Stability of urea in astrophysical ices. A laboratory study of VUV irradiation and high-energy electron bombardment

Abstract

The recent detection of urea in the interstellar medium raises questions about its stability in different astronomical environments. In this work, we have studied the stability of urea ices and urea/water ice mixtures under vacuum-ultraviolet (VUV; 6.3-10.9 eV) irradiation and high-energy (5 keV) electron bombardment at 30, 100, and 200 K. The evolution of the ices was monitored with infrared spectroscopy. CO2, HNCO, and OCN- were identified as reaction products in the 30 K samples. At the higher temperatures CO2 and HNCO were hardly found in the processed ices. The measurements provided destruction cross-sections and allowed the derivation of radiation yields, G100, and half-life doses for urea. G100 values were found to be low (≈3.6-0.3 molecules/100 eV) both for VUV photons and high-energy electrons with electrons being slightly more efficient for the destruction of the molecule. These low G100 values are likely due to favourable mechanisms of energy dissipation or urea recombination. The stability of urea under irradiation increases with temperature which suggests that higher mobility improves the repair mechanisms. Estimates based on these laboratory data indicate that urea should be stable (≈108-109 yr) against irradiation in cold dense clouds and hot cores. It would not survive long (≈103-104 yr) on the bare surface of a Kuiper belt object, but would be well protected (≈109 yr) against radiation below a 30 $$m ice layer. The high resistance of the molecule to radiation damage makes it a good candidate for prebiotic chemistry.