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We have investigated the thermally induced proton/deuteron exchange in mixedamorphous H$_2$O:D$_2$O ices by monitoring the change in intensity ofcharacteristic vibrational bending modes of H$_2$O, HDO, and D$_2$O with timeand as function of temperature. The experiments have been performed using anultra-high vacuum setup equipped with an infrared spectrometer that is used toinvestigate the spectral evolution of homogeneously mixed ice uponco-deposition in thin films, for temperatures in the 90 to 140 K domain. Withthis non-energetic detection method we find a significantly lower activationenergy for H/D exchange -- $3840 \pm 125$ K -- than previously reported. Verylikely this is due to the amorphous nature of the interstellar ice analoguesinvolved. This provides reactive timescales ($\tau<10^4$ years at $T$ $>70$ K)fast enough for the process to be important in interstellar environments.Consequently, an astronomical detection of D$_2$O will be even more challengingbecause of its potential to react with H$_2$O to form HDO. Furthermore,additional experiments, along with previous studies, show that proton/deuteronswapping also occurs in ice mixtures of water with other hydrogen bondedmolecules, in particular on the OH and NH moieties. We conclude that H/Dexchange in ices is a more general process that should be incorporated into icemodels that are applied to protoplanetary disks or to simulate the warming upof cometary ices in their passage of the perihelion, to examine the extent ofits influence on the final deuteron over hydrogen ratio.

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