Enhancement of deuterated ethane on Jupiter

We report laboratory measurements of cross sections of CH₃D and C₂H₅D in the extreme ultraviolet. The results are incorporated in a photochemical model for the deuterated hydrocarbons up to C2 in the upper atmosphere of Jupiter, taking into account the fast reactions for exchanging H and D atoms between H₂ and CH₄, H + HD ↔ D + H₂, CH₃ + D ↔ CH₂D + H. Since there is no reliable kinetics measurement for the reaction, CH₂D + H → CH₃ + D, we use Yung et al.'s estimate for its rate constant. The strong temperature dependence for this reaction leads to large isotopic fractionation for CH₃D and C₂H₅D in the upper atmosphere of Jupiter, where their production rates depend on the abundance of deuterated methyl radical. The model predicts that the D/H ratio in deuterated ethane is about 15 times that of the bulk atmosphere. A confirmation of this result would provide a sensitive test of the photochemistry of hydrocarbons in the atmosphere of Jupiter.