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

Abstract

Organic volatiles and water in Oort Cloud comets were investigated at infrared wavelengths. The detected species include H ₂O, CO, CH ₃OH, CH ₄, C ₂H ₂, C ₂H ₆, OCS, HCN, NH ₃, and H ₂CO. Several daughter fragments (CN, OH, NH ₂, 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 CH ₄). 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.