The Radiolytic Stability of Thymine in Water Ice

Nucleobases are present in all known forms of life and are essential for encoding genetic information. The detection and characterization of nucleobases in extraterrestrial materials may be important for understanding the origin of life and for the detection of current of past life. Both biological and non-biological nucleobases have been detected in samples of carbonaceous chondrites (Folsome et al. 1971; Hayatsu etal. 1975; Stoks & Schwartz 1979; Callahan et al. 2011) and may be present in a range of Solar System ices. Nucleobases may form abiotically through the radiolysis or photolysis of ices before and during the formation of the Solar System. On icy worlds such as Europa or Enceladus, extant or past life in their subsurface oceans could leave detectable biomarkers, including nucleobases, that wind up in ices that are accessible to future missions. Because of the potential importance of nucleobases as a target for future missions to icy bodies, it is important to assess their radiolytic stability. In this work, we present data on the radiolytic destruction of the nucleobase thymine, both neat and embedded in water ice, over a range of concentrations and temperatures. This work follows our previous efforts to quantify the radiolytic destruction of amino acids (Gerakines and Hudson 2013, 2015) in ices. In our experiments, we subject micrometer-thin films of pure and diluted mixtures of these compounds in H2O at low temperatures to ~0.8–1.0 MeV protons and monitor their destruction in situ using infrared spectroscopy. Our results demonstrate that both the composition of the ices and the temperatures at which they are irradiated play significant roles in the stability of thymine.

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