The detection of HCN on Jupiter

ICARUS 48, 283--289 (1981)

The Detection of HCN on Jupiter A. T. TO KUNAGA 1 Institute for Astronomy, University of Hawaii, 2680 Woodlawn Drive, Honolulu, Hawaii 96822

S. C. B E C K 2 Department of Physics, UniversiO' of California, Berkeley, California 94720

T. R. G E B A L L E 2 Mt. Wilson and Las Campanas Observatory, 813 Santa Barbara St., Pasadena, Califi)rnia 91101

J. H. LACY 2 Department of Physics, California Institute of Technology, Pasadena, California 91125 AND

E. SERABYN 2 Department of Physics, University of California, Berkeley, California 94720 Received July 13, 1981; revised S e p t e m b e r 8, 1981 We report the detection of H C N on Jupiter. Three R-branch lines o f the v2 f u n d a m e n t a l o f H C N near 13.5/~m were o b s e r v e d in absorption, from w h i c h the H C N c o l u m n density is inferred to be 5 × 10-s c m - a m with an uncertainty o f a factor o f 2. If e m i s s i o n from the stratosphere exists, then the derived c o l u m n density is only a lower limit. W e s u g g e s t that the Jovian H C N m o s t likely originates from the photolysis o f CH4 and NHa in the lower stratosphere and upper troposphere. In addition, an upper limit o f 2.5 × 10 -2 c m - a m w a s established for the c o l u m n density o f H C N on Saturn.

INTRODUCTION

HCN (hydrogen cyanide) is an important molecule in the synthesis of organic molecules, and therefore it could have a major role in the chemical evolution of the atmospheres of the outer planets. HCN may also be important in the production of chromophores, as suggested by Woeller and Ponnamperuma (1969). It is also potentially useful as a tracer of disequilibrium processes such as lightning or transport of ma1 Staff A s t r o n o m e r at the Infrared T e l e s c o p e Facility, which is operated by the U n i v e r s i t y o f Hawaii u n d e r contract from the National A e r o n a u t i c s and Space Administration. 2 Visiting A s t r o n o m e r at the Infrared T e l e s c o p e Facility.

terial from the deep atmosphere (Lewis, 1980). We report the first detection of HCN on Jupiter. Three R-branch lines of the u2 fundamental of HCN near 740 cm -1 (13.5/zm) were observed. In addition, an upper limit to the HCN column density is established for Saturn. These results complement the recent detection of HCN on Titan by Voyager 1 (Hanel et al., 1981). OBSERVATIONS

Spectra of Jupiter and Saturn were obtained with a cryogenic 10-/zm F a b r y - P e r o t spectrometer (Lacy, 1 9 7 9 ) that was mounted at the Cassegrain focus of the Infrared Telescope Facility. We observed Jupiter and Saturn during three periods from 283 0019-1035/81 / 110283-07502.00/0 Copyright © 1981by AcademicPress, Inc. All rights of reproduction in any form reserved.

284

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ET AL.

TABLE I LOG OF OBSERVATIONS AND C A L C U L A T E D M I X I N G RATIOS

Line

Vacuum frequency (cm -1)

R6 R7 R11 Rll

732.66 735.62 747.41 747.41

Date observed

1 Dec. 25 June 12 Jan. 12 Jan.

1980 1980 1981 1981

Object

Observed equivalent width (cm-q

Line strength [cm-l(cm-am)-q

Jupiter Jupiter Jupiter Saturn

0.0050 0.0024 0.0046 <0.011

4.70 4.81 4.11 4.11

f.cN°

1.7 0.9 4.2 <7

× x × ×

10-9 10-~ 10-9 10-9

a A s s u m i n g the H C N is present only in the troposphere.

June 1980 to January 1981. A log of the observations is given in Table I, and the data are shown in Figs. 1-4. For all three periods the aperture used was 6 arcsec and the spectral resolution was 0.09 cm -1 (the full width at half maximum o f the Lorentzian profile). The aperture was centered on the disks of Jupiter and Saturn, with the exception of the June 1980 observation, when the aperture was set 5 arcsec north of the equator• The scans were oversampled and therefore the noise in the data has features that are smaller than a single spectral element. The frequency calibration of each spectrum was obtained at the telescope by comparison to spectra o f NH3 lines that were obtained by inserting an NH3 gas cell and blackbody source into the field o f view of the spectrometer. In addition, telluric absorption lines were used for frequency calibration when possible. The uncertainty in the frequency scale is _0.01 cm -1. The telluric absorption was difficult to correct since a nearby bright standard was not available. H o w e v e r , an estimate of the telluric absorption can be obtained from a spectrum o f the sky chopped against an ambient temperature blackbody. During June and December 1980, we obtained suitable sky spectra and divided the Jupiter data by them. The correction is not complete, however, as is evident in Figs. 1 and 2. For January 1981, no useful sky spectra were obtained, and the data shown in Figs.

3 and 4 are shown undivided. These spectra also contain the transmission o f the grating m o n o c h r o m a t o r which is peaked on the line with a symmetric and gradual drop toward either end o f the spectrum. H C N was detected on Jupiter but not on Saturn. There was sufficient time to observe only three lines on Jupiter, and the lines were chosen to be in regions o f relatively high telluric atmospheric transmission. We also chose the lines to be as close

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0.7

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JUPITER

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733.O( '

Frequency cm-l:

FIG. 1. O b s e r v a t i o n s of the R6 line on Jupiter. The c o n t i n u u m a s s u m e d in deriving the equivalent width is s h o w n as a light dashed line. The f r e q u e n c y scale is given in v a c u u m w a v e n u m b e r s and is based on the N H a line frequencies by Garing e t al. (1959). The Doppler shift of Jupiter therefore appears in the spectrum. Strong telluric CO2 lines at 732.20 and 733.20 c m -1 are present.

HCN ON JUPITER

Jupiter spectra is 20 times that of Saturn; thus the noise level is much more evident in the Saturn spectra. The absolute calibration was not sufficiently accurate to warrant inclusion into the figures.

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ANALYSIS

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FIG. 2. O b s e r v a t i o n o f the R7 line on Jupiter. Telluric CO2 absorption lines at 735.36 and 736.18 c m -~ affect the c o n t i n u u m .

as possible to the Q branch of the v2 fundamental. The lines on Jupiter are extremely weak, but we are confident that the detection is real because: (1) three different rotational lines were observed on Jupiter; (2) the lines are approximately at the Doppler shift expected (+0.06 cm -~ in June 1980; -0.06 cm -1 in December 1980); and (3) the lines were carefully chosen not to coincide with known telluric or Jovian lines. In addition, the integration times were sufficiently long to achieve a signal-to-noise of -> 100 in the continuum. The lines do not appear in the solar atlas by Mige6tte et al. (1956), nor do they appear in the sky spectra obtained with the Fabry-Perot spectrometer. We note that HCN has been recently detected in the Earth's stratosphere by Coffey et al. (1981), but the observed column density is approximately 100 times smaller than the HCN column density observed on Jupiter as derived in the next section. Since the equivalent width of the lines are of interest in this paper, the data shown in Figs. 1-4 have been normalized to the peak flux. The determination of the equivalent width is affected by uncertainty in the placement of the continuum and in the distortion of the line by telluric absorption. The R7 line is greatly affected by the telluric absorption, and this probably accounts for its narrower width compared to the other lines. The continuum flux in the

A radiative transfer program was used to determine the abundance of HCN on Jupiter and an upper limit to the abundance of HCN on Saturn. This program had been used by Tokunaga et al. (1979) on the analysis of the 10-/zm spectrum of Jupiter, and it divides the atmosphere into at least eight layers per decade in pressure between 1.0 and 10-a bar. The pressure-induced absorption of H~ and the NHa and HCN lines are included although the NHa absorption was negligible in the spectral regions observed. A Voigt profile was used for all levels in the atmosphere. The frequency step size used was 0.01 cm -1 for the analysis of the tropospheric absorption and 0.002 cm -1 for the analysis of possible stratospheric emission. The HCN line intensities were calculated using the total band intensity of the Vz fundamental of 204 cm -2 atm -1 (Hyde and Hornig, 1952). The calculated line strengths

1.0

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Frequency Icm-11

FIG. 3. O b s e r v a t i o n s o f the R I I line on Jupiter. Telluric CO2 absorption lines at 746.83 and 747.90 c m -1 affect the c o n t i n u u m .

286

TO KUNAGA ET AL.

SATURN >, "~

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FIG. 4. Observation of the R11 line on Saturn. None of the features in the spectrum are significant; the peak-to-peak fluctuation of the features is indicative of the noise level. and the v a c u u m line frequencies obtained from Yin and Rao (1972) are shown in Table I. The t e m p e r a t u r e d e p e n d e n c e of the line strength was taken into account in the radiative transfer program using the theoretical expression for a linear molecule. A . The H C N A b u n d a n c e on J u p i t e r

The a t m o s p h e r i c p r e s s u r e - t e m p e r a t u r e profile used in the analysis was an average of the V o y a g e r 1 profiles for latitudes + 10 and +20 ° (Hanel et al., 1979; V. Kunde, private communication). If a uniform mixing ratio of H C N throughout the atmosphere is a s s u m e d , the H C N line core appears in emission despite the very low abundance o f H C N . This emission arises f r o m the stratospheric region as does emission from the 10-/xm bands of CH4 and C2H6 (Hanel et al., 1979, 1981). Therefore, we have fitted the H C N absorption by first assuming that all of the H C N is in the troposphere and uniformly mixed. The ratio n ( H C N ) / n ( H ~ ) (hereafter fncN) was determined by the following iterative process: fHcy was chosen, the s p e c t r u m calculated, the calculated equivalent width was compared to the o b s e r v e d equivalent width, a n d f n c y was adjusted for the next iteration. The calculation a s s u m e d an opaque cloud deck at the 140°K level (0.6 bar) in the a t m o s p h e r e as had been found earlier for the equatorial region (Tokunaga et al., 1979). H o w e v e r , the results were not sensitive to this a s s u m p t i o n since extending the

calculation d o w n to the 1-bar level increased the equivalent width by only 12%. The values offncN obtained are given in Table I. We adoptfncN = 2 × 10 -9 as the best estimate of the H C N mixing ratio with an uncertainty o f a factor of 2. The H C N column density is 5 × 10 -3 c m - a m , using an H2 column density of 24 k m - a m to the 140°K level. We note that u p p e r limits for H C N from previous w o r k have ranged between 1 and 50 c m - a m (Cruikshank and Binder, 1969; Gillett et al., 1969; Treffers et a/., 1978). Limits can be placed on the a m o u n t of H C N that could exist in the stratosphere. WithfHcN equal to 2 × 10-9 throughout the stratosphere, the flux in the line core emission would be nearly equal to the flux rem o v e d from the continuum in the absorption line, and the line would be barely observable in this case. Therefore we conclude thatfHcN must be less than 2 × 10 -9 in the stratosphere. Since the line core emission would have a Doppler width of 0.004 cm -1, we cannot rule out the possibility of some emission that is not o b s e r v e d at our spectral resolution. I f substantial H C N emission exists, then the column density derived a b o v e is a lower limit. If, however, the emission in the line core affects the o b s e r v e d equivalent width by less than 20%, then fHCN< 5 × 10 -1° in the stratosphere, corresponding to an H C N column density of < 3 × 10 -4 c m - a m in the stratosphere. B. A n U p p e r L i m i t to the H C N A b u n d a n c e ol7 S a t u r n

The data in Fig. 4 do not show any feature that m a y be attributed unambiguously to H C N , but we can establish a new upper limit to the H C N column density. We adopted a 3o" upper limit to the H C N equivalent width that was three times larger than a noise feature at the H C N line position. The model a t m o s p h e r e used for Saturn was an average of the Voyager 1 p r e s s u r e t e m p e r a t u r e profiles presented by Hanel et al. (1981). The lower bound of the model

HCN ON JUPITER was set at the 98°K level (0.4 bar). The brightness temperature of the continuum in this model was 95°K, which is a few degrees lower than that observed by Voyager 1 (Hanel et al., 1981). However, at the Voyager 1 spectral resolution we expect the observed continuum brightness temperature to be higher than its true brightness temperature because of the unresolved C2H2 emission lines in the spectrum. As in the analysis of the Jupiter data, the maximum mixing ratio was determined by iteration and the tropospheric and stratospheric upper limits were established separately. With the assumption of no stratospheric emission, we find an upper limit tofncN of 7 × 10-9. Since the column density of H2 in the troposphere down to the 0.4-bar level is 35 km-am, the upper limit to the HCN column density is 2.5 x 10-z cm-am. This upper limit is significantly lower than a previous upper limit of 0.1 cm-am derived by Larson et al. (1980) from 5-/zm observations. Changing the lower limit of the model from 0.4 to 1 bar increases the upper limit column density by only 20%. The maximum amount of HCN in the stratosphere was determined by calculating the value forfNcN that would give rise to an emission feature comparable to the noise features in the spectrum. An upper limit of 8 × 10-9 was found forfHcN, corresponding to a column density of 5 × 10-a cm-am. This upper limit assumes no HCN absorption exists. DISCUSSION The low HCN mixing ratio observed on Jupiter and Saturn is in striking contrast to the relatively high mixing ratio on Titan of 2 x 10-7 derived from the HCN emission observed by Voyager 1 (Hanel et al., 1981). This could be explained by different HCN production mechanisms existing on Jupiter and Titan. HCN is probably produced on Titan from the dissociation of N2 by magnetospheric electron impact (Broadfoot et al., 1981), whereas on Jupiter it could be produced by photolysis and lightning, or it

287

could be transported from the deep atmosphere to the upper troposphere. In this section we discuss these mechanisms and suggest that photolysis of CH4 and NHa is the most likely source of HCN production in the Jovian atmosphere. Experiments by Raulin et al. (1979) show that observable amounts of HCN can be produced photochemically in a mixture of H2, NHa, and CH4 with ultraviolet light. They point out that the NHa abundance is low in the Jovian stratosphere and that most of the HCN production by photolysis would therefore occur in the lower stratosphere and upper troposphere. Observations of Jovian NHa in the ultraviolet by Combes et al. (1980) indicate that the NHa stratospheric mixing ratio is indeed low: approximately 10-7 at the 0.25-bar level and decreasing linearly with pressure or as the square of the pressure. High-resolution infrared observations also indicate low NH3 stratospheric mixing ratios (Tokunaga et al., 1979). In addition, calculations by Visconti (1981) indicate that the maximum absorption of ultraviolet radiation by NHa does occur in the upper troposphere (see Fig. 6 of his paper). Neither experiments nor theoretical modeling can predict a mixing ratio by photolysis at the present time. Although the reaction that produces HCN is not reliably known, it could be produced by the photolysis of CHaNH2 (Gardner and McNesby, 1980). Kuhn et al. (1977) have calculated the distribution of CH3NH2 in the Jovian atmosphere, and they find that the maximum CH3NH2 production rate occurs at a pressure of approximately 0.03 bar, corresponding to the lower stratosphere. Unfortunately, the CHaNH2 mixing ratio they obtain is about 100 times less than the observed HCN mixing ratio. Therefore, CHaNH2 is not the source of the HCN unless the CHaNH2 abundance is considerably higher than calculated by Kuhn et al. However, other photochemical reactions may produce HCN, in which case the observations constrain the HCN production

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to be restricted to the lower stratosphere and the upper troposphere. A second possible mechanism that might explain the observed abundance of HCN on Jupiter is the transport of material by convection from the deep atmosphere. However, the abundance of HCN in chemical equilibrium model calculations equals that observed only at the atmospheric level where the temperature is 1500°K. This is much deeper than the 1000°K level from which PH3, CO, and GeH4 are believed to be brought to the upper troposphere by convection (Barshay and Lewis, 1978; Larson et al., 1978; Fink et al., 1978). If the observed PHa and the CO mixing ratios at 5 /~m are used, the level from which PHa and CO rises is at a temperature of 900-1100 ° K. Barshay and Lewis (1978) calculate that at the 1000°K level the HCN mixing ratio is only 2 × l0 -xa. The available evidence thus suggests that HCN is not transported from the deep atmosphere, otherwise much higher PHa and CO mixing ratios would be observed. A third mechanism for HCN production is synthesis by lightning. However, Lewis (1980) argues that this mechanism can yield an HCN mixing ratio of at most 3 × l0 -12 even with the most optimistic assumptions possible. This is a factor of l0 a times less than the observed HCN abundance. In summary, it appears that photolysis of NHa and CH4 is the most likely source of the observed HCN in the Jovian troposphere compared to other possible sources. Further photochemical modeling is necessary to establish the feasibility of HCN production by photochemistry. If this mechanism is valid, then the HCN abundance on Saturn is likely to be many orders of magnitude less than on Jupiter because of the considerably smaller NHa abundance on Saturn (Encrenaz et al., 1974; Tokunaga et al., 1980). We cannot determine the distribution of HCN at the lower atmospheric levels. However, center-to-limb observations of the lines could put constraints on the HCN distribution at least to the extent of establishing whether the HCN exists primarily in

the troposphere or lower stratosphere. Finally, we note that HCN plays an extremely important role in the formation of amino acids and nucleic acid components in a reducing atmosphere. This has been reviewed by Lemmon (1970) and at a less formal level by Dickerson (1978). Laboratory experiments show that complex polymers can be produced with HCN in a reducing atmosphere (Woeller and Ponnamperuma, 1969). Although the abundance of HCN gas detected in the Jovian atmosphere is extremely low, the HCN might occur at higher concentrations in airborne droplets and be converted there into complex organic molecules. Very complex molecules such as those found by Sanchez et al. (1967) in aqueous HCN solutions could form. In this way significant organic synthesis involving HCN could occur despite the lack of a surface on Jupiter. Such droplets could be the Jovian analog of the terrestrial "primordial soup" out of which life on Earth may have arisen. SUMMARY

HCN has been detected on Jupiter for the first time. We estimate the column density of HCN to be 5 × 10-a cm-am with an uncertainty of a factor of 2. Since HCN appears in absorption, the bulk of the HCN must reside in the Jovian troposphere or in the lower stratosphere. If emission from the stratosphere exists, then the derived column density is only a lower limit. We also place an upper limit on the HCN column density on Saturn of 2.5 × 10-2 cm-am. We suggest that the Jovian HCN most likely originates from the photolysis of CH~ and NH3 in the lower stratosphere and upper troposphere, although further photochemical modeling is necessary to establish this. ACKNOWLEDGMENTS AT acknowledges the support of NASA Contract NASW 3159, and SCB, JHL, and ES acknowledge support from the NSF and NASA. SCB is grateful to the Institute for Theoretical Physics, U.C. Santa Barbara, for their hospitality while this work was in progress. We thank V. Kunde for useful discussions and D. Strobel for an informative discussion of HCN production via photochemistry.

HCN ON JUPITER Note added in proof. We would be remiss not to mention the works by Sagan and Miller (1960, Astron. J. 65, 499) and Sagan, Lippincott, Dayhoff, and Eck (1967, Nature 213, 273) which showed on experimental and theoretical grounds that HCN is likely to be present on Jupiter.

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