Thermal H/D exchange in polar ice - deuteron scrambling in space

We have investigated the thermally induced proton/deuteron exchange in mixed amorphous H2O:D2O ices by monitoring the change in intensity of characteristic vibrational bending modes of H2O, HDO, and D2O with time and as function of temperature. The experiments have been performed using an ultrahigh vacuum setup equipped with an infrared spectrometer that is used to investigate the spectral evolution of homogeneously mixed ice upon co-deposition in thin films, for temperatures in the 90-140K domain. With this non-energetic detection method, we find a significantly lower activation energy for H/D exchange -3840 +/- 125K - than previously reported. Very likely this is due to the amorphous nature of the interstellar ice analogues involved. This provides reactive time-scales (tau < 10(4) yr at T > 70 K) fast enough for the process to be important in interstellar environments. Consequently, an astronomical detection of D2O will be even more challenging because of its potential to react with H2O to form HDO. Furthermore, additional experiments, along with previous studies, show that proton/deuteron swapping also occurs in ice mixtures of water with other hydrogen-bonded molecules, in particular on the OH and NH moieties. We conclude that H/D exchange in ices is a more general process that should be incorporated into ice models that are applied to protoplanetary discs or to simulate the warming up of cometary ices in their passage of the perihelion, to examine the extent of its influence on the final deuteron over hydrogen ratio.