Vertical profiles of HCN, HC3N, and C2H2 in Titan's atmosphere derived from Cassini/CIRS dataIntroductionForward modelReference atmospheric modelSpectroscopic parametersDataHigh spectral resolution data (0.5 cm-1)Low spectral resolution data (13.5 cm-1)Pointing accuracyAveraging multiple limb observations using splinesMethodHigh resolution retrieval methodStage 1: Temperature and tangent heightStage 2: CompositionLow resolution retrieval methodStage 1: Temperature and tangent heightStage 2: CompositionResultsHigh resolutionTemperatureCompositionHC3NHCNC2H2Low resolution examplesLow resolution limb mapsTemperatureCompositionDiscussionConsistency of high and low resolution resultsEffect of horizontal gradientsTemperatureHC3NHCNC2H2ConclusionsAcknowledgmentsReferencesIcarus 186 (2007) 364–384www.elsevier.com/locate/icarusVertical profiles of HCN, HC3N, and C2H2in Titan’s atmosphere derivedfrom Cassini/CIRS dataN.A. Teanbya,∗, P.G.J. Irwina,R.deKoka, S. Vinatierb, B. Bézardb,C.A.Nixonc, F.M. Flasard,S.B. Calcutta,N.E.Bowlesa, L. Fletchera, C. Howetta,F.W.TayloraaAtmospheric, Oceanic & Planetary Physics, Department of Physics, University of Oxford, Clarendon Laboratory, Parks Road, Oxford, OX1 3PU, UKbObservatoire de Paris, LESIA, Meudon F-92195, FrancecDepartment of Astronomy, University of Maryland, College Park, MD 20742, USAdNASA/Goddard Space Flight Center, Code 693, Greenbelt, MD 20771, USAReceived 13 July 2006; revised 18 September 2006Available online 17 November 2006AbstractMid-infrared limb spectra in the range 600–1400 cm−1taken with the Composite InfraRed Spectrometer (CIRS) on-board the Cassini spacecraftwere used to determine vertical profiles of HCN, HC3N, C2H2, and temperature in Titan’s atmosphere. Both high (0.5 cm−1) and low (13.5 cm−1)spectral resolution data were used. The 0.5 cm−1data gave profiles at four latitudes and the 13.5 cm−1data gave almost complete latitudinalcoverage of the atmosphere. Both datasets were found to be consistent with each other. High temperatures in the upper stratosphere and mesospherewere observed at Titan’s northern winter pole and were attributed to adiabatic heating in the subsiding branch of a meridional circulation cell.On the other hand, the lower stratosphere was much colder in the north than at the equator, which can be explained by the lack of solar radiationand increased IR emission from volatile enriched air. HC3N had a vertical profile consistent with previous ground based observations at southernand equatorial latitudes, but was massively enriched near the north pole. This can also be explained in terms of subsidence at the winter pole.A boundary observed at 60◦N between enriched and un-enriched air is consistent with a confining polar vortex at 60◦NandHC3N’s shortlifetime. In the far north, layers were observed in the HC3N profile that were reminiscent of haze layers observed by Cassini’s imaging cameras.HCN was also enriched over the north pole, which gives further evidence for subsidence. However, the atmospheric cross section obtained from13.5 cm−1data indicated a HCN enriched layer at 200–250 km, extending into the southern hemisphere. This could be interpreted as advectionof polar enriched air towards the south by a meridional circulation cell. This is observed for HCN but not for HC3N due to HCN’s longerphotochemical lifetime. C2H2appears to have a uniform abundance with altitude and is not significantly enriched in the north. This is consistentwith observations from previous CIRS analysis that show increased abundances of nitriles and hydrocarbons but not C2H2towards the north pole.© 2006 Elsevier Inc. All rights reserved.Keywords: Titan; Atmospheres, composition1. IntroductionThe Cassini spacecraft entered orbit around Saturn on 1stJuly 2004. Since then it has made numerous flybys of Sat-urn’s largest moon Titan. The Composite InfraRed Spectrom-eter (CIRS) on board the Cassini orbiter records spectra in thefar- and mid-IR (10–1400 cm−1) and was designed to provideinformation on the atmospheric temperature and composition of*Corresponding author. Fax: +44 1865 272923.E-mail address: [email protected] (N.A. Teanby).Saturn and its moons (Kunde et al., 1996; Flasar et al., 2004).CIRS is a Fourier transform spectrometer and the full spec-tral range of the instrument is covered by three separate focalplanes, which use the same telescope and scan mechanism: FP110–600 cm−1(far-IR); FP3 600–1100 cm−1(mid-IR); and FP41100–1400 cm−1(mid-IR).The apodised spectral resolution of CIRS is adjustable be-tween 0.5 and 15.5 cm−1. We use both high (0.5 cm−1) andlow (13.5 cm−1) spectral resolution data in this paper in orderto exploit the best features of each dataset and provide a consis-tency check.0019-1035/$ – see front matter © 2006 Elsevier Inc. All rights reserved.doi:10.1016/j.icarus.2006.09.024Profiles of HCN, HC3N, and C2H2in Titan’s atmosphere 365The mid-IR focal planes of CIRS were designed to havea very small field-of-view (FOV) so that limb measurementsof Titan’s atmosphere could be used to determine tempera-ture and composition with vertical resolutions of less thanan atmospheric scale height (20–40 km). FP3 and FP4 eachcomprise a linear array of 10 square pixels with a full-widthhalf-maximum (FWHM) of 0.27 × 0.27 mrad—over 10 timessmaller than the FOV of the Voyager-IRIS instrument.The aim of this paper is to obtain vertical profiles of thetwo main nitrile species in Titan’s atmosphere—HCN andHC3N—which have prominent spectral features at 712.25 and663.25 cm−1, respectively. Therefore, we focus on datasetswith sub-scale-height vertical resolution from the CIRS mid-IRarrays. Emission at these wavenumbers is affected by temper-ature, haze, C2H2, and CO2—which must also be retrieved inorder to properly fit the spectra. Additional vertical profiles ob-tained from CIRS for other species are discussed by de Kok etal. (2007) (CO2vertical profile and CO abundance) and Vinatieret al. (2006) (C2H2,C2H4,C2H6,C3H4,C3H8,C4H2,C6H6,and HCN vertical profiles).Nitriles are produced by photochemical reactions above300 km (0.1 mbar) in the mesosphere and thermosphere (Laraet al., 1996; Wilson and Atreya, 2004). Deeper in the at-mosphere they are removed by condensation. Hence, the up-per atmosphere source combined with a sink in the lower at-mosphere sets up a concentration gradient and implies that vol-ume mixing ratio (VMR) should increase with