Icarus 191 (2007) 397–400 www.elsevier.com/locate/icarus
Note
Detection of 13CH3D on Titan Bruno Bézard a,∗ , Conor A. Nixon b , Isabelle Kleiner c , Donald E. Jennings d a LESIA, Observatoire de Paris, CNRS, UPMC, Université Paris-Diderot, 5 Place Jules Janssen, 92190 Meudon, France b Department of Astronomy, University of Maryland, College Park, MD 20742, USA c LISA, Universités Paris 12, Paris 7 and CNRS, 61 Ave Général de Gaulle, 94100 Créteil Cedex, France d NASA/Goddard Space Flight Center, Code 693, Greenbelt, MD 20771, USA
Received 6 April 2007; revised 1 June 2007 Available online 14 July 2007
Abstract We report the detection of 13 CH3 D in Titan’s stratosphere from Cassini/CIRS infrared spectra near 8.7 µm. Fitting simultaneously the ν6 bands of both 13 CH3 D +0.15 and 12 CH3 D and the ν4 band of CH4 , we derive a D/H ratio equal to 1.32−0.11 × 10−4 and a 12 C/13 C ratio in deuterated methane of 82+27 −18 , consistent with that measured in normal methane. © 2007 Elsevier Inc. All rights reserved. Keywords: Titan; Atmospheres, composition
1. Introduction
Table 1 CIRS spectral selections
The Composite Infrared Spectrometer (CIRS) aboard Cassini is currently monitoring Titan’s thermal emission spectrum through its three focal planes (FP1: 10–600 cm−1 , FP3: 600–1100 cm−1 , FP4: 1100–1500 cm−1 ). Signatures from many hydrocarbons, nitriles and oxygen compounds are detected and used to retrieve the horizontal and vertical distributions of these gases (Flasar et al., 2005). In addition to the main ones, several rarer isotopes have been detected in CIRS spectra: the deuterated isotope of methane (12 CH3 D), the 13 C isotopes of methane (13 CH4 ), acetylene (13 C12 CH2 ), and hydrogen cyanide (H13 CN), and the 15 N isotope of hydrogen cyanide (HC15 N). The analyses of these emission features in terms of isotopic ratios are presented in Coustenis et al. (2007), Nixon et al. (2007) and Vinatier et al. (2007b). Measurements of isotopic ratios are particularly important because they can provide information on the material that accreted to form Titan and on subsequent evolutionary processes, such as atmospheric escape. We present here the first detection of a doubly-substituted isotope in Titan’s atmosphere, the 13 C isotope of CH D (13 CH D) and we derive the 12 C/13 C ratio in this mole3 3 cule.
Latitude range
2. Observations We made a large selection of CIRS FP4 spectra at the highest spectral resolution available, 0.52 cm−1 (apodized). We first focused on latitudes around 15◦ S because precise information on the CH4 mixing ratio and temperature profile in the stratosphere is available from the Huygens descent (Niemann
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[email protected] (B. Bézard). 0019-1035/$ – see front matter © 2007 Elsevier Inc. All rights reserved. doi:10.1016/j.icarus.2007.06.004
Mean latitude
(−20, −10) −16 (−10, 0) −5 (0, 10) 5
Number of spectra
Mean airmass
12 C/13 C
D/H (×104 )
1281 3242 4973
1.17 1.07 1.05
90 (+30, −18) 75 (+23, −14) 86 (+24, −16)
1.34 ± 0.05 1.24 ± 0.06 1.37 ± 0.05
Error bars correspond to the 1-SD uncertainty due to noise level. et al., 2005) and limb-viewing CIRS measurements (Vinatier et al., 2007a). We averaged spectra recorded during all flybys between Tb and T10, having their field of view (0.27 × 0.27 mrad) centered between 10◦ S and 20◦ S and an emission angle less than 50◦ . The final selection incorporates 1281 individual spectra and the mean airmass is 1.17. The spectral range between 1130 and 1190 cm−1 is shown in Fig. 1. The most prominent feature at 1156 cm−1 is due to the ν6 band of 12 CH3 D; the other emission features originate from this band and from the P-branch of the ν4 band of CH4 centered at 1304 cm−1 . A continuum emission due to stratospheric haze opacity also contributes to the observed spectrum. For verification purposes, we performed two other independent selections in the latitude bands 0◦ –10◦ S and 0◦ –10◦ N using the same criteria (Table 1). 3. Radiative transfer analysis Spectra were generated from a line-by-line radiative transfer program, taking into account collision-induced opacity from various combinations of N2 – CH4 –H2 pairs, rovibrational bands from CH4 , CH3 D, C2 H2 and C2 H4 , and haze continuum opacity. Details are given in Vinatier et al. (2007a).
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Fig. 1. Comparison between a CIRS spectral selection centered at 15◦ S (symbols) and synthetic spectra calculated with (red line) and without (green line) 13 CH3 D line opacity, assuming a 12 C/13 C ratio of 89. Spectral resolution is 0.52 cm−1 (apodized). Both calculations use the same haze model, a CH4 mole fraction of 1.4% and a CH3 D mole fraction corresponding to D/H = 1.34 × 10−4 . (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) While a linelist for 12 CH3 D is available in the Geisa or Hitran catalogs, this is not the case for 13 CH3 D. We thus produced a complete line-by-line prediction in the range 952–1694 cm−1 by calculating the line positions and intensities for the triad composed of the ν3 , ν5 and ν6 fundamental bands. The first step was to obtain eigenvectors describing the lower and upper states of the transition. To avoid problems connected with phase conventions and matrix elements definitions, we refitted the line positions and assignments of the ν3 /ν5 /ν6 transitions as derived from Tables 2–3 of Ulenikov et al. (2000). The lower state energy levels were calculated using the ground state parameters from their Table 1 and included as an input constraint in our line position fit. We decided to limit our calculation up to J = K = 18 as these are the maximum quantum numbers covered by the experimental rovibrational terms values published by Ulenikov et al. The set of programs for both the energy fitting and the intensity calculation was written by G. Tarrago and has been used in a series of papers related to C3v symmetric top molecules including PH3 (Tarrago et al., 1992; Brown et al., 2002). The root mean square deviation of our energy fit is 0.0029 cm−1 for 1086 lines going up to J = 18 using 41 parameters. Having obtained the eigenvectors for the triad, taking into account various rovibrational interactions between the three bands, we calculated the line intensities, assuming vibrational transition dipole moments equal to those of 12 CH3 D as determined by Brown et al. (2004, see their Table 2) and setting the vibration–rotation factors (so-called Herman–Wallis terms) to zero. The partition function was evaluated from the calculated energy levels: Z = 1598 at T = 296 K. No additional line intensity measurements were performed for the present study. Nitrogen-broadened linewidths were calculated according to the analysis of Malathy Devi et al. (2002) and assuming a temperature exponent of −0.75 as did Brown et al. (2003) for 12 CH D. 3 The temperature profile in the stratosphere was retrieved from the spectral range 1215–1309 cm−1 in the ν4 methane band using a linear constrained inversion algorithm (Conrath et al., 1998). We assumed a CH4 mixing ratio of 1.4%, uniform with height (Niemann et al., 2005). The haze opacity profile is the same as in Vinatier et al. (2007a) but the integrated optical depth was scaled by a factor of 2/3, which minimizes the residuals with the observations. The starting temperature profile in the inversion process is that retrieved by Vinatier et al. (2007b) from a combination of nadir and limb spectra around 15◦ S. Information from the present spectra is limited to the range ∼0.1–10 mbar and the temperature profile outside this range essentially reflects the initial estimate. The formal error due to noise propagation in the retrieval process amounts to 0.13 K in the range 1–5 mbar, increasing to 0.3 K at 0.1 and 10 mbar.
4. Modeling of the CH3 D emission We used the derived temperature profile to analyze the 1080–1180 cm−1 region where the ν6 band of CH3 D is the dominant source of gas opacity. Two parameters were simultaneously retrieved from this spectral range: the CH3 D mole fraction (assumed to be constant with height in the stratosphere) and the haze opacity at 1100 cm−1 , assuming the same vertical dependence as in Vinatier et al. (2007a). The inversion algorithm is the one developed by Conrath et al. (1998) and described in Vinatier et al. (2007a). Wavenumbers used in the inversion are those in the ranges 1080–1100 cm−1 , where haze opacity dominates, and 1130–1180 cm−1 where CH3 D dominates. As concerns CH3 D opacity, we initially used the Geisa 2003 linelist (Jacquinet-Husson et al., 2005) that incorporates only the 12 C isotope. The retrieved CH3 D mixing ratio, that minimizes the residuals with our nominal selection at 15◦ S, is then 7.8 × 10−6 , which corresponds to a D/H ratio of 1.39 × 10−4 . While the agreement between observed and calculated spectra is generally very good, a significant discrepancy remains around 1148.3 cm−1 . This location coincides with the center of the ν6 band of 13 CH3 D, shifted by −7.8 cm−1 with respect to the 12 CH3 D band (Ulenikov et al., 2000). Adding the opacity brought by this isotope resolves the discrepancy as illustrated in Figs. 1 and 2. Fig. 1 shows two calculations with the same haze opacity and CH3 D mixing ratio (7.5 × 10−6 ), one including only 12 CH3 D opacity (Geisa 2003), and the other one with the 13 CH3 D opacity added. Fig. 2 shows the differences between the observed and synthetic spectra in these two cases. Around 1148.3 cm−1 , the no-13 CH3 D spectrum exceeds the observed one by about 5 times the standard deviation of the residuals, a discrepancy that disappears when 13 CH3 D is included. The same feature is seen on our two other selections at 5◦ S and 5◦ N. Note that both the 12 CH3 D and 13 CH3 D emissions originate from a region centered at 8 mbar (0.2–20 mbar at half maximum of the inversion kernels). The best fit values for the ratios 12 C/13 C = 12 CH3 D/13 CH3 D and D/H = 1 12 CH D/12 CH ratio are given in Table 1 for each spectral selection along 3 4 4 with the 1-SD noise uncertainty. Including 13 CH3 D opacity lowers the best fit (in a least square sense) D/H ratio by about 3% (0.04 × 10−4 ) and decreases the reduced χ 2 by about 20% over the whole spectral range (140 independent spectral data points), which is quite significant from a statistical point of view. All three selections yield isotopic ratios consistent with each other, considering the 1-SD error bars. An analysis of the average of the three selections in −4 Table 1 yields 12 C/13 C = 82+21 −14 and D/H = 1.32 ± 0.05 × 10 . These error bars are somewhat larger than one could expect from combining analyses
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Fig. 2. Residuals between the observed and calculated spectra shown in Fig. 1. At 1148.3 cm−1 , the difference between the observed spectrum and the synthetic one with no 13 CH3 D (green line) amounts to about 5 times the rms of the residuals. Including 13 CH3 D line opacity with a terrestrial 12 C/13 C ratio (red line) removes this discrepancy. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) of independent spectra, which likely points to a systematic error in the modeling. A possibility is the presence of propane bands in the relevant spectral range (Giver et al., 1984), not included in the present calculations due to lack of spectroscopic data. The D/H determination is also affected by uncertainties in the temperature profile in the lower stratosphere, due to noise propagation in the retrievals and uncertainties in the haze opacity around 1200–1300 cm−1 , the region used to derive temperature information. Tests indicate that this uncertainty induces a possible error of about (−0.06/ + 0.12) × 10−4 on the D/H ratio. The 12 C/13 C ratio in CH3 D is not significantly affected as the 12 CH3 D and 13 CH3 D peak emissions occur at very close frequencies and originate from the same atmospheric region. The uncertainty of the 12 CH3 D line intensities is about 3% (Brown et al., 2004) and that of the N2 -broadened linewidths (especially through the temperature exponent) can be estimated to about 5%, which adds an uncertainty of ±0.08 × 10−4 on the D/H ratio. As concerns 13 CH3 D, our linelist was generated assuming a vibrational transition dipole moment equal to that of 12 CH3 D. If this is not the case, the retrieved 12 C/13 C will be in error, being essentially proportional to the square of the dipole moment. From comparison with other isotopic bands, an upper limit of 15% can reasonably be set on any difference between the 12 CH3 D and 13 CH3 D band strengths, and accordingly on the error induced on the derived 12 C/13 C ratio. Combining all the +0.15 above error sources, we conclude that D/H = 1.32−0.11 × 10−4 and 12 C/13 C (in CH3 D) = 82+27 −18 .
5. Conclusions We have detected the doubly-substituted 13 CH3 D isotope in Titan’s stratosphere near 1148.3 cm−1 in different selections of Cassini/CIRS spectra. Fitting simultaneously the 13 CH3 D, 12 CH3 D and CH4 emission features +0.15 × 10−4 and a 12 C/13 C indicates a D/H ratio in methane equal to 1.32−0.11
13 ratio in deuterated methane equal to 82+27 −18 . Accounting for CH3 D opacity lowers the best fit D/H ratio by about 3%. The D/H ratio we infer agrees with the recent analysis of the same 12 CH3 D band observed from the ground by Penteado et al. (2005) at very high spectral resolution (D/H = 1.25 ± 0.25 × 10−4 ). On the other hand, it is significantly larger than the value derived from observations of this band by the Infrared +3.2 Space Observatory (ISO) that yielded D/H = 8.7−1.9 × 10−5 (Coustenis et al., 2003). This low value might result from calibration uncertainties or from the fact that the ISO observations are a disk average and are thus more difficult to model, given the important contribution from the limb and the inhomogeneity of the emission across the disk. Our result is also slightly larger than that de-
+0.16 rived by Coustenis et al. (2007) from Cassini/CIRS spectra (1.17−0.21 × 10−4 ). This is probably because those authors used a temperature profile at 15◦ S that is slightly too warm and does not reproduce exactly the intensities of the weak methane lines, as can be seen in their Fig. 6. As discussed in Coustenis et al. (2007), the large enhancement of the D/H value in Titan’s methane with respect to the protosolar value (in hydrogen) was probably acquired in the presolar cloud and can be explained by the trapping of this deuterium-enriched methane in icy grains subsequently incorporated in the planetesimals that formed Titan. In contrast, Fischer–Tropsch synthesis of CH4 from CO and H2 , either in Saturn’s sub-nebula or in Titan’s interior, would not enhance the D/H ratio in methane over the protosolar value. Therefore this mechanism is likely not the main source of Titan’s methane. The 12 C/13 C we infer in deuterated methane is consistent with that measured from CIRS spectra in methane and acetylene (Nixon et al., 2007) with a higher precision, and also in hydrogen cyanide (Vinatier et al., 2007b). The 12 C/13 C ratio in all these molecules agrees with that measured in situ by the GCMS aboard Huygens 82.3 ± 1 (Niemann et al., 2005) and with the Cassini/INMS measurement of Waite et al. (2005) extrapolated to the lower atmosphere (12 C/13 C = 81). Therefore, within error bars, no isotopic fractionation of carbon is detected in CH3 D, C2 H2 or HCN with respect to the main reservoir, CH4 . Also, the value we derived here is consistent with those reported for the outer Solar System: in ethane for Jupiter, Saturn and Neptune (Sada et al., 1996; Orton et al., 1992), in methane for Jupiter (Niemann et al., 1996), and in hydrogen cyanide for comets (Hutsemékers et al., 2005), all of which agree with the terrestrial value. However, the more precise in situ measurements of the 12 C/13 C ratio in Titan (Niemann et al., 2005; Waite et al., 2005) clearly point to a value that is ∼8% lower than the terrestrial and solar values (89.9). This difference might result from atmospheric escape, with strong implications on the supply rate of methane to the atmosphere. To improve our determination of the 12 CH3 D/13 CH3 D ratio, laboratory measurements of the 13 CH3 D ν6 band strength as well as a spectroscopic analysis of the propane bands occurring in the same spectral region (ν7 , ν20 ) would be desirable.
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