THE IMPORTANCE OF BEING ORGANIC–SOURCES AND CONSEQUENCES OF CARBONACEOUS MATTER IN THE OUTER SOLAR SYSTEM

The following propositions will be defended: 1) Organic molecules are ubiquitous in interstellar space, in both gas and solid form. More than 120 molecules have been identified in interstellar and circumstellar gas, most of them organic. Fewer condensed phases (ices) have been identified to date, but abundances of the major species, CO, carbon dioxide, and methanol, are similar to those found in comets. 2) Comets are neither pristine agglomerations of interstellar matter nor wholly processed and recondensed solar nebula material. Abundances of volatile species in comets parallel but do not precisely match interstellar abundances. Conditions in the outer solar nebula were not conducive to wholesale chemical reequilibration of material accreted from the natal molecular cloud, but molecular evolution due to ion-molecular reactions in the cold, dark gas was probable. 3) Comets are rich in refractory organics. This constraint comes primarily from spacecraft observations of Halley in 1986 (whereas our overall chemical knowledge owes a tremendous debt to Hyakutake and Hale-Bopp). Transneptunian bodies such as Kuiper Belt Objects (KBOs), Pluto-Charon, and Triton were likely all initially well endowed with refractory organics (~20% by mass). The organic fraction in the larger of these bodies may have substantially influenced their structural and chemical evolution. 4) The solid matter that Jupiter accreted into its atmosphere came mainly from cold (<50 K) regions of the solar nebula. This surprising result comes from detailed analysis of Galileo Probe data, and implies either that a) Jupiter formed farther from the Sun, b) the nebula near Jupiter was colder than previously thought, or c) the later bombardment of Jupiter was dominated by KBOs. 5) Substantial organic-rich matter was incorporated into the protosatellite nebula around Jupiter, and thus into Europa. This is contrary to the traditional view that the relatively high-temperature and pressure nebula around Jupiter converted most organic matter to uncondensable methane. Europa has an ocean, but whether submarine hydrothermal systems exist is unknown. Prospects for biogeochemistry on Europa will be discussed.