Comets : Heterogenous Cometary Composition – Evidence for Variable Processing in the Pre-planetary Nebula

s are listed by topic, in order of discussion at the Workshop: INTERSTELLAR MEDIUM :. 1 COMETS : . 3 PLANETARY SURFACES : . 4 INTERPRETING PLANETARY SPECTRA : . 4 ICE STRUCTURES : . 5 STRUCTURAL ALTERATIONS :. 8 CHEMICAL ALTERATIONS :. 12 THE PHYSICAL STATE OF ICE :. 14 LABORATORY MEASUREMENTS APPLIED TO MISSIONS :. 19 LIMITATIONS OF LABORATORY SIMULATIONS : . 20 Interstellar Medium : Energetic and Thermal Processing of Interstellar Ices Pascale Ehrenfreund Raymond and Beverly Sackler Laboratory for Astrophysics at Leiden Observatory P.O. Box 9513 2300 RA Leiden The Netherlands Tel: (31) 715275812 / Fax: (31) 715275819 pascale@strw.leidenuniv. nl Silicate and carbonaceous particles produced in stellar outflows act as condensation nuclei when they pass through a dense molecular cloud. In cold dense environment such particles acquire icy grain mantles which are formed by efficient accretion of atoms and molecules from the interstellar gas. An active surface chemistry takes place on the grains that, in principle, provides many possible alternative routes to the formation of complex interstellar molecules. Regulatory mechanisms such as selective desorption, sublimation and grain explosion desorb material and return molecules back in the gas phase. The evolution of interstellar ices is determined by processes in the protostellar PROSSI 2000 Abstracts page 2 environment, which can be quiescent or dominated by shocks. When molecular clouds evolve from an initial cold quiescent phase to warm, dense and active proto-stellar regions processes (such as ultraviolet irradiation, cosmic ray bombardment and termerature variations) may become important and alter the grain surface composition. Dust particles with icy mantels produced by these elementary processes are interspersed in dense interstellar gas with an abundance of around 10 per H atom. Despite their low abundance they act as important catalysts in the interstellar medium. Icy grain mantles have been recently revisited with the Infrared Space Observatory (ISO). ISO allowed for the first time to observe the complete wavelength range between 2.5-200 :m, free of any telluric contamination. Several new molecules could be detected and accurate abundances of ice species could be determined. Astronomical observations indicate the existence of different types of ices in proto-stellar environments and toward field stars. Hydrogen-rice ices (polar ices), dominated by H20 ice, are formed when H is abundant in the interstellar gas. Besides water ice they contain: CO, CO2, CH4, NH3, CH3OH, and possibly traces of HCOOH and H2CO. Apolar or hydrogen-poor ices are formed far away from the proto-star or in cold dens clumps and are composed of molecules with high volatility (evaporation temperatures of < 20 K) such as CO, O2, and N2. Thermally processed ice mantels, in which CO2 ice is locked in intermolecular complexes with CH3OH ice, have been discovered with the ISO satellite towards high-mass protostars. The role of UV energetic processing of interstellar ices remains uncertain. The UV radiation field of a young protostar is strongly attenuated within its close environment, and only cosmic rays can penetrate throughout the cloud, leading to an induced UV radiation field of only ~ 10 photons s cm. Laboratory experiments show that irradiation processes and thermal processing can indeed lead to the formation of radicals, complex molecules and even organic refractory material, but it is not clear if the experimental simulation accurately represent the conditions appropriate to the clumpy structure of molecular clouds. The formation of CO2 in dense clouds in large quantities (namely 20% relative to water ice) remains a mystery. CO2 may be formed by grain surface reactions, namely by oxydation of CO. Energetic processing (UV irradiation or cosmic rays) or gas phase production in shocks and subsequent accretion on grains are alternative formation routes. Recent ground-based observations indicate large amounts of CH3OH in the line of sight toward some high massive protostars, but rather low CH3OH ice abundances are observed toward low-mass protostars. CH3OH may be either formed by grain surface reactions or by energetic processing. To study the formation and evolution of the key molecules CO2 and CH3OH in the laboratory may therefore help to reconstruct the temperature and irradiation conditions in protostellar environments. Recent ISO results show extensive ice segregation in the vicinity of protostars and evidence for acid-base reactions occurring on interstellar grain surfaces. This clearly indicates that thermal processing is a dominant factor in interstellar ice chemistry. This review presents a schematic outline of energetic and thermal processes which lead to the chemical diversity of interstellar ices PROSSI 2000 Abstracts page 3 and the formation of complex molecules in interstellar ice and gas. Comets : Heterogenous Cometary Composition – Evidence for Variable Processing in the Pre-planetary Nebula Mike J. Mumma Laboratory for Extraterrestrial Physics NASA Goddard Space Flight Center Greenbelt, MD 20771 USA Mmumma@lepvax.gsfc. nasa.gov Cometary nuclei appear to contain the least processed material remaining from the era of planetary formation, and their compositions reflect their origin and history. The oxidation state of volatile carbon and the crystallinity of silicates both seem to differ among comets, suggestive of variable processing in the pre-planetary disk. The simplest members of the (CHO) homologous series of volatile carbon have been detected in recent comets: CO, CO2, HCOOH, H2CO, CH3OH, and CH4. A few higher order members (e.g. C2H6) have also been found. Methanol appears with abundances ranging from ~0.5% to 5% that of water and is sometimes more abundant than CO, thus the oxidation state of volatile carbon is variable in comets. CO is itself highly variable, and the ratio of ethane to CO (both hypervolatiles) also varies among comets. The organic ices are most easily modified and thus are most sensitive to origin-related processes, but most cometary carbon resides in refractory grains. Elemental compositions were measured for CHON grains in P/Halley but their chemical structures and abundances are unknown, hence the competing influences of kinetic, thermodynamic, and other factors affecting their end chemistry PROSSI 2000 Abstracts page 4 remain obscure. When compared with information gleaned from the 500-plus organic species isolated from Murchison matrix, it is clear that the clues contained in cometary organics have barely been touched. A deep understanding will depend upon analysis of samples taken directly from the near surface layers of the nucleus, such as is planned on the ROSETTA mission, and on laboratory analysis of dust returned by the STARDUST mission. Our current understanding of cometary organics and their relation to interstellar and nebular processing will be discussed. Planetary Surfaces :