Study of the Atmosphere of Titan from Millimeter and Submillimeter Observations

Over the last decade ground-based radiotelescopes have provided significant new information on the chemical composition of Titan, the largest satellite of Saturn. Many rotational transitions have been observed in the millimeter and submillimeter ranges, allowing us to retrieve disk-averaged vertical profiles of molecular compounds. A review of spectroscopic measurements undertaken between 1995 and 1999 with the IRAM 30-m Telescope and the JCMT is presented. Our study was mainly concerned with the abundances of nitriles and oxygen-bearing compounds in the stratosphere of Titan. New results regarding HCN, HC₃N, CH₃CN and CO have been obtained. Observations of isotopic components, such as H¹³CN, HC¹⁵N and ¹³CO, led to the determination of the isotopic ratios ¹²C/¹³C and ¹⁴N/¹⁵N. A comparison of our abundance results with predictions of various photochemical models is outlined. A search for trace species, i.e., HC₅N, C₂H₃CN, C₂H₅CN, CH₃OH, H₂CO, is also discussed. A step forward in the microwave investigation of Titan is expected from the use of newly-developed interferometric techniques yielding disk-resolved measurements.