The origin of water in chondritic meteorites and the Earth

The chondritic meteorites formed between ~2 Myr and ~4 Myr after Solar System formation and provide records of processes that were operating in the solar protoplanetary disk before and during that time. However, the fidelity of that record varies because all chondrites were subject to internal heating by the decay of short-lived radionuclides. This internal heating drove aqueous and metamorphic lithification processes that converted unconsolidated ‘sediments’ into rocks that could survive impact excavation from their asteroidal parent bodies and atmospheric entry. The effects of lithification can be reduced to some extent by studying the least altered meteorites, but water and organics have been modified even in these samples. Water, for instance, was involved in the oxidation of Fe-metal. If the H2 that was generated and lost approached isotopic equilibrium with the water, it was probably very deuterium-(D)-poor as is seen in low temperature serpentines on Earth. As a result, the remaining water became increasingly D-rich. The water was also involved in the alteration of silicates to hydrated minerals. Indeed, it is the H2O/OH in these hydrated minerals that preserves a record of the water isotopic compositions, albeit modified by fluid-silicate H and O isotopic exchange. Any ice that may once have been present will have sublimed away in interplanetary space before atmospheric entry. It also seems likely that there was H isotopic exchange between the water and D-rich organic matter during lithification. Various approaches can be used to try to ’see’ through the parent body modification. It seems that the carbonaceous chondrites, which are generally thought to have formed beyond the orbit of Jupiter, accreted ices (CO/CO2– and NH3/HCN-bearing) and refractory organic matter that resembled in many ways those in comets. Both the water and organics in comets and chondrites may have at least partial interstellar heritages as indicated by their high D/H ratios relative to the solar ratio. However, the D/H of the water and, perhaps, the organics are lower in chondrites than comets, probably because of heating in the disk prior to accretion. Counterintuitively, the water accreted by the chondrites, which are thought to have formed in the warmer terrestrial planet region sunward of Jupiter, may have had higher D/H than in the carbonaceous chondrites, but this remains controversial. The bulk of the Earth’s building blocks were probably early-formed (<2 Myr) planetesimals that had melted and differentiated due to the abundance of short-lived radioactive heat sources. Our measurements of the silicate mantles of such objects, achondrites, indicates that they contain very little H. Hence, the Earth’s H, along with C, N, noble gases and other highly volatile elements, was probably largely delivered by a few weight percent of planetesimals that resembled CI/CM carbonaceous chondrites and was accreted before the Moon-forming impact. However, noble gases require small contributions from comets and solar gas. There is also some noble gas evidence that, counter to the current paradigm, most volatiles were neither degassed from nor equilibrated with a global magma ocean following formation of the Moon, or that impact induced blow off of early atmospheres can account for the non-chondritic BSE relative abundances of volatiles elements like N.