- © The Mineralogical Society Of America
Earth is the water planet. Liquid water covers more than 70% of the surface and dominates all surface processes, geological, meteorological, and biological. However the hydrosphere composes only about 0.025% of the planet’s mass, so that small amounts of H incorporated into the oxygen minerals of the interior may constitute the majority of Earth’s total water. The Earth is thought to be generally similar in composition to the chondrite meteorites which average about 0.10% by weight H2O. So if the Earth were strictly chondritic in its H content, about 75% of that H as water would have either been tied up in the minerals of the interior or lost to space. Understanding how H behaves at the atomic scale in these materials will help us to understand how the Earth balances and retains its water and may help us to understand how water planets develop and how common they might be.
In addition to the surface processes, water also controls the processes of the interior. Water dramatically reduces the melting temperature of rocks controlling igneous processes. Even trace amounts of hydrogen have a major effect on some physical properties such as deformation strength and electrical conductivity (Karato 1990). The nominally anhydrous minerals of the Earth’s interior are capable of incorporating many times the amount of water in the hydrosphere, and these phases would need to be saturated before stoichiometrically hydrous minerals could be stable. Hydrogen in amounts reported in olivine, wadsleyite, and ringwoodite by Kohlstedt et al. (1996) as recalibrated by Bell et al. (2003), if present in the Earth, would constitute a significant fraction of the total water budget of the planet. The amounts that can be incorporated into the nominally anhydrous minerals of the Transition Zone (410–660 km depth) may constitute the largest reservoir of …