Mulit-Isotopic Fractionation of Water From Sublimation at Low Temperatures
Abstract
Understanding how astrophysical processes like diffusion, evaporation, and condensation of volatile ices affects the ratios of isotopes can help us better understand observations of isotopic variation in astrophysical systems. For example, the sublimation of water-ice from astrophysical and planetary surfaces such as interstellar dust, comets and asteroids at low temperatures is expected theoretically to result in significant shifts in the isotopic composition of the residual water ice. Using a UHV system with closed-cycle He cryostat and a cavity ring-down isotope spectrometer (CRD), we developed a protocol for measuring changes in the ratios of D/H, 18O/16O, and 17O/16O that result from the sublimation of water ice at 155 K. In this presentation, we discuss the challenges and lessons learned to achieving experimental conditions that mimic astrophysical environments. Results from our experiments, including instantaneous fractionation factors associated with hydrogen and oxygen isotopes will be presented. The role that experimental conditions such as sample size and surface temperature and composition play will also be discussed. To assess whether the experimental conditions present in the UHV chamber are adequate for simulating sublimation into the near perfect vacuum of space, we developed a detailed model of the isotopic fluxes associated with the water-ice surface, vacuum chamber walls, and pumping provided by a magnetically levitated turbo pump. This model incorporates a transition-state-theory (TST) based representation of surface potential energies and associated isotopic fractionation factors. Using this model, we estimated the magnitude of water recondensation onto the cold-surface and calculated the associated effects on the isotopic composition of the residual water ice compared to the theoretical values expected for pure sublimation.
- Publication:
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American Astronomical Society Meeting Abstracts #233
- Pub Date:
- January 2019
- Bibcode:
- 2019AAS...23345502T