!ts MONODEUTERATED METHANE IN THE OUTER SOLAR SYSTEM 

III. ITS ABUNDANCE ON TITAN 

C. de Bergh 

Observatoire de Paris 

B. L. Lutz 

Lowell Observatory 

T. Owen 

State University of New York at Stony Brook 

and 

J. Chauville 

Observatoire de Paris 

(bASA-Cii-180254) BCNCDEUIEEEIEC CJETifANE IN N87- 19326 

31E OUTER SOLAR SY,CIEC. PAEl C: IIS 

ABUNZAhCE CH Il!IdL (Ctservatoire de Paris 

(Irance).) Ib E; CSCL 03B Unclas 

63/91 U38G8 
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ABSTRACT 

The 3u2 band of CH3D has been detected in spectra of Titan recorded at 

1.6 pm with the Fourier Transform Spectrometer (FTS) at the 4 m telescope of 

the Kitt Peak National Observatory (NOAO). We have obtained a value of the 

CH3D/CH4 mixing ratio of 6.6'::; x lo-' from a comparison between the observed 

Titan spectra and synthetic spectra. This value is -2 times higher than the value 

measured on Uranus (de Bergh et al. 1986) and -6 times higher than on Jupiter 

and on Saturn (Courtin et al. 1984; de Bergh et al. 1986). It corresponds to D/H of 

1.65$i:i5 x nominally 8 times higher than the most commonly accepted value 

for the protosolar D/H = 2 x (Geiss and Reeves 1981). The value we find on 

Titan for D/H in methane is comparable to the D/H ratio measured in terrestrial 

H20. 

I. INTRODUCTION 

The study of CHsD on Titan presented here is the third in a series of investigations of the 3u2 band of this molecule in spectra of the outer planets. Lutz et al. 

(1981) reported the probable detection of this band in the spectrum of Uranus. Paper I1 (de Bergh et al. 1986) gave an analysis of the Uranus absorptions, leading to 

a value of CH3D/ CH4 = 3.62::; x lo-'. 

In 1975, there was a tentative detection of CHsD on Titan at 8.6 pm by Gillett 

(1975). In 1981, the analysis of Voyager spectra at 8.6 pm revealed the presence 

of CsHs emissions at the same wavelength as those of the presumed CH3D band. 

Hanel et al. (1981) suggested that perhaps there was no contribution of CHsD to 
.. 

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the emission observed in the Voyager spectra at that wavelength. Subsequently 

Kim and Caldwell (1982) made a more extensive analysis of Voyager spectra and 

evaluated the relative contributions of C3H8 and CH3D to the 8.6 pm emission by 

using an emission feature of C3H8 at 13.4 pm for comparison. They obtained a 

CHsD/CH4 ratio of 1.7 X 

At about the same time, we reported the detection of absorptions from the 324 

band of monodeuterated methane in FTS spectra of Titan recorded in the 1.6 pm 

region at KPNO with a spectral resolution of 2 cm-' (de Bergh et al. 1981). Our 

preliminary measurements provided values of the CHsD/CH4 ratio that were about 

ten times lower than the value of Kim and Caldwell (1982). Since then, Kim and 

Caldwell (private communication) have revised their original estimate downward, 

and we have recorded new spectra of Titan with an improved spectral resolution 

and made a correction for a numerical error in the CH3D partition function that 

we had used previously (Paper 11). The spectrum used in this analysis is the same 

as the one in which CO was discovered in Titan's atmosphere (Lutz, de Bergh, and 

Owen 1983). Here we present the results of our analysis of the CH3D absorption in 

these spectra. 

11. PLANETARY OBSERVATIONS 

The set of observations from which the present measurements have been derived 

consists of spectra of Titan recorded with the FTS at the 4 m Mayall telescope 

of KPNO for time blocks of about one hour each during the nights of U.T. 1982 

June 2-7 with an unapodized FWHM resolution of 1.2 cm-'. We used a circular 
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aperture 3.75" in diameter. Several spectra have been coadded for the total effective 

integration time of 7.5 hours, corresponding to a total signal-to-noise ratio of 40:l 

(S/a). In addition, a spectrum of diffused solar radiation was recorded at the 

same spectral resolution during the same observing run in order to evaluate the 

contribution from terrestrial and solar absorptions in the Titan spectra. 

The spectra cover a usable spectral range of 6100-6700 cm-', but the present 

analysis is restricted to 6413-6483 cm-'. Since there was little or no contribution 

from telluric absorption, the coadded spectrum of Titan could simply be divided by 

the Doppler-shifted solar spectrum to remove all extraneous lines. 

111. ATMOSPHERIC MODEL FOR TITAN 

We first computed synthetic spectra of Titan which included only CHI absorptions. We searched for atmospheric models that would provide a reasonable 

agreement with the observations of these CHI features. The synthetic spectra of 

Titan included the 610 lines of CHI detected in our laboratory spectra. The intensity measurements for these lines and the 'kotational assignments" are the same 

as the ones which were used to generate the spectra of Uranus (Paper 11). The 

pressure-broadening coefficients for the CHI-NZ collisions were assumed to be 0.064 

cm-'/atm for all CHI lines with a temperature dependence of T-' (Varanasi, Giver, 

and Valero 19834. Since no region in the Titan spectrum is entirely devoid of absorptions, the definition of the continuum level in the observed spectrum is difficult 

and somewhat subjective. In each of our spectra we defined the continuum level as 

the minimum level above the region of the weak absorption near 6345 cm-' that 
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provided a best fit between the computed and observed spectra. 

In our search for the simplest atmospheric model that would best reproduce the 

observed spectrum, we found that neither a clear atmosphere above a reflecting layer, 

nor a homogeneous scattering atmosphere, would provide a satisfactory agreement 

at 1.6 pm. Assuming a continuum albedo of 0.7 (Fink and Larson 1979), we obtained 

a best fit for isothermal, uniform-pressure models with an isotropically scattering 

cloud layer of optical thickness 7 = 0.23 in an otherwise clear atmosphere. No 

CHI was required above the cloud layer. CHI absorptions are quite broad and 

tend to favor atmospheric models in which the average pressure is on the order of 

0.7 atm or more. The best agreement between synthetic and observed spectra was 

obtained with an integrated methane column abundance of 2200 m-amagat at an 

“average” temperature of 85 K in the atmosphere between the cloud layer and the 

reflecting surface. For comparison, the methane column abundance derived by Fink 

and Larson (1979) from the integrated band absorptions at 6400 and 7900 cm-l 

under the assumption of a clear reflecting layer model is 1700 f 400 m-amagat. 

We also computed synthetic spectra for models in which the pressure and 

temperature were allowed to vary with altitude. As in the case of the constant 

temperature and pressure models, we used various cloud properties and methane 

abundances. The partial pressure of methane was constrained at each atmospheric 

level by its saturated vapor pressure. The temperature-pressure structure we assumed is the one derived from the Voyager observations by Linda1 et al. (1983). 

Whatever percentage of CHI wag used, the line profiles in the synthetic spectra were 
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always much shallower than the observed ones. We found that this effect was caused 

by the absorption which occurs above the methane condensation level; Le., a part 

of the atmosphere that we assumed free of any particles that scatter significantly 

at 1.6 pm. Allowing for subsaturation of CHI above the condensation level does 

improve the fit with the observations. The less CHI we assume above that altitude 

level, the better is the agreement. However, we know that a minimum amount of 

CH4 must be present above the condensation level, since it was detected in emission at 8 pm by the Voyager IRIS experiment (Hanel et al. 1981). Furthermore, 

its presence is required by the observation of hydrocarbons produced by methane 

photochemistry in the satellite’s upper atmosphere (Strobe1 1982). Therefore, it is 

probably necessary to assume not only subsaturation of methane above the methane 

cloud, but also the possibility of an even more complex altitude distribution for the 

scattering particles throughout the atmosphere. 

A more thorough study of various scattering models with different vertical 

distributions of CHI and of scattering particles must be undertaken. For such a 

study, it will be extremely useful to investigate also the profiles of the CO lines 

which we detected in the same Titan spectrum (Lutz, de Bergh, and Owen 1983). 

This simultaneous study of two constituents of the Titan troposphere, one with a 

uniform vertical distribution (CO) and the other with a nonuniform distribution 

(CHI), should be most informative. 

Independent of the true structure of the atmosphere, the line formation processes for CHI and CHsD and their respective vertical distributions must be very 
Page 7 

similar. Consequently, a CHsD/CHI mixing ratio derived from the absorptions 

of these two gases is relatively insensitive to any adopted atmospheric model. In 

practice, however, it is also important that the absorptions of the two gases are 

in adjacent spectral regions so that a common continuum level can be adopted for 

both, as is the case here. Thus it is sufficient to optimize the fit for methane alone, 

and then to adjust the CH3D/CH4 mixing ratio (as is discussed in the next section) 

until a good agreement between observed and synthetic spectra is obtained in the 

spectral range where the absorptions occur. We have indeed verified by calculation 

that different atmospheric models which provide reasonable fits with the observed 

Titan spectrum lead to the same D/H ratio, regardless of the differences among 

the models. Of the various models we have studied so far, the one that provides 

the best agreement with the observed CH4 absorptions in the Titan spectrum is 

actually the simplest one described above: a single cloud with constant pressure 

and temperature between the cloud and the surface, with no CH4 above the cloud. 

The synthetic spectrum of Titan shown in Figure 1 is based on this model. 

IV. MEASUREMENTS OF THE CH3D/CH4 RATIO 

AB in the case of Uranus (see Paper II), CH4 alone could not provide a satisfactory fit with the spectrum of Titan near 6427 and 6452 cm-' in any of the 

atmospheric models considered. These are the regions of the Q-branch and R2 

manifold, respectively, of the 34 band of CHsD. We therefore computed synthetic 

spectra corresponding to different CHsD/CH4 ratios, using the rotational assignments for the 3vz band worked out by Lutz, de Bergh, and Maillard (1983) and the 
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same line intensities used in paper 11. The mean CHJD-N~ pressure-broadening coefficient was assumed to be the same as that adopted for CH4-N2. This assumption 

is supported by our own preliminary laboratory measurements. The temperature 

dependence of this coefficient was also assumed to be T-' (Varanasi, Giver, and 

Valero 1983b). With our single cloud model, we find the best fit to the observed 

Titan spectrum for a mixing ratio of CHsD/CH4 = 6.6 x and this fit is shown 

in Figure 1. 

The major sources of uncertainty in this determination lie in the line intensities 

and broadening coefficients for the CHI and CH3D lines, in the rotational assignments for the lines of CH4 (see Paper 11), and in the placement of the continuum 

level for the Titan spectrum and the choice of an appropriate atmospheric model. 

Another source of uncertainty is the difficulty we have in accounting for extremely 

weak lines of methane that are undetectable in our room-temperature laboratory 

spectra. These lines nevertheless contribute to the absorptions observed in the spectrum of Saturn (see Paper 11) and must, therefore, also contribute to the absorptions 

in the spectra of Titan. Given all the uncertainties listed above, we believe that we 

have measured the CHsD/CH4 ratio on Titan to within a factor of 2. 

We have found that the wavelength ranges where the fits between observed 

and synthetic spectra of Titan are not very good (6433-6450 cm-', for instance) 

are the same as the ranges where the fits between the observed and synthetic spectra 

of Uranus and of Saturn are not very good. This gives us some hope that we will 

be able to improve the precision of our CHsD abundance measurements on Titan 
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(and also on Uranus and Saturn) when we have a better knowledge of the extremely 

weak CHI absorptions. This knowledge should be achievable by studying very high 

signal-to-noise ratio laboratory spectra of methane obtained at low temperatures 

with very long optical paths, if and when such data become available. 

V. DISCUSSION 

The value of D/H in Titan’s atmosphere is derived directly from the 

CH3D/CH4 ratio using the stoichiometric relationship: 

D/H=;[CHsD/CH4] = 1.65+t::5 x lo-‘. 

This ratio is distinctly higher than the range of values measured on Jupiter and Saturn, D/H = (0.6 - 5.0) x lo-’ (Owen, Lutz, and de Bergh 1986). Uranus represents 

an intermediate case with a range of D/H = (4.5- 18) x lo-’ (de Bergh et al. 1986). 

All of these values are based on ratios of CH3D/CH4. The situation for HD/H2 is 

not yet well defined. Published measurements (for a summary, see Table 2 of de 

Bergh et al. 1986, and the discussion therein), as well as new measurements (Smith, 

Schempp, and Macy 1987) suggest that on Jupiter and Saturn the D/H ratios derived from this abundance ratio are systematically higher compared with the values 

derived from CHSD/CHd. We have proposed that part of this apparent discrepancy 

may be traceable to contamination by very weak methane absorptions in the regions of the HD lines (Owen, Lutz, and de Bergh 1986), and preliminary laboratory 

studies by Smith (private communication) seem to support this possibility. 

The only other determination of D/H on Titan was made by Kim and Caldwell 

(1982), using the ug band of CHsD at 8.6 pm by the Voyager IRIS instrument (Hanel 
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et al. 1981). At this wavelength the spectrum is formed predominantly in Titan’s 

stratosphere. Deriving a mixing ratio for CH4/N2 = 3.6 f 1.1 x they found 

CH3D/CHr = 1.7 x Subsequent to their analysis, 

a new temperature-pressure profile for Titan’s atmosphere was published by Linda1 

et al. (1983) and new broadening coefficients for Yg of CH3D were obtained by 

Varanasi, Giver, and Valero (19834. The net effect of these two changes, plus a 

small error in their computer code, reduces their original estimate of D/H by about a 

factor of 2 (Kim and Caldwell, private communication). Their work is thus actually 

in good agreement with the determination we have made using a completely different 

band, in absorption instead of emission. 

and D/H = 4t:.5 x 

We have discussed the general problem of deuterium abundance in the outer 

solar system elsewhere in more detail (Paper I1 and Owen, Lutz, and de Bergh 

1986). We agree with Kim and Caldwell (1982) that the enrichment of D/H on 

Titan must have occurred prior to the formation of the satellite. We believe that 

Pinto et al. (1986) provide quantitative support for this conclusion. They found that 

all the processes acting to increase the value of CHsD/CH4 during the evolution of 

the planet’s atmosphere could only achieve an enrichment of 2.2 times the original 

value. Another factor of 2 may be possible through 500 K catalysis on metallic 

grains in the proto-Saturnian nebula prior to Titan’s formation. This process seems 

unlikely and in any case fails to produce the observed ratio except for the lower 

range in the error bars. 

Our general conclusion is that bodies in the outer solar system had access 
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to at least two distinct reservoirs of deuterium: one in hydrogen gas and one in 

methane frozen on or trapped in solid material. Water may and other hydrogencontaining compounds constitute additional reservoirs, depending on how closely 

the value of D/H which we have measured in primitive ice agrees with the D/H in 

Titan’s atmosphere when the latter is corrected for the possible enrichment by a 

factor of about 2 that may have occurred since the satellite formed (Pinto et al. 

1986) and for possible dilution with equilibrated methane in the proto-Saturnian 

nebula (Prinn and Fegley 1981). The fact that D/H on Titan is so close to the 

value of 1.5 x lo4 found in standard mean ocean water (SMOW) [which may also 

be enriched by about a factor 2 through several escape processes (Hunten, private 

communication)] suggests that the primordial methane and water reservoirs may 

actually have very similar deuterium-to-hydrogen ratios. 

ACKNOWLEDGMENTS 

This research is supported in part by NASA grants NSG-7499 and NGR33015141. 
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REFERENCES 

Courtin, R., Gautier, D., Marten, A., BCzard, B., and Hanel, R. 1984, Ap. J., 287, 

899. 

de Bergh, C., Lutz, B. L., Lockwood, G. W., Owen, T., Brault, J., and Buriez, J. 

C. 1981, Bull. Amer. Astron. SOC., 13, 703. 

de Bergh, C., Lutz, B. L., Owen, T., Brault, J., and Chauville, J. 1986, Ap. J., 

311, 501. 

Fink, U., and Larson, H. P. 1979, Ap. J., 233, 1021. 

Gillett, F. C. 1975, Ap. J. (Letters), 201, L41. 

Geiss, J., and Reeves, H. 1981, Astron. Ap., 93, 189. 

Hanel, R. A., Conrath,B. J., Flasar, F. M., Kunde, V. G., Maguire, W., Pearl, J. 

C., Pirraglia, J. A., Samuelson, R., Hearth, L., Allison, M., Cruikshank, D. P., 

Gautier, D., Gierasch, P., Horn, L., Koppany, R., and Ponnamperuma, C. 1981, 

Science, 212, 192. 

Kim, S. J., and Caldwell, J. 1982, Icarus, 52, 473. 

Lindal, G. F., Wood, G. E., Hotz, H. B., and Sweetnam, D. N. 1983, Icarus, 53, 

348. 

Lutz, B. L., de Bergh, C., Maillard, J. P., Owen, T., and Brault, J. 1981, Ap. J. 

(Letters), 248, L141. 

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Lutz, B. L., de Bergh, C., and Owen, T. 1983, Science, 220, 1374. 

Owen, T., Lutz, B. L., and de Bergh, C. 1986, Nature, 320, 244. 
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Pinto, J. P., Lunine, J. I., Kim, S. J., and Yung, Y. L. 1986, Nature, 319, 388. 

Prinn and Fegley 1981, Ap. J., 249, 308. 

Smith, W. H., Schempp, W., and Macy, W. 1987, Icarus, in press. 

Strobel, D. F. 1982, Planet. Space Sci., 30, 839. 

Varanasi, P., Giver, L. P., and Valero, F. P. J. 1983a, J.Q.S.R. T., 30, 481. 

Varanasi, P., Giver, L. P., and Valero, F. P. J. 1983b, J.Q.S.R. T., 30, 511. 

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I 
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FIGURE CAPTION 

Figure 1. Comparison between observed and synthetic Titan spectra. (a) and (b) 

Full line: Spectrum of Titan recorded in 1982 June at Kitt Peak National Observatory (NOAO) with the FTS at the 4-m telescope (spectral resolution: 1.2 cm-', 

S/N: 40). This spectrum has been divided by the spectrum of diffuse solar radiation 

recorded during the same run. 

(a) Dashed line: Synthetic spectrum of Titan computed as explained in the 

text for only CHI absorptions, based on our nominal atmospheric model (a temperature of 85 K, and a total methane abundance of 2.2 km-atm, at an average pressure 

of 1 atm, between a cloud layer of optical thickness 7 = 0.23 and the suriace; no 

CHI above the cloud). 

(b) Dashed line: Synthetic spectrum of Titan as shown in (a) but with 

CH3D absorptions included: CH3D/CH4 = 6.6 x The regions of the main 

cH3D-3~2 absorptions are indicated at the top of the figure. 
I I I I I I I I I 1 1 I I 

fT (a) 

I 1 

I I I I I I I I , Wyvenumber cm-’ 

6420 643 0 644 0 6450 6460 6470 6480 

I I I I I I I I I Wav en umber cm1 

6420 6430 6440 645 0 6460 6470 6480 
ADDRESSES OF AUTHORS 

J. Chauville 

Laboratoire d’Evolution Stellaire et Galactique 

Observatoire de Paris 

92195 Meudon Principal Cedex 

France 

C. de Bergh 

Laboratoire d’Astronomie Infrarouge 

Observatoire de Paris 

92195 Meudon Principal Cedex 

France 

Barry L. Lutz 

Lowell Observatory 

Mars Hill Road, 1400 West 

Flagstaff, Arizona 86001 

U.S.A. 

Tobias Owen 

Department of Earth and Space Sciences 

State University of New York 

Stony Brook, New York 11794 

U.S.A.