Isotopic ratios in Titan's stratosphere from Cassini CIRS and implications for evolution of the atmosphere

Isotopic ratios in planetary atmospheres are valuable sources of information regarding the formation and evolution of the body. However, deriving a history of the atmosphere from contemporary values requires detailed modeling and a unique interpretation may not always be possible. The present-day values of isotopic ratios reflect the sum total of many processes that alter them from primordial: physical and chemical processes in the nebula, during primary or secondary atmospheric formation, and processes at work today. In order to constrain models it is therefore critical to gain the most accurate measurements possible to use as inputs.

Since Cassini's entry to Kronian orbit in 2004 the spacecraft has made more than 60 close flybys of Titan, during which the Composite Infrared Spectrometer (CIRS, Flasar et al. 2004) instrument has been active in mapping the thermal emissions from stratospheric gases (Flasar et al. 2005). The large improvement in spectral resolution and sensitivity over Voyager IRIS, combined with the ability to make repeated long-path limb observations to increase signal-to-noise has proved CIRS to be a powerful tool for measuring isotopic variants of stratospheric gases (Bézard 2009).

These measurements provide an ideal complement to those of the Huygens probe mass spectrometer (GCMS, Niemann et al. 2005). Whereas GCMS provided a single, high-precision value for the isotopic ratios in abundant species (e.g. D/H in H₂), CIRS instead provides multiple, lower-precision measurements of the same isotopic ratios in multiple abundant and trace gases. So far, CIRS has published values for four ratios (D/H, ¹²C/¹³C, ¹⁴N/¹⁵N, ¹⁸O/¹⁶O) in six gases (CH₄, C₂H₂, C₂H₆, HCN, HC₃N, CO₂), totaling ten different molecular ratios in all (six ¹³C variants, two deuterated gases, and one of each ¹⁸O and ¹⁵N) (Vinatier et al. 2007, Bézard et al. 2007, Nixon et al. 2008a, 2008b, Jennings et al. 2008, Coustenis t al. 2008). Divergence of the values between species (e.g. D/H is different in H₂ and CH₄; ¹⁴N/¹⁵N is different in HCN than in N₂) can then be used to constrain models of fractionation (Liang et al. 2007, Mandt et al. 2009).

Regarding the ¹²C/¹³C, both CIRS and GCMS have found evidence for enrichment of methane, the primary carbon-carrying species, in the heavier isotope by about 10%. Intriguingly, preliminary measurements by CIRS of the same ratio in ethane, the main gaseous product of methane photochemistry, show no such enhancement, and the ratio is close to terrestrial. This value has been confirmed by recent ground-based measurements (Jennings et al. 2009), indicating that a Kinetic Isotopic Effect (KIE) fractionation mechanism may be at work.

In this scenario, the amount of carbon being removed from the atmosphere, mostly in the form of ethane, is countered by replenishment of methane to the atmosphere from an internal source. The ¹²C/¹³C ratio in this reservoir is terrestrial, but the KIE effect in the production of ethane, largely during the production of methyl (CH₃) from methane (CH₄) by abstraction of ethynyl: C₂H + CH₄ → C₂H₂ + CH₃, preferentially partitions ¹²C into ethane leaving behind ¹³C-enriched methane. In equilibrium, the amount of ¹²C and ¹³C removed must exactly balance the input amounts, therefore the ¹²C/¹³C in ethane (mostly removed by condensation in the lower stratosphere is the same as the incoming reservoir methane. The stable isotopic ratio in the atmosphere methane however is enhanced by the same ratio as the KIE effect in the dominant reaction: about 10%.

In this paper we summarize the molecular isotopic measurements by CIRS to date including the recent search for H¹³CCCCH, and review the status of current interpretations. We also outline which other ratios will potentially be detected by CIRS during the Cassini extended mission.

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