Origin of Water Ice in the Solar System

The origin and early distribution of water ice and more volatile compounds in the outer solar system is considered. The origin of water ice during planetary formation is at least twofold: It condenses beyond a certain distance from the proto-Sun — no more than 5 AU but perhaps as close as 2 AU — and it falls in from the surrounding molecular cloud. Because some of the infalling water ice is not sublimated in the ambient disk, complete mixing between these two sources was not achieved, and at least two populations of icy planetesimals may have been present in the protoplanetary disk. Added to this is a third reservoir of water ice planetesimals representing material chemically processed and then condensed in satellite-forming disks around giant planets. Water of hydration in silicates inward of the condensation front might be a separate source, if the hydration occurred directly from the nebular disk and not later in the parent bodies. The differences among these reservoirs of icy planetesimals ought to be reflected in diverse composition and abundance of trapped or condensed species more volatile than the water ice matrix, although radial mixing may have erased most of the differences. Possible sources of water for Earth are diverse, and include Mars-sized hydrated bodies in the asteroid belt, smaller “asteroidal” bodies, water adsorbed into dry silicate grains in the nebula, and comets. These different sources may be distinguished by their deuterium-to-hydrogen ratio, and by predictions on the relative amounts of water (and isotopic compositional differences) between Earth and Mars.