IR detection of H2O2 at 80 K in ion-irradiated laboratory ices relevant to Europa

We present the first in situ laboratory detection of H2O2 in H+ irradiated ices at temperatures relevant to the icy Galilean satellites. These experiments were motivated by the recent Galileo NIMS detection on Europa of a 3.5-mu m band identified with H2O2 (R. W. Carlson et al. 1999, Science 283, 2062-2064). In our laboratory experiments, the IR signature of H2O2 was easily observed after irradiation of pure H2O at 16 K, but it was not seen after irradiation at 80 K. Radiolysis of mixtures of H2O with O-2 or CO2 at 80 K did produce H2O2 These results show that ices more complex than pure H2O are involved in the radiolysis pathway to form H2O2 On Europa. We also report the intrinsic band strength and radiation yield of H2O2 in ice mixtures, along with possible formation and destruction mechanisms. Sufficient concentrations of H2O2 can be formed in H2O ices containing O-2 to explain the NIMS observation. (C) 2000 Academic Press.