Ice processes and organic matter in space: observations and laboratory simulations

Abstract

The formation of ice mantles on interstellar and circumstellar dust grains is governed by the accretion and desorption processes. Thermal processing of astrophysical ices takes place in circumstellar environments, near protostars, or already formed stars (comets and other icy bodies). Temperature-programmed desorption experiments of ice composed of various molecular components (H_2O, CO, CO_2, CH_3OH, and NH_3) were dedicated to reproduce the heating of circumstellar ices in hot cores, and serve to interpret the measurements of volatiles that mass spectrometers on board the ESA-Rosetta cometary mission are performing on comet 67P/Churyumov-Gerasimenko. But the presence of gas-phase molecules in the coldest regions, such as dark interstellar clouds, remains unexplained. UV-photodesorption is a plausible non-thermal desorption process. The UV-absorption cross-section and the photo-desorption rate of these species were measured to estimate the effect of photo-processing of ice in interstellar clouds and the solar nebula. Certain species detected in the gas were probably formed in dust grains. The synthesis of UV, X-ray and ion irradiation products in pre-cometary ice was studied experimentally. The organic matter of prebiotic interest formed by irradiation and heating of ice mantles might be preserved to some degree in Halley-like comets and some asteroids. Indeed, organic matter accounts for about 20% of the total mass in comet Halley, more recently, the infrared features of organic compounds were detected in asteroids. The on-going Rosetta mission is being crucial to characterize this elusive material in a comet nucleus.