Abundances of C3Hx Hydrocarbons in Titan’s Stratosphere from Cassini CIRS

Abstract:

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 C₂H_x molecules where C₂H₄ is less abundant than C₂H₂ and C₂H₆. Therefore a pattern emerges whereby: alkanes > alkynes > alkenes within the C₂H_x and C₃H_x 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 C₃H_x isomers (allene: CH₂CCH₂, and cyclopropane: c-C₃H₆) and C₄H_x species using CIRS.