれまで考えられていたよりもずっと低く、惑星形成後期における太陽系内の初期地球と他の物体との高速衝突で容易に到達可能であることを見つけた。著者らは、このような物体の鉄の核は衝突により生成される衝撃圧により蒸発し、その結果生じる蒸気のプリュームは地球全体に分布することを提案している。冷却後には、蒸気は凝結して鉄の雨となり、まだ溶けていた地球のマントルに混合された。


関連するNews & Viewsの記事でWilliam Andersonは、「この研究は地球や他の惑星の進化で高速衝突過程が果たす役割を強調している」と記している。

Vaporization of iron in violent collisions between the early Earth and other objects in the Solar System may explain the iron-rich composition of the Earth's mantle, reports a study published online in Nature Geoscience. It was previously assumed that iron from these impacts would melt and quickly sink into the Earth's core.

High-speed impacts between two solid objects produce high-pressure shockwaves that can compress solid material. After the shockwave has passed, if the pressure was sufficiently high, the material will vaporize.

Richard Kraus and colleagues subjected iron samples to extremely high shock pressures by using the Sandia National Laboratories Z-machine, the world's most powerful radiation source, to slam aluminium plates into iron samples at extremely high velocities. They found that the shock pressure required to vaporize iron is much lower than previously thought, and could be readily achieved in the high-speed impacts between the early Earth and other objects in the Solar System towards the end of planet formation. The authors propose that the iron cores of these objects were vaporized by shockwaves generated on impact and the resulting plume of vapour was distributed around the Earth. After cooling, the vapour would then have condensed into an iron rain that then mixed into the Earth’s still-molten mantle.

This process may also explain why the Moon, which is thought to have formed by this time, lacks iron-rich material despite being exposed to similarly violent collisions. The authors suggest that the Moon's reduced gravity could have prevented it from retaining most of the vaporized iron.

In an accompanying News and Views article, William Anderson writes that "the study emphasizes the role that high-velocity impact processes played in the evolution of the Earth and planets."

doi: 10.1038/ngeo2369


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