【惑星科学】前駆天体の溶融と蒸発によって決まった地球の化学組成

Earth’s distinctive chemical composition may have been shaped by the evaporation of molten rock from planetesimals (the precursors of planets), according to two independent studies published in Nature this week. These findings could help to explain how Earth, Mars and other rocky bodies in our Solar System formed.

Earth and other rocky planets have different compositions from chondritic meteorites, which are thought to represent the primitive conditions in the Solar System and are considered to have been the building blocks of Earth. Although some explanations for the variations have been proposed, not all differences have been accounted for. The evaporation of molten rock from growing planetesimals seems to fill some of the gaps in our understanding. Remco Hin and colleagues find that this process can explain the differences they observe in magnesium isotope abundances between differentiated bodies, such as Earth, and chondritic meteorites. In a separate study, Ashley Norris and Bernard Wood also conclude that the depletion of certain elements on Earth could have resulted from the evaporation of molten rock on precursor bodies. They recreated processes that occurred during Earth’s formation by melting rock in a furnace, and found that some elements could escape in vapour released from the molten rock in proportions similar to that observed within Earth.

Both groups suggest that collisions between planetesimals, such as the Moon-forming impact, may have driven the melting and evaporation of the growing bodies. Understanding the physical chemistry of melting evaporation could be an arbiter of competing models of planet formation, writes Edward Young in an accompanying News & Views article. He adds that although these studies are not the first to suggest a role for collisions in the depletion of certain elements, they may encourage further studies of how collisions may have shaped the chemical makeup of planets.