The condensation and vaporization behavior of H 2O: CO ices and implications for interstellar grains and cometary activity

Abstract

A number of parameters associated with the physical behavior of CO, H 2O, and H 2O: CO ices have been determined using infrared spectroscopy. The COCO surface binding energy in pure CO ices is found to be ( ΔH s k )=960±10 K , in good agreement with a previously determined value of 955 K. In H 2O-rich ices containing minor amounts of CO, the H 2OH 2O surface binding energy is founnd to be ( Δh s k )=4815±15 K and 5070±50 K for unannealed and annealed ices, respectively. The determined COH 2O volume binding energies are ( ΔH v k )=5550±350 K and 5800±350 K for unannealed and annealed ices, respectively. CO is observed to condense in substantial quantities into H 2O-rich ices at temperatures twice as high as those required for condensation in pure CO. Under interstellar conditions, CO can condense onto H 2O-rich ice grains at temperatures up to 50 K. When H 2O:CO ices condensed at 10 K are warmed they lose substantial amounts of CO between 30 and 65 K, 125 and 150 K, and 150 and 175 K. These ranges are associated with processes involving molecular rearrangement and sublimation of the H 2O matrix and diffusion of CO through the H 2O matrix. The presence of CO in H 2O ice modestly increases the “effective volatility” of the H 2O. The implications of these data for cometary models and our understanding of cometary formation are considered. The amounts of CO and CO 2 observed in Comet Halley are consistent with ices produced in the interstellar phase. When the nucleus is warmed upon approaching the Sun, volatiles such as CO can filter through porous, amorphous H 2O ice of the comet and escape, resulting in activity further from the Sun than expected for pure H 2O ices. Some of the mobile volatiles can collect in pockets and be released suddenly, producing outburst and jets. The dominant behavior, however, will be mediated by H 2O as long as the concentrations of more volatile species is less than 30%.