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 H-2 and CH4,H + HD <-> D + H-2, 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.