Radiolytic Destruction of Uracil in Interstellar and Solar System Ices

Uracil is one of the four RNA nucleobases and a component of meteoritic organics. If delivered to the early Earth, uracil could have been involved in the origins of the first RNA-based life, and so this molecule could be a biomarker on other worlds. Therefore, it is important to understand uracil's survival to ionizing radiation in extraterrestrial environments. Here we present a study of the radiolytic destruction kinetics of uracil and mixtures of uracil diluted in H₂O or CO₂ ice. All samples were irradiated by protons with an energy of 0.9 MeV, and experiments were performed at 20 and 150 K to determine destruction rate constants at temperatures relevant to interstellar and Solar System environments. We show that uracil is destroyed much faster when H₂O ice or CO₂ ice is present than when these two ices are absent. Moreover, destruction is faster for CO₂-dominated ices than for H₂O-dominated ones and, to a lesser extent, at 150 K compared with 20 K. Extrapolation of our laboratory results to astronomical timescales shows that uracil will be preserved in ices with half-lives of up to ∼10⁷ years on cold planetary bodies such as comets or Pluto. An important implication of our results is that for extraterrestrial environments, the application of laboratory data measured for the radiation-induced destruction of pure (neat) uracil samples can greatly underestimate the molecule's rate of destruction and significantly overestimate its lifetime, which can lead to errors of over 1000%.