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Oxygen compounds in Titan's stratosphere as observed by Cassini CIRS
Icarus 186 (2007) 354–363 www.elsevier.com/locate/icarus
Oxygen compounds in Titan’s stratosphere as observed by Cassini CIRS R. de Kok a,∗ , P.G.J. Irwin a , N.A. Teanby a , E. Lellouch b,c , B. Bézard b,c , S. Vinatier b,c , C.A. Nixon d , L. Fletcher a , C. Howett a , S.B. Calcutt a , N.E. Bowles a , F.M. Flasar e , F.W. Taylor a a Atmospheric, Oceanic & Planetary Physics, Department of Physics, University of Oxford, Clarendon Laboratory, Parks Road, Oxford, OX1 3PU, UK b Observatoire de Paris, LESIA, F-92195 Meudon, France c Université Pierre et Marie Curie-Paris 6, UMR 8109, F-75005 Paris, France d Department of Astronomy, University of Maryland, College Park, MD 20742, USA e NASA/Goddard Space Flight Center, Code 693, Greenbelt, MD 20771, USA
Received 31 March 2006; revised 3 August 2006 Available online 20 November 2006
Abstract We have investigated the abundances of Titan’s stratospheric oxygen compounds using 0.5 cm−1 resolution spectra from the Composite Infrared Spectrometer on the Cassini orbiter. The CO abundance was derived for several observations of far-infrared nadir spectra, taken at a range of latitudes (75◦ S–35◦ N) and emission angles (0◦ –60◦ ), using rotational lines that have not been analysed before the arrival of Cassini at Saturn. The derived volume mixing ratios for the different observations are mutually consistent regardless of latitude. The weighted mean CO volume mixing ratio is 47 ± 8 ppm if CO is assumed to be uniform with latitude. H2 O could not be detected and an upper limit of 0.9 ppb was determined. CO2 abundances derived from mid-infrared nadir spectra show no significant latitudinal variations, with typical values of 16 ± 2 ppb. Midinfrared limb spectra at 55◦ S were used to constrain the vertical profile of CO2 for the first time. A vertical CO2 profile that is constant above the condensation level at a volume mixing ratio of 15 ppb reproduces the limb spectra very well below 200 km. This is consistent with the long chemical lifetime of CO2 in Titan’s stratosphere. Above 200 km the CO2 volume mixing ratio is not well constrained and an increase with altitude cannot be ruled out there. © 2006 Elsevier Inc. All rights reserved. Keywords: Titan; Atmospheres, composition
1. Introduction Three oxygen compounds have previously been detected in Titan’s stratosphere: CO2 , H2 O and CO. Carbon dioxide in Titan’s stratosphere was first detected by the Voyager IRIS instrument (Samuelson et al., 1983). More detailed analysis of Voyager data and analysis of data from the Infrared Space Observatory (ISO) and from Cassini Composite Infrared Spectrometer (CIRS) show a mixing ratio of 15–20 ppmv (Coustenis et al., 1989, 2003; Flasar et al., 2005). The temperature and pressure at Titan’s tropopause are such that condensation occurs, providing a sink for CO2 . Therefore, the equilibrium stratospheric abundance of CO2 would be expected to be the same as the * Corresponding author. Fax: +44 1865 272923.
E-mail address: [email protected] (R. de Kok). 0019-1035/$ – see front matter © 2006 Elsevier Inc. All rights reserved. doi:10.1016/j.icarus.2006.09.016
saturation level at the tropopause if CO2 originates from below the tropopause. Instead, the observed values are at least a thousand times larger, indicating that CO2 is replenished in the stratosphere. The most effective way to produce CO2 is via the following chemical reaction: CO + OH → CO2 + H.
(1)
This scheme requires (i) a sufficient abundance of CO, and (ii) a source of oxygen in the form of OH. The OH radical can be formed from photolysis of H2 O, or from reactions involving CO and CH4 (Samuelson et al., 1983). CO is one of the more abundant trace gases in Titan’s atmosphere (see below), so the first condition is satisfied. Also, water ice is common in the Solar System and a continuous stream of icy dust particles falling into the atmosphere could provide the oxygen. This mechanism was previously suggested to explain the oxygen source for the giant planets (Feuchtgruber et al., 1997). These dust particles
Titan’s stratospheric oxygen compounds
heat up in the atmosphere, leaving a trail of gas along their way. An attempt to model the effect of infalling dust particles is presented by English et al. (1996). They show that most of the resulting vapours are deposited high in Titan’s atmosphere over a broad range of altitudes centred around 750 km. At these high altitudes H2 O can be readily destroyed by ultraviolet radiation, creating the OH radical needed to produce CO2 . CO2 is only slowly destroyed in the stratosphere, with a chemical lifetime of several hundred years (Wilson and Atreya, 2004). Not all water molecules are destroyed and converted to CO2 . According to photochemical models (e.g., Lara et al., 1996; Wilson and Atreya, 2004) a significant amount of water should remain high in the atmosphere, with the abundance quickly dropping with decreasing altitude. A dedicated observation with the Earth-orbiting infrared telescope ISO, using its high spectral resolution and high sensitivity, confirmed the existence of water vapour in Titan’s stratosphere at 0.4 ppb, assuming that H2 O is constant above the saturation level (Coustenis et al., 1998). Like CO2 , H2 O also condenses at the tropopause. However, unlike CO2 , H2 O has a relatively short lifetime against photolysis (of the order of 5 years; Coustenis et al., 1998) and thus a continuous influx is needed to achieve a steady state abundance that is consistent with observations. CO is very stable in Titan’s atmosphere: it has a chemical lifetime of 500 Myr (Lellouch et al., 2003) and it does not condense at the tropopause. CO is thus expected to be uniformly mixed throughout the entire atmosphere in an equilibrium situation, since the timescale of mixing is much smaller than that of the destruction of CO. Previous observations of CO in Titan’s atmosphere (summarised in Table 1) have not conclusively confirmed such a uniform profile. Despite the multitude of observations at various wavelengths, there is no consensus on the mixing ratio and vertical profile of the gas. Although most of the observations that probe the stratosphere show a volume mixing ratio (VMR) of at least 50 ppm, Hidayat et al. (1998) derived a vertical profile that decreases with altitude. However, this profile does not fit subsequent 5 µm data from Lopez-Valverde et al. (2005). It is also inconsistent with CO concentrations derived by Gurwell and Muhleman (1995, 2000) and Gurwell (2004), which use the same CO rotational lines. The most reliable tropospheric CO concentration is most likely given by Lellouch et al. (2003), since they use improved lineshapes and improved 5 µm observations compared to earlier tropospheric measurements, according to the authors. However, their tropospheric VMR of 32 ± 10 ppm is lower than that derived for the stratosphere, which is unexpected given that CO should not condense in Titan’s atmosphere. Lopez-Valverde et al. (2005) find that a higher stratospheric CO mixing ratio (∼60 ppm) is required to match CO fluorescence emission at 5 µm. Thus, a depletion of CO in the troposphere cannot be ruled out, based on observations made so far. However, a physical mechanism that would explain a decrease in the troposphere has not yet been put forward. The Composite Infrared Spectrometer (CIRS) on board the Cassini orbiter has the capability to study all three of the previously detected oxygen compounds (Flasar et al., 2004). CO2 has its ν2 band in the mid-infrared (at 667.75 cm−1 ),
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Table 1 Previous determinations of CO abundances in Titan’s atmosphere [extended from Table 1 in Gurwell and Muhleman (2000)] Altitude
CO VMR (ppm)
Troposphere 48+100 −32
Wavelength
Reference
1.57 µm
Lutz et al. (1983)
Troposphere 10+10 −5
4.8 µm
Noll et al. (1996)
Troposphere
4.8 µm
Lellouch et al. (2003)
2.6 mm
Muhleman et al. (1984)
2.6 mm
Marten et al. (1988)
2.6 mm
Gurwell and Muhleman (1995)
Stratosphere Stratosphere Stratosphere Stratosphere Stratosphere
32+10 −10 60+40 −40 2+2 −1 50+10 −10 27+5 −5 52+6 −6 51+4 −4
Stratosphere Stratosphere 60 Stratosphere 45+15 −15
2.6, 1.3, 0.9 mm Hidayat et al. (1998) 1.3 mm 0.9 mm 4.8 µm 150–500 µm
Gurwell and Muhleman (2000) Gurwell (2004) Lopez-Valverde et al. (2005) Flasar et al. (2005)
Note. The table shows a wide range of CO VMR values, giving rise to debates about the CO abundance and vertical profile.
whereas CO and H2 O have rotational lines in the far-infrared (between 10 and 300 cm−1 ). CIRS has three separate focal planes that together measure the infrared spectrum between 10 and 1400 cm−1 . FP1 covers the far-infrared (10–600 cm−1 ) and has a single circular field-of-view (FOV) with a fullwidth half-maximum of 2.5 mrad. The mid-infrared is covered by FP3 (600–1100 cm−1 ) and FP4 (1100–1400 cm−1 ), which each comprise of an array of 10 pixels, each having a 0.27 × 0.27 mrad FOV. In this paper we use CIRS data with an apodised resolution of 0.5 cm−1 , which is the maximum resolution obtainable by CIRS. We use downward viewing (nadir) observations, taken on multiple close approaches of Cassini with Titan, to obtain abundances and distributions with latitude of Titan’s stratospheric oxygen compounds. We also use a horizontal viewing (limb) observation to constrain the vertical profile of CO2 . Analysis of Titan’s oxygen compounds with both a high spectral resolution and a high spatial resolution was not presented before in literature. Such an analysis gives a better view of the threedimensional distribution of oxygen compounds on Titan. 2. Observations The FP3 nadir data that were used to obtain the latitudinal CO2 distribution consist of sequences of maximum 5330 spectra taken from observations lasting less than 16 h. Table 2 summarises the data used from each Titan flyby. For each observation sequence, spectra were averaged in bins with a width of 10◦ latitude and a spacing of 5◦ latitude to achieve Nyquist sampling. The FP1 sequences used for determining CO and H2 O abundances are summarised in Table 3. These sequences last up to 4 h. Their observation numbers refer to the flybys listed in Table 2. To obtain information about the vertical profile of CO2 , limb observations from the T6 flyby on 28 August 2005 were used. Limb observations probe the atmosphere at different altitudes.
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Table 2 FP3 observations used to derive the latitudinal CO2 distribution, grouped by Titan flyby Flyby
Date
Latitude
No. of spectra
Emission angle
FOV size
T0 Tb T4 T5 T6 T7a
2–3 Jul. 04 11–13 Dec. 04 31 Mar.–1 Apr. 05 16 Apr. 05 21–22 Aug. 05 24–25 Sep. 05
75◦ 10◦ 45◦ 30◦ 70◦ 15◦
1455 7529 2190 783 1786 1918
31◦ –43◦ 7◦ –50◦ 42◦ –54◦ 30◦ 27◦ –60◦ 25◦ –33◦
5◦ 2◦ –7◦ 2◦ 2◦ 2◦ 7◦
S–65◦ S S–25◦ N S–35◦ S; 30◦ N–35◦ N N S–60◦ S; 20◦ N–25◦ N S–5◦ S
Note. The last set of observations was taken from a distant flyby. Since these observations were taken after the T7 flyby, the name T7a was adopted here. FOV sizes are in degrees of latitude. Table 3 Properties of the selected FP1 observations and the retrieved CO VMR Observation
Latitude
No. of spectra
Emission angle
CO VMR (ppm)
FOV size
Tb-1 Tb-2 T4-2 T4-3 T5-2 T6-3 T6-4
17◦ 18◦ 41◦ 48◦ 55◦ 30◦ 54◦
226 129 241 239 166 191 248
46◦ 16◦ 43◦ 52◦ 51◦ 52◦ 45◦
46 ± 20 41 ± 24 43 ± 19 51 ± 21 40 ± 19 48 ± 20 60 ± 20
19◦ 14◦ 19◦ 20◦ 21◦ 18◦ 18◦
N N S N N N S
Note. Observation numbers refer to the flyby numbers of Table 2. There are often more than one observation sequences per flyby and the second numbers in the observation names point at the individual sequences. FOV sizes are in degrees of latitude. The retrieved values for the CO VMR are consistent across all latitudes and emission angles and have a weighted mean of 47 ± 8 ppm.
If the opacity is small enough, a limb observation will probe the atmosphere at the point along the line of sight that is closest to the surface—the tangent height of the observation. The T6 limb sequence consists of 198 FP3 spectra and 288 FP4 spectra, taken in a 1 h time interval at a latitude of 55◦ S. The tangent heights of the spectra lie between 50 and 300 km and the average FOV size is 11 km at the limb. FP4 spectra are used to derive temperature at 55◦ S, FP3 spectra are used to constrain the CO2 vertical profile. All the above data have an apodised spectral resolution of 0.5 cm−1 . We derive temperature profiles from nadir data with a resolution of 2.5 cm−1 obtained at the T0, Tb and T4 flybys as a basis for retrievals that use higher spectral resolution. These lower resolution data are used, because 0.5 cm−1 resolution FP4 data do not always cover the same latitudes as the FP3 data used to derive the gas VMR due to a 0.94 mrad offset between FP3 and FP4. In this case, temperature cannot be determined for the correct latitude. On the other hand, the 2.5 cm−1 data has good spatial coverage at all latitudes up to 60◦ N (see Teanby et al., 2006). 3. Model The radiative transfer model used here to analyse the CIRS spectra is based on the model used for Jupiter CIRS analysis by Irwin et al. (2004). It models the atmosphere as a number of spherical layers, each with independently adjustable parameters. In this case 99 layers were used, covering altitudes up to 790 km.
Line data for methane were taken from Brown et al. (2003), HCN data from Maki et al. (1996), as compiled in the HITRAN2004 database by Rothman et al. (2005). HC3 N and C2 N2 data were taken from the GEISA97 database (JacquinetHusson et al., 1999) and line data for the oxygen compounds and all other gases were extracted from the HITRAN2000 database (Rothman et al., 2003). From the line data, the k-distributions were calculated and used in a correlated-k model (Lacis and Oinas, 1991). This method drastically decreases computing time for the radiative transfer model, while maintaining enough of its accuracy for analysis of CIRS data (Irwin et al., 2004). Titan’s far-infrared spectrum is most notably influenced by collision-induced absorption of nitrogen and methane molecules, with a smaller contribution from hydrogen molecules. All six collision pairs involving these molecules are included in the radiative transfer model. Absorption coefficients were calculated using the subroutines described in Borysow and Frommhold (1986a, 1986b, 1986c, 1987), Borysow (1991), Borysow and Tang (1993). Aerosol absorption can also be included in the model. Its optical properties are assumed to be similar to laboratoryproduced tholins and the optical constants of Khare et al. (1984) were used to model the haze. The extinction cross-section of the haze is determined by Mie theory, assuming a particle size of 0.2 µm, as determined by Nixon (1998). We use an a priori haze profile that has a constant number of particles per gram of atmosphere above 90 km. Below 90 km, the density decreases exponentially, as described in Teanby et al. (2006). Surface emission is modelled as a blackbody source with a uniform temperature of 94 K. The largest errors in the model are due to the correlated-k approximation, which results in maximum errors of 2 nW cm−2 sr−1 (cm−1 )−1 for nadir observations and 5 nW cm−2 sr−1 (cm−1 )−1 for limb observations. Errors in the line intensities of the molecules are in the order of 2–5% (Rothman et al., 2003). All these errors are smaller than the noise equivalent spectral radiance (NESR) in all cases presented here. Errors due to CIA should not influence the results presented here, since CIA is merely used to fit the continuum and has its origin mainly in the troposphere. Forward model errors of 5 nW cm−2 sr−1 (cm−1 )−1 at maximum are added quadratically to the measurement errors to take into account the correlated-k approximation errors and the line data errors. The measured spectra are fitted using an optimal estimation method that minimises a cost function (Rodgers, 1976). The
Titan’s stratospheric oxygen compounds
Fig. 1. (a) Temperature contribution functions (dR/dT ) for two wavenumbers in the CH4 ν4 band for a nadir observation. (b) Temperature contribution functions for limb observations at different tangent heights. Note that for low altitudes, the contribution functions do not peak at the tangent height.
cost function includes the misfit to the spectrum and the deviation from an a priori estimate. The use of a priori information prevents the appearance of highly unphysical solutions in the retrieval process, caused by the underconstrained nature of the problem. The distance that the solution is allowed to deviate from its a priori estimate depends on the specified a priori error. Care must be taken when choosing the a priori errors. Too large or small an error will result in an under- or over-constrained solution. To find a value between the two extremes, we balanced the measurement error with the a priori error in such a way that both errors constrain the retrieval equally, resulting in smooth retrieved profiles that can deviate significantly from the a priori profile, while providing a satisfactory fit to the data. The model is the same as in Teanby et al. (2006) and more details can be found there. The retrieval of each molecule will be discussed separately here. 4. Temperature retrievals Emission from gases in the stratosphere is highly dependent on the temperature structure of the atmosphere. Therefore, a temperature profile must be determined before retrieving any mixing ratios of trace species. Temperature can be inferred from the ν4 band of methane in FP4, assuming methane is uniformly mixed in the stratosphere. Methane is expected to be uniformly mixed in the stratosphere and Huygens results show that this is indeed the case (Niemann et al., 2005). The assumption of uniformity thus seems reasonable. A stratospheric methane VMR of 1.6% is assumed, as measured with CIRS by Flasar et al. (2005). For nadir observations, temperatures were derived from 2.5 cm−1 resolution data from 1240 to 1360 cm−1 . Contribution functions for two representative wavenumbers are shown in Fig. 1. They peak in the stratosphere, between 0.5 and 10 mbar (100–250 km). A temperature profile was derived at intervals of 5◦ latitude by averaging spectra in bins 10◦ latitude wide, from 90◦ S to 50◦ N. Only spectra with emission angles within 20◦ of
357
Fig. 2. (a) The smooth radiance profile (dotted line) derived from the raw data points (symbols) for one wavenumber in the spectrum used to derive temperature from limb observations. The solid line shows the fit to this curve for this wavenumber for the temperature retrieval. (b) Fits (dotted lines) to the interpolated spectra (solid lines) at three altitudes for a selected wavenumber range.
each other were taken into consideration to prevent averaging over a wide range of emission angles. The retrieved temperature profiles are in agreement with those presented in Flasar et al. (2005) and Teanby et al. (2006), which were obtained using data of the same resolutions at the T0, Tb and T4 flybys. Nadir observations can only retrieve temperature over a limited altitude range. However, to accurately retrieve gas abundances we require temperatures over the full range of altitudes probed by the limb data. Again, we used FP4 spectra for this purpose, taken from the limb sequence at 55◦ S described previously. There are a number of points to consider when analysing CIRS limb data. Firstly, at the CIRS resolution, below 120 km all methane lines have optical depths larger than unity, which means that they do not provide information at the tangent height itself, but somewhere above it (see Fig. 1). It is therefore not possible to derive the temperature below 120 km from FP4 limb data. Secondly, the limb sequence we used consists of more than 100 spectra, each taken at a different altitude. To increase the signal-to-noise, we fit smooth curves through the data with a vertical resolution of 20 km. For each wavenumber, a curve is fitted through the radiance versus altitude points (the radiance profile). This curve is then sampled at equal altitude steps, resulting in a reduced number of spectra. An example of such a curve is given in Fig. 2. This method reduces computation time for retrievals of the temperature profile (and the CO2 vertical profile) from limb data. It also reduces noise in a similar way as averaging spectra does for nadir observations. The error on the interpolated spectra is derived by mapping the NESR of the individual spectra into the curve-fitting coefficients. A third issue is that the Cassini spacecraft has a pointing uncertainty that translates into an altitude uncertainty for limb observations. This uncertainty can be as large as seven times the FP3 field-of-view size (Flasar et al., 2004). For the observations we used, this amounts to an uncertainty larger than an
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Fig. 3. The derived temperature profiles from limb observations at 55◦ S (solid line) and the a priori temperature profile (dotted line).
atmospheric scale height. It is therefore necessary to correct for this uncertainty. The method we used for this involves retrieving temperature with the FP4 limb data and relies on the fact that the shape of the methane band there changes with pressure. The spectra that were sampled from the smooth curves were used simultaneously to retrieve a temperature profile. Then the presumed altitude of these spectra were repeatedly shifted and other temperature profiles were retrieved. The altitude shift that minimises the residuals with the observed spectra for the temperature retrieval yields the correction in altitude. The temperature retrieved with this shift is then used for the CO2 retrievals, using FP3 spectra that are shifted with the same altitude correction. The temperature profile retrieved with this method for 55◦ S is shown in Fig. 3. 5. CO abundance To determine the CO VMR, the rotational lines between 30 and 60 cm−1 were analysed. This spectral range has not yet been used to analyse Titan’s CO before the arrival of Cassini at Saturn and can thus provide important independent results about stratospheric CO. Another advantage of the CIRS data is the high spatial resolution. This is the first time spatially resolved CO measurements are taken. The data used come from long integration time observations, where FP1 is pointed at a fixed location on Titan for a number of hours. Parameters like latitude and emission angle thus stay constant during the observation. This allowed us to simply average the spectra produced to increase the signal-to-noise ratio. Only observations that have more than 100 spectra were used here to ensure a high signal-to-noise ratio. The T0 flyby did not have an observation with sufficient spectra and hence no observations from this flyby were included for the analysis of FP1 data. The chosen spectral region is mainly affected by N2 –N2 collision-induced absorption and is sensitive to the temperature near the tropopause (see Fig. 4). This part of the spectrum also includes the rotational lines of CO and HCN. Because some
Fig. 4. The contribution functions for temperature (dR/dT , solid lines) and CO [dR/dq(CO), dashed lines] at two wavenumbers in the far-infrared. The contribution function for H2 O is shown by the dotted line.
of the rotational lines of these two molecules blend, the HCN VMR must also be accurately determined. The temperature retrievals mentioned previously are not sensitive to temperatures near the tropopause, and therefore, in order to fit the spectrum, tropospheric temperature was retrieved. This was done using the part of the spectrum between 30 and 60 cm−1 that has no rotational lines of CO or HCN (i.e., the continuum) in order to prevent any bias in the retrievals caused by variations in gaseous abundances. Note that this temperature retrieval has only the purpose of fitting the continuum, since the contribution function of the CO rotational lines peaks in the stratosphere, where temperature is determined from FP4 observations (Fig. 4). After fitting the continuum, temperature was fixed and the full spectrum between 30 and 60 cm−1 was fitted by retrieving CO. Horizontal and vertical variations of the HCN VMR were fixed to those deduced by Teanby et al. (2006). Note that these vary significantly with latitude. After fixing temperature and HCN at each latitude, an optimal scaling factor was then determined for a CO profile that is constant with height. Our a priori estimate for the CO VMR was 50 ppm. Fig. 5 shows a typical fit to an averaged spectrum. Fits are reasonable, although the CO and HCN lines are not much larger than the noise level in the spectrum. This gives rise to the reasonably large error bars on the retrieved CO VMR values given in Table 3. This table also shows the observations used, their properties and the retrieved CO VMR. The results show that CO is uniform within error over the latitude range analysed here. The CO VMR derived from these observations has a weighted average of 47 ± 8 ppm, assuming that CO is uniform with latitude. This value is consistent with Gurwell (2004). 6. H2 O abundance The method used to determine the CO VMR can also be used to constrain the H2 O abundance in Titan’s stratosphere as this
Titan’s stratospheric oxygen compounds
359
Fig. 6. Measured spectrum (solid line) and fit to the spectrum (dashed line) for a typical nadir observation between 660 and 675 cm−1 . The 1σ error bar on the spectrum is shown in the top right. This observation is taken at the equator with an emission angle of 50◦ .
Fig. 5. Measured spectrum (solid line) and fit to the spectrum (dashed line) for a typical nadir observation at (a) 30–60 cm−1 and (b) 110–190 cm−1 . Positions of the strongest rotational lines of CO, HCN, CH4 and H2 O are indicated by vertical lines. The 1σ error bar on the spectrum is shown in the bottom right of each plot. These spectra come from the T5-2 observation in Table 3.
molecule has strong rotational lines in the far-infrared region covered by CIRS. The observations in Table 3 can be used for this purpose. However, the data suggest an absence of H2 O lines above the noise level (see Fig. 5). Therefore only an upper limit can be determined. Spectra between 110 and 190 cm−1 were fitted by retrieving temperature, assuming no H2 O. Wavenumbers where H2 O lines were expected were not taken into account for this retrieval. The misfit to the measured spectra, defined as: χ2 =
n (x(˜νi ) − y(˜νi ))2 i=1
σ (˜νi )2
(2)
was calculated for the spectrum with n wavenumber points. Here, y(˜νi ) is the measured radiance for wavenumber ν˜ i with variance σ (˜νi ) and x(˜νi ) is the calculated spectrum for that wavenumber. Then, an H2 O profile was introduced that is constant above the condensation level and the value for χ 2 was again calculated. χ 2 values were calculated for a range of stratospheric H2 O VMRs and the VMR corresponding to the 3σ upper limit was determined (χ 2 = 9; Press et al., 1992). The observation with the least noise in the spectra (T5-2 in Table 3) yielded a 3σ upper limit of 0.9 ppb. This upper limit is consistent with a H2 O abundance of 0.4 ppb derived by Coustenis et al. (1998) for an H2 O profile that is constant above the saturation level. 7. Latitudinal distribution of CO2 Since FP3 consists of an array of 10 individual detectors that are relatively small, a range of latitudes is covered for every observation. This allows us to sample the atmosphere at a higher spatial resolution compared to FP1 observations and get a better view of any variations with latitude. The advantage of the high
Fig. 7. The contribution functions for CO2 [dR/dq(CO2 )] for nadir (0◦ emission angle, dashed line) and limb observations at different tangent heights.
spectral resolution data used here compared to the 2.5 cm−1 data used in Flasar et al. (2005) is that spectral features are better separated and less smooth. This allows a better distinction between the many emission lines that influence FP3 spectra and the haze contribution. Also, more spectra are averaged together for the 0.5 cm−1 resolution data compared to the 2.5 cm−1 data, because the spacecraft is pointed to a single point on Titan during the observation sequence instead of scanning across the moon’s sphere. Furthermore, data with two different resolutions provide an opportunity to check the results that are obtained, since they should be consistent. The disadvantage of the high spectral resolution data is the limited spatial coverage: the observations available so far extend from 75◦ S to only as far north as 35◦ N (see Table 2). We used spectra between 660 and 675 cm−1 to retrieve CO2 . For all nadir spectra used here, the Q branch of the ν2 band of CO2 is the most prominent feature in this part of the spectrum (see, e.g., Fig. 6). Its contribution function peaks at the stratosphere, around 100 km (see Fig. 7) Besides CO2 , the spectrum is also influenced by weak lines of C2 H2 , HCN and HC3 N and by haze.
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Fig. 8. Retrieved variations of CO2 VMR with latitude.The retrieved values are grouped by Titan flyby number. Any variation of the CO2 VMR with latitude is entirely within the error bars.
CO2 is retrieved by scaling a profile that is constant above the condensation level. The effects of C2 H2 , HCN and HC3 N, are taken into account by assuming the profiles and variations derived by Teanby et al. (2006) from 2.5 cm−1 resolution data. A scaling factor for haze is left as a free parameter to fit the continuum. It is solved for, together with the CO2 scaling factor, by the retrieval algorithm. Fig. 8 shows the retrieved stratospheric CO2 VMR for the various observations, grouped by Titan flyby. It is clear from this that there is very little variation with latitude in stratospheric CO2 , if at all. Any variation lies completely within the error bars. The retrieved values from different observations are almost identical when there is an overlap in latitude. This is very encouraging and shows that the retrievals are consistent, because no large temporal variations are expected for the CO2 VMR in the period covered by the data. The results are also consistent with those from Coustenis and Bezard (1995) and Flasar et al. (2005). Note that the results presented here are derived from data with three times higher spectral resolution than Coustenis and Bezard (1995) and Flasar et al. (2005). 8. Vertical distribution of CO2 With FP3 limb spectra at 55◦ S, a vertical profile of CO2 was derived with spectra between 660 and 675 cm−1 . The method for deriving this vertical profile is similar to that for the temperature retrievals that use FP4 limb observations. First the measured data was interpolated and sampled at equal altitude steps. Then, the altitude information of these spectra were corrected with the shift obtained from the temperature retrieval. Finally, C2 H2 , HCN, HC3 N and haze profiles were retrieved simultaneously to fit the parts of the spectra not significantly affected by CO2 . The retrieved C2 H2 profile is roughly uniform with a VMR of 2.5 ppb, which is close to the profile derived by Teanby et al. (2005). The retrieved HCN profile has a similar gradient to the profile derived by Marten et al. (2002) and is consistent with the retrieved HCN profile using CIRS data at wavenumbers where HCN has strong emission lines (Teanby et
Fig. 9. (a) Measured radiances at 667.75 cm−1 (symbols), where CO2 contributes most to the radiance. Solid lines are the radiance profiles for CO2 profiles that are constant above the condensation level, with a VMR of 10 (low radiances), 15 and 20 ppb (high radiances). (b) Parts of the spectra obtained by interpolating the measurements (thick dashed line) at three altitudes across the range of altitudes covered by the observations. Thin lines show synthetic spectra for CO2 profiles that are constant above the condensation level, with a VMR of 10 (low radiances), 15 and 20 ppb (high radiances). Aerosol opacity, HCN, C2 H2 and HC3 N VMR are kept constant at the retrieved values. The 1σ error bars on the spectra are shown in the top right.
al., 2005). So, although the lines of C2 H2 and HCN between 660 and 675 cm−1 are not ideal for retrieving these gases, the retrieved profiles seem reasonable. This is important, because these two gases have lines that overlap with the CO2 peak at 667.75 cm−1 . These lines are much weaker than the CO2 peak at southern latitudes, but can still introduce small offsets in the radiance at 667.75 cm−1 and hence influence conclusions about the vertical distribution of CO2 . With the retrieved gases and haze profile kept fixed, synthetic spectra were calculated for three CO2 profiles. These profiles are constant above the condensation level with VMR levels of 10, 15 and 20 ppb. Any significant variation of CO2 with altitude will then be visible as a deviation of the data from the three synthetic radiance profiles. Fig. 9 shows the radiance profiles and spectra for the three scenarios, compared to the measured data (the spectra are interpolated from the data as described above). At altitudes between 100 and 200 km, the data at 667.75 cm−1 is very well fitted with the profile of 15 ppb. This profile is preferred above the other two profiles between these altitudes. Below 100 km this profile still fits the data very well, but since the contribution functions do not peak at tangent heights this low (see, e.g., Figs. 1 and 7), variables at these altitudes are not constrained. Instead, observations at these low tangent heights probe higher altitudes. In any case, a CO2 profile that is constant above the condensation level at 15 ppb is consistent with these low-altitude data.
Titan’s stratospheric oxygen compounds
Above 200 km a profile with 15 ppb seems to underestimate the data (especially around 230 km). However, at these high altitudes the CO2 VMR is poorly constrained, since radiance changes only moderately with changing CO2 VMR and the spread of the data is large. In other words, a wide range of CO2 VMR is consistent with the data at these altitude. The profile with 15 ppb gives radiances that lie within the data spread. In summary, a constant CO2 VMR of 15 ppb above the condensation level is consistent with all the data taken at this limb observation sequence at 55◦ S. 9. Discussion and conclusions The CIRS results presented here can be compared with previous results and expectations, discussed in the introduction. Firstly, the CIRS measurements present another independent determination of the CO VMR, using a wavelength range not used for this purpose before the arrival of Cassini at Saturn. Also, for the first time, spatially resolved measurements of CO are presented here. The analysis shows that CO is uniform with latitude within error over the observed latitude range. Because CO has a very long lifetime in Titan’s stratosphere it is expected to be uniform with latitude. This analysis shows that this may indeed be the case, although variations in the order of 20 ppm are still within the error bars. The weighted average stratospheric CO VMR of 47 ± 8 ppm, which assumes that CO is uniform with latitude, agrees well with the results from Gurwell (2004) and Gurwell and Muhleman (2000). On the other hand, this result is higher than the derived stratospheric VMR of Hidayat et al. (1998), which use the same rotational lines as Gurwell (2004) and Gurwell and Muhleman (2000). The derived stratospheric VMR is also consistent with the tropospheric value derived by Lellouch et al. (2003) (32 ± 10 ppm). Hence, the CIRS results presented here can argue for a uniform CO profile. However, a profile that increases slightly with altitude cannot be ruled out. The question about the origin of Titan’s CO, however, is still open. Where the abundances of CO2 and H2 O can be explained well with an external oxygen source consisting mostly of water, such a model fails to reproduce the high CO VMR in Titan’s atmosphere. Maintaining the high level of CO in a steady state requires an extra source of CO (Lara et al., 1996). If the meteorites that are providing the oxygen are made not purely out of water, but have a fraction of CO or organics in them, this might provide the extra source, as pointed out by Lellouch et al. (2003). Another possibility is that the extra CO is provided by some internal source, bringing CO from below the surface into the atmosphere. A third possibility is simply that CO is not in equilibrium and that most of the CO we see today is the remains of a much larger CO abundance from the past. Results from Wong et al. (2002) suggest that CO was as much as 14 times more abundant in Titan’s early atmosphere, based on arguments dealing with Titan’s isotope ratios. The timescale for this decay is estimated to be comparable to the chemical lifetime of 500 Myr. A final option is that CO is periodically replenished by comets. The oxygen released in a comet impact comes mostly in the form of CO (Zahnle, 1996). The extra CO
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flux needed to maintain the VMR we see today can be provided by a major impact every 100,000 years (Lellouch et al., 2003). This is much shorter than the chemical lifetime of CO, and hence CO will never be in chemical equilibrium is this case. Stratospheric H2 O could not be detected here and an upper limit of 0.9 ppb was determined. This upper limit is not inconsistent with the detection by Coustenis et al. (1998). They derive a VMR of 0.4 ppb for a profile that is constant above the condensation level. Variations of CO2 with latitude have been determined from data with both high spectral resolution and high spatial resolution. All previous analysis of CO2 either had lower spectral resolution (Coustenis and Bezard, 1995; Flasar et al., 2005) or lower spatial resolution (Coustenis et al., 2003). This analysis shows that CO2 does not vary noticeably with latitude. The retrieved VMR values around 16 ppb and the latitudinal variations agree very well with retrievals from Voyager spectra (Coustenis and Bezard, 1995) and 2.5 cm−1 resolution CIRS spectra (Flasar et al., 2005). For the first time the vertical profile of CO2 on Titan has been constrained here. Limb observations were used that probe Titan’s stratosphere at a range of altitudes. All limb spectra are well fitted with a CO2 profile that is constant with altitude and a VMR of 15 ppb. This is especially clear below 200 km, where also nadir observation probe. The VMR from limb data is consistent with the values we find for nadir observations. Unfortunately, the CO2 VMR above 200 km is poorly constrained and a small increase there cannot be ruled out. The lack of variation with latitude and altitude is possibly caused by the long chemical lifetime of CO2 [of the order of several hundred years at 300 km (Wilson and Atreya, 2004)]. The chemical lifetime is much larger than the timescale for atmospheric motion, resulting in a vertical profile that does not increase rapidly with altitude. The limb observations presented here indicate that this is indeed the case. Models show that the amount of latitudinal variation of a certain gas is tied with the steepness of its vertical profile, combined with vertical atmospheric transport (Lebonnois et al., 2001; Hourdin et al., 2004). Such a connection is also shown by the (at least qualitative) correlation between the polar gas enrichments found by Flasar et al. (2005) and the relative steepness of the newly derived vertical profiles of hydrocarbons near the equator of Vinatier et al. (2006). Nadir and limb observations thus give consistent results that agree with the idea of vertical transport at the north pole, which brings enriched air down to levels where they are observed in nadir observations. If the vertical profile is steep the atmosphere will be highly enriched at higher altitudes and downward transport will thus result in a higher polar enrichment. If the vertical profile is roughly constant, downward transport will not affect retrieved abundances from nadir observations significantly, as is the case for CO2 here. The Cassini spacecraft will continue to make observations until at least 2008. In this time many more CIRS spectra will be taken. By co-adding large numbers of nadir spectra it should be possible to detect stratospheric H2 O with CIRS and decrease errors of the retrieved CO abundance. Also, there will be increased latitudinal coverage of high spectral resolution observa-
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tions. After the end of 2006, Cassini will be in a high inclination orbit, which allows it to observe Titan’s north pole from a good vantage point. The north pole is likely surrounded by a polar vortex. The effect of a vortex on the oxygen compounds can be studied then. Finally, many more limb observations will be made. By analysing limb observations with more spectra than analysed here, or by combining multiple limb observations, the CO2 profile can be better constrained, also above 200 km, and the CO2 vertical profile can be analysed at a range of latitudes. It may also be possible to obtain a rough vertical profile of CO and perhaps H2 O by analysing FP1 limb data. The vertical resolution will be poor for these gases since the FP1 FOV is relatively large. All such analyses will give unprecedented information about Titan’s oxygen compounds. Acknowledgments We would like to acknowledge the many people involved in building and operating CIRS and in calibrating the data. Athena Coustenis, Mark Gurwell and an anonymous reviewer provided useful comments, which helped improve the manuscript. R.d.K. would like to thank the Prins Bernhard Cultuurfonds and the Pieter Beijer Fonds for financial support. Furthermore, this research was funded by the UK Particle Physics and Astronomy Research Council and the NASA Cassini Project. References Borysow, A., 1991. Modelling of collision-induced infrared-absorption spectra of H2 –H2 pairs in the fundamental band at temperatures from 20 K to 300 K. Icarus 92 (2), 273–279. Borysow, A., Frommhold, L., 1986a. Collision-induced rototranslational absorption spectra of N2 –N2 pairs for temperatures from 50 to 300 K. Astrophys. J. 311, 1043–1057. Borysow, A., Frommhold, L., 1986b. Theoretical collision-induced rototranslational absorption spectra for modelling Titan’s atmosphere: H2 –N2 pairs. Astrophys. J. 303, 495–510. Borysow, A., Frommhold, L., 1986c. Theoretical collision-induced rototranslational absorption spectra for the outer planets: H2 –CH4 pairs. Astrophys. J. 304, 849–865. Borysow, A., Frommhold, L., 1987. Collision-induced rototranslational absorption spectra of CH4 –CH4 pairs at temperatures from 50 to 300 K. Astrophys. J. 318, 940–943. Borysow, A., Tang, C., 1993. Far infrared CIA spectra of N2 –CH4 pairs for modelling of Titan’s atmosphere. Icarus 105, 175–183. Brown, L.R., Benner, D.C., Champion, J.P., Devi, V.M., Fejard, L., Gamache, R.R., Gabard, T., Hilico, J.C., Lavorel, B., Loete, M., Mellau, G.C., Nikitin, A., Pine, A.S., Predoi-Cross, A., Rinsland, C.P., Robert, O., Sams, R.L., Smith, M.A.H., Tashkun, S.A., Tyuterev, V.G., 2003. Methane line parameters in HITRAN. J. Quant. Spectrosc. Radiat. Trans. 82, 219–238. Coustenis, A., Bezard, B., 1995. Titan’s atmosphere from Voyager infrared observations. 4. Latitudinal variations of temperature and composition. Icarus 115, 126–140. Coustenis, A., Bezard, B., Gautier, D., 1989. Titan’s atmosphere from Voyager infrared observations. I. The gas composition of Titan’s equatorial region. Icarus 80, 54–76. Coustenis, A., Salama, A., Lellouch, E., Encrenaz, T., Bjoraker, G.L., Samuelson, R.E., de Graauw, T., Feuchtgruber, H., Kessler, M.F., 1998. Evidence for water vapour in Titan’s atmosphere from ISO/SWS data. Astron. Astrophys. 336, L85–L89. Coustenis, A., Salama, A., Schulz, B., Ott, S., Lellouch, E., Encrenaz, T.H., Gautier, D., Feuchtgruber, H., 2003. Titan’s atmosphere from ISO midinfrared spectroscopy. Icarus 161, 383–403.
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Oxygen compounds in Titan’s stratosphere as observed by Cassini CIRS R. de Kok a,∗ , P.G.J. Irwin a , N.A. Teanby a , E. Lellouch b,c , B. Bézard b,c , S. Vinatier b,c , C.A. Nixon d , L. Fletcher a , C. Howett a , S.B. Calcutt a , N.E. Bowles a , F.M. Flasar e , F.W. Taylor a a Atmospheric, Oceanic & Planetary Physics, Department of Physics, University of Oxford, Clarendon Laboratory, Parks Road, Oxford, OX1 3PU, UK b Observatoire de Paris, LESIA, F-92195 Meudon, France c Université Pierre et Marie Curie-Paris 6, UMR 8109, F-75005 Paris, France d Department of Astronomy, University of Maryland, College Park, MD 20742, USA e NASA/Goddard Space Flight Center, Code 693, Greenbelt, MD 20771, USA
Received 31 March 2006; revised 3 August 2006 Available online 20 November 2006
Abstract We have investigated the abundances of Titan’s stratospheric oxygen compounds using 0.5 cm−1 resolution spectra from the Composite Infrared Spectrometer on the Cassini orbiter. The CO abundance was derived for several observations of far-infrared nadir spectra, taken at a range of latitudes (75◦ S–35◦ N) and emission angles (0◦ –60◦ ), using rotational lines that have not been analysed before the arrival of Cassini at Saturn. The derived volume mixing ratios for the different observations are mutually consistent regardless of latitude. The weighted mean CO volume mixing ratio is 47 ± 8 ppm if CO is assumed to be uniform with latitude. H2 O could not be detected and an upper limit of 0.9 ppb was determined. CO2 abundances derived from mid-infrared nadir spectra show no significant latitudinal variations, with typical values of 16 ± 2 ppb. Midinfrared limb spectra at 55◦ S were used to constrain the vertical profile of CO2 for the first time. A vertical CO2 profile that is constant above the condensation level at a volume mixing ratio of 15 ppb reproduces the limb spectra very well below 200 km. This is consistent with the long chemical lifetime of CO2 in Titan’s stratosphere. Above 200 km the CO2 volume mixing ratio is not well constrained and an increase with altitude cannot be ruled out there. © 2006 Elsevier Inc. All rights reserved. Keywords: Titan; Atmospheres, composition
1. Introduction Three oxygen compounds have previously been detected in Titan’s stratosphere: CO2 , H2 O and CO. Carbon dioxide in Titan’s stratosphere was first detected by the Voyager IRIS instrument (Samuelson et al., 1983). More detailed analysis of Voyager data and analysis of data from the Infrared Space Observatory (ISO) and from Cassini Composite Infrared Spectrometer (CIRS) show a mixing ratio of 15–20 ppmv (Coustenis et al., 1989, 2003; Flasar et al., 2005). The temperature and pressure at Titan’s tropopause are such that condensation occurs, providing a sink for CO2 . Therefore, the equilibrium stratospheric abundance of CO2 would be expected to be the same as the * Corresponding author. Fax: +44 1865 272923.
E-mail address: [email protected] (R. de Kok). 0019-1035/$ – see front matter © 2006 Elsevier Inc. All rights reserved. doi:10.1016/j.icarus.2006.09.016
saturation level at the tropopause if CO2 originates from below the tropopause. Instead, the observed values are at least a thousand times larger, indicating that CO2 is replenished in the stratosphere. The most effective way to produce CO2 is via the following chemical reaction: CO + OH → CO2 + H.
(1)
This scheme requires (i) a sufficient abundance of CO, and (ii) a source of oxygen in the form of OH. The OH radical can be formed from photolysis of H2 O, or from reactions involving CO and CH4 (Samuelson et al., 1983). CO is one of the more abundant trace gases in Titan’s atmosphere (see below), so the first condition is satisfied. Also, water ice is common in the Solar System and a continuous stream of icy dust particles falling into the atmosphere could provide the oxygen. This mechanism was previously suggested to explain the oxygen source for the giant planets (Feuchtgruber et al., 1997). These dust particles
Titan’s stratospheric oxygen compounds
heat up in the atmosphere, leaving a trail of gas along their way. An attempt to model the effect of infalling dust particles is presented by English et al. (1996). They show that most of the resulting vapours are deposited high in Titan’s atmosphere over a broad range of altitudes centred around 750 km. At these high altitudes H2 O can be readily destroyed by ultraviolet radiation, creating the OH radical needed to produce CO2 . CO2 is only slowly destroyed in the stratosphere, with a chemical lifetime of several hundred years (Wilson and Atreya, 2004). Not all water molecules are destroyed and converted to CO2 . According to photochemical models (e.g., Lara et al., 1996; Wilson and Atreya, 2004) a significant amount of water should remain high in the atmosphere, with the abundance quickly dropping with decreasing altitude. A dedicated observation with the Earth-orbiting infrared telescope ISO, using its high spectral resolution and high sensitivity, confirmed the existence of water vapour in Titan’s stratosphere at 0.4 ppb, assuming that H2 O is constant above the saturation level (Coustenis et al., 1998). Like CO2 , H2 O also condenses at the tropopause. However, unlike CO2 , H2 O has a relatively short lifetime against photolysis (of the order of 5 years; Coustenis et al., 1998) and thus a continuous influx is needed to achieve a steady state abundance that is consistent with observations. CO is very stable in Titan’s atmosphere: it has a chemical lifetime of 500 Myr (Lellouch et al., 2003) and it does not condense at the tropopause. CO is thus expected to be uniformly mixed throughout the entire atmosphere in an equilibrium situation, since the timescale of mixing is much smaller than that of the destruction of CO. Previous observations of CO in Titan’s atmosphere (summarised in Table 1) have not conclusively confirmed such a uniform profile. Despite the multitude of observations at various wavelengths, there is no consensus on the mixing ratio and vertical profile of the gas. Although most of the observations that probe the stratosphere show a volume mixing ratio (VMR) of at least 50 ppm, Hidayat et al. (1998) derived a vertical profile that decreases with altitude. However, this profile does not fit subsequent 5 µm data from Lopez-Valverde et al. (2005). It is also inconsistent with CO concentrations derived by Gurwell and Muhleman (1995, 2000) and Gurwell (2004), which use the same CO rotational lines. The most reliable tropospheric CO concentration is most likely given by Lellouch et al. (2003), since they use improved lineshapes and improved 5 µm observations compared to earlier tropospheric measurements, according to the authors. However, their tropospheric VMR of 32 ± 10 ppm is lower than that derived for the stratosphere, which is unexpected given that CO should not condense in Titan’s atmosphere. Lopez-Valverde et al. (2005) find that a higher stratospheric CO mixing ratio (∼60 ppm) is required to match CO fluorescence emission at 5 µm. Thus, a depletion of CO in the troposphere cannot be ruled out, based on observations made so far. However, a physical mechanism that would explain a decrease in the troposphere has not yet been put forward. The Composite Infrared Spectrometer (CIRS) on board the Cassini orbiter has the capability to study all three of the previously detected oxygen compounds (Flasar et al., 2004). CO2 has its ν2 band in the mid-infrared (at 667.75 cm−1 ),
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Table 1 Previous determinations of CO abundances in Titan’s atmosphere [extended from Table 1 in Gurwell and Muhleman (2000)] Altitude
CO VMR (ppm)
Troposphere 48+100 −32
Wavelength
Reference
1.57 µm
Lutz et al. (1983)
Troposphere 10+10 −5
4.8 µm
Noll et al. (1996)
Troposphere
4.8 µm
Lellouch et al. (2003)
2.6 mm
Muhleman et al. (1984)
2.6 mm
Marten et al. (1988)
2.6 mm
Gurwell and Muhleman (1995)
Stratosphere Stratosphere Stratosphere Stratosphere Stratosphere
32+10 −10 60+40 −40 2+2 −1 50+10 −10 27+5 −5 52+6 −6 51+4 −4
Stratosphere Stratosphere 60 Stratosphere 45+15 −15
2.6, 1.3, 0.9 mm Hidayat et al. (1998) 1.3 mm 0.9 mm 4.8 µm 150–500 µm
Gurwell and Muhleman (2000) Gurwell (2004) Lopez-Valverde et al. (2005) Flasar et al. (2005)
Note. The table shows a wide range of CO VMR values, giving rise to debates about the CO abundance and vertical profile.
whereas CO and H2 O have rotational lines in the far-infrared (between 10 and 300 cm−1 ). CIRS has three separate focal planes that together measure the infrared spectrum between 10 and 1400 cm−1 . FP1 covers the far-infrared (10–600 cm−1 ) and has a single circular field-of-view (FOV) with a fullwidth half-maximum of 2.5 mrad. The mid-infrared is covered by FP3 (600–1100 cm−1 ) and FP4 (1100–1400 cm−1 ), which each comprise of an array of 10 pixels, each having a 0.27 × 0.27 mrad FOV. In this paper we use CIRS data with an apodised resolution of 0.5 cm−1 , which is the maximum resolution obtainable by CIRS. We use downward viewing (nadir) observations, taken on multiple close approaches of Cassini with Titan, to obtain abundances and distributions with latitude of Titan’s stratospheric oxygen compounds. We also use a horizontal viewing (limb) observation to constrain the vertical profile of CO2 . Analysis of Titan’s oxygen compounds with both a high spectral resolution and a high spatial resolution was not presented before in literature. Such an analysis gives a better view of the threedimensional distribution of oxygen compounds on Titan. 2. Observations The FP3 nadir data that were used to obtain the latitudinal CO2 distribution consist of sequences of maximum 5330 spectra taken from observations lasting less than 16 h. Table 2 summarises the data used from each Titan flyby. For each observation sequence, spectra were averaged in bins with a width of 10◦ latitude and a spacing of 5◦ latitude to achieve Nyquist sampling. The FP1 sequences used for determining CO and H2 O abundances are summarised in Table 3. These sequences last up to 4 h. Their observation numbers refer to the flybys listed in Table 2. To obtain information about the vertical profile of CO2 , limb observations from the T6 flyby on 28 August 2005 were used. Limb observations probe the atmosphere at different altitudes.
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Table 2 FP3 observations used to derive the latitudinal CO2 distribution, grouped by Titan flyby Flyby
Date
Latitude
No. of spectra
Emission angle
FOV size
T0 Tb T4 T5 T6 T7a
2–3 Jul. 04 11–13 Dec. 04 31 Mar.–1 Apr. 05 16 Apr. 05 21–22 Aug. 05 24–25 Sep. 05
75◦ 10◦ 45◦ 30◦ 70◦ 15◦
1455 7529 2190 783 1786 1918
31◦ –43◦ 7◦ –50◦ 42◦ –54◦ 30◦ 27◦ –60◦ 25◦ –33◦
5◦ 2◦ –7◦ 2◦ 2◦ 2◦ 7◦
S–65◦ S S–25◦ N S–35◦ S; 30◦ N–35◦ N N S–60◦ S; 20◦ N–25◦ N S–5◦ S
Note. The last set of observations was taken from a distant flyby. Since these observations were taken after the T7 flyby, the name T7a was adopted here. FOV sizes are in degrees of latitude. Table 3 Properties of the selected FP1 observations and the retrieved CO VMR Observation
Latitude
No. of spectra
Emission angle
CO VMR (ppm)
FOV size
Tb-1 Tb-2 T4-2 T4-3 T5-2 T6-3 T6-4
17◦ 18◦ 41◦ 48◦ 55◦ 30◦ 54◦
226 129 241 239 166 191 248
46◦ 16◦ 43◦ 52◦ 51◦ 52◦ 45◦
46 ± 20 41 ± 24 43 ± 19 51 ± 21 40 ± 19 48 ± 20 60 ± 20
19◦ 14◦ 19◦ 20◦ 21◦ 18◦ 18◦
N N S N N N S
Note. Observation numbers refer to the flyby numbers of Table 2. There are often more than one observation sequences per flyby and the second numbers in the observation names point at the individual sequences. FOV sizes are in degrees of latitude. The retrieved values for the CO VMR are consistent across all latitudes and emission angles and have a weighted mean of 47 ± 8 ppm.
If the opacity is small enough, a limb observation will probe the atmosphere at the point along the line of sight that is closest to the surface—the tangent height of the observation. The T6 limb sequence consists of 198 FP3 spectra and 288 FP4 spectra, taken in a 1 h time interval at a latitude of 55◦ S. The tangent heights of the spectra lie between 50 and 300 km and the average FOV size is 11 km at the limb. FP4 spectra are used to derive temperature at 55◦ S, FP3 spectra are used to constrain the CO2 vertical profile. All the above data have an apodised spectral resolution of 0.5 cm−1 . We derive temperature profiles from nadir data with a resolution of 2.5 cm−1 obtained at the T0, Tb and T4 flybys as a basis for retrievals that use higher spectral resolution. These lower resolution data are used, because 0.5 cm−1 resolution FP4 data do not always cover the same latitudes as the FP3 data used to derive the gas VMR due to a 0.94 mrad offset between FP3 and FP4. In this case, temperature cannot be determined for the correct latitude. On the other hand, the 2.5 cm−1 data has good spatial coverage at all latitudes up to 60◦ N (see Teanby et al., 2006). 3. Model The radiative transfer model used here to analyse the CIRS spectra is based on the model used for Jupiter CIRS analysis by Irwin et al. (2004). It models the atmosphere as a number of spherical layers, each with independently adjustable parameters. In this case 99 layers were used, covering altitudes up to 790 km.
Line data for methane were taken from Brown et al. (2003), HCN data from Maki et al. (1996), as compiled in the HITRAN2004 database by Rothman et al. (2005). HC3 N and C2 N2 data were taken from the GEISA97 database (JacquinetHusson et al., 1999) and line data for the oxygen compounds and all other gases were extracted from the HITRAN2000 database (Rothman et al., 2003). From the line data, the k-distributions were calculated and used in a correlated-k model (Lacis and Oinas, 1991). This method drastically decreases computing time for the radiative transfer model, while maintaining enough of its accuracy for analysis of CIRS data (Irwin et al., 2004). Titan’s far-infrared spectrum is most notably influenced by collision-induced absorption of nitrogen and methane molecules, with a smaller contribution from hydrogen molecules. All six collision pairs involving these molecules are included in the radiative transfer model. Absorption coefficients were calculated using the subroutines described in Borysow and Frommhold (1986a, 1986b, 1986c, 1987), Borysow (1991), Borysow and Tang (1993). Aerosol absorption can also be included in the model. Its optical properties are assumed to be similar to laboratoryproduced tholins and the optical constants of Khare et al. (1984) were used to model the haze. The extinction cross-section of the haze is determined by Mie theory, assuming a particle size of 0.2 µm, as determined by Nixon (1998). We use an a priori haze profile that has a constant number of particles per gram of atmosphere above 90 km. Below 90 km, the density decreases exponentially, as described in Teanby et al. (2006). Surface emission is modelled as a blackbody source with a uniform temperature of 94 K. The largest errors in the model are due to the correlated-k approximation, which results in maximum errors of 2 nW cm−2 sr−1 (cm−1 )−1 for nadir observations and 5 nW cm−2 sr−1 (cm−1 )−1 for limb observations. Errors in the line intensities of the molecules are in the order of 2–5% (Rothman et al., 2003). All these errors are smaller than the noise equivalent spectral radiance (NESR) in all cases presented here. Errors due to CIA should not influence the results presented here, since CIA is merely used to fit the continuum and has its origin mainly in the troposphere. Forward model errors of 5 nW cm−2 sr−1 (cm−1 )−1 at maximum are added quadratically to the measurement errors to take into account the correlated-k approximation errors and the line data errors. The measured spectra are fitted using an optimal estimation method that minimises a cost function (Rodgers, 1976). The
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Fig. 1. (a) Temperature contribution functions (dR/dT ) for two wavenumbers in the CH4 ν4 band for a nadir observation. (b) Temperature contribution functions for limb observations at different tangent heights. Note that for low altitudes, the contribution functions do not peak at the tangent height.
cost function includes the misfit to the spectrum and the deviation from an a priori estimate. The use of a priori information prevents the appearance of highly unphysical solutions in the retrieval process, caused by the underconstrained nature of the problem. The distance that the solution is allowed to deviate from its a priori estimate depends on the specified a priori error. Care must be taken when choosing the a priori errors. Too large or small an error will result in an under- or over-constrained solution. To find a value between the two extremes, we balanced the measurement error with the a priori error in such a way that both errors constrain the retrieval equally, resulting in smooth retrieved profiles that can deviate significantly from the a priori profile, while providing a satisfactory fit to the data. The model is the same as in Teanby et al. (2006) and more details can be found there. The retrieval of each molecule will be discussed separately here. 4. Temperature retrievals Emission from gases in the stratosphere is highly dependent on the temperature structure of the atmosphere. Therefore, a temperature profile must be determined before retrieving any mixing ratios of trace species. Temperature can be inferred from the ν4 band of methane in FP4, assuming methane is uniformly mixed in the stratosphere. Methane is expected to be uniformly mixed in the stratosphere and Huygens results show that this is indeed the case (Niemann et al., 2005). The assumption of uniformity thus seems reasonable. A stratospheric methane VMR of 1.6% is assumed, as measured with CIRS by Flasar et al. (2005). For nadir observations, temperatures were derived from 2.5 cm−1 resolution data from 1240 to 1360 cm−1 . Contribution functions for two representative wavenumbers are shown in Fig. 1. They peak in the stratosphere, between 0.5 and 10 mbar (100–250 km). A temperature profile was derived at intervals of 5◦ latitude by averaging spectra in bins 10◦ latitude wide, from 90◦ S to 50◦ N. Only spectra with emission angles within 20◦ of
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Fig. 2. (a) The smooth radiance profile (dotted line) derived from the raw data points (symbols) for one wavenumber in the spectrum used to derive temperature from limb observations. The solid line shows the fit to this curve for this wavenumber for the temperature retrieval. (b) Fits (dotted lines) to the interpolated spectra (solid lines) at three altitudes for a selected wavenumber range.
each other were taken into consideration to prevent averaging over a wide range of emission angles. The retrieved temperature profiles are in agreement with those presented in Flasar et al. (2005) and Teanby et al. (2006), which were obtained using data of the same resolutions at the T0, Tb and T4 flybys. Nadir observations can only retrieve temperature over a limited altitude range. However, to accurately retrieve gas abundances we require temperatures over the full range of altitudes probed by the limb data. Again, we used FP4 spectra for this purpose, taken from the limb sequence at 55◦ S described previously. There are a number of points to consider when analysing CIRS limb data. Firstly, at the CIRS resolution, below 120 km all methane lines have optical depths larger than unity, which means that they do not provide information at the tangent height itself, but somewhere above it (see Fig. 1). It is therefore not possible to derive the temperature below 120 km from FP4 limb data. Secondly, the limb sequence we used consists of more than 100 spectra, each taken at a different altitude. To increase the signal-to-noise, we fit smooth curves through the data with a vertical resolution of 20 km. For each wavenumber, a curve is fitted through the radiance versus altitude points (the radiance profile). This curve is then sampled at equal altitude steps, resulting in a reduced number of spectra. An example of such a curve is given in Fig. 2. This method reduces computation time for retrievals of the temperature profile (and the CO2 vertical profile) from limb data. It also reduces noise in a similar way as averaging spectra does for nadir observations. The error on the interpolated spectra is derived by mapping the NESR of the individual spectra into the curve-fitting coefficients. A third issue is that the Cassini spacecraft has a pointing uncertainty that translates into an altitude uncertainty for limb observations. This uncertainty can be as large as seven times the FP3 field-of-view size (Flasar et al., 2004). For the observations we used, this amounts to an uncertainty larger than an
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Fig. 3. The derived temperature profiles from limb observations at 55◦ S (solid line) and the a priori temperature profile (dotted line).
atmospheric scale height. It is therefore necessary to correct for this uncertainty. The method we used for this involves retrieving temperature with the FP4 limb data and relies on the fact that the shape of the methane band there changes with pressure. The spectra that were sampled from the smooth curves were used simultaneously to retrieve a temperature profile. Then the presumed altitude of these spectra were repeatedly shifted and other temperature profiles were retrieved. The altitude shift that minimises the residuals with the observed spectra for the temperature retrieval yields the correction in altitude. The temperature retrieved with this shift is then used for the CO2 retrievals, using FP3 spectra that are shifted with the same altitude correction. The temperature profile retrieved with this method for 55◦ S is shown in Fig. 3. 5. CO abundance To determine the CO VMR, the rotational lines between 30 and 60 cm−1 were analysed. This spectral range has not yet been used to analyse Titan’s CO before the arrival of Cassini at Saturn and can thus provide important independent results about stratospheric CO. Another advantage of the CIRS data is the high spatial resolution. This is the first time spatially resolved CO measurements are taken. The data used come from long integration time observations, where FP1 is pointed at a fixed location on Titan for a number of hours. Parameters like latitude and emission angle thus stay constant during the observation. This allowed us to simply average the spectra produced to increase the signal-to-noise ratio. Only observations that have more than 100 spectra were used here to ensure a high signal-to-noise ratio. The T0 flyby did not have an observation with sufficient spectra and hence no observations from this flyby were included for the analysis of FP1 data. The chosen spectral region is mainly affected by N2 –N2 collision-induced absorption and is sensitive to the temperature near the tropopause (see Fig. 4). This part of the spectrum also includes the rotational lines of CO and HCN. Because some
Fig. 4. The contribution functions for temperature (dR/dT , solid lines) and CO [dR/dq(CO), dashed lines] at two wavenumbers in the far-infrared. The contribution function for H2 O is shown by the dotted line.
of the rotational lines of these two molecules blend, the HCN VMR must also be accurately determined. The temperature retrievals mentioned previously are not sensitive to temperatures near the tropopause, and therefore, in order to fit the spectrum, tropospheric temperature was retrieved. This was done using the part of the spectrum between 30 and 60 cm−1 that has no rotational lines of CO or HCN (i.e., the continuum) in order to prevent any bias in the retrievals caused by variations in gaseous abundances. Note that this temperature retrieval has only the purpose of fitting the continuum, since the contribution function of the CO rotational lines peaks in the stratosphere, where temperature is determined from FP4 observations (Fig. 4). After fitting the continuum, temperature was fixed and the full spectrum between 30 and 60 cm−1 was fitted by retrieving CO. Horizontal and vertical variations of the HCN VMR were fixed to those deduced by Teanby et al. (2006). Note that these vary significantly with latitude. After fixing temperature and HCN at each latitude, an optimal scaling factor was then determined for a CO profile that is constant with height. Our a priori estimate for the CO VMR was 50 ppm. Fig. 5 shows a typical fit to an averaged spectrum. Fits are reasonable, although the CO and HCN lines are not much larger than the noise level in the spectrum. This gives rise to the reasonably large error bars on the retrieved CO VMR values given in Table 3. This table also shows the observations used, their properties and the retrieved CO VMR. The results show that CO is uniform within error over the latitude range analysed here. The CO VMR derived from these observations has a weighted average of 47 ± 8 ppm, assuming that CO is uniform with latitude. This value is consistent with Gurwell (2004). 6. H2 O abundance The method used to determine the CO VMR can also be used to constrain the H2 O abundance in Titan’s stratosphere as this
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Fig. 6. Measured spectrum (solid line) and fit to the spectrum (dashed line) for a typical nadir observation between 660 and 675 cm−1 . The 1σ error bar on the spectrum is shown in the top right. This observation is taken at the equator with an emission angle of 50◦ .
Fig. 5. Measured spectrum (solid line) and fit to the spectrum (dashed line) for a typical nadir observation at (a) 30–60 cm−1 and (b) 110–190 cm−1 . Positions of the strongest rotational lines of CO, HCN, CH4 and H2 O are indicated by vertical lines. The 1σ error bar on the spectrum is shown in the bottom right of each plot. These spectra come from the T5-2 observation in Table 3.
molecule has strong rotational lines in the far-infrared region covered by CIRS. The observations in Table 3 can be used for this purpose. However, the data suggest an absence of H2 O lines above the noise level (see Fig. 5). Therefore only an upper limit can be determined. Spectra between 110 and 190 cm−1 were fitted by retrieving temperature, assuming no H2 O. Wavenumbers where H2 O lines were expected were not taken into account for this retrieval. The misfit to the measured spectra, defined as: χ2 =
n (x(˜νi ) − y(˜νi ))2 i=1
σ (˜νi )2
(2)
was calculated for the spectrum with n wavenumber points. Here, y(˜νi ) is the measured radiance for wavenumber ν˜ i with variance σ (˜νi ) and x(˜νi ) is the calculated spectrum for that wavenumber. Then, an H2 O profile was introduced that is constant above the condensation level and the value for χ 2 was again calculated. χ 2 values were calculated for a range of stratospheric H2 O VMRs and the VMR corresponding to the 3σ upper limit was determined (χ 2 = 9; Press et al., 1992). The observation with the least noise in the spectra (T5-2 in Table 3) yielded a 3σ upper limit of 0.9 ppb. This upper limit is consistent with a H2 O abundance of 0.4 ppb derived by Coustenis et al. (1998) for an H2 O profile that is constant above the saturation level. 7. Latitudinal distribution of CO2 Since FP3 consists of an array of 10 individual detectors that are relatively small, a range of latitudes is covered for every observation. This allows us to sample the atmosphere at a higher spatial resolution compared to FP1 observations and get a better view of any variations with latitude. The advantage of the high
Fig. 7. The contribution functions for CO2 [dR/dq(CO2 )] for nadir (0◦ emission angle, dashed line) and limb observations at different tangent heights.
spectral resolution data used here compared to the 2.5 cm−1 data used in Flasar et al. (2005) is that spectral features are better separated and less smooth. This allows a better distinction between the many emission lines that influence FP3 spectra and the haze contribution. Also, more spectra are averaged together for the 0.5 cm−1 resolution data compared to the 2.5 cm−1 data, because the spacecraft is pointed to a single point on Titan during the observation sequence instead of scanning across the moon’s sphere. Furthermore, data with two different resolutions provide an opportunity to check the results that are obtained, since they should be consistent. The disadvantage of the high spectral resolution data is the limited spatial coverage: the observations available so far extend from 75◦ S to only as far north as 35◦ N (see Table 2). We used spectra between 660 and 675 cm−1 to retrieve CO2 . For all nadir spectra used here, the Q branch of the ν2 band of CO2 is the most prominent feature in this part of the spectrum (see, e.g., Fig. 6). Its contribution function peaks at the stratosphere, around 100 km (see Fig. 7) Besides CO2 , the spectrum is also influenced by weak lines of C2 H2 , HCN and HC3 N and by haze.
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Fig. 8. Retrieved variations of CO2 VMR with latitude.The retrieved values are grouped by Titan flyby number. Any variation of the CO2 VMR with latitude is entirely within the error bars.
CO2 is retrieved by scaling a profile that is constant above the condensation level. The effects of C2 H2 , HCN and HC3 N, are taken into account by assuming the profiles and variations derived by Teanby et al. (2006) from 2.5 cm−1 resolution data. A scaling factor for haze is left as a free parameter to fit the continuum. It is solved for, together with the CO2 scaling factor, by the retrieval algorithm. Fig. 8 shows the retrieved stratospheric CO2 VMR for the various observations, grouped by Titan flyby. It is clear from this that there is very little variation with latitude in stratospheric CO2 , if at all. Any variation lies completely within the error bars. The retrieved values from different observations are almost identical when there is an overlap in latitude. This is very encouraging and shows that the retrievals are consistent, because no large temporal variations are expected for the CO2 VMR in the period covered by the data. The results are also consistent with those from Coustenis and Bezard (1995) and Flasar et al. (2005). Note that the results presented here are derived from data with three times higher spectral resolution than Coustenis and Bezard (1995) and Flasar et al. (2005). 8. Vertical distribution of CO2 With FP3 limb spectra at 55◦ S, a vertical profile of CO2 was derived with spectra between 660 and 675 cm−1 . The method for deriving this vertical profile is similar to that for the temperature retrievals that use FP4 limb observations. First the measured data was interpolated and sampled at equal altitude steps. Then, the altitude information of these spectra were corrected with the shift obtained from the temperature retrieval. Finally, C2 H2 , HCN, HC3 N and haze profiles were retrieved simultaneously to fit the parts of the spectra not significantly affected by CO2 . The retrieved C2 H2 profile is roughly uniform with a VMR of 2.5 ppb, which is close to the profile derived by Teanby et al. (2005). The retrieved HCN profile has a similar gradient to the profile derived by Marten et al. (2002) and is consistent with the retrieved HCN profile using CIRS data at wavenumbers where HCN has strong emission lines (Teanby et
Fig. 9. (a) Measured radiances at 667.75 cm−1 (symbols), where CO2 contributes most to the radiance. Solid lines are the radiance profiles for CO2 profiles that are constant above the condensation level, with a VMR of 10 (low radiances), 15 and 20 ppb (high radiances). (b) Parts of the spectra obtained by interpolating the measurements (thick dashed line) at three altitudes across the range of altitudes covered by the observations. Thin lines show synthetic spectra for CO2 profiles that are constant above the condensation level, with a VMR of 10 (low radiances), 15 and 20 ppb (high radiances). Aerosol opacity, HCN, C2 H2 and HC3 N VMR are kept constant at the retrieved values. The 1σ error bars on the spectra are shown in the top right.
al., 2005). So, although the lines of C2 H2 and HCN between 660 and 675 cm−1 are not ideal for retrieving these gases, the retrieved profiles seem reasonable. This is important, because these two gases have lines that overlap with the CO2 peak at 667.75 cm−1 . These lines are much weaker than the CO2 peak at southern latitudes, but can still introduce small offsets in the radiance at 667.75 cm−1 and hence influence conclusions about the vertical distribution of CO2 . With the retrieved gases and haze profile kept fixed, synthetic spectra were calculated for three CO2 profiles. These profiles are constant above the condensation level with VMR levels of 10, 15 and 20 ppb. Any significant variation of CO2 with altitude will then be visible as a deviation of the data from the three synthetic radiance profiles. Fig. 9 shows the radiance profiles and spectra for the three scenarios, compared to the measured data (the spectra are interpolated from the data as described above). At altitudes between 100 and 200 km, the data at 667.75 cm−1 is very well fitted with the profile of 15 ppb. This profile is preferred above the other two profiles between these altitudes. Below 100 km this profile still fits the data very well, but since the contribution functions do not peak at tangent heights this low (see, e.g., Figs. 1 and 7), variables at these altitudes are not constrained. Instead, observations at these low tangent heights probe higher altitudes. In any case, a CO2 profile that is constant above the condensation level at 15 ppb is consistent with these low-altitude data.
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Above 200 km a profile with 15 ppb seems to underestimate the data (especially around 230 km). However, at these high altitudes the CO2 VMR is poorly constrained, since radiance changes only moderately with changing CO2 VMR and the spread of the data is large. In other words, a wide range of CO2 VMR is consistent with the data at these altitude. The profile with 15 ppb gives radiances that lie within the data spread. In summary, a constant CO2 VMR of 15 ppb above the condensation level is consistent with all the data taken at this limb observation sequence at 55◦ S. 9. Discussion and conclusions The CIRS results presented here can be compared with previous results and expectations, discussed in the introduction. Firstly, the CIRS measurements present another independent determination of the CO VMR, using a wavelength range not used for this purpose before the arrival of Cassini at Saturn. Also, for the first time, spatially resolved measurements of CO are presented here. The analysis shows that CO is uniform with latitude within error over the observed latitude range. Because CO has a very long lifetime in Titan’s stratosphere it is expected to be uniform with latitude. This analysis shows that this may indeed be the case, although variations in the order of 20 ppm are still within the error bars. The weighted average stratospheric CO VMR of 47 ± 8 ppm, which assumes that CO is uniform with latitude, agrees well with the results from Gurwell (2004) and Gurwell and Muhleman (2000). On the other hand, this result is higher than the derived stratospheric VMR of Hidayat et al. (1998), which use the same rotational lines as Gurwell (2004) and Gurwell and Muhleman (2000). The derived stratospheric VMR is also consistent with the tropospheric value derived by Lellouch et al. (2003) (32 ± 10 ppm). Hence, the CIRS results presented here can argue for a uniform CO profile. However, a profile that increases slightly with altitude cannot be ruled out. The question about the origin of Titan’s CO, however, is still open. Where the abundances of CO2 and H2 O can be explained well with an external oxygen source consisting mostly of water, such a model fails to reproduce the high CO VMR in Titan’s atmosphere. Maintaining the high level of CO in a steady state requires an extra source of CO (Lara et al., 1996). If the meteorites that are providing the oxygen are made not purely out of water, but have a fraction of CO or organics in them, this might provide the extra source, as pointed out by Lellouch et al. (2003). Another possibility is that the extra CO is provided by some internal source, bringing CO from below the surface into the atmosphere. A third possibility is simply that CO is not in equilibrium and that most of the CO we see today is the remains of a much larger CO abundance from the past. Results from Wong et al. (2002) suggest that CO was as much as 14 times more abundant in Titan’s early atmosphere, based on arguments dealing with Titan’s isotope ratios. The timescale for this decay is estimated to be comparable to the chemical lifetime of 500 Myr. A final option is that CO is periodically replenished by comets. The oxygen released in a comet impact comes mostly in the form of CO (Zahnle, 1996). The extra CO
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flux needed to maintain the VMR we see today can be provided by a major impact every 100,000 years (Lellouch et al., 2003). This is much shorter than the chemical lifetime of CO, and hence CO will never be in chemical equilibrium is this case. Stratospheric H2 O could not be detected here and an upper limit of 0.9 ppb was determined. This upper limit is not inconsistent with the detection by Coustenis et al. (1998). They derive a VMR of 0.4 ppb for a profile that is constant above the condensation level. Variations of CO2 with latitude have been determined from data with both high spectral resolution and high spatial resolution. All previous analysis of CO2 either had lower spectral resolution (Coustenis and Bezard, 1995; Flasar et al., 2005) or lower spatial resolution (Coustenis et al., 2003). This analysis shows that CO2 does not vary noticeably with latitude. The retrieved VMR values around 16 ppb and the latitudinal variations agree very well with retrievals from Voyager spectra (Coustenis and Bezard, 1995) and 2.5 cm−1 resolution CIRS spectra (Flasar et al., 2005). For the first time the vertical profile of CO2 on Titan has been constrained here. Limb observations were used that probe Titan’s stratosphere at a range of altitudes. All limb spectra are well fitted with a CO2 profile that is constant with altitude and a VMR of 15 ppb. This is especially clear below 200 km, where also nadir observation probe. The VMR from limb data is consistent with the values we find for nadir observations. Unfortunately, the CO2 VMR above 200 km is poorly constrained and a small increase there cannot be ruled out. The lack of variation with latitude and altitude is possibly caused by the long chemical lifetime of CO2 [of the order of several hundred years at 300 km (Wilson and Atreya, 2004)]. The chemical lifetime is much larger than the timescale for atmospheric motion, resulting in a vertical profile that does not increase rapidly with altitude. The limb observations presented here indicate that this is indeed the case. Models show that the amount of latitudinal variation of a certain gas is tied with the steepness of its vertical profile, combined with vertical atmospheric transport (Lebonnois et al., 2001; Hourdin et al., 2004). Such a connection is also shown by the (at least qualitative) correlation between the polar gas enrichments found by Flasar et al. (2005) and the relative steepness of the newly derived vertical profiles of hydrocarbons near the equator of Vinatier et al. (2006). Nadir and limb observations thus give consistent results that agree with the idea of vertical transport at the north pole, which brings enriched air down to levels where they are observed in nadir observations. If the vertical profile is steep the atmosphere will be highly enriched at higher altitudes and downward transport will thus result in a higher polar enrichment. If the vertical profile is roughly constant, downward transport will not affect retrieved abundances from nadir observations significantly, as is the case for CO2 here. The Cassini spacecraft will continue to make observations until at least 2008. In this time many more CIRS spectra will be taken. By co-adding large numbers of nadir spectra it should be possible to detect stratospheric H2 O with CIRS and decrease errors of the retrieved CO abundance. Also, there will be increased latitudinal coverage of high spectral resolution observa-
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tions. After the end of 2006, Cassini will be in a high inclination orbit, which allows it to observe Titan’s north pole from a good vantage point. The north pole is likely surrounded by a polar vortex. The effect of a vortex on the oxygen compounds can be studied then. Finally, many more limb observations will be made. By analysing limb observations with more spectra than analysed here, or by combining multiple limb observations, the CO2 profile can be better constrained, also above 200 km, and the CO2 vertical profile can be analysed at a range of latitudes. It may also be possible to obtain a rough vertical profile of CO and perhaps H2 O by analysing FP1 limb data. The vertical resolution will be poor for these gases since the FP1 FOV is relatively large. All such analyses will give unprecedented information about Titan’s oxygen compounds. Acknowledgments We would like to acknowledge the many people involved in building and operating CIRS and in calibrating the data. Athena Coustenis, Mark Gurwell and an anonymous reviewer provided useful comments, which helped improve the manuscript. R.d.K. would like to thank the Prins Bernhard Cultuurfonds and the Pieter Beijer Fonds for financial support. Furthermore, this research was funded by the UK Particle Physics and Astronomy Research Council and the NASA Cassini Project. References Borysow, A., 1991. Modelling of collision-induced infrared-absorption spectra of H2 –H2 pairs in the fundamental band at temperatures from 20 K to 300 K. Icarus 92 (2), 273–279. Borysow, A., Frommhold, L., 1986a. Collision-induced rototranslational absorption spectra of N2 –N2 pairs for temperatures from 50 to 300 K. Astrophys. J. 311, 1043–1057. Borysow, A., Frommhold, L., 1986b. Theoretical collision-induced rototranslational absorption spectra for modelling Titan’s atmosphere: H2 –N2 pairs. Astrophys. J. 303, 495–510. Borysow, A., Frommhold, L., 1986c. Theoretical collision-induced rototranslational absorption spectra for the outer planets: H2 –CH4 pairs. Astrophys. J. 304, 849–865. Borysow, A., Frommhold, L., 1987. Collision-induced rototranslational absorption spectra of CH4 –CH4 pairs at temperatures from 50 to 300 K. Astrophys. J. 318, 940–943. Borysow, A., Tang, C., 1993. Far infrared CIA spectra of N2 –CH4 pairs for modelling of Titan’s atmosphere. Icarus 105, 175–183. Brown, L.R., Benner, D.C., Champion, J.P., Devi, V.M., Fejard, L., Gamache, R.R., Gabard, T., Hilico, J.C., Lavorel, B., Loete, M., Mellau, G.C., Nikitin, A., Pine, A.S., Predoi-Cross, A., Rinsland, C.P., Robert, O., Sams, R.L., Smith, M.A.H., Tashkun, S.A., Tyuterev, V.G., 2003. Methane line parameters in HITRAN. J. Quant. Spectrosc. Radiat. Trans. 82, 219–238. Coustenis, A., Bezard, B., 1995. Titan’s atmosphere from Voyager infrared observations. 4. Latitudinal variations of temperature and composition. Icarus 115, 126–140. Coustenis, A., Bezard, B., Gautier, D., 1989. Titan’s atmosphere from Voyager infrared observations. I. The gas composition of Titan’s equatorial region. Icarus 80, 54–76. Coustenis, A., Salama, A., Lellouch, E., Encrenaz, T., Bjoraker, G.L., Samuelson, R.E., de Graauw, T., Feuchtgruber, H., Kessler, M.F., 1998. Evidence for water vapour in Titan’s atmosphere from ISO/SWS data. Astron. Astrophys. 336, L85–L89. Coustenis, A., Salama, A., Schulz, B., Ott, S., Lellouch, E., Encrenaz, T.H., Gautier, D., Feuchtgruber, H., 2003. Titan’s atmosphere from ISO midinfrared spectroscopy. Icarus 161, 383–403.
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