- © The Mineralogical Society Of America
The origin of water is a long standing problem that has fascinated generations of philosophers and scientists since the dawn of humanity. It has to do with processes that took place in the nascent solar system, but, unfortunately, we lack record of what really happened during this dark age. On Earth, the tectonic activity has erased completely any record dating back from this period, and the oldest rocks preserved on Earth crystallized about 600 Ma after start of solar system condensation (ASSC). What we observe today does not represent necessarily what was present when the solar system formed, and it may well be possible that the true water ancestors could have had chemical and isotopic characteristics that are not observed in any reservoir of the present-day solar system. However, the extraterrestrial objects that escaped planetary differentiation (chondrites, comets, interplanetary dust) are probably the best available precursor candidates for the source of volatile elements in Earth. In this context, the nature of such potential contributions can be estimated in view of chemical and isotopic mass balance, taking into account astrophysical and/or thermodynamic constraints on planetary system formation.
The Earth is not a water-rich body. Water at the Earth’s surface (1.5 × 1024 g), mostly in the oceans, makes about 0.025% H2O over the whole Earth (5.97 × 1027 g). Most estimates for the mantle water content advocate a few ocean masses, so that the bulk water content of Earth may be <0.2% H2O (see the “Potential Water Contributors” section in this chapter). For comparison, carbonaceous chondrites contain up to 10% equivalent water (Boato 1954; Robert and Epstein 1982; Kerridge 1985) and comets have up to 50% H2O (Delsemme 1988). Thus the contribution of only a small fraction of such water-rich bodies …