Carbonaceous Compounds in Comets: Infrared Observations

Abstract

AbstractThe Comet Halley observations showed that carbon is a major component of the comet nucleus, with mass spectroscopic data giving near-cosmic C/O ratios. Gaseous and solid compounds were also observed with infrared spectroscopy, which gave detections of CO and CO2, probable detections or upper limits of H2CO and CH4, and a tentative detection of OCS. The CH4/CO ratio of less than unity in Comet Halley points to a CO-rich solar nebula; however, the ratio is higher than in interstellar clouds. A broad, complicated emission feature near 3.4 μm is evidence for carbonaceous compounds containing C-H groups in gas or solid phases. Analysis of radiation mechanisms and abundance constraints suggests that thermal emission or transient heating by single photons can account for the 3.4-μm emission feature. The band resembles (but is not identical to) bands of carbonaceous chondrite organic material, synthetic materials, and interstellar carbonaceous bands. Direct connections among these materials are possible, but have not been established.