An Etymological Dictionary of Astronomy and Astrophysics

The → enrichment of deuterium (D) with respect to → hydrogen (H) in
→ Solar System molecules
when compared with the D/H ratio in the → interstellar → solar nebula. H-bearing molecules in → comets, → planets, and → chondrite 
→ meteorites show a systematic D enrichment relative to the → molecular hydrogen of the solar nebula. Because there is no nuclear source for D in the Universe,
the observed → isotopic enrichment must have its origin in chemical reactions having faster reaction rates for D than for H.

In the Solar nebula the → isotopic fractionation of D between → water and H followed the reversible reaction:

H₂O + HD ⇔ HDO + H₂.

At low temperatures, this reaction favors the concentration of D in HDO. In the → interstellar medium grain chemistry plays a crucial role in D enrichment. See also → enrichment factor.

Apart from → deuterium fractionation, D could be enriched through another mechanism. Since molecular hydrogen (H₂) is more → volatile than molecular deuterium (D₂), D/H ratio could increase in certain planets that orbit near their star.

See also: → deuterium; → enrichment.