ACCRETION DISK ORIGIN OF EARTH'S WATER: LABORATORY EXPERIMENTS

The presence of water on the Earth is an enigma. It is generally agreed that it was too hot at 1 AU for hydrous minerals to be stable in the accretion disk. Thus, Earth's water is conventionally believed to be delivered by comets or wet asteroids after Earth formed. However, wet asteroids and comets have elemental and isotopic properties that are inconsistent with those of Earth [1] limiting the amount of water derived from comets and wet asteroids. Drake [1] proposed that water was introduced during planet growth in the accretion disk in a form stable under high temperature conditions. This hypothesis is supported by the presence of water in the disks around young stars [2] and by numerical simulations of water adsorption on silicate grains under conditions corresponding to those in the accretion disk [3-5] which showed that molecular chemisorption of water on forsterite (a main constituent of the dust grains) might account for the formation of several Earth oceans [4]. Here we show by laboratory experiments (see Figure) that water adsorbs dissociatively on the olivine {100} surface at the temperature (approximately 500-1500 K) and water pressure (approximately 10)8 bar) expected for the accretion disk, leaving an OH adlayer that is stable at least up to 900 K. This high temperature stability may result in the accretion of many Earth oceans to Earth and other terrestrial planets, provided that a viable mechanism to produce water from hydroxyl exists.