Carbonaceous compounds in comets - Infrared observations

The 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 microns 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-micron emission feature. The band resembles (but is not identical to) bands of carbonaceous chondrite organic material, synthetic materials, and interstellar carbonaceous bands.