Solar System Deuterium/Hydrogen Ratio

The D/H variations in chondritic meteorites are reviewed and compared with astronomical observations in the interstellar medium. The observed D enrichment in organic molecules — and to a lesser extent in water — is a general rule in the universe. The D is preferentially transferred from the universal reservoir (H2) by low-temperature reactions (10–100 K) from neutral molecules (H2 or H) to hot radicals or ions. The signature of this process is preserved in meteorites and comets. Although this D-enrichment procedure is well understood, in this review a special emphasis is placed on the unsolved issues raised by solar system D/H data. Specifically, it is shown that (1) the usual interstellar interpretation for the origin of the D-rich compounds in chondrites is not quantitatively substantiated and (2) the isotopic heterogeneity observed in LL chondrites is difficult to reconcile with any known natural reservoir or process.