Longitudinal variability of methane and ammonia bands on Jupiter

ICARUS 42, 102--110 (1980)

Longitudinal Variability of Methane and Ammonia Bands on Jupiter WILLIAM D. COCHRAN AND ANITA L. COCHRAN McDonald Observatory and Department o f Astronomy, University o f Texas at Austin, Austin, Texas 78712 Received N o v e m b e r 26, 1979; revised F e b r u a r y 14, 1980 W e have obtained a set o f 78 spectra o f the Jovian Equatorial Zone central meridian on 10 a n d 11 F e b r u a r y 1979. E a c h s p e c t r u m , covering the region from 6000 to 6600 A, w a s obtained in 9 min o f o b s e r v a t i o n and represents an average over 16° o f longitude a n d 11° o f lattitude. Jupiter w a s o b s e r v e d for over 9 hr each night, so we achieved complete longitudinal coverage o f the equator. T h e spectra were reduced to reflectivities. Equivalent widths o f the m e t h a n e 6190 A and a m m o n i a 6450-A b a n d s were m e a s u r e d for e a c h s p e c t r u m using a blind, a u t o m a t e d , self-consistent c o n t i n u u m determination system. Both b a n d s have large equivalent widths n e a r longitudes 35 and 340 ° , and small equivalent widths near longitude 150° . T h e m e t h a n e and a m m o n i a equivalent widths are well correlated with e a c h other. T h e c o n t i n u u m reflectivity is slightly anticorrelated with the equivalent width o f both bands. Calculations b a s e d on the m o d e l of M. Sato a n d J. E. H a n s e n ( 1979, J. Atmos. Sci. 36, 113-167), using a doubling-adding radiative transfer code, indicate that the o b s e r v e d longitudinal variability o f equivalent width and c o n t i n u u m reflectivity are the result of c h a n g e s in the altitude of the a m m o n i a cloud. I. I N T R O D U C T I O N

Previous studies of spatial and temporal variations of molecular absorptions on planetary disks have made a number of simplifying assumptions. One of the most crucial assumptions has been that of longitudinal homogeneity of the atmosphere. In most of the studies of center-to-limb variations of absorption lines (e.g., Bergstralh, 1972; Cochran et al., 1976; Avery et al., 1974) observations were made at various values of/x in a belt or zone without regard to the longitude observed. In most studies of long-term temporal variability (e.g., Trafton, "1977; Cess and Caldwell, 1979) spectra taken over several-year intervals at random longitudes were compared to determine seasonal climatic trands. A very clear danger in this approach is the possibility that longitudinal variations in molecular absorption might exist which could accidentally be interpreted as center-to-limb variations or as long-term seasonal variations. In fact, longitudinal variability of Jupiter is quite evident in the Voyager images (Smith et al., 1979) and in the 5-/zm maps of Terrile and Westphal (1977).

In this paper we present results of a systematic quantitative study of longitudinal variations in methane and ammonia absorption on Jupiter. A similar study was undertaken earlier by Hunt and Bergstralh (1977) who searched for variability in the H2 quadrupole lines. Their photographic spectra of the central meridian taken over a several month interval revealed substantial variability. In our study, we concentrated on obtaining high signal-to-noise ratio photoelectric spectra in a rapid time sequence to obtain complete longitudinal coverage of Jupiter on each night. Details of the observations are presented in Section II and the data set is discussed in Section III. The atmospheric models used to interpret the results are presented in Section IV, and our conclusions and recommendations for future observing procedures are given in Section V. II. O B S E R V A T I O N S A N D D A T A R E D U C T I O N

All data were obtained on the nights of 10 and 11 February, 1979 using the Big Cassegrain Scanner on the 2.1-m Struve reflector of McDonald Observatory. Each spectrum 102

0019-1035/80/040102-09502.00/0 Copyright © 1980by AcademicPress, Inc. All fights of reproduction in any form reserved.

JOVIAN LONGITUDINAL VARIABILITY was the sum o f m a n y rapid forward and reverse spectral scans b e t w e e n 6000 and 6600/~. E a c h s p e c t r u m with signal-to-noise ratio in excess of 300 was obtained in nine min of integration. The exit slit corresponded to 8-/k resolution, and the spectra were sampled e v e r y 3 A. The entrance aperture was a circular diaphragm with a diameter which projected to 4.02 arcsec on the sky. The aperture was centered on the Jovian disk, so the region o b s e r v e d was the central meridian of the Equatorial Zone (latitudes b e t w e e n +5.5 and -5°.5). During a 9-min observation, the central meridian would rotate by 50.5 of longitude, so each s p e c t r u m represents an average o v e r about 16° o f longitude. A total of 78 Jovian spectra were obtained on these two nights. We also regularly o b s e r v e d the M o o n to obtain solar c o m p a r i s o n spectra at matching airmasses. The s p e c t r o p h o t o m e t r i c standard stars H y a and 0 Crt were also o b s e r v e d to determine instrumental response. The standard stars were o b s e r v e d with an entrance aperature large enough to admit all of the light from the star. The airmass at which each longitude was o b s e r v e d is shown in Fig. 1. S y s t e m I

<~

103

longitudes are used throughout this p a p e r since we are dealing only with the Equatorial region. Nearly c o m p l e t e longitudinal coverage o f the planet was obtained on each night. Due to the rotational period of Jupiter, we were able to obtain at least one s p e c t r u m of each longitudinal region at low airmass. Standard s p e c t r o p h o t o m e t r i c techniques were used to reduce the spectra. We rem o v e d the a t m o s p h e r i c extinction f r o m all spectra using m e a n seasonal extinction coefficients for M c D o n a l d O b s e r v a t o r y . The o b s e r v e d fluxes were then c o n v e r t e d to magnitudes. The instrumental r e s p o n s e was determined for each night from the standard star observations using the recalibration of the s p e c t r o p h o t o m e t r i c standard stars by Cochran (1979). The instrumental response was then r e m o v e d f r o m all Jovian and lunar spectra, resulting in Jovian and lunar fluxes a b o v e the a t m o s p h e r e . Each Jovian s p e c t r u m was divided by the lunar s p e c t r u m which m o s t nearly matched its airmass. We then needed to determine the lunar reflectivity so that it could be r e m o v e d from these Jovian data. To do this, the lunar fluxes were divided by the solar irradiances

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COCHRAN AND COCHRAN

determined by Arvesen et al. (1969) to yield relative reflectivities of the lunar areas we observed. This method of determining the lunar reflectivity by division by published solar irradiances introduces high spatial frequency noise into the reflectivities from mismatch of solar line profiles due to differing instrumental profiles. Since the work of McCord et al. (1972) indicates that the lunar reflectivity varies slowly through this spectral region, we calculated the leastsquares fit o f a cubic polynomial to our lunar reflectivities. We multiplied the J u p i t e r / M o o n reflectivities by this polynomial to remove the lunar albedo features. This left relative Jovian reflectivities. This final step was to put these reflectivities on an absolute scale. To do this, we calculated the reflectivity at 6360 using the relation I / F = (TrFJTrFo)(R/a)2~r/A

where ~rF~ and zrFo are the Jovian and solar fluxes above the terrestrial atmosphere, R and a are the J u p i t e r - S u n and E a r t h - S u n distances at the times of the Jovian and solar observations, and A is the projected solid angle of the aperture. This value o f the reflectivity was then used to scale the rest o f the spectrum onto an I / F scale. The results of the reduction to reflectivities are shown for two spectra in Fig. 2. These two spectra were taken on different nights at the same longitude and at about the same airmass.

IIl. T H E DATA SET

The major goal of the observational program was to search for possible longitudinal variations in the equivalent width o f the CH4 6190 /~ and NH3 6450 /~ absorption bands. A major difficulty in accurate mea-

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FIG. 2. Representative spectra of the CH4 6190-2%and NH3 6450-,~ bands taken on different nights at the same longitude and airmass. The upper spectrum was obtained on 10 February 1979at 11= 272° and airmass 1.33. The lower spectrum was obtained on 11 February 1979at 11= 273° and airmass 1.25. The regions used to define the continuum for each are indicated by the vertical lines.

JOVIAN

LONGITUDINAL

surement of equivalent widths is the proper placement of the continuum level. This problem is even more important in this investigation since we are searching for variations in equivalent width. We want to be certain that any detected variations in Wx are intrinsic to Jupiter and not the result of subjective biases in continuum placement. We, therefore, needed a totally blind, automated, self-consistent method of continuum determination that gives reliable measurements of equivalent width variations. We first defined a wavelength region of continuum on each side of each band, delimited by the vertical lines in Fig. 2. These continuum regions are most likely not true continuum (i.e., no molecular absorption), but they are well outside of the absorption which may be clearly attributed to the band in the Jovian spectrum. It should be noted, however, that the longest wavelengths in the red continuum of the NH3 band are in the wing of a weak CH4 band centered at 6670 A. We then computed a least-squares fit of a parabola through the continuum regions to define the band continuum. The same wavelength regions were used to determine the contin20

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105

VARIABILITY

uum for all spectra. Numerical integration across the observed band profile then gave a value for the equivalent width which was unaffected by subjective interpretation of continuum position and shape. Examples of derived continuum and calculated equivalent widths are shown in Fig. 2. The variation of equivalent width with longitude for both bands is shown in Fig. 3. The scatter in the data is no more than would be expected from the known sources of uncertainty. Several trends are clearly evident. Both bands exhibit systematically large equivalent widths near longitudes 35 and 340° and small equivalent widths near longitude 150° . This behavior is evident in the data from both nights. The calculation of the uncertainty in equivalent widths has been discussed by Breger et al. (1979). Their formula, slightly modified for our method of continuum placement, gives an uncertainty in each equivalent width of ---0.2 A for the NH3 band and _+0.25/~ for the CH4 band due to photon statistics. Slight inaccuracies in placement of the spectrometer entrance aperture result in an additional uncertainty. In order to illustrate more clearly the i

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FIG. 3. Equivalent width vs longitude. T h e u p p e r data points are for the m e t h a n e 6190-A b a n d and the lower data points are for the a m m o n i a 6450-/~ band. The + s y m b o l s represent the data from 10 F e b r u a r y 1979 and the dots r e p r e s e n t the data from 11 February 1979.

106

COCHRAN AND COCHRAN

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FIG. 4. Equivalent width of the NH3 6450-/~ band vs equivalent width of the CH4 6190-/~ band. The symbols are the same as in the previous figures.

relationship between methane and ammonia absorption, Fig. 4 shows the equivalent width of the CH4 6190-• band plotted against the equivalent width of the NH3 6450-/k band for each spectrum. A very strong correlation is evident. Regions with strong CH4 absorption also have strong NH3 absorptions, and conversely, weak CH4 absorption is accompanied by weak NH3 absorption. 0.8

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The variations of continuum (h6060-/k) reflectivity with longitude is shown in Fig. 5. The reflectivity displays little longitudinal variation. The data between 1~ of 200 and 250 ° from 10 February were observed at high airmass (c.f. Fig. 1) and the scatter in this part of Fig. 5 is probably due to incomplete removal of extinction. Comparison of Figs. 3 and 5 indicates that there may be a slight tendency for areas of strong

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FIG. 6. Relationships between continuum reflectivity and equivalent width of the CH4 6190-A band (upper panel) and NH3 6450-.~ band (lower panel). The symbols are the same as in the previous figures.

molecular absorption to be accompanied by slightly lower reflectivity and vice-versa. This trend is more evident in Fig. 6, in which the methane and ammonia equivalent widths are plotted against continuum reflectivity. This weak anticorrelation of equivalent width with reflectivity is statistically significant. The relationship for the NH3 6450-,~ band has a Pearson correlation coefficient o f - 0 . 2 7 2 and a significance of 0.008, while for CH4 6190-,~ band the correlation coefficient is - 0 . 3 5 1 and the significance is 0.001.

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have calculated a number of theoretical models of the Jovian atmosphere. We adopted the basic Jovian model atmosphere derived by Sato and Hansen (1979), which is shown in Fig. 7. There are obviously a large number of free parameters in the model: the optical thickness and singlescattering albedo of the haze layer and NH3 cloud, the albedo of the thick lower cloud, and the cloud and haze altitudes (which determine the gas abundances in various regions of the atmosphere). Parameters of Sato and Hansen's model are given in Table I. This model was c h o s e n because it is the most recent and most thorough analysis of all relevant observational data. Our conclusions will therefore be based on the validity of the Sato and Hansen model. We started GAS

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IV. THEORETICAL INTERPRETATION

In order to determine the physical cause longitudinal variations, we

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FiG. 7. Jovian model atmosphere (after Sato and Hansen, 1979). The methane is located in all of the gas regions while the ammonia is mostly confined to the region between the NHa cloud and the lower cloud.

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COCHRAN AND COCHRAN

our analysis by making the assumption that the observed variations are the result o f longitudinal variation o f a single model parameter rather than simultaneous variation o f several parameters. While this may not be strictly correct, it is the only approach which makes the problem tractable. Radiative transfer calculations were made using a doubling and adding technique. We adopted the CH4 6190-/~ band absorption coefficients measured by Giver (1978) and NH3 6450-/~ band absorption coefficients from Lutz and Owen (1980). A family o f profiles for each band was calculated in which one model parameter at a time was varied. Equivalent widths o f the theoretical profiles were then calculated using the same continuum fitting routine as was used for the actual observations. We first attempted to vary the properties o f the high-altitude haze layer. Variation of the haze single-scattering albedo has a detectable influence on the continuum reflectivity but a negligible effect on the CH4 and NH3 equivalent widths. For instance, changing ~ from 1.00 to 0.95 decreases I / F by 6% but decreases Wx of the CH4 6190-A band by less than 1%. Such a result clearly contradicts the observations in Fig. 6. Variation o f the haze optical thickness encounters a similar problem. Varying r between 0.1 and 0.9 increases I / F significantly but causes a decrease in Wx which is much too small to fit the observations. The next likely candidate for the cause of the variation is the upper NH3 cloud. We varied both the optical thickness and singlescattering albedo o f this cloud in our model calculations. Variation of both r and t~ produce significant changes in Wx and I / F , but the calculations show that in these models there is a direct relationship between I / F and Wx. The sense of the relationship for both bands is opposite to what we have found. Variation of the single-scattering albedo o f the lower cloud gives a similar result to

variation of t~ o f the upper cloud. Larger t~ results in increased I / F and Wx for both bands, which is ruled out by the observations. Variation of any one o f the scattering properties of the various aerosols is insufficient in itself to cause the observed longitudinal variations. Therefore, we tried variations o f the altitude of the cloud layers. This seemed to work quite well. As the NH3 cloud moves upward, the amount of molecular absorption between the clouds increases, causing the band equivalent width to increase. This motion will also cause the CH4 absorption outside the 6190/~ band to increase slightly, causing a small decrease in the continuum reflectivity. A maximum cloud altitude excursion of _+4 km is sufficient to cause the observed variations. This corresponds to a variation in the pressure level of the NH3 cloud o f -+0.11 bar. It seems then, that the observed longitudinal variation of methane and amonia absorption and continuum reflectivity may be attributed to longitudinal variations in the NH3 cloud altitude.

V. DISCUSSION AND CAVEATS FOR FUTURE OBSERVATIONS Significant longitudinal variation of methane and ammonia absorption has been found in the Equatorial Zone of Jupiter. There are distinct regions several degrees of longitude across which have either slightly elevated or depressed upper NH3 cloud layers. Since the vertical distribution of H2 is nearly identical to that of CH4, it is reasonable to assume that these variations would also be evident in the hydrogen quadrupole lines. It is therefore incorrect to assume longitudinal homogeneity in studies of spatial and temporal variations of absorption features on the Jovian disk. Such an assumption could easily lead to incorrect conclusions on the structure of the Jovian atmosphere. In the study of center-to-limb variations,

JOVIAN LONGITUDINAL VARIABILITY models are most tightly constrained by the d a t a o b t a i n e d n e a r e s t t h e l i m b s . I f t h e long i t u d e s o b s e r v e d n e a r t h e l i m b h a p p e n to be regions of either elevated or depressed c l o u d s , t h e n t h e o b s e r v e d v a r i a t i o n in a b s o r p t i o n will b e d u e to b o t h t h e c h a n g i n g v a l u e s o f / ~ a n d / z 0 a n d t h e different longitude. If one has assumed longitudinal h o m o g e n e i t y t h e d e r i v e d m o d e l will b e c o n t o r t e d t o a c c o u n t f o r all o f t h e o b s e r v e d v a r i a t i o n a n d will p r o b a b l y n o t r e p r e s e n t the t r u e s t r u c t u r e o f a n y l o n g i t u d e . A p r o p e r w a y to o b s e r v e c e n t e r - t o - l i m b v a r i a t i o n s o f a b s o r p t i o n l i n e s o r b a n d s is to p i c k a c e r t a i n l o n g i t u d e a n d t h e n to o b t a i n a s e r i e s o f s p e c t r a as this s p o t m o v e s f r o m t h e w e s t l i m b to t h e c e n t r a l m e r i d i a n t o t h e east limb. The danger of being fooled by p o s s i b l e l o n g i t u d i n a l v a r i a t i o n s is t h u s elimi n a t e d b y t h e o b s e r v a t i o n o f single longitude. Longitudinal variations of molecular abs o r p t i o n p o s e a s i m i l a r d a n g e r in t h e interpretation of possible long-term temporal v a r i a t i o n s . It is c l e a r l y insutficient t o observe long-term variations by obtaining s p e c t r a at r a n d o m l o n g i t u d e s o v e r m a n y years. The possibility of accidentally obs e r v i n g a l o n g i t u d e o f a n o m a l o u s l y high o r l o w a b s o r p t i o n d u r i n g o n e o f t h e s p e c t r a is q u i t e large. A p o s s i b l e a p p r o a c h m i g h t b e to o b s e r v e t h e s a m e l o n g i t u d e at t h e s a m e v a l u e s o f / z a n d /z0 in all o f t h e s p e c t r a . H o w e v e r , t h e r e is no e v i d e n c e t h a t a longitudinal feature such as we have detected will r e m a i n fixed in p l a c e . In f a c t , t h e V o y a g e r 1 images of Jupiter (Smith et al., 1979) s h o w c o n s i d e r a b l e l o n g i t u d i n a l m o v e ment of features on a timescale of weeks. T h e r e f o r e , this a p p r o a c h w o u l d n o t w o r k either. To study properly long-term variability of molecular absorption, one must determine a complete longitudinal average e q u i v a l e n t w i d t h for e a c h t i m e p e r i o d . T h i s is o b v i o u s l y a v e r y t i m e - c o n s u m i n g o b s e r v a t i o n a l p r o g r a m , b u t u n f o r t u n a t e l y it is d i c t a t e d b y t h e c o m p l e x n a t u r e o f t h e Jovian atmosphere.

109

ACKNOWLEDGMENT This research was supported by NASA Grant NGR 44-012-152. REFERENCES ARVESEN, J. C., GRIFFIN, R. N., AND PEARSON, B.

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MORRISON, D., BRIGGS, G. A., AND SUOM|, V. E. (1979). The Jupiter system through the eyes of Voyager I. S c i e n c e 204, 951-972. TERRILE, R. J., AND WESTPHAL, J. A. (1977). The

vertical cloud structure of Jupiter from 5 p,m measurements. Icarus 30, 274-281. TaAFTON, L. (1977). Saturn: Long-term variations of H2 and CH4 absorptions. Icarus 31, 369-384.