An ISO View on Interstellar and Cometary Ice Chemistry

The ISO-SWS instrument offering a large wavelength coverage and a resolution well adapted to the solid phase has changed our knowledge of the physical-chemical properties of ices in space. The discovery of many new ice features was reported and the comparison with dedicated laboratory experiments allowed the determination of more accurate abundances of major ice components. The presence of CO₂ ice has recently been confirmed with the SWS (Short Wavelength Spectrometer) as a dominant ice component of interstellar grain mantles. The bending mode of CO₂ ice shows a particular triple-peak structure which provides first evidence for extensive ice segregation in the line-of-sight toward massive protostars. A comparison of interstellar and cometary ices using recent ISO data and ground-based measurements has revealed important similarities but also indicated that comets contain, beside pristine interstellar material, admixtures of processed material. The investigation of molecules in interstellar clouds is essential to reveal the link between dust in the interstellar medium and in the Solar System. This revised version was published online in June 2006 with corrections to the Cover Date.     

The Ice Survey Opportunity of ISO

The instruments on board the Infrared Space Observatory have for the first time allowed a complete low (PHOT, CVF) to medium resolution (SWS) spectroscopic harvest, from 2.5 to 45 μm, of interstellar dust. Amongst the detected solids present in starless molecular clouds surrounding recently born stellar and still embedded objects or products of the chemistry in some mass loss envelopes, the so-called “ice mantles” are of specific interest. They represent an interface between the very refractory carbonaceous and silicates materials that built the first grains with the rich chemistry taking place in the gas phase. Molecules condense, react on ices, are subjected to UV and cosmic ray irradiation at low temperatures, participating efficiently to the evolution toward more complex molecules, being in constant interaction in an ice layer. They also play an important role in the radiative transfer of molecular clouds and strongly affect the gas phase chemistry. ISO results shed light on many other species than H₂O ice. The detection of these van der Waal's solids is mainly performed in absorption. Each ice feature observed by ISO spectrometer is an important species, with abundance in the 10⁻⁴–10⁻⁷ range with respect to H₂. Such high abundances represent a substantial reservoir of matter that, once released later on, replenishes the gas phase and feeds the ladder of molecular complexity. Medium resolution spectroscopy also offers the opportunity to look at individual line profiles of the ice features, and therefore to progressively reveal the interactions taking place in the mantles. This article will give a view on selected results to avoid to overlap with the numerous reviews the reader is invited to consult (e.g. van Dishoeck, in press; Gibb et al., 2004.). Based on observations with ISO, an ESA project with instruments funded by ESA Member States (especially the PI countries: France, Germany, The Netherlands, and the United Kingdom), and with the participation of ISAS and NASA.     

Origin and stability of lunar polar volatiles

Temperature regime at the LCROSS impact site is studied. All detected species in the Cabeus crater as well as CH₄ and CO clathrate hydrates except H₂, CO, and CH₄ are stable against evaporation at the LCROSS impact site. CO and CH₄ can be chemisorbed at the surface of the regolith particles and exist in the form of clathrate hydrates in the lunar cold traps. Flux rates of delivery of volatile species by asteroids, micrometeoroids, O-rich, C-rich, and low-speed comets into the permanently shadowed regions are estimated. Significant amounts of H₂O, CO, H₂, H₂S, SO₂, and CO₂ can be impact-produced during collisions between asteroids and O-rich comets with the Moon while CH₃OH, NH₃ and complex organic species survive during low-speed comet impacts as products of disequilibrium processes. C-rich comets are main sources of CH₄, and C₂H₄.