Remote infrared observations of parent volatiles in comets: A window on the early solar system Original Research Article

Organic volatiles and water in Oort Cloud comets were investigated at infrared wavelengths. The detected species include H2O, CO, CH3OH, CH4, C2H2, C2H6, OCS, HCN, NH3, and H2CO. Several daughter fragments (CN, OH, NH2, etc.) are also measured, and OH prompt emission provides a proxy for water. Long-slit spectra are taken at high spectral dispersion and high spatial resolution, eliminating several sources of systematic error. The resulting parent volatile production rates are highly robust, permitting a sensitive search for compositional diversity among comets. Here, seven OC comets are compared. Six (including Halley) exhibit similar compositions (excepting CO and CH4). Their low formation temperatures (∼30 K) suggest this group probably formed beyond 30 AU from the young sun. However, C/1999 S4 is severely depleted in hypervolatiles and also in methanol, and it likely formed near 5–10 AU. C/2001 A2 is discussed briefly to illustrate future prospects.