ORIGIN OF TERRESTRIAL WATER

A key scientific motivation for studying small solar system bodies—comets, asteroids and small satellites, is that they can provide information about the initial conditions, processes and stages of solar system formation, as well as the primordial sources of organics and volatiles for planets in the habitable zone. This is becoming increasingly interesting given the recent explosion in the number of known exoplanets, many in potentially habitable zones around their stars. Nevertheless, how the Earth received its water is an important unsolved question. We can place constraints on when water was delivered: sometime after the formation of CAIs, but earlier than 4.38 Gyr when zircons provide evidence for a veneer of water. Possible scenarios for water delivery include capture of gases adsorbed onto refractory dust grains and then incorporation into planetesimals, chemical reactions on the early earth, and possible delivery by planetary building blocks that formed outside the snow line – including comets and asteroids. Recent focus has been placed on the identification of a chemical fingerprint, namely D/H ratios, that could be used to trace the origin of water on Earth and in the inner solar system.

We will present a summary of D/H information obtained from both ground- and space observations for comets and place this into the context of a new understanding of solar system formation dynamical models, and new insights into protoplanetary disk chemical models and observations to assess this chemical marker for the origin of water on Earth. We will also discuss efforts from the geochemical side to assess the primordial composition of water on Earth. Understanding the origin and early solar system distribution of volatiles is not only important for understanding how Earth became habitable, and relevant to extrasolar planetary system habitability, but is also important for in-situ resource utilization. Finally, we will present some ideas for Discovery class missions to address some of the questions of early solar system volatile distribution.