A planetary perspective on the deep Earth

Abstract

Earth is an engine, tending to obliterate some of the evidence of events that are distant in time, but a memory is retained in its chemistry, its isotopes, the presence of the Moon, perhaps also in geophysical observables such as the temperature of the core and the nature of the mantle immediately above the core, and maybe even in the existence of plate tectonics and life. The remarkable growth in the study and understanding of Earth has happened in parallel with a spectacular era of planetary exploration, relevant astronomical discoveries and computational and theoretical advances, all of which help us to place Earth and its interior in a perspective that integrates the Earth sciences with extraterrestrial studies and basic sciences such as condensed-matter physics. However, progress on the biggest challenges in understanding the deep Earth continues to rely mainly on looking down rather than looking up. A planetary perspectiveEarth is a planet -one of many. There is nothing particularly remarkable about our home, except perhaps that it is suitable for life like us -arguably a tautology. It happens to be the largest of its type in the Solar System, but as there are only three others of the terrestrial type (Mercury, Venus and Mars) this is not particularly significant. Among planets in general, it is small.In the past decade, we have seen an astonishing explosion in our catalogue of planets outside the Solar System to about 250 so far (see the Extrasolar Planets Encyclopaedia, http://exoplanet.eu/ catalog.php). These are mostly planets that we suspect are like Jupiter, very different from Earth. But as time goes on and detection methods improve, we can expect to find bodies that are Earth-like at least to the extent of being made predominantly of rock and iron, the primary constituents of our planet. Some would claim we might already be finding such bodies 1 , initially those that are more massive than Earth.If planets were like atoms or molecules, or even crystals, we could speak of their characteristics (their DNA, so to speak) in a very compact way, just as a handbook might list the properties of a material. Planets are richer, more complex and more resistant to reductionist thinking. Genetics is the science of heredity and variation in biological systems. By analogy, we can speak of the genetics of a planet such as Earth, while also acknowledging that environment has a role in its evolution and its current state.Cosmologists are familiar with thinking about time logarithmically: a lot happened in a very short period of time back near the Big Bang. To some extent, it helps to think about planet formation in a similar way (Fig. 1). The events that defined Earth's formation and the initial conditions for its subsequent evolution are squeezed into an epoch that may have already been over within 100 million years of the formation of the Solar System. In this epoch more happened inside Earth and more energy was dissipated from within the planet than throughout all of subsequent geological time. We have no direct ...

Composition and fate of short-period super-Earths

“…Molten silicate and water are known to be miscible with each other at pressures above a few GPa and temperatures > ∼1000 K (Shen & Keppler 1997;Mibe et al 2007). Above ∼10 000 K, silicate and iron are also no longer immiscible (Stevenson 2008). Hydrogen, helium and water will mix as well at high enough temperatures.…”

Partitioning experiments in the laser-heated diamond anvil cell: volatile content in the Earth's core

“…Hartmann & Davis 1975) evolved with substantial debate on an impact-triggered origin of the Earth-Moon system (for an overview, see Canup & Righter (2000)) that continues to the present day (Stevenson 2007(Stevenson , 2008. Recently, the possibility of more energetic collisions has led to possible extension of the magma ocean depth through high-temperature impactor core implantation (Canup 2004;Rubie et al 2007;Solomatov 2007) and early tidal heating.…”

High-pressure experimental geosciences: state of the art and prospects

“…Applications of these combined techniques are numerous, as illustrated byafe we xamplesbelo w . Anotheras yet unresolved issue is the scenario of core formation, and whether the percolation of molten Fealloys occurred through a solid or molten silicate matrix (Stevenson, 2008). Percolation has been studied mainly by analysis of the texture of quenched samples (Minariketal.,1996;Shannonand Agee, 1998), which showed non-connected Fe-alloys islands among the silicate matrix.…”