Methane and its isotopologues on Saturn from Cassini/CIRS observations

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2008.09.019 Byline: L.N. Fletcher (a), G.S. Orton (a), N.A. Teanby (b), P.G.J. Irwin (b), G.L. Bjoraker (c) Keywords: Saturn; Atmospheres; composition; Atmospheres; structure Abstract: High spectral resolution observations from the Cassini Composite Infrared Spectrometer [Flasar, F.M., and 44 colleagues, 2004. Space Sci. Rev. 115, 169-297] are analysed to derive new estimates for the mole fractions of CH.sub.4, CH.sub.3D and.sup.13CH.sub.4 of (4.7[+ or -]0.2)x10.sup.-3, (3.0[+ or -]0.2)x10.sup.-7 and (5.1[+ or -]0.2)x10.sup.-5 respectively. The mole fractions show no hemispherical asymmetries or latitudinal variability. The analysis combines data from the far-IR methane rotational lines and the mid-IR features of methane and its isotopologues, using both the correlated-k retrieval algorithm of Irwin et al. [Irwin, P., and 9 colleagues, 2008. J. Quant. Spectrosc. Radiat. Trans. 109, 1136-1150] and a line-by-line approach to evaluate the reliability of the retrieved quantities. C/H was found to be enhanced by 10.9[+ or -]0.5 times the solar composition of Grevesse et al. [Grevesse, N., Asplund, M., Sauval, A., 2007. Space Sci. Rev. 130 (1), 105-114], 2.25[+ or -]0.55 times larger than the enrichment on Jupiter, and supporting the increasing fractional core mass with distance from the Sun predicted by the core accretion model of planetary formation. A comparison of the jovian and saturnian C/N, C/S and C/P ratios suggests different reservoirs of the trapped volatiles in a primordial solar nebula whose composition varies with distance from the Sun. This is supported by our derived D/H ratio in methane of (1.6[+ or -]0.2)x10.sup.-5, which appears to be smaller than the jovian value of Lellouch et al. [Lellouch, E., Bezard, B., Fouchet, T., Feuchtgruber, H., Encrenaz, T., de Graauw, T., 2001. Astron. Astrophys. 370, 610-622]. Mid-IR emission features provided an estimate of C12/C13= 91.8.sub.-7.8.sup.+8.4, which is consistent with both the terrestrial ratio and jovian ratio, suggesting that carbon was accreted from a shared reservoir for all of the planets. Author Affiliation: (a) Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109, USA (b) Atmospheric, Oceanic & Planetary Physics, Department of Physics, University of Oxford, Clarendon Laboratory, Parks Road, Oxford, OX1 3PU, UK (c) NASA/Goddard Spaceflight Center, Greenbelt, MD 20771, USA Article History: Received 19 June 2008; Revised 23 September 2008; Accepted 28 September 2008