Abundances of C3Hx Hydrocarbons in Titan’s Stratosphere from Cassini CIRS
Wednesday, 17 December 2014
During the ten years since entry into Saturn orbit in 2004, the Cassini spacecraft has made more than 100 close flybys of Titan, measuring the properties of the atmosphere by both in situ and remote sensing techniques. Cassini’s Composite Infrared Spectrometer (CIRS) senses the infrared spectrum from 7-1000 μm (1400-10 cm-1), a region which exhibits the vibrational emissions of many different molecular species. CIRS has therefore been able to map the spatial distributions and temporal variations of hydrocarbons, nitriles and other gas species in Titan’s atmosphere, yielding information about the chemistry and dynamics. Recently, Nixon et al. (2013) made the first detection of a new stratospheric gas species from Cassini using CIRS – the C3H6 molecule (propene). This filled in a long-time missing link in the chemical picture of Titan’s lower atmosphere, since the C3H4 (propyne) and C3H8 (propane) molecules had been detected in 1981 by Voyager 1 IRIS. The inferred abundance of C3H6 is less than both C3H8 and C3H4, and this pattern is repeated also in the C2Hx molecules where C2H4 is less abundant than C2H2 and C2H6. Therefore a pattern emerges whereby: alkanes > alkynes > alkenes within the C2Hx and C3Hx chemical families in the lower stratosphere. We comment on how this trend compares to published photochemical model predictions, and also give updates on the search for C3Hx isomers (allene: CH2CCH2, and cyclopropane: c-C3H6) and C4Hx species using CIRS.