Infrared polar brightening on Jupiter

ICARUS 64, 233--248 (1985)

Infrared Polar Brightening on Jupiter III. Spectrometry from the Voyager 1 IRIS Experiment SANG J. KIM, 1 J O H N CALDWELL, A. R. RIVOLO, AND R. W A G E N E R Earth and Space Sciences Department, State University of New York, Stony Brook, New York 11794-2100 AND

G L E N N S. ORTON Jet Propulsion Laboratory 183-301, California Institute of Technology, Pasadena, California 91109 Received April 29, 1985; revised June 20, 1985 Spectra from the Voyager 1 IRIS experiment confirm the existence of enhanced infrared emission near Jupiter's north magnetic pole in March 1979. The spectral characteristics of the enhanced emission are consistent with a Planck source function. A temperature-pressure profile is derived for the region near the north magnetic pole, from which quantitative abundance estimates of minor species are made. Some species previously detected on Jupiter, including CH3D, C2H2, and C2H6, have been observed again near the pole. Newly discovered species, not previously observed on Jupiter, include C2H4, C3H4, and C6H6. All of these species except CH3D appear to have enhanced abundances at the north polar region with respect to midlatitudes. Upper limits are determined for C4H2 and C3H8. The quantitative results are compared with model calculations based on ultraviolet results from the IUE satellite. The plausibility of the C6H6 identification is discussed in terms of the literature on C2H2 polymerization. The relation of C6H6 to cuprene is also discussed. © 1985AcademicPress, Inc.

INTRODUCTION

A fortuitous discovery of transient polar limb brightenings at 8-ttm wavelength was reported by Caldwell et al. (1980; hereafter referred to as Paper I). A subsequent systematic investigation of this phenomenon confirmed that it persisted over more than one Earth year and that the polar brightenings were highly correlated with the respective magnetic polar regions in both hemispheres (Caldwell et al., 1982; hereafter referred to as Paper II). These observations were made mostly with the NASA Infrared Telescope Facility 3-m telescope on Mauna Kea. 1Present address: National Space Science Data Center, Code 633, Goddard Space Flight Center, Greenbelt, Md. 20771.

The data in Papers I and II were obtained through a narrowband filter with an effective wavelength of 7.8/zm, with FWHM = 0.6/xm. By design, this filter includes primarily radiation from the strong v4 fundamental of methane (CH4), centered at 7.7 /zm. Because of these spatial and spectral characteristics of the brightenings, it was suggested that they are due to an infrared aurora on Jupiter, in which CH4 molecules are collisionally excited, either directly or indirectly, by precipitating high-energy magnetospheric particles. The Voyager 1 encounter with Jupiter, which occurred for several days near 5 March 1979, provided a potentially unique opportunity to investigate the phenomenon further. It achieved spatial resolution that was unmatched by any Earth-based telescope, and its Infrared Interferometer

233 0019-1035/85 $3.00 Copyright© 1985by AcademicPress. Inc. All rightsof reproductionin any formreserved.

234

KIM ET AL.

The new information in this paper comes from the wide spectral coverage achieved 60 by the IRIS. In the following sections, we 50 t,i show that the brightenings are not confined t~ to CH4, but are also observed in the emisI--° 0 • 1 sions of other molecules. This and other '~" -30 "' i ?'"'I ':'1"~, '"':" / w characteristics of the radiation indicate that it is predominantly thermal, probably resulting from local heating of the upper atmosphere by energetic particles precipitat0 90 leo ' 270 ' ~so ing near the magnetic poles. A second SYSTEM "Ill" (1965) LONGITUDE result of this investigation is that the heated FIG. 1. A m e r c a t o r map o f Jupiter, with latitude and polar stratosphere uniquely exhibits very System III (1965) longitude. The solid curves are the auroral zones of Connerney et al. (1981). The broken small emission features, which are attribcurves are the footprints of the magnetic field lines uted to molecules not observed elsewhere through the Io torus. The dots are the locations of IRIS on Jupiter: ethylene (C2H4), methyl acetyspectra used in this study, separated into four regions lene (C3H4), and benzene (C6HG). These as described in the text. NI is the prime region of identifications are further tested by cominterest. paring their ultraviolet absorptions with results from the International Ultraviolet S p e c t r o m e t e r (IRIS) instrument was sensi- Explorer (IUE) satellite. tive o v e r the spectral range from approxiTHE IRIS DATA mately 200 to 2300 cm -= (50 to 4.4/zm). This paper reports the results of an invesFigure 1 is a mercator projection of Jupitigation we made of the Voyager 1 IRIS ter in System III (1965) coordinates. On it data. Because Voyager preceded the dis- are shown the centers of the IRIS fields of c o v e r y of the infrared polar brightenings on view corresponding to the spectra used Jupiter, its pointing was not optimized for herein. The data consist of all Voyager 1 studying them. Fortunately, some observa- points with spatial resolution greater than tions o f the auroral regions planned for the 1/10 of the Jovian disk and which fall within Ultraviolet S p e c t r o m e t e r (UVS) also gave one of four regions on Jupiter: information on the infrared properties of (i) NI: latitude > 48°; 120° < system III these areas, because the IRIS operated (1965) longitude < 240 ° (87 points); (ii) N2: latitude > 50°; 0 ° < longitude < continuously throughout the encounter. Although the enhanced polar infrared emis- 30 ° (24 points); sion and the ultraviolet aurorae on Jupiter (iii) W: - 3 0 ° < latitude < 0°; excluding may have different morphologies, there longitudes 120-220 ° (164 points); was enough overlap in the two instruments' (iv) S: latitude < -60°; all longitudes (90 fields o f view that we were able to confirm points). infrared brightenings in the IRIS data at the Also shown in Fig. 1 are the north and north polar limb in early 1979. south " f o o t p r i n t s " of the magnetic flux Our effective spatial resolution was de- tube which intersects the Io orbital torus graded by the need to average many spectra (broken lines) and the north and south aurotaken near the prime area of interest, to im- ral arcs (solid curved lines). The latter are prove the signal to noise ratio in the IRIS the boundaries within which magnetic field data. In fact, the present study is less useful lines map into the magnetotail. These with respect to spatial resolution than the curves are taken from Connerney e t al. systematic ground-based observations (Pa- (1981). per II and work in progress). The primary field of interest is N I . It is 9O

-6o1_9o

IR JOVIAN POLAR BRIGHTENING

235

Also present in Fig. 2 between 900 and 1200 cm -1 are absorptions due to tropoVOYAGER FDSC NUMBERS FOR SPECTRA INCLUDED spheric PH3 (phosphine) and NH3 (ammoIN FIELD NI nia). The broad adsorption feature due to 16371.52 16371.53 16371.54 16371.55 molecular hydrogen (H2) from 800 to 600 16371.56 16372.56 16372.57 16372.58 cm -~ is also noticeable. The absorption fea16372.59 16373.04 16373.05 16411.53 tures originate between 100 and 300 mbar in 16411.54 16411.55 16411.56 16413.10 the troposphere. All of these molecules 16415.34 16415.35 16415.36 16415.37 mentioned here have previously been iden16415.39 16415.40 16415.41 16415.42 16415.43 16415.44 16415.45 16415.46 tified by Hanel et al. (1979). At frequencies 16415.47 16415.48 16415.49 16415.50 above 1350 cm -1, noise becomes dominant 16415.51 16415.52 16440.54 16452.41 in these spectra, as the signal decreases ex16452.42 16452,43 16452.45 16452.46 ponentially. 16452.47 16452.48 16452.49 16452.50 The excess stratospheric brightening of 16452.51 16452.52 16452.53 16452.54 16452.55 16452,56 16452.57 16452.58 field N1 is visible in Fig. 2 in the Q branch 16452.59 16453.00 16453.01 16453.02 of C2H2 at 737 cm -~, where the peak bright16453.03 16453.04 16453.05 16453.06 ness temperature is 137°K, compared to 16453.07 16453.08 16453.09 16453.10 126°K for the fields N2 and S and 129°K for 16453.11 16453.12 16453.13 16453.14 W. Less obvious from that figure, but 16453.15 16453.16 16453.17 16453.18 16453.19 16453.20 16453.21 16453.22 equally significant, is the enhancement in 16453.23 16453.24 16453.25 16453.26 the Q branch of CH4 at 1307 cm -~, where 16453.27 16453.28 16453.29 16453.30 the peak brightness temperature is 178°K, 16453.31 16453.32 16453.33 compared to 166, 164, and 160°K, respectively, for N2, W, and S. actually defined to incorporate the IRIS obIn Fig. 3, we present flux ratios of spectra servations that are closest to the auroral from the previous figure. The top of Fig. 3 arcs and Io torus footprints, and it is not is the ratio of N1 to S fluxes and the bottom readily specified accurately by simple limits is the ratio of N1 to N2. Also shown in Fig. as given above. To avoid any possible con- 3 are several new molecular identifications fusion, the individual IRIS spectra compris- that are discussed in a later section. Figure ing field N1 are listed explicitly in Table 1 3 further demonstrates the enhancement of according to their Voyager 1 Flight Data stratospheric emission from various hydroSubsystem Count (FDSC) numbers. The other fields are defined arbitrarily 250 and are used for reference. In Fig. 1, the paucity of points in field S between longitudes 0 and 90 ° is regrettable in view of the subsequent discovery (Paper II) of strong infrared activity there. Figure 2 shows the averaged spectra of 1 5 0 ~ these fields. They have been separated vertically for clarity. In all four spectra, prominent emission features are seen due to the 6oo coo Iooo 12oo 140o following stratospheric molecular species: WAVENUMBER(cm"~) C2H2 (acetylene), 737 cm-1; C2H6 (ethane), - 8 2 0 cm-1; CH4, 1307 cm -1. These emisFIG. 2. IRIS spectra of the four regions from Fig. 1, sion features originate at altitudes above separated vertically for clarity. Prominent features inthe - 2 - m b a r pressure level, where, as is clude C2H2 (737 cm-1), C2H6 (~820 cm-~), NH3 and shown below, the stratosphere is warmest. PH3 (900-1100 cm-l), and CH4 (1200-1350 cm-D. TABLE I

•

,

,

,

-

•

,

•

,

*

,

,

'

- -

v

4 0 0

~

,

,

236

KIM ET AL. THE NATURE OF THE EXCESS INFRARED

RADIATION C2H2v5 C2H4V7 f) ~ ~C3H41/9 ~ C2H6v9 ~

CH3D/'~

~11 ~

TI " I II

o

o

6oo

8oo

~ooo 12oo WAVENUMBER (cm -I)

~4oo

FIG. 3. Flux ratios of N1/S (top) and NI/N2 (bottom), showing molecular features enhanced near the north magnetic pole.

carbons in region N 1 compared to the other regions. The CH4 enhancement of N1 is clearly greater with respect to S than with respect to N2, even though the small number of available spectra in N2 (24) causes the bottom ratio in Fig. 3 to have a much lower signal to noise ratio at higher frequencies. This confirms the CH4 brightness temperatures quoted above from Fig. 2 and shows that the northern polar stratosphere away from the magnetic pole was warmer than the southern one in 1979. Caldwell et al. (1979) have presented an explanation for this in terms of slight seasonal effects on Jupiter at the time of Voyager encounter. In summary, the Voyager IRIS data confirm an infrared brightening in the N 1 field, both with respect to similar southern latitudes (S) and with respect to similar northern latitudes at different longitudes (N2). In the following section, we present arguments that the excess emission is mainly thermal, even though it is associated with the magnetic features of Jupiter. In the subsequent section we present quantitative models of molecular abundances determined from the enhanced emission evident in Fig. 3. Finally, we discuss the plausibility of one of the newly discovered trace molecule identifications, C6H6.

An implicit assumption of this series of papers is that the infrared polar brightenings are due to the precipitation of energetic particles from the Jovian magnetosphere. This is founded on the observation that the effect is confined to fixed longitude ranges in System III (1965) coordinates, near the magnetic poles in respective hemispheres. An alternate class of models, with an interhal energy source that is fixed in System III longitude and that somehow deposits energy in the Jovian polar stratosphere, cannot be ruled out on the basis of our observations to date. However, we have no means to evaluate such models, and will continue to follow our working hypothesis that the energy source is the particle flux from the Jovian magnetosphere. The excess emission could be due to direct collisional excitation of hydrocarbon molecules in the Jovian stratosphere by the incident particles, or to secondary excitation through collisions between the hydrocarbons and more numerous H2 molecules, after the latter had undergone collisions with the high-energy particles. In the second alternative, multiple collisions would lead to the expectation that the excess energy has been thermalized, whereas in the former, this would not be so. As demonstrated in the previous section, C2H2 and CH4 both show excess emission near the north magnetic pole. C2H2 is a nonequilibrium substance that is undoubtedly derived from dissociation of CH4. Infrared observations (this work; Orton and Aumann, 1977), ultraviolet observations (Wagener et al., 1985), and theoretical considerations (Strobel, 1973) all indicate that it should be less abundant than CH4 by approximately five orders of magnitude in the stratosphere. Figure 3 indicates that the enhanced flux of CH4 emission is indeed greater than that of C2H2, but not by five orders of magnitude. Assuming that the collision cross sec-

IR JOVIAN POLAR BRIGHTENING

.01

tions for C2H2 and C H 4 a r e similar, we therefore suspect that the C2H2 emission per molecule is being favored over

N|

(This~.

CH4.

The most simple mechanism by which this can occur is that the source function for the radiation be the Planck function. At the frequency of the C2H2 Q branch, the Planck function for a typical Jovian stratospheric temperature of 170°K is a factor of 10 greater than at the CH4 Q branch. To proceed further with quantitative calculations, we must make some reasonable assumptions and approximations about the chemical composition and thermal structure of Jupiter's atmosphere. We begin by adopting a value for the CH4 mixing ratio of 2 x 10-3 (Gautier et al., 1982), which we assume is valid everywhere on the planet. That is, we assume that the concentration of CH4 is not altered appreciably by chemical activity or by condensation. We derive a temperature-pressure model for the atmosphere consistent with the N1 spectrum of Fig. 2 as follows. We neglect any spatial variation within N1, a major but necessary approximation, and we assume an effective emission angle of 60° . We compute emission spectra for various candidate temperature profiles. We then make a constrained fit, in spectral regions controlled by H2 and CH4 opacities, of these computed spectra with the N1 data shown in Fig. 2. The best fitting profile is shown in Fig. 4. That profile has subsequently been verified by inversion of the N 1 IRIS spectrum using direct temperature sounding techniques. The computed spectra are based on the initial assumption (investigated below) that the atmosphere is in LTE, so that the source function for the emitted radiation is the Planck. function. The isothermal character of the stratosphere above the 1-mbar level is assumed; a temperature of 185°K fits the observations (with an uncertainty of a few °K). The upper region of the profile is determined by the CH4 opacity from 1200 to 1350 cm -t, and the lower region is deter-

237

I E

Ld n(~ IC UJ D~ DIOC

=~ i o l .

1979)

tO0080 I00 120 140 160 t80 200 220 TEMPERATURE (K)

FIG. 4. Temperature profile derived from the N I spectrum of Fig. 2, with CH4 and H2 mixing ratios of 2 x 10-3 and 0.90, respectively. Also shown is a midlatitude profile derived by Hanel et al. (1979).

mined by the H2 pressure-induced S(0) and S(1) lines from 300 to 600 cm -l. An He mixing ratio of 0.10 is adopted (Gautier et al., 1981). We assumed an equilibrium mixture of ortho- and para-H2. Also shown in Fig. 4 is the published profile for the latitude 15° south (Hanel et al., 1979) which corresponds reasonably well to our region W. We next compute the expected flux ratio for the regions N 1 and W, which we compare to the observed ratio in Fig. 5. There is a pronounced asymmetry in the band ratio, in the sense that the higher frequency wing is relatively brighter than the low-frequency one. In the model, this results from the spectral nature of the assumed Planck source function. Because of the higher temperature associated with the polar stratosphere (185°K) compared to the near-equatorial one (170°K), the decrease of the Planck function with increasing frequency is not as rapid for the numerator in Fig. 5 (N1) as it is for the denominator (W). Figure 5 and the earlier discussion in this section therefore suggest that the assumption that the radiative source function is

238

KIM ET AL.

has been possible to detect thermal emission from trace constituents in the stratosphere (e.g., Hanel et al., 1979). Because the polar stratosphere is warmer than the equatorial stratosphere, the polar strato_0 )-. sphere provides a better medium for detecting trace constituents. The nonlinear charx acter of the Planck function at Jovian stratospheric temperatures and in the spectral region from 600 to 1400 cm -1, where many interesting molecules have bands, ~ooo .oo 12oo laoo ~4oo further enhances molecular detectability in WAVENU~BER lore-l) the polar stratosphere of Jupiter with reFIG. 5. Flux ratio for the regions NI/W. Tempera- spect to other latitudes. ture profilesfrom Fig. 4 and a CH4mixingratio of 2 × In deriving the temperature profile in Fig. 10-3 have been used to compute line-by-line spectra 4, we have already adopted a CH4 mixing for this ratio spectrum. ratio of 2 × 10-3 (Gautier et al., 1982). given by the Planck function is c o n s i s t e n t Therefore, there is no further information with the available observations. In the fol- available regarding CH4 in our data. The lowing section, we continue with this as- first molecule for which we derive indepensumption and with the derived temperature dent quantitative information is CH3D. CH3D has different molecular symmetry profile in Fig. 5 to determine molecular than CH4 has, so it has additional vibration abundances in the polar stratosphere of rebands which are not superimposed on CH4 gion N1. spectral features. Its chemical and physical One could test the assumption of LTE with ultrahigh spectral resolution observa- properties are otherwise similar to CH4. It tions of individual rotational lines in the should therefore be uniformly mixed molecular bands, because fully resolved through the Jovian atmosphere, as c n 4 is, lines in thermal emission have well-defined and it should be independent of the details half-widths. Kostiuk et al. (1983) made of CH4 chemistry, contrary to C2H2 and such an attempt for C2H6with their ground- other molecules. Since the abundance of based heterodyne spectrometer. Unfortu- CH3D has been extensively investigated innately, they found that differential Doppler dependently by others (Knacke et al., 1982; broadening within their field of view due to Kunde et al., 1982), it can be used as a test Jovian rotation was the dominant broaden- of our temperature profile in Fig. 4. As indicated in Fig. 3, there is a small ing mechanism at 830 cm -1 and latitude 70° south. In their analysis, they ultimately emission feature at 1156 cm -1, correspondadopted an LTE formulation, as we did. ing to the v6 fundamental of CH3D. We Note, however, that the same group had have computed the expected emission from earlier interpreted high-resolution observa- CH3D for various trial mixing ratios, using tions of individual NH3 lines as indicating a line-by-line radiative transfer calculation. nonthermal emission from that molecule Our calculation was performed at sufficiently high resolution to resolve the molec(Kostiuk et al., 1977). ular lines, and the results were then deMOLECULARABUNDANCES IN THE POLAR graded to IRIS resolution (4.3 cm -1) for STRATOSPHERE comparison to the observations. Molecular Because the Jovian stratosphere is parameters for CH3D and for other moleviewed against the background of the cules are summarized in Table II and in the opaque and relatively cold troposphere, it Appendix. .

.

.

.

i

,

,

,

,

i

,

,

,

.

J

.

.

.

.

239

IR JOVIAN POLAR BRIGHTENING TABLE II MOLECULAR BAND PARAMETERS

Molecular band

Band center (cm ~)

Band intensity (cm-l atm t at 300°K)

Lorentzian linewidth (cm-1)

Reference

CH4 v4 CH3D v6 CzH4 v7

1305 1156 949 633 674 737 820 628 748

134 62 356 -+ 4 300 -4-_20 378 -+ 15 729 -+ 28 31 -+ 1.5 -2500 9.1 - 0.4

0.075 (300//) 0.5 0.075 (300//) °'5 0.09 (300//) 0.5 0.09 (300//) 0.5 0.09 (300//3°'5 0.09 (300//) °5 0.09(300//) 0.5 0.09 (300//) °'s 0.06(300//) °5

Orton and Robiette (1980) Pinkley et al. (1977) Smith and Mills (1964) Thomas and Thompson (1968) Kauppinen et al. (1980) Palmer et al. (1972) Daunt et al. (1981) Hardwick et al. (1979) Gayles and King (1965); Kondo and Saeki (1973)

C3H4 C6H6 C2H2 C2H6 C4H2

1'9 /-'11 /:5 /'9 /-'g

C3H8 uz6

W e derived a CH3D/H2 mixing ratio of 5 •r~+5.0 u-3.0 × 10 -7. With our a s s u m e d CH4 mixing ratio o f 2 × 10 -3 and with a chemical fractionation factor of 1.37, this corresponds to a D / H ratio o f 4.6-2~8 +4 6 × 10 -5. Using the s a m e molecular band and similar a s s u m p t i o n s , K u n d e e t al. (1982) derived D / H = 3.6+_~° × 10 -5 for the Jovian e q u a t o r f r o m I R I S data, and K n a c k e e t al. (1982) derived 3.8 --- 0.6 × 10 -5 f r o m ground-based spectroscopy. The relatively large errors in our result include uncertainties in the t e m p e r a t u r e profile and the effects o f strong underlying absorption lines of t r o p o s p h e r i c NH3. While these effects were also present for the other investigators cited above, our result has an important difference. T h e y o b s e r v e d the p l a n e t ' s e q u a t o r and found CH3D to be in a b s o r p t i o n . This is b e c a u s e the mixing ratio of CH3D is so low that it is almost t r a n s p a r e n t in the stratosphere. In the t r o p o s p h e r e , where the column abundances are greater and the individual lines are m o r e p r e s s u r e b r o a d e n e d , the molecular opacity b e c o m e s greater. H o w e v e r , since the t e m p e r a t u r e is increasing with depth in the t r o p o s p h e r e , an a b s o r p t i o n feature with r e s p e c t to the adjacent continu u m results. In our o b s e r v a t i o n of the N1 region, enhanced stratospheric emission f r o m CH3D

fills in the underlying absorption by tropospheric CH3D and p r o d u c e s a small residual emission feature (Fig. 3). The enhancem e n t of stratospheric emission is due to two factors: the higher stratospheric temperature in N1 and the greater optical depth for near-limb observations. The spectral c o n t r a s t is reduced greatly, so our uncertainties are relatively m u c h larger. O u r results, therefore, do not i m p r o v e o u r k n o w l e d g e of the v e r y important D/H ratio, but they do indicate that our procedure is consistent with previous work. The polar t e m p e r a t u r e profile of Fig. 4 is thus verified to the extent possible with these data. F o r o t h e r molecules, such as C2H2, uncertainties in the details of the chemical or p h o t o c h e m i c a l p r o c e s s e s b y which they are created and d e s t r o y e d , and also the possibility o f spatial variations of such molecules on Jupiter, preclude independent tests of the t e m p e r a t u r e profile without additional information. In the following, we retain the uncertainties o f our t e m p e r a t u r e profile within the error bars of c o m p u t e d a b u n d a n c e s of other molecules and search for spatial variations o f molecular abundances. Our results are s u m m a r i z e d in Table III. F o r C2H2, we derive a mixing ratio of 9 --2 × 10 -8 for the north polar stratosphere n e a r the magnetic polar (NI region). W. M.

240

KIM ET AL. T A B L E 111 S P A T I A L V A R I A T I O N IN M I X I N G R A T I O

North polar

South polar

auroral

nonauroral zone

Midlatitude

Remarks

zone

CH 4

2 × 10 3

C2H2

9 -+ 2 x 10 -8

C2H6

5 +- 1 × 10 6

2×10

3 × 10 -8 (IRIS) 1-3 × 10 6 (IRIS and ground-based)

2.5+~ × 10 9 <3 × 10 l0

C3H8

< 6 × 10 7

2×10

3

3 × 10 -8 (IRIS and IUE) 2-3 × 10 6 (IRIS and

uvs) <1 × 10 9 (IUE) <1 × 10 to (IUE)

C2H4 7 +- 3 × 10 9 C6FI6 2+~ × 10 9 C3H4 C4H2

3

m

<7 × 10 l0 3 -+ 2 × 10 l0 (IUE)

Maguire (1985, private communication) has calculated a mixing ratio for C2H2 at midlatitude and south polar regions of 3 × 10 -8 from his study of IRIS data. Furthermore, Wagener et al. (1985) have derived an equatorial stratospheric mixing ratio of - 3 × l0 -8 from their I U E satellite observations of Jupiter below 200-nm wavelength, in excellent agreement with Maguire's results, and significantly below the derived value for N I. Also using I U E data, Clarke et al. (1982) found more absorption at the Jovian north polar limb than was evident at the equator in the spectral range (170-200 nm) where C 2 H 2 is a strong absorber. Since their spatially resolved observations were made to observe auroral features, their longitude range was probably similar to our field N 1. This is further independent evidence that the relative abundance of C 2 H 2 n e a r the north magnetic pole is greater than elsewhere on Jupiter. The north magnetic polar enhancement o f C 2 H 2 emission thus appears to be a real abundance effect, not merely an artifact of the temperature profile, since the ultraviolet results are essentially independent of temperature.

Gautier et al. (1982); A s s u m e d to be uniform over the planet, as is CH3D Maguire (1985, private communication) for IRIS values Maguire (1985, private communication) for IRIS values

Error bars include systematic effects; IR detection exceeds noise by 10tr Both values are from this work UV detection; Gladstone and Yung (1983)

F o r C2H6, our derived mixing ratio is 5 - 1 x 10 -6 for N1. A t r e y a et al. (1981) have found a C 2 H 6 mixing ratio of 2.5+-2~° × 10 -6 at midlatitudes at a pressure level of 5/.,bar using the Voyager UVS stellar occultation results. Kostiuk et al. (1983) have made ultrahigh spectral resolution infrared observations of C2H6 at southern polar latitudes and find C2HdH2 = *.; , ~+1.2 o.6 x 10 -6. Maguire (1985, private communication) has also determined the C2H6 mixing ratio to be from 2 to 3 × l0 -6, with some indication of an increase from equator to latitude -+60 ° . The totality o f the infrared and UVS information suggests that C2H6 is enhanced in the field N 1 with respect to other regions on Jupiter by a relative amount similar to the e n h a n c e m e n t of C 2 0 2 . Ultraviolet reflection observations (as opposed to occultations) are not as useful for C2H6 as for C2H2 because C2H6 absorption occurs at shorter wavelengths, where the solar continuum flux is very low and where H2 emissions dominate. We now turn to a molecule which has not previously been observed on Jupiter: C2H4. In Fig. 3, particularly in the top ratio, there is strong evidence for emission from the C2H4 /)7 fundamental at 949 cm-J in the N l

IR JOVIAN POLAR BRIGHTENING llg

a

"

'

"

'

"

'

"

'

I19

"

.

,

.

,

.

,

.

118 #

'

j

117

i 1 7

116 600

,

b

C6H6 u,,.~ 2/c,~ 118

•

241

620

640 660 Frequency (cm -1)

680

700

116 600

' 620

' . . . . . 640 660 680 Frequency (cm -1)

700

FIG. 6. (a) The IRIS spectrum for region N1, with the features due to C3H4 and C6H6. Model spectra corresponding to abundances listed in Table III are shown with dashed lines. A very tentative identification of emission by CH3 at 607 cm 1 is also shown. The latter must be regarded as extremely uncertain because of the existence of other comparable, unidentified features. (b) The IRIS spectrum for a subset of low-latitude spectra, selected to have emission angle >-60°. There is no indication of real emission features at 633 or 674 cm-L Upper limit calculations, as given in Table III for C3H4 and C6H6, are shown by dashed lines.

region. This fundamental vibration produces a perpendicular type band; to model it we include K-type doubling appropriate for a slightly asymmetric top molecule. Our derived mixing ratio for N1 is 7 + 3 x 10 -9. This abundance is consistent with an upper limit of 3.3 × 10-8 derived from high spectral but low spatial resolution spectroscopy of Jupiter near 5/zm by Treffers et al. (1978). Using our mixing ratio with the midlatitude Voyager profile shown in Fig. 4, we do not produce a recognizable spectral feature in synthetic spectra. We set I x 10-8 as the upper limit there with great uncertainty because of the predominance of tropospheric NH3 absorption near 949 cm-I.

A stronger limit for the C2H4 abundance at low latitudes comes from the IUE results of Wagener et al. (1985) between 165 and 175 nm. They set an upper limit of - 1 × 10 -9 for the C2H4 mixing ratio near the equator. It therefore appears as if C2H4 is alSO enhanced in N1 with respect to other regions, and that the relative enhancement may be somewhat greater than for C2H2 and C2H6.

We next investigate two small features apparent in Fig. 2 at 633 and at 674 c m -1 . This part of the spectrum of field N1 is replotted as a brightness temperature spec-

trum in Fig. 6a. There, the features are much more apparent and are clearly seen to surpass the noise in the data. The strong increase near 700 cm -~ is due to the v5 band of C2H2 discussed above. The error bars in Fig. 6a represent empirical ---lo- deviations from the mean of the 87 spectra. They are calculated from the noise-equivalent spectral radiances of Voyager I. The feature at 633 cm -I is identified as the v9 band of methyl acetylene, C3H4, a probable result of CH4-C2H2 dissociation chemistry. We do not see the vm band at 327 cm -~, but since its intrinsic strength is a factor of 5 below that of the v9 band, this does not represent a contradiction. The v9 band has been tentatively identified in emission at the south pole of Saturn, and, with much greater certainty, on Titan (Hanel et al., 1981). We have modeled the emission in a fashion similar to C2H4 and CH3D. We find a mixing ratio of C3H4/H2 = 2.5+-2 x 10-9. Uniform vertical distribution throughout the stratosphere is assumed. This fit is shown by a dashed line in Fig. 6a. Again, the uncertainty in the mixing ratio includes the effects of possible errors in the temperature profile of Fig. 4, so that the reality of the feature itself is much better than the

242

KIM ET AL.

t o t a l error in the mixing ratio. The derived

mixing ratio is compatible with the high spectral resolution, ground-based spectroscopy of Treffers e t al. at 5 p,m, which established an upper limit of 8 × l0 -9. No other infrared studies of this molecule on Jupiter have been made. Furthermore, the W field is not optimal for quantitative study because it contains a variety of emission angles. To improve quantitative results at midlatitudes, we expand our data set to include all Voyager 1 spectra consistent with the previously stated spatial resolution criterion and which also satisfy the restrictions (i) Ilatitudel -<40°, (ii) emissson angle ---60°, (iii) Tb at 1306 cm -~ --> 160°K. The first condition effectively increases the number of available spectra, while excluding polar regions, to maximize the signal to noise ratio in the average spectrum for midlatitudes. The second condition mimics the high-emission angle for field NI and enhances the detectability of trace species. The third excludes spectra which accidentally include deep space in their fields because of small pointing errors near the limb. A total of 332 spectra meet these conditions. The average of these spectra is plotted in Fig. 6b, together with noise-equivalent spectral radiance errors (_ lo-). To test whether this increase in the latitude range of included spectra compromises the information, we have segregated the spectra into smaller latitude ranges not shown here. This does not produce systematic differences between average spectra over the spectral range of Fig. 6b, so we conclude that the midlatitude profile from Fig. 4 can be used to compute trace gas emission spectra for Fig. 6b. Also shown in Fig. 6b is a model emission spectrum for a mixing ratio of 7 × 10-~° for C3H4, which we regard as an upper limit in the midlatitudes of Jupiter. Because of the kink in the continuum just above 630 cm -1, this upper limit cannot be reduced further, but it is still significantly below the

value for the field N1. Thus, C3H4 also seems to be enhanced in N1 with respect to other parts of Jupiter. The ultraviolet midlatitude upper limit of Wagener et al. (1985) for C3H4 is 2 × 10-9, which is not as sensitive as our infrared limit. Next, we identify the feature at 674 cm as the uH fundamental band of benzene (C6H6). This band has not previously been identified in any extraterrestrial site to our knowledge. It is n o t at the frequency of the nearby strong band of COz (667 cm-1). C6H6 has a hexagonal planar configuration, thereby resembling an oblate symmetric top. The vii band is a parallel band and the strongest infrared band of C6H6. Using a line-by-line computer simulation of this band, we derive a mixing ratio of 2+] × 10-9 for C6H6 for N1. The fit is shown by a dashed line in Fig. 6a. As is the case for C3H4, the reality of the feature itself greatly surpasses the formal uncertainty in the derived abundance, which includes possible errors in the temperature profile. The dashed line in Fig. 6b shows the emission for a mixing ratio of 2 × 10-1°, which we regard as the midlatitude infrared upper limit for C6H6. The midlatitude ultraviolet upper limit of Wagener et al. (1985) is 1 × 10-~°. Clearly, C6H6 is another example of enhanced abundance in the N I field. We also sought evidence for diacetylene (C4H2) and propane (C3H8) in the N1 spectrum at 629 and 748 cm -1, respectively. The latter has been tentatively identified on both Saturn and Titan by Hanel et al. (1981). The former is a possible by-product of C2H2 photochemistry (Okabe, 1981), and it has been observed in the north polar region of Titan (Hanel et al., 1981). Neither was found in the atmosphere of Jupiter. The upper limits for the mixing ratios were 3 × 10-l° for C4H2 and 6 × 10-7 for C3H8. There are two reasons for the great difference in these upper limits. First, C3H8 overlaps the strong u5 band of C2H2 centered at 737 cm -I. Second, the intrinsic strength of the C3H8 band is very low (see Table II).

IR JOVIAN POLAR BRIGHTENING

243

These upper limits are included in Table ter (Hanel et al., 1980), and ~ is the number III, which summarizes all the quantitative of included spectra (87). spectroscopy discussed in this section. The We also calculated the standard deviaupper limit for C4H2 is consistent with the tion from the mean, using data from the ind e t e c t i o n at a mixing ratio of 3 ± 2 × 10-1° dividual spectra. The second method gave for midlatitudes claimed by Gladstone and standard deviations that were 10 to 20% Yung (1983) using IUE data at 160 nm. bigger than the first method did, but the secNote that Wagener et al. (1985) have dis- ond method also includes the effect of poscussed alternate interpretations of similar sible small changes in the background level. IUE data. Since the background in this region is due Wagener et al. (1985) have found a mix- to the wing of the very broad, pressure-ining ratio for allene (an isomer of C3H4) of duced S(1) line of H2, it cannot introduce 0.7 ± 0.4 × 10 -9 for midlatitude regions narrow features that could be mistaken for using the IUE. They also claim that a mix- vibration-rotation bands of trace species. ing ratio of 8 x 10-9 for cyclopropane However, background shifts from one (C3H6) improves the model fit to their data spectrum to the next could increase the calnear 160 nm, but it is not unique. These culated standard deviation. molecules are not well suited to an infrared We therefore used the noise-equivalent study of trace features because of the inter- radiance method for plotting the error bars ference of more abundant molecules. The in Figs. 6a and b, although the two methods strongest band of allene is confused with are in such good agreement that this choice the probable (H2)2 dimer feature at 350 cm -1 does not significantly influence the question (see Fig. 9 of Wagener et al.) and the of whether a particular feature is real or due strongest band of C3H6 is obscured by to noise. It is of some interest to note that the size C2H6. We have not attempted quantitative analyses of these two molecules here. of the small features in Fig. 6a are someWe conclude this discussion of molecular what larger with respect to those of Fig. 6b abundances by noting that there are several than would be expected from the respective other small peaks evident in Fig. 6a, in ad- sizes of the error bars in each. Furtherdition to those for C3H4 and C6H6. These more, one of the largest of these small feaadditional features are generally compara- tures in Fig. 6a coincides with the freble in size to the typical error bars (± lo-) quency of the v2 fundamental of the methyl shown in the figure, so that it would be ex- radical (CH3) at 607 cm -l, as measured by tremely risky to draw conclusions from Tan et al. (1972). We emphasize the uncerthem. For example, in the region from 640 tainty of this suggestion by including the to 660 cm -~, it would be equally possible to question mark in the figure. claim small emission peaks at 647 and 658 Photochemical models by Strobel (1973) cm- l, or an absorption feature at 651 cm- 1. indicate that at the pressure level -0.03 The error bars in Figs. 6a and b were esti- mbar, the abundance ratio [CHa]/[C2H6] mated in two ways. First, the instrumental 10-3 (see his Fig. 3). According to our Table noise was determined from the formula III, this would correspond to a [CH3]/[Hz] mixing ratio - 5 x 10 -9. The concentration NER of CH3 is extremely dependent on pressure. AI=-,01/2 , At 0.003 mbar in Strobel's models, where the more abundant trace molecules such as after which the intensity, I, was converted C2H6 are beginning to be depleted by phototo a temperature using the Planck function. dissociation and by diffusive separation In this formula, NER is the noise-equiva- (Atreya et ai. 1981), CH3 is one of the most lent radiance, determined by observing abundant hydrocarbons. At 3 mbar, its deep space near the time of Jupiter encoun- abundance relative to C2H6 has decreased

244

KIM ET AL.

to ~ I 0 -6 and thus [CH3]/[H2] - 5 × 10-j2. In summary, its abundance is expected to be comparable to those of C3H4 and C6Hr, as given in Table III, over much of the vertical extent of the warm stratosphere in our model atmosphere (Fig. 4). Because of the extreme reactivity of CH3, there have been no quantitative measurements of its infrared band strengths. However, if the strength of the v2 fundamental is comparable to the other bands listed in Table I, then the discussion in the previous paragraph would lead to the expectation that its emission feature would be comparable to the emissions from C3H4 and C6H6. Within a factor of 3, this is entirely consistent with Fig. 6a. Other hydrocarbon radicals, such as CH, CH2, C2H3, and C2H5, are probably too scarce to be detectable (Strobel, 1973).

at least a factor of 7 (Table III). If the relative abundances of the smaller precursors is a reliable indicator, then the Jovian polar aerosol may be predominantly cuprene. Experimental support for this conjecture of linkage between C6H6 and cuprene is described below. The earliest suggestion that cuprene is present in the atmospheres of the outer planets was by Urey (1959). The composition of the aerosol is of wider interest than the environments of the outer planets. It is important with respect to such diverse topics as the fundamental properties of hydrocarbons, the microphysics of clouds, and the origin of life. However, it may be extremely difficult to determine the aerosol composition directly, because of the lack of observed spectral features. It therefore seems worthwhile to examine the chemical plausibility of the claimed detection of C6H6 near Jupiter's north magnetic pole. THE CASE FOR BENZENE The presence of C6H6 on Jupiter has not been predicted by chemical modeling, but, Of all the species discussed in the precedin retrospect, it is at least qualitatively coning section, the most intriguing is C6H6. It is comparable in molecular weight to the larg- sistent with other aspects of Jovian chemisest known interstellar molecules. It is the try. There is an extensive literature, over largest molecule ever identified in a plane- the past six decades, which describes the tary atmosphere, except for the ubiquitous natural production of C6H6, at high effiaerosol that darkens the ultraviolet reflec- ciency, from the polymerization of C2H2. It tivities of Jupiter, Saturn, and Titan. Since is well documented that when C2H2 molethe aerosol is more concentrated at polar cules in a concentrated mixture become exlatitudes on Jupiter than at lower latitudes cited, trimerization to C6H6 is one of the (West et al., 1981), as are C6H6 and several most common results. Furthermore, virtually any conceivable other molecules at some longitudes (Table III), these substances may all be generically energy source suffices to start the chemistry. Among the published techniques are direlated. The abundance of C6H6 relative to C4H2 rect photolysis; indirect photolysis (i.e., is potentially of great importance in under- production of hot H atoms first, which substanding the detailed chemistry of the aero- sequently drive the reaction); bombardment sol. This is because these two molecules by electrons, protons, or alpha particles; are the smallest stable respective examples surface catalysis; y- or X-irradiation; and of polymers of the forms (C2H2)n, also pyrolysis. Of these, the first four at least known as cuprene, and CznH2. Either of may be relevant to Jupiter. Many uncertainties remain in underthese types of particles may result from the polymerization of CzHz, which is present in standing this phenomenon. For example, significant amounts on Jupiter (Fig. 2). Our there is still some question about the spefinding is that the abundance of C6H6 at high cific form of the excited C2H2 molecule altitudes exceeds that of C4H2 in field N 1 by which initiates the reactions. Furthermore,

IR JOVIAN POLAR BRIGHTENING there has been very little work on the chemistry of C2H2 polymerization in dilute mixtures, such as the Jovian stratosphere. The probability of quenching the excited CEH2 before it can encounter a second C2H2 is high, and means that one cannot make a general application of existing laboratory data to the Jovian environment. In the remainder of this section, we review a few of the papers which have described C6H6 formation from C2H2, with emphasis on those that may be of interest for planetary atmospheres. Dunicz (1941) found that pure C2H2 irradiated with ultraviolet light below 235-nm wavelength produced a yellow powder, similar to cuprene, and a small amount of gaseous C6H6. Both by-products increase steadily with time during the experiment. Intermediate products, which do not increase with time, are also observed spectroscopically. Other photolysis experiments, primarily using light at shorter ultraviolet wavelengths, give diverse results. Some find evidence for C6H6 (Stief et al., 1965) and some emphatically do not Orion and Kompa, 1982; Okabe, 1981, 1983a, 1983b). When C6H6 is not found, or when it is a minor product, C4H2 is generally observed prominently. The process clearly depends critically on the details of the experiment. From the wide range of published ultraviolet results, it is quite difficult to identify the path that would occur for Jupiter's atmosphere. However, if ultraviolet photons are the agent which causes C2H2 polymerization, then it is clear that a process driven by photons above 200-nm wavelength will be preferred over a process initiated at shorter wavelengths, because of the shape of the solar spectrum. In this regard, the laboratory procedure of Dunicz (1941) may approximate the Jovian environment better than the others. We next consider chemistry initiated by energetic particles. It has long been known that energetic charged particles can induce polymerization of C2H2 (e.g., Garrison, 1947, and references therein), to form patti-

245

cles of the cuprene type. Also, hot hydrogen atoms can initiate a variety of interesting chemistry, starting with very simple molecules (Hong et al., 1974). Either process could be relevant to the polar stratospheric environment of Jupiter, as direct or secondary energy sources from the magnetosphere. Just as the products of photochemistry depend on the details of the experimental procedure, so do the products of particle irradiation. For example, Willis et al. (1977) report that, at relatively high dose rates, the dominant products of C2H2 irradiation by 600-keV electrons are C4H2 and C4H4, with significant but smaller yields of hydrocarbons from C3 to C9. This is in contrast to low flux irradiation, where C6H6 and cuprene are the e x c l u s i v e products. It is of interest to note that both the photochemical experiment of Dunicz (1941) and the particle experiment of Willis et al. (1977) link C6H6 and cuprene, supporting the conjecture on this question earlier in this section. Field (1964) has investigated the results of diluting C2H2 with both H2 and Ar and bombarding the mixtures with mega-electronvolt electrons. Again, C6H6 is a primary product, and remains so as the dilution approaches 94% H2 + Ar, the lowest concentration of C2H2 that Field considered. A nondetection of C6H6 was reported by Scattergood and Owen (I 977). In an explicit attempt to mimic the natural environment of the outer Solar System, they bombarded mixtures of CH4 and H2 with 3-MeV protons. They succeeded in detecting various hydrocarbons from C2 to C5, including C4H2. They did not detect C6H6 and place an upper limit on its concentration of several tens of parts per million, about four orders of magnitude higher than the concentration found in this paper (Table III). It appears that the work of Scattergood and Owen does not represent the environment of N1 on Jupiter, not because they do not detect C6H6, but because they do produce so many compounds, for which we find no

246

KIM ET AL.

evidence, that are more abundant in their apparatus than C 6 H 6 . Further efforts to simulate Jupiter would clearly be of great interest. Of the work published to date, it appears that that of Field (1964) and of Willis e t al. (1977) are most relevant to Jupiter's polar atmosphere. It may be very significant that both used approximately mega-electronvolt electrons as their energy source. Additional theoretical studies are also required. The present paper does not address quantitatively such questions as chemical kinetics, exothermicity of relevant reactions, and the vertical distribution of energy sources in Jupiter's polar stratosphere. Work is currently in progress to attempt a direct confirmation of the C6H6 result with IUE polar spectra of Jupiter. Unfortunately, this will be hampered by the enhanced absorption at Jupiter's poles by other, more abundant molecules, such as C2H2 (Clarke e t al., 1982), which have overlapping ultraviolet absorption bands. If the IUE spectra are inadequate, it should be possible to attempt confirmation with the Hubble Space Telescope within 1 or 2 years. Observations with it should be significantly superior to those of the IUE with respect to both spatial and spectral resolution and signal to noise quality. Finally, an infrared confirmation would be very helpful. It must be emphasized that the detection illustrated in Fig. 6a is based only on the central frequency and width of the feature. Sufficient spectral resolution to show band structure is needed, together with at least modest spatial resolution, of order 6 arcsec. Such an observation would require a sophisticated space facility, such as the proposed NASA Space Infrared Telescope Facility, which unfortunately will not be available for many years. APPENDIX: MOLECULAR BAND MODELS

For the 1'21 band of C3Hs, the v5 band of C2H2, the 1"4band of CH4, and the 1,'6 band of CH3D, we used the molecular band models from Table I of Kim and Caldwell (1982).

See the Appendix II of Kim and Caldwell for the construction of the C3Hs bands. We directly adopted line-by-line band models for the Vl~ band of C6H6 and the v9 band of C2H6 in Kauppinen e t al. (1980) and Daunt e t al. (1981), respectively. For the v7 band of C2H4, the b'9 band of C3H4, and the v8 band of C4H2, however, satisfactory band models for our purpose are not in the literature. We first constructed the v7 band of C2H4 using molecular constants of Smith and Mills (1964). This band is a perpendicular type band of a slightly asymmetric top molecule and has a similar structure to the v2t band of C3H8. Therefore, we constructed the C2H4 band using the same computer code as that of the C3H8 band. The center of the C2H4 band, however, may be significantly influenced by hot bands at room temperature. Since the hot bands are suppressed in the cold environment of Jupiter, high-resolution and low-temperature measurements of this band will be particularly important to separate the fundamental band from the hot bands. The hot band problems in the v4 band of C3H4 and the v8 band of C4H2 are more significant than in the C2H4 band, because hot band intensity is proportional to the Boltzman factor and the bands of C3H4 and C4H2 are located at longer wavelength than the C2H4 band. Ultimately, the uncertain hot band influences on these molecules have been considered for estimating the uncertainties of the molecular mixing ratios. The v9 band of C3H4 is a perpendicular band of a symmetric top molecule. The band has been constructed using molecular constants of Thomas and Thompson (1968), except a constant aB = B' - B" = 0.001 cm -1. C4H2 is a linear molecule. The v8 band has a well-developed Q branch, which is, however, contaminated by hot bands. (Hardwick e t al., 1979). The least wellknown quantity of this band is total band intensity, which is thought to be greater than 2000 cm -2 atm -j. The band intensity

IR JOVIAN POLAR BRIGHTENING 2500 cm -2 atm -~ has been used to estimate the upper limit mixing ratio of C4H2 in the north pole of Jupiter. ACKNOWLEDGMENTS SJK, JC, and ARR received support for this work under the NASA Jupiter Data Analysis Program through Grant NAGWI60. RW was supported at Stony Brook by NASA Grant NAG5275 from the IUE project. GSO was supported by the Galileo Project, the NASA Outer Planet Data Analysis Program, and by NASA Contract NAS 7-100 at JPL. SJK is now with Sigma Data Corp., a M/A-Corn Information System, Inc. We thank an anonymous graduate student for discussions on the C2H 2 polymerization problem. REFERENCES ATREYA, S. K., T. M. DONAHUE, AND M. C. FESTOU (1981 ). Jupiter: Structure and composition of the upper atmosphere. Astrophys. J. 247, L43-L47. CALDWELL, J., R. D. CESS, B. E. CARLSON, A. T. TOKUNAGA, F. C. GILLETT, AND I. G. NOLT (1979). Temporal characteristics of the Jovian atmosphere. Astrophys. J. 243, L155-L158. CALDWELL, J., A. T. TOKUNAGA, AND F. C. G1LLETT (1980). Possible infrared aurorae on Jupiter. Icarus 41, 667-675 (Paper I). CALDWELL, J., A. T. TOKUNAGA, AND G. S. ORTON (1982). Further observations of 8/zm polar brightenings of Jupiter. Icarus 53, 133-140 (Paper II). CLARKE, J. T., H. W. Moos, AND P. D. FELDMAN (1982). The far-ultraviolet spectra and geometric albedos of Jupiter and Saturn. Astrophys. J. 255, 806818. CONNERNEY, J. E. P., M. H. ACUNA, AND N. F. NESS (1981). Modeling the Jovian current sheet and inner magnetosphere. J. Geophys, Res. 86, 8370-8384. DAUNT, S. J., W. E. BLASS, G. W. HALSEY, K. Fox, AND R. J. LOVELL (1981). High-resolution infrared spectrum and analysis of the v9 band of ethane at 12.17 I~m. J. Mol. Spectrosc. 86, 327-343. DUNICZ, B. L. (1941). The mechanism of the photochemical change of acetylene. J. Amer. Chem. Soc. 63, 2461-2472. FIELD, F. H. (1964). Benzene production in the radiation chemistry of acetylene. J. Phys. Chem. 68, 1039-1045. GARRISON, W. i . (1947). On the polymerization of unsaturated hydrocarbons by ionizing radiation. J. Chem. Phys. 15, 78-79. "GAUTIER, D., B. BEZARD, A. MARTEN, J. P. BALUTEAU, N. SCOTT, A. CHEDIN, V. KUNDE, AND R. HANEL (1982). The C/H ratio in Jupiter from the Voyager infrared investigation. Astrophys. J. 257, 901-912. GAUTIER, D., B. CONRATH, M. FLASAR, R. HANEL, V. KUNDE, A. CHEDIN, AND N. SCOTT (1981). The

247

hefium abundance of Jupiter from Voyager. J. Geophys. Res. 86, 8713-8720. GAYLES, J. N., AND W. "F. KING (1965). The infrared spectrum of propane. Spectrochim. Acta. 21, 543557. GLADSTONE, (3. R., A N D Y. i. Y U N G (1983).A n analysis of the reflection spectrum of Jupiter from 1500 A to 1740 A. Astrophys. J. 266, 415-424. HANEL, R., D. CROSBY, L. HERATH, D. VANOOS, D. COLLINS, H. CRESWICK, C. HARRIS, AND M. RHODES (1980). Infrared spectrometer for Voyager. Appl. Opt. 19, 1391-1400. HANEL, R., et al. (1979). Infrared observations of the Jovian system from Voyager 1. Science (Washington, D.C.) 204, 972-976. HANEL, R., et al. (1981). Infrared observations of the Saturnian system from Voyager 1. Science (Washington, D.C.) 212, 192-200. HARDWICK, J. L., D. A. RAMSAY, J. M. GARNEAU, J. LAVlGNE, AND A. CABANA (1979). The infrared spectrum of diacetylene. J. Mol. Spectrosc. 76, 492-505. HONG, K,-Y., J.-H. HONG, AND R. S. BECKER (1974). Hot hydrogen atoms: Initiators of reactions of interest in interstellar chemistry and evolution. Science (Washington, D.C.) 184, 984-987. ImoN, M. P., AND K. L. KOMPA (1982). UV laser photochemistry of acetylene at 193 nm. Appl. Phys. B 27, 183-183. KAUPPINEN, J., P. JENSEN, AND S. BRODERSEN (1980). Determination of the B0 constant of CrHr. J. Mol. Spectrosc. 83, 161-174. KIM, S. J., AND J. CALDWELL (1982). The abundance of CH3D in the atmosphere of Titan, derived from 8- to 14-#m thermal emission. Icarus 52, 473-4.82. KNACKE, R. F., S. J. KIM, S. T. RIDGWAY, AND A. T. TOKUNAGA (1982). The abundances of CH4,, CH3D, NH3, and PH3 in the troposphere of Jupiter derived from high-resolution 1100-1200 cm -~ spectra. Astrophys. J. 262, 388-395. KONDO, S., AND S. SAEKI (1973). Infrared absorption intensities of ethane and propane. Spectrochim. Acta, Part A 29, 735-751. KOSTIUK, T., M. J. MUMMA, F. ESPENAK, D. DEMING, D. E. JENNINGS, W. MAGUIRE, AND D. ZlPOY (1983). Measurements of stratospheric ethane in the Jovian south polar region from infrared heterodyne spectroscopy of the 1,'9band near 12/zm. Astrophys. J. 265, 564-569. KOSTIUK, T., M. J. MUMMA, J. J. HILLMAN, D. BUHL, L. W. BROWN, J. L. FARIS, AND D. L. SPEARS (1977). NH3 spectral line measurements on Earth and Jupiter using a 10 /~m superheterodyne receiver. Infrared Phys. 17, 431-439. KUNDE, V., R. HANEL, W. MAGUIRE, D. GAUTIER, J. P. BALUTEAU, A. MARTEN, A. CHEDIN, N. HUSSON, AND N. SCOTT (1982). The tropospheric gas composition of Jupiter's North Equatorial Belt

248

KIM ET AL.

(NH3, PH3, CH3D, GeH4, H20) and the Jovian D/H ratio. Astrophys. J. 263, 443-467. OKABE, H. (1981). Photochemistry of acetylene. J. Chem. Phys. 75, 2772-2776. OKABE, H. (1983a). Photochemistry of acetylene at 1849 A. J. Chem. Phys. 78, 1312-1317. OKABE, H. (1983b). Photochemistry of acetylene. Canad. J. Chem. 61, 850-855. ORTON, G. S., AND G. AUMMAN (1977). The abundance of acetylene in the atmosphere of Jupiter. Icarus 32, 431-436. ORTON, G. S., AND A. G. ROBIETTE (1980). A line parameter list for the v2 and v4 bands of IzCH4 and I3CH4 extended to J' = 25 and its application to planetary atmospheres. J. Quant. Spectrosc. Radiat. Transfer 24, 81-95. PALMER, K. F., M. E. MICKELSON, AND K. N. RAO (1972). Investigations of several infrared bands of 12C2H2 and studies of the effects of vibrational-rotational interactions. J. Mol. Spectrosc. 44, 131-144. PINKLEY, L. W., K. N. RAO, G. TARRAGO, G. POUSSIGUE, AND M. DANG-NHu (1977). Analysis of the v6 band of 12CH3D at 8.6 izm. J. Mol. Spectrosc. 68, 195-222. SCATTERGOOD, T., AND T. OWEN (1977). On the sources of ultraviolet absorption in spectra of Titan and the outer planets. Icarus 30, 780-788. SMITH, W. L., AND I. M. MILLS (1964). Coriolis perturbations in the infrared spectrum of ethylene. J. Chem. Phys. 40, 2095-2109.

STRIEF, L. J., V. J. DE CARLO, AND R. J. MATALONI (1965). Vacuum-ultraviolet photolysis of acetylene. J. Chem. Phys. 42, 3113-3121. STROBEL, D. F. (1973). The photochemistry of hydrocarbons in the Jovian atmosphere. J. Atmos. Sci. 30, 489-498. TAN, L. Y., A. M. WINER, AND G. C. PIMENTEL (1972). Infrared spectrum of gaseous methyl radical by rapid scan spectroscopy. J. Chem. Phys. 57, 4028-4037. THOMAS, R. K., AND H. W. THOMPSON (1968). Vibration-rotation band of methyl acetylene. Spectrochim. Acta, Part A 24, 1337-1352. TREFFERS, R. R., H. P. LARSON, U. FINK, AND T. N. GAUTIER (1978). Upper limits to trace constituents in Jupiter's atmosphere from an analysis of its 5-/xm spectrum. Icarus 34, 331-343. UREY, H. C. (1959). The atmospheres of the planets. In Handbuch der Physik (S. Flugge, Ed.), Vol. 52, pp. 363-418. WAGENER, R., J. CALDWELL, T. OWEN, S. J. KIM, M. COMBES, AND TH. ENCRENAZ (1985). The Jovian stratosphere in the ultraviolet. Icarus 63, 222-236. WEST, R. A., C. W. HORD, K. E. SIMMONS, D. L. COFFEEN, M. SATO, AND A. L. LANE (1981). Near ultraviolet scattering properties of Jupiter. J. Geophys. Res. 86, 8783-8792. WILLIS, C., R. A. BACK, AND R. H. MORRIS (1977). Radiation chemistry of acetylene at high intensity: The initial product distribution. Canad. J. Chem. 55, 3288-3293.