地球型惑星内部物理学の最近の進展

Abstract

Great advances on observational studies of terrestrial planets including our own Earth since the 1990s suggest that we should fundamentally revise our view on the evolution of planetary interiors. It has been implicitly assumed that the mantle of a terrestrial planet evolves quasi-statically due to heat extraction by mantle convection. Tomographic studies on the Earth's mantle together with geologic studies, however, show that the overturn time of mantle convection is on the order of a billion years, which is comparable to the age of the Earth. So the evolution of the Earth's mantle is a dynamic process and the quasi-static model of mantle evolution is no longer tenable. Images of terrestrial planets recently sent from spacecraft suggest that the evolution of terrestrial planets depends systematically on their size, and that the evolution of the Earth should be understood in the context of this systematic relationship. The finding of super-Earths implies that we must figure a way to infer the evolution of these planets from limited information, such as the planetary mass, radius, and the composition of the atmosphere, on the basis of studies on terrestrial planets in our solar system. Geologic studies on Archean continents older than 2.5 billion years show that the tectonic regime of early Earth is qualitatively different from that of modern Earth, so the magmatism-mantle convection system in the Earth must have experienced a regime-transition at some point in its history. More dynamically oriented models of mantle evolution would be useful for predicting how the Earth's mantle evolved at its earliest stages on the basis of observations of other celestial bodies such as the Moon, where the old surface is still preserved. Further refinement of seismic studies on the Earth's interior combined with studies on the properties of mantle materials under high pressures are crucial to improve our understanding of the history of the Earth's interior.