Titan's Isotopic Menagerie: The Cassini CIRS Perspective

Saturn's long-mysterious moon Titan is gradually yielding up its secrets under the intense scrutiny of the Cassini spacecraft, which has just completed a 4-year prime mission including 45 close flybys of the giant satellite. We here focus on the isotopic composition of the stratosphere, which since Voyager 1 in 1980 has been known to comprise a surprisingly rich mixture of hydrocarbons, nitriles and several oxygen species. These molecules are now understood to originate in the upper atmosphere by chemical processes initiated by the dissociation of the most abundant native species - methane and nitrogen - with some oxygen added from externally-supplied water. Measurements of isotopic ratios in these compounds are important and can provide valuable information on the formation and evolution of Titan's atmosphere. E.g. Chemical processes can cause isotopic fractionation via the 'kinetic isotope effect' (KIE).

Cassini's Composite Infrared Spectrometer (CIRS), which is sensitive to thermal infrared radiation from 10-1500 cm^-1 (7-1000 micron), is an ideal tool for measuring molecular concentrations and can distinguish between isotopologues due to the shifts in the molecular bands. CIRS has now identified at least eleven isotopologue species in our spectra, with multiple new detections in the past year (¹³CO₂, CO¹⁸O, HC¹³CCCN). CIRS has measured the ratios ¹²C/¹³C in a total of seven species, D/H in two species, and ¹⁴N/¹⁵N and ¹⁶O/¹⁸O each in one species - the best measurement so far of the important ratio ¹⁶O/¹⁸O on Titan (346±110). In this presentation we will summarize all our results to date on isotopic ratios, including comparison with Huygens GCMS and other determinations, a discussion of possible isotopic separation in hydrocarbon chains, and formation/evolution implications of these measurements for Titan.