The Astrophysical Journal, 681: L101–L103, 2008 July 10 䉷 2008. The American Astronomical Society. All rights reserved. Printed in U.S.A.
ISOTOPIC RATIOS IN TITAN’S ATMOSPHERE FROM CASSINI CIRS LIMB SOUNDING: CO2 AT LOW AND MIDLATITUDES C. A. Nixon,1,2 D. E. Jennings,2 B. Be´zard,3 N. A. Teanby,4 R. K. Achterberg,1,2 A. Coustenis,3 S. Vinatier,3 P. G. J. Irwin,4 P. N. Romani,2 T. Hewagama,1,2 and F. M. Flasar2 Received 2008 March 27; accepted 2008 June 4; published 2008 June 25
ABSTRACT This Letter reports on a search for infrared emissions of isotopologues of CO2 in the atmosphere of Titan using spectral data recorded by the Cassini Composite Infrared Spectrometer (CIRS). We have made a successful 6.5 j detection of 13CO2 at a fraction CO2/13CO2 p 84 Ⳳ 17, consistent with measurements of 12C/13C in other species, and also the terrestrial value (89). We also find a probable 3.5 j detection of C16O18O at a fraction CO2/ C 16O18O p 173 Ⳳ 55, slightly lower than the terrestrial value (253) and consistent with the twofold enhancement in 18O reported previously in CO, or with an intermediate value as suggested by chemistry. These isotopic ratios provide important constraints on models of the formation, evolution, and current processes in Titan’s atmosphere. Subject headings: infrared: solar system — planets and satellites: formation — planets and satellites: individual (Titan) — radiative transfer 1. INTRODUCTION
2. OBSERVATIONS
CIRS is a dual interferometer, consisting of a far-IR polarizing interferometer covering the spectral range 10–600 cm⫺1, and a mid-IR Michelson interferometer sensitive to 600–1100 cm⫺1 and 1100–1500 cm⫺1 with two parallel 1 # 10 HgCdTe detector arrays. The spectral resolution is variable, from 15 to 0.5 cm⫺1. For further instrumental details see Flasar et al. (2004). From 5 to 9 hr from closest approach to Titan, CIRS is tasked to make mid-IR limb composition measurements at 0.5 cm⫺1 (highest) spectral resolution, in which the two detector arrays are placed perpendicular to the limb, successively at two different altitudes, to gain a vertical composition profile at a single latitude. This observation is repeated on many flybys at different latitudes, to build up a latitude-altitude 2D picture of the trace gas species. These include various hydrocarbons, from CH4 up to C6H6, nitriles (containing the -C{N group), and also CO2, the only oxygen species detected in the mid-IR range (CO and H2O are seen in the far-IR). The limb observation geometry is ideal for detecting the faintest possible species, due to the much longer path length through the atmosphere compared to viewing toward the surface. However, the time period in which CIRS can make these observations with scale-height or better vertical resolution is heavily subscribed among Cassini instrument teams, and therefore limited. To improve the signal-to-noise ratio beyond that achievable on a single flyby, we co-add spectra at similar latitudes and thereby attempt to detect fainter gas species and isotopologues. For the purposes of this study, we added spectra from within a restricted altitude range (100–150 km in Titan’s stratosphere, where we can expect our best signal-to-noise ratio) and also across no more than 20⬚ of latitude. Four sets were produced in this way, derived from eight flybys, at mean latitudes of ⫺53, ⫺26, ⫹9, and ⫹29 (see Table 1). We did not use data from farther north, because HC3N, which overlaps with CO2, increases dramatically above 40⬚ north.
Carbon dioxide was first detected in Titan’s atmosphere by the Voyager IRIS infrared spectrometer (Samuelson et al. 1983) via the n2 band at 667 cm⫺1, seen in emission in the stratosphere. CO2 condenses rapidly and thus requires constant resupply to the upper atmosphere, where it is thought to be produced by OH ⫹ CO r CO2 ⫹ H, with the OH radical originating from photolyzed water. Subsequently Coustenis & Be´zard (1995) reanalyzed the Voyager data searching for meridional trends in abundance: none was found to within the error bars. Since 2004, the Cassini spacecraft has been orbiting Saturn and making repeated close flybys of Titan. On board Cassini is the Composite Infrared Spectrometer (CIRS; Flasar et al. 2004), which has a spectral resolution up to 9 times higher than IRIS. CIRS has been used to determine the latitudinal variation of CO2, once again finding little evidence of any trend in latitude (Coustenis et al. 2007; de Kok et al. 2007). CIRS has also detected the isotopologues of many of the stratospheric species, including those containing D (in CH4 and C2H2), 13C (in CH4, CH3D, C2H6, C2H2, HCN, and HC3N), and 15N (in HCN) (Vinatier et al. 2007; Be´zard et al. 2007; Nixon et al. 2008; Coustenis et al. 2008; Jennings et al. 2008). The isotopic ratios can be used to put important constraints on the formation and evolution of the atmosphere, and to shed light on current physical and chemical processing. These successes prompted us to search for 13CO2, which has a strong Q-branch emission at 648.75 cm⫺1, and also the Q-branch of C16O18O at 662.5 cm⫺1. The terrestrial ratio of 12C/13C is ∼89, compared with ∼82 on Titan (Niemann et al. 2005), while the terrestrial ratio of 16O/18O is ∼500, implying a CO2/C16O18O of ∼250 on Titan if the isotopic ratio is similar to Earth.
1 University of Maryland, College Park, MD 20742;
[email protected]. 2 NASA Goddard Space Flight Center, Code 693, Greenbelt, MD 20771. 3 LESIA, Observatoire de Paris, 5 Place Jules Janssen, 92195 Meudon Cedex, France. 4 Atmospheric, Oceanic, and Planetary Physics, University of Oxford, Clarendon Laboratory, Parks Road, Oxford OX1 3PU, UK.
3. DATA ANALYSIS
Our analysis closely follows that of Nixon et al. (2008). First, a model atmosphere is constructed based on known low-latitude L101
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TABLE 1 Observations and Results Flybys (1)
Latitude Range (deg) (2)
T15 (2006 Jul 2), T39 (2007 Dec 20) . . . . . . . . T27 (2007 Mar 26), T28 (2007 Apr 10) . . . . . . T21 (2006 Dec 12), T23 (2007 Jan 13) . . . . . . . T19 (2006 Oct 9), T24 (2007 Jan 29) . . . . . . . . Mean . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
⫺60, ⫺35, 00, ⫹25, …
data, including the temperature profile and profiles of the major gases, N2, CH4, and H2, following the results of the Huygens probe (Fulchignoni et al. 2005; Niemann et al. 2005). Other gases relevant to the 15 mm spectral region of interest here, C2H2, C2H6, HCN, HC3N, and CO2, are initially given stratospheric profiles that are constant with altitude, at abundances derived by Coustenis et al. (2007). In addition to the major isotopologues of each chemical species, we included as separate gas species the 13C and 18O isotopologues of CO2, with initial profiles scaled relative to CO2 by 1/82 and 1/250, respectively. In the lower stratosphere cold trap, the abundances are constrained to follow a saturation vapor curve, and fall to low levels in the troposphere (see Fig. 1 of Nixon et al. 2008). In addition, we add a haze absorber that has a constant number density above the tropopause (45 km) and spectral characteristics as measured in laboratory tholin by Khare et al. (1984). The model atmosphere is used as an input to a radiative transfer code (NEMESIS; Irwin et al. 2008), which computes the emerging spectrum based on collision-induced absorption of the major gases, haze opacity, and spectral line opacity based on information available in the HITRAN and GEISA atlases (Rothman et al. 2005; Jacquinet-Husson et al. 2005; as detailed in Nixon et al. 2008), except for HC3N, where we substituted the recent measurements of Jolly et al. (2007). The model computes a radiance spectrum at the mean altitude of the spectral set, which is then compared to the data. Model parameters are then adjusted and the spectrum recalculated iteratively in a nonlinear least-squares minimization to find the best match
Fig. 1.—Comparison between CIRS spectral data and radiative transfer models. Top: Weighted mean spectrum from eight Titan mid-infrared limb observation sequences (black line), best-fit model with all gases and isotopic variants included (red line), model with 13CO2 and C16O18O removed (green line), and model with HC3N removed (blue line). Bottom: Residual emission after data minus model subtraction for three model cases.
⫺40 ⫺15 ⫹20 ⫹35
ALatitudeS (deg) (3)
Nspec (4)
CO2/13CO2 (5)
⫺53 ⫺26 ⫹9 ⫹29 …
226 183 162 224 …
89 96 80 78 84
Ⳳ Ⳳ Ⳳ Ⳳ Ⳳ
41 36 33 27 17
CO2/C16O18O (6) 143 185 190 224 173
Ⳳ Ⳳ Ⳳ Ⳳ Ⳳ
85 107 128 153 55
between model and data, subject to smoothing (a priori) constraints (for details of the formalism see Rodgers 2000). The fitting proceeded in two steps. First, a spectral range containing the n4 band of methane between 1225 and 1325 cm⫺1 (P- and Q- branches) was modeled to retrieve temperatures from 5 mbar to less than 0.1 mbar in the stratosphere, using a constant-with-height stratospheric mixing ratio of 1.4%. The retrieved temperature profile was then fixed and used to constrain the abundances of the minor gases and CO2 isotopologues. 4. RESULTS AND DISCUSSION
Figure 1 shows CIRS data from the spectral range near 15 mm compared to model calculations, as described in the figure caption. These spectra are weighted averages of the four modeled latitudes, where each data spectrum and model was given a weight equal to the number of spectra in the latitudinal subaverage (Table 1, col. [4]). The bottom panel shows the residual (data ⫺ model) for each of the three models, making evident the location of the 13CO2 and C16O18O emissions at 648.75 and 662.5 cm⫺1, respectively, and also demonstrating that the latter isotope is resolved from HC3N at 663.25 cm⫺1. The rms residual from the best model is ∼1.77 nW cm⫺2 sr⫺1/cm⫺1, yielding detections of 6.5 and 3.5 j respectively for 13 CO2 and C16O18O, if the residual is considered purely random noise. However, close inspection suggests that the residual is probably not random (or uniform) across the spectral interval, and that systematic errors may remain in the fit. In particular, we note that the residual appears to increase above 652 cm⫺1. The cause of this is undetermined at the present time; the mismatch may arise from deficiencies in fitting one of the more abundant trace gases such as the P-branch of CO2 or C2H2, from errors in the line strengths in the line atlas, from errors in our temperature profile, or even due to emissions from unknown gases, condensates, or haze particles. In view of the fact that there are features in the spectral residual that approach the amplitude of the C16O18O emission, we therefore regard the detection of the 18O isotope as “probable,” pending confirmation, e.g., from higher resolution spectroscopy. The calculated ratios of CO2/13CO2 and CO2/C16O18O at each latitude and the variance-weighted mean across all latitudes (see § 5.1 of Bevington 1969) are given in Table 1. There is no evidence for latitude variation of the ratios as the values all agree to within 1 j error bars, calculated based on instrumental NESR and a priori errors as described in Nixon et al. (2008). The mean value found for 12C/13C in CO2 is 84 Ⳳ 17, in good agreement with determinations in other Titan molecular species: including the Huygens GCMS value of 82.3 Ⳳ 1 in CH4 (Niemann et al. 2005), but also with those measured by CIRS in CH4, CH3D, C2H2, C2H6, and HCN (Vinatier et al. 2007; Be´zard et al. 2007; Nixon et al. 2008). It is also consistent with the outer planets average (88 Ⳳ 7), and the Saturn value in
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particular (99⫹43 ⫺23 ; Sada et al. 1996). We note that the most accurate value (GCMS) is lower by some ∼8% than the terrestrial inorganic standard value (88.9), implying that either the initial fraction of 13C in the Saturn subnebula was greater than that at 1 AU, or else subsequent processes have driven the 12C/ 13 C lower, such as atmospheric escape (Strobel 2008). Exchange between all carbon species appears to be rapid, as the various ratios are in close agreement, excepting a possible slight depletion in 13C in the C2 compounds C2H2 and C2H6. Our preliminary retrieved value for CO2/C16O18O p 173 Ⳳ 55 is significantly lower than the terrestrial ratio of 253 at the 1 j level of random noise, but consistent at a 2 j error level. Our implied 16O/18O p 346 Ⳳ 110 is consistent however with the twofold enhancement in 18O over terrestrial in the CO molecule reported by Owen et al. (1999) (16O/18O ⯝ 250), via detection of a submillimeter line. A simple explanation therefore would be that the 16O/18O in CO2 is close to the mean value of the 16O/18O in CO and a higher (near terrestrial) value in cometary H2O, which would agree with the simplest chemical production mechanism. Wong et al. (2002) have used a photochemical model to study isotope exchange between CO and other species, including CH2
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radicals derived from CH4, and OH (from water) which reacts with CO to form CO2. They find that any original enhancements in 13CO are diluted in a timescale of ∼800 Myr, however C18O is diluted more slowly in the model, possibly preserving any original enhancement over 4.6 Gyr. That study makes no prediction about C16O18O however, which is implicitly assumed to be terrestrial at the present epoch. Finally, we note that Noll et al. (1995) have reported a factor of 1–3 enhancement in 18O in Jupiter’s water, although it is hard to draw any conclusions from this comparison. In future, the CIRS team is planning to implement changed observing strategies in which the mid-IR arrays are positioned for longer periods at the stratospheric altitudes where S/N is highest, in the hope of improving the detection of weak species and isotopologues such as C16O18O. A large number of people contributed to the operations, commanding, calibration, navigation, and databasing of CIRS data, including J. C. Brasunas, M. H. Elliott, S. B. Calcutt, R. Carlson, E. Guandique, E. Lellouch, A. Mamoutkine, P. J. Schinder, M. E. Segura, and J. S. Tingley. The US-based authors acknowledge the ongoing support of the NASA Cassini Project.
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