Kinetics of hydrogen/deuterium exchanges in cometary ices
Résumé
The D/H composition of volatile molecules composing cometary ices brings key constraints on the origin of comets, on the extent of their presolar heritage, as well as on the origin of atmospheres and hydro-spheres of terrestrial planets. Nevertheless, the D/H composition may have been modified to various extents in the nucleus when a comet approaches the Sun and experiences deep physical and chemical modifications in its subsurface. We question here the evolution of the D/H ratio of organic species by proton exchanges with water ice. We experimentally studied the kinetics of D/H exchanges on the ice mixtures H 2 O:CD 3 OD, H 2 O:CD 3 ND 2 and D 2 O:HCN. Our results show that fast exchanges occur on the –OH and –NH 2 chemical groups, which are processed through hydrogen bonds exchanges with water and by the molecular mobility triggered by structural changes, such as glass transition or crystallization. D/H exchanges kinetic is best described by a second-order kinetic law with activation energies of 4300 ± 900 K and 3300 ± 100 K for H 2 O:CD 3 OD and H 2 O:CD 3 ND 2 ice mixtures, respectively. The corresponding pre-exponential factors ln(A(s À1)) are 25 ± 7 and 20 ± 1, respectively. No exchange was observed in the case of HCN trapped in D 2 O ice. These results strongly suggest that upon thermal heating (1) –OH and –NH 2 chemical groups of any organic molecules loose their primordial D/H composition and equilibrate with water ice, (2) HCN does not experience proton transfer and keeps a primordial D/H composition and (3) C–H chemical groups are not isotopically modified.