| In operation since July 2004, the Cassini/Huygens mission has significantly improved our perspective of Saturn’s largest moon, Titan. Indeed, numerous publications have already revealed its uniqueness within the Saturnian System and the Solar System in general [e.g. 1, and references therein]. Focusing on its dense nitrogen atmosphere, rich in organic constituents, Cassini’s Composite InfraRed Spectrometer (CIRS) [2] has measured its detailed stratospheric composition both in medium (2.5 cm-1) and high (0.5 cm-1) resolution [e.g. 3, 4]. In this paper, we will present a recent analysis of CIRS spectra in two ways: We study the variations of Titan’s stratospheric structure during the Cassini/Huygens mission and we search for new species by averaging all the spectra obtained from the beginning of the Cassini tour until late 2010.
Our work method consists in first making averages of spectra to derive the temperature profiles as a function of latitude by using the n4 methane band at 1306 cm-1 [5]. We then inject these temperature profiles in our line-by-line radiative transfer code to infer a model simulation that we compare to the observations. Once the best possible fit is obtained in the emission bands and the continuum, we infer the abundances of these trace gases as a function of time or latitude as in previous studies [e.g. 3, 4, 6, 7, 8].
We then also compare these results with other inferences from the Voyager missions [e.g. 9, 10] and other ground/space –based observations [e.g. 11] to obtain information as to the seasonal effects on Titan. From the large averages we try to identify new species in Titan’s stratosphere (or establish upper limits) and to improve on the isotopic ratios [e.g. 12, 13, 14]. We will present a progress report on our results.
Studying Titan’s organic budget by the Cassini-Huygens instrumentation will enable scientists not only to understand the origin, evolution and dynamics of its atmosphere, but also to investigate its astrobiological potential [e.g. 15].
References
[1] Braun, R.H. et al. (eds), Titan from Cassini-Huygens, Springer, pp.535, 2009.
[2] Flasar, F. M., et al., Exploring the Saturn system in the thermal infrared: The Composite Infrared Spectrometer, SSR, 115, pp. 169-297, 2004.
[3] Coustenis, A., et al., Titan trace gaseous composition from CIRS at the end of the Cassini-Huygens prime mission, Icarus, 207, pp. 461-476, 2010.
[4] Coustenis, A., et al., The composition of Titan’s stratosphere from Cassini/CIRS mid-infrared spectra, Icarus, 189, pp. 35-62, 2007.
[5] Achterberg, R.K., et al., Titan's middle-atmospheric temperatures and dynamics observed by the Cassini Composite Infrared Spectrometer, Icarus, 194, p. 263-277, 2008.
[6] Flasar, F. M., et al., Titan’s atmospheric temperatures, winds, and composition, Science, 308, pp. 975-978, 2005.
[7] Jennings, D., et al., Isotopic Ratios in Titan's Atmosphere from Cassini CIRS Limb Sounding: HC3N in the North, Astrophys. J. Let., 681, pp. L109-L111, 2008.
[8] Teanby, N., et al., Dynamical implications of seasonal and spatial variations in Titan’s stratospheric composition, PhTrRS, 367, pp. 697-711, 2009.
[9] Coustenis, A., et al., Titan’s atmosphere from Voyager infrared observations: I. The gas composition of Titan’s equatorial region, Icarus, 80, pp. 54-76, 1989.
[10] Coustenis, A., Bezard, B., Titan’s atmosphere from Voyager infrared observations: IV. Latitudinal variations of température and composition, Icarus, 115, pp. 126-140, 1995.
[11] Coustenis, A., et al., Titan’s atmosphere from ISO mid-infrared spectroscopy, Icarus, 161, pp. 383-403, 2003.
[12] Coustenis, A., et al., Detection of C2HD and the D/H ratio on Titan, Icarus, 197, pp. 539-548, 2008.
[13] Nixon, C.A., et al., Isotopic ratios in Titan’s atmosphere from Cassini limb sounding: CO2 at low altitudes and midlatitudes, The Astroph. Journal, 681, L101-L103, 2008.
[14] Nixon C.A., et al., The 12C/13C isotopic ratio in Titan hydrocarbons from Cassini/CIRS infrared spectra, Icarus, 195, 778-791, 2008.
[15] Coustenis A., et al., Life in the Saturnian neighborhood, Cellular Origin, book chapter in Life in Extreme Habitats and Astrobiology (COLE) series, J. Seckbach (eds.), in press. |