THE ROLE OF NEBULAR AND PLANETARY H20 BETWEEN THE PROTOSTELLAR DISK TO PLANET FORMATION TIMES AND LATER

The role of water in the earliest solar system is increasingly recognized in meteorites of all types from the heavily hydrated CI - chondrites (the average solar system proxy) to nominally anhydrous achondrites produced during planetary differentiation (e.g. the H.E.D. suite). Oxygen isotope signatures (d¹⁷O, d¹⁸O & D¹⁷O), the fundamental properties of almost all meteoritic components vary dramatically from near solar compositions (refractory CAI’s at D¹⁷O ~ -30) to ¹⁶O depleted hydration products (D¹⁷O ~+100 to +150). In contrast, the terrestrial planets appear to have a narrow range of D¹⁷O =0±2. The source of the oxygen isotope diversity and homogenization almost certainly reflects the role of water from protostellar times to the epoch of planet building.

The range of the three- isotope diversity in oxygen probably reflects photochemical and isotopic self-shielding phenomena that occurred as the protosun increased mass and radiative output prior to, and at the onset of, solar nuclear fusion in the central star. The radial temperature gradient of the solar nebula from the very high temperatures near the star, reflected in the formation of refractory inclusions (CAI), to low interstellar temperatures at the outer limits of the prestellar gas/dust cloud provides a context for hydration – dehydration ‘alteration’ reactions that depend on localized a_H20.

A ‘frost-line’ where water ice becomes stable has existed in the solar system at all times from protosolar to the present day. The position of the frost line has varied with time but must have expanded outward dramatically at the onset of stellar nuclear fusion. The limited range of planetary oxygen signatures reflects homogenization of nebular diversity controlled by the stoichiometry of hydration reactions for the accreted silicates in most planets (esp. olivine). Solids condensed near the sun (D¹⁷O ~ -30) when transported across the frost line can react with outer solar system water (D¹⁷O as positive as +100 to +150). Mass balance of such reactions results produces D¹⁷O ~ -2 to +2 the signature of known planets and most meteorites. The role of water, as constrained by the frost line, therefore provides a fundamental constraint on meteorite and planet formation and indicates that ‘alteration’ is the most widespread and ubiquitous nebular process.