Biology
Scientific paper
Dec 2011
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011agufm.p33a1741e&link_type=abstract
American Geophysical Union, Fall Meeting 2011, abstract #P33A-1741
Biology
[5200] Planetary Sciences: Astrobiology, [6020] Planetary Sciences: Comets And Small Bodies / Ices
Scientific paper
Among D-bearing molecules, water is especially interesting from an astrophysical point of view. Although the deuterium content of water in astronomical environments is relatively small as compared with other molecules, it holds most valuable information, still largely undeciphered, on the dynamics of formation and evaporation of ice grain mantels in protostellar regions [1], and is crucial for the understanding of the formation of the Solar System and the Earth [2]. In this work, we have used the OD stretching bands of HDO and D2O molecules in various ice mixtures formed by vapor deposition on a cold substrate (see ref [3] for a description of the experimental set-up) to study the sensitivity of the IR technique for the detection of HDO in ice samples, and to monitor processes of H/D isotope exchange in these solids. It is found that the detection sensitivity is strongly dependent on the ice structure. The OD band is extremely broad and tends to disappear into the absorption continuum of H2O for low temperature amorphous samples. Detectable HDO/H2O ratios with this technique may range from a few per cent for amorphous samples to a few per thousand in crystalline ice. These relatively high upper limits and the appreciable dependence of the band shape on temperature, complicating the interpretation of data from many lines of sight, may question the usefulness of this technique. Isotopic H/D exchange in mixed ices of H2O/D2O is found to start at ~ 120 K and is greatly accelerated at 150 K, as crystallization proceeds in the ice. The process is mainly driven by proton transfer assisted by orientational defect mobility. Annealed amorphous samples are more favourable for isotope exchange than samples directly formed in the crystalline phase. The annealing process seems to lead to polycrystalline ice morphology with a higher defect activity. The present data emphasize the relevance of a depletion mechanism for D atoms in hydroxylic bonds in the solid state, recently advanced by Ratajczak et al.[4] in a study with deuterated methanol. This mechanism can be of relevance for deuterium fractionation in star forming regions. This work has been funded by the Spanish Ministry of Science under Grants FIS2007-61686 and FIS2010-16455. RE acknowledges Sabbatical grant PR2010-0012 from the same Ministry, at Department of Chemistry, University of British Columbia, Vancouver V6T 1Z1, Canada. OG acknowledges also funding from the MCINN RyC program. [1] Lui, F. et al. 2011, A&A, 527, A19. [2] a) Robert, F. et al. 2000, Space Sci. Rev., 92, 201,b) Robert, F., 2001, Science 293, 1056. [3] a) Carrasco, E. et al. 2002, rev. Sci. Instrum. 73, 3469, b) Maté, B. et al. 2003, J. Phys. Chem. B, 107, 11098, c) Gálvez, O. et al., 2008, Icarus, 197, 599. [4] Ratajczak, A. et al. 2009, A&A, 496, L21.
Escribano R. M.
Gálvez O.
Herrero Víctor J.
Maté Belén
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