Spectropolarimetry of the methane and ammonia bands of Jupiter between 6800 and 8200 Å

ICARUS38, 155--165 (1979)

Spectropolarimetry of the Methane and Ammonia Bands of Jupiter between 6800 and 8200 R A M O N D. W O L S T E N C R O F T

Royal Observatory, Edinburgh, Scotland AND

R O B E R T J. S M I T H

Department of Astronomy, University of Edinburgh, Scotland Received August 17, 1978; revised December 4, 1978 Spectropolarimetry of Jupiter at resolutions between 22 and 35 )` reveals a strong increase of linear polarization in the 7250-)` CH4 band. This is very probably due to the decreasing contribution toward the band center of the higher orders of scattering, which have a smaller net polarization than the first few orders. The linear polarization is also enhanced in the band at 7900 )` comprising the 7920-). NHa and 7600- to 8200-)` CH4 bands. The normalized circular polarization shows a feature at 7250 )` with a dispersion shape. This is most probably produced in a double-scattering process involving either a solid or liquid aerosol with an absorption at 7250 )`. Methane aerosols, the obvious candidates from a spectroscopic point of view, are, however, forbidden if current estimates of the Jovian atmospheric temperature are correct. INTRODUCTION Observations of the wavelength dependence of linear polarization of Jupiter have so far been confined to measurements in a few relatively broad bands. For example, Gehrels et al. (1969) observed at eight wavelengths between 3310 and 9 4 3 5 A with bandwidths Ah between 380 and 1400 A, and Morozhenko and Yanovitskii (1973) at seven wavelengths between 3730 and 8000 A with Ak between 155 and 290 A. I n interpreting these and other observations to yield parameters of a t m o spheric models [-see T o m a s k o (1976) for a recent review] it is tacitly assumed t h a t the b r o a d b a n d polarizations represent continuum values, or alternately t h a t the polarization is the same in and out of the numerous bands in the Jovian spectrum. Bugaenko et al. (1971) found no detectable

difference in and out of the 6190- and 7250-A methane bands of Saturn, which suggests t h a t this might also be the case for Jupiter. However, Coffeen found the polarization in the 8900-A methane band at the pole of Saturn to be twice the cont i n u u m value at 9200 A [-unpublished observation described in Coffeen and Hansen (1974)]. We have studied the linear and circular polarization of Jupiter between 6800 and 8200/~ at resolutions between 22 and 35 A and have found t h a t the polarization does indeed increase in the bands, as found b y Coffeen in the case of Saturn. OBSERVATIONS The measurements were taken with the R O E (Royal Observatory, Edinburgh) spectropolarimeter at the Cassegrain focus

155 0019-1035/79/050155-11502.00/0 Copyright O 1979 by Academic Press, Inc. All rights of reproduction in any form reserved.

156

WOLSTENCROFT AND SMITH

IOO0 "

'

'

'

I

'

'

'

'

I

'

'

'

'

I

JUPITER January 25•26. 1978 AA = 22•

7250~ ~ / ~ OO ;~

~2

I~

_~.~

BbondO2

7920• 750018200A NH3 CH4

76ooh

~nd02

CH&

@

8~

7000

10001

'

7500

'

--

I

'

-

8000 WAVELENGTH A '

~

'

8500

'

.

A ~ L

,

'

I

JUPITER

Jonuary21122.1978

~I~

S ~o z~ D~

8g

&h6

,

,

,

I

,

,

7000 b

,

,

I

7500 WAVELENGTH

FIG. 1. Observed spectrum of Jupiter without correction for atmospheric transmission or instrumental response (a) at 22o-~resolution and 0a) at 10-9~resolution.

of the 1-m reflector of the University of Arizona at Mount Lemmon during 4 nights between January 22/23 and 26/27, 1978.

The spectropolarimeter comprises a photoelastic polarimeter placed ahead of the entrance slit of a crossed Czerny-Turner

0

005

'

i

i

'

I

i

L~.~=22~

I

I

I 7000

January 25/26. ~

'

i

I

+

I

I

++

7500

WAVELENGTH

i

I

{I

'

i

i

1

I

70O0

I

,~)1

'

÷

i

i

,

'

+

+

J

7500

I

{ I

'

I

I

1'

I

Tt

8000

! i

I

I

I

I

Fro. 2. Degree of linear polarization on the 4 nights. Symbols distinguish different runs on a given night. Lines in the M S T column of Table I separate the individual runs.

°7

u.I I.LI n" n'- ' < 0.10

~ 0.,5

N n," ,<

_o

Z

0 05

0 ~0

0 15

..lanu~'y 22123.197~ A~,=3~

¢91 '-3

2

©

N

©

©

~g

C~

158

W O L S T E N C R O F T AND S M I T H TABLE I MEASUREMENTS OF PO~RIZATION

h (1~)

ah (_~) p (%)

a(p) %

0(°) a

a(O)(°)

q (%)

a(q) (%)

MST b

1978 J a n u a r y 2 2 / 2 3 7000 7125 7250 7375 7500

35 35 35 35 35

0.041 0.046 0.131 0.047 0.072

0.018 0.016 0.021 0.021 0.018

131.0 175.6 135.2 151.9 138.2

12.8 16.6 4.8 15.7 7.3

-O.001 -0.015 -0.014 +0.008 -0.017

0.007 0.007 0.009 0.008 0.007

2218 2229 2240 2251 2302

7170 7270

35 35

0.017 0.105

0.012 0.017

145.3 131.4

23.4 6.5

--O.OOl -}-0.005

0.006 0.009

2315 2350

6770 6960 7260 7280 7300 7320 7340

23 23 23 23 23 23 23

0.006 0.013 0.123 0.098 0.118 0.086 0.066

0.009 0.010 0.015 0.014 0.015 0.012 0.012

26.9 114.2 130.9 128.1 140.4 133.3 126.9

45.5 22.3 3.3 4.5 3.4 3.8 5.0

1950 2001 2059 2106 2114 2121 2128

7790 7825 7860 7895 7930 7965 8000 8035

23 23 23 23 23 23 23 23

0.133 0.113 0.109 0.174 0.163 0.123 0.184 0.200

0.022 0.028 0.030 0.035 0.036 0.035 0.041 0.040

143.9 138.2 139.5 142.2 123.1 127.7 146.9 137.3

5.4 5.7 8.0 4.8 6.1 8.8 6.0 8.1

2215 2221 2227 2233 2239 2245 2251 2257

7070 7150

23 23

0.055 0.076

0.011 0.012

130.6 139.0

5.3 4.3

2318 2328

7195 7465

23 23

0.098 0.060

0.012 0.011

142.0 124.4

3.6 5.2

2345 2355

7410 7720

23 23

0.020 0.052

0.018 0.037

103.4 147.2

25.5 18.6

0034 0042

8105

23

0.021

0.136

68.3

185.4

0054

7050 7120 7190 7260 7330 7400 7470

22 22 22 22 22 22 22

0.050 O. 065 0.098 0.150 0.105 0.069 0.067

0.009 O. 009 0.011 0.013 0.012 0.010 0.011

134.8 141.0 137.4 134.5 131.1 132.0 136.6

5.4 4.7 3.2 2.8 3.0 4.3 4.8

-0.009 -0.015 --0.020 --0.010 --0.008 --0.Oll --0.017

0.003 O. 003 0.003 0.004 0.004 0.003 0.003

1956 2021 2046 2111 2136 2201 2238

7155 7225

22 22

0.062 0.128

0.011 0.015

141.1 136.6

4.8 3.3

--0.006 --0.003

0.003 0.004

2316 2341

1978 J a n u a r y 2 4 / 2 5

1978 January 25/26

SPECTROPOLARIMETRY OF JUPITER

159

TABLE I--Continued X ().) 7295 7365

A~ (~.) p (%)

a(p) (%)

0(°) a

a(0)(°)

q (%)

~(q) (%)

MST b

+0.011 -0.009

0.005 0.004

0006 0031

22 22

0.141 0.078

0.014 0.012

128.8 150.3

3.0 4.7

7190 7260 7330

35 35 35

0.084 0.146 0.114

0.012 0.015 0.012

145.9 141.4 138.8

4.1 3.2 3.3

7720 7825 7930 8035 8140

35 35 35 35 35

0.078 0.102 0.195 0.177 0.157

0.017 0.023 0.032 0.039 0.061

150.4 141.1 134.0 148.9 152.9

6.8 5.9 4.8 6.5 10.8

--0.014 --0.005 +0.001 +0.018 +0.036

0.006 0.008 0.011 0.014 0.021

2117 2138 2159 2220 2251

7755 7860 7965 8070 8175

35 35 35 35 35

0.122 0.175 0.137 0.127 0.100

0.018 0.021 0.033 0.051 0.085

145.5 137.4 130.6 148.5 17.3

4.0 4.7 7.5 9.3 23.4

--0.010 +0.001 +0.014 --0.001 +0.208

0.006 0.010 0.014 0.020 0.070

2323 2347 0011 0035 0059

7790 7895 8000 8105

35 35 35 35

0.147 0.161 0.159 0.160

0.016 0.022 0.026 0.040

125.7 136.2 135.8 169.1

3.0 4.6 5.0 6.6

-0.006 -0.007 --0.015 --0.071

0.006 0.008 0.010 0.016

0136 0154 0212 0229

1978 January26/27 1955 2012 2029

a e (equatorial system) = 221 ° -- e (table). bMST = U T - 7 h r . s c a n n i n g s p e c t r o m e t e r ( M c P h e r s o n 218). T h e p o l a r i m e t e r c o n s i s t s of a p h o t o e l a s t i c m o d u l a t o r of f u s e d silica in f r o n t of w h i c h can be placed various retardation plates and polarization calibrators mounted on a w h e e l ; f o l l o w i n g t h e m o d u l a t o r is a n a n a l y z e r of H N P ' B P o l a r o i d w h i c h c a n b e c h o p p e d b e t w e e n t w o p o s i t i o n s 90 ° a p a r t a t a t y p i c a l f r e q u e n c y of 0.1 H z . The photoelastic modulator was chosen p r i n c i p a l l y b e c a u s e (1) i t s r a p i d m o d u l a t i o n f r e q u e n c y (50 k H z ) p e r m i t s successful discrimination against atmospheric scintillat i o n noise a n d (2) its i n s t r u m e n t a l l i n e a r a n d c i r c u l a r p o l a r i z a t i o n is v e r y low. P u l s e s f r o m a G a - A s p h o t o m u l t i p l i e r ( R C A 31034) p l a c e d b e y o n d t h e e x i t slit of t h e s p e c t r o m e t e r a r e c o u n t e d b y a P A R 1110 d u a l c h a n n e l c o u n t e r , w i t h t h e g a t i n g for t h e t w o c h a n n e l s s l a v e d to a r e f e r e n c e signal from the modulator. Data acquisition and

some instrument control functions are h a n d l e d b y a n H P 9810 c a l c u l a t o r . A full d e s c r i p t i o n of t h e i n s t r u m e n t a n d t h e c a l i b r a t i o n a n d o b s e r v i n g p r o c e d u r e s will b e p r e s e n t e d elsewhere. S p e c t r a of J u p i t e r (whole p l a n e t ) were o b t a i n e d o n e a c h n i g h t of o b s e r v a t i o n a n d t w o of t h e s e a r e s h o w n in F i g . 1. N e i t h e r s p e c t r u m is c o r r e c t e d for i n s t r u m e n t response or atmospheric transmission. The two main features, shown at a resolution of 22/~ in F i g . l a , a r e t h e 7250-A CH4 b a n d a n d t h e b a n d a t 7900 A, w h i c h is a b l e n d of a n N H 3 b a n d a t 7920 A a n d a n a s y m m e t r i c CH4 b a n d c e n t e r e d a t 7980 A ext e n d i n g f r o m a b o u t 7600 to 8200/~ (see D i c k a n d F i n k , 1977). T h e 7250-A CH4 b a n d is s h o w n a t 10-A r e s o l u t i o n in Fig. l b . T h e d e p e n d e n c e o n w a v e l e n g t h of t h e d e g r e e of l i n e a r p o l a r i z a t i o n , p, is s h o w n in Fig. 2 for i n d i v i d u a l n i g h t s , a n d in F i g . 5

160

WOLSTENCROFT AND SMITH '

'

'l

I

I

I

,

I

Jonuory 22/23, 1978

150

++ { {

ax:ssk I 120 90

I

I

I

I

1

I

L

I

I

I

Jar~a~y 2/-425. 1978

150 23.&

t/) f,,_

120 90

I.IJ d

C.'3 Z < Z 0 I-U') 0 13_

150

+,k

+

i

I

I

I

I

i

I

I

January 25/26,19?8

4

+4 ,0 , ¢ 4

AA: 22£

120 90

*4**4

I

I

I

I

I

I

I

I

I

I

4 1 '

I

Januery 26/27, 1978

150

+*

¢

+

120 90

7000

7500

8OO0 WAVELENGTH /3,

Fro. 3. Position angle of the polarization plane on the individual nights. Symbols for different r u n s on a given n i g h t are the same as in Fig. 2.

for all nights. Only whole-planet observations were made, and thus the polarization is quite small. Nevertheless at X < 7600 A the errors of 0.01 to 0.02% are small enough to show a well-defined polarization peak of 0.14% centered on the 7250-A CH4 band. On the short-wavelength side of the b a n d there is a shoulder between 6950 and 7125/~ t h a t m a y be associated with the weak CH4 b a n d at 7000/~. On the longwavelength side p falls to a m i n i m u m at a b o u t 7400 A a n d t h e n rises t o w a r d 7500 A. This latter rise is not associated with other CH~ b a n d s since CH4 is a t its most trans-

p a r e n t at 7500/~ (Dick and Fink, 1977); nor is it associated with the closest b a n d of NH3 at 7350 A, which is too narrow and too weak (McBride and Nicholls, 1972). A connection with the terrestrial A b a n d at 7600 A is possible b u t unlikely. At X < 7600/~ the position angle of polarization, 0, with one exception, shows no clearcut trend for either individual scans (Fig. 3) or the grand average (Fig. 5). I T h e exception occurs for the two scans on J a n u a r y 1 N o t e t h a t the average value of 0 of 135 ° corres p o n d s to negative polarization.

S P E C T R O P O L A R I M E T R Y OF J U P I T E R

005

I

I

I

l

161

I

Jonuory 22/23.197B AZ=36A

+ +++ ++ -0.05 0-05

I

I

I

I

I

I

I

I

I

I

I

I

+

+ 0

,:, ÷ ,I,+ +,I,+ ++

N

i! °.+

-0.05

+

i

J

I

i

I

I

I

i

r

I

I

I

f

I

.lom,m'y Zmt~ +@'PB~=3r~k

I

t

I

7000

i

t +++++++t I

I

J

i

7500

i

I

8000

WAVELENGTH FIG. 4. Degree of circular polarization on the individual nights. Symbols for different runs on a given night are the same as in Fig. 2.

162

WOLSTENCROFT AND SMITH '

'

I

'

_~ 0.15

January26/27 x

N

~n

'

f

'

1

'

I

'

i

r x

JUPITER 197B Symbot A ~ January22/23 o 35

_

X x

35 ~

•

x

x

x~e~e

0.10

.I'.L

n,t~

z

S

._1

~

oo5

/~

o I

......

oXJ

W W

~

/

I

" °

• ,

L

I

I

J

i

I

o x

150

x

"~

o

+,°i

"l'e

+

~

+

x 0

•

+o

,,,:

•

x i

X

i

I

.~

+

÷

x

o X

+

+

I x

x o o

x

•

i

S

x

x.

÷

o+

4-

X

+

+

x

I-o

X

X

X

x

o -0.05 ~5 W

x

0 L 7000

i

I

I

I

I

i

I

7500

I

t

I

8000 WAVELENGTH ~,

F r o . 5. P o s i t i o n a n g l e a n d d e g r e e s of l i n e a r

25/26 for which e (X) has a dispersion shape centered on 7250 ~, with a peak-to-peak amplitude of 10 ° . Longward of the A band, where the average polarimetric error is 0.035v~v, p rises from a value of about 0.065% at 7720 A to peak values of about 0.18% between 7900 and 8000 A. T h e errors are too large to discern any clear-cut structure either on individual scans or in the grand average. However, 8 does show a definite minimum at 7950/~ (see J a n u a r y 26/27, Fig. 2 ; and Fig. 5) which may be associated

and circular polarization for all nights. with the 7920-A NH3 band and the 7600/ 8200-A CH4 band. Beyond 7900 A the dichroic efficiency of the H N P ' B Polaroid analyzer begins to decline rapidly, and beyond 8200 ,k the Polaroid becomes detectably birefringent. T h e measurements are calibrated with a circular polarizer, one component of which is an H N P ' B Polaroid, and this permits reliable measurements to be made up to 8200 A even though the apparent (uncalibrated) polarization is only 53% of the true polarization. T h e birefringence effects

SPECTROPOLARIMETRY OF JUPITER

163

in both the analyzer and the calibrator prevent useful measurements of either .S 002 ~ o linear or circular polarization from being made beyond 8200 A. The degree of circular polarization, q, is much smaller than p and required corre- " o o I "t'x~'~O~ spondingly longer integration times (10 to 20 min for each wavelength) yielding an average error of 0.005% at ~ < 7600 A. o -0.o211 , , I 7000 7500 The first scans across the methane band Wavelength ~, (see Fig. 4) on each of the 2 nights that this FIG. 6. Normalizeddegree of circular polarization, feature was observed showed a consistent dispersion shape to q(h). However, the q(k) - q(7200/i.). Open circles and crosses refer to the first and second runs on January 22/23, and second scans on those 2 nights were dis- filled circles and plus signs to the first and second placed upward by 0.015%. The origin of runs on January 25/26. the shift in the average value of q(k) is not known, but it may come about be- polarization across the same band. The incause the continuum values are slowly crease in p as the band center is approached changing as the planet rotates during the is very probably associated with the changlong integrations needed to determine q. ing contribution to the emergent flux by We have partially removed the variable the higher orders of scattered light. Since average level by normalizing the circular the higher orders produce a smaller net polarization, and the normalized value polarization than the first few orders of q'(~,) = q(X) -- q (7200A) is shown in scattering, the continuum, which has the Fig. 6. Only 2 of the 18 points deviate greatest contribution from the higher from a tightly defined q' (k) curve within the orders, should show the least polarization error bars, and this shows rather clearly and the band center, which has the smallest the dispersion-like behavior of the circular contribution, should show the greatest polarization. It is interesting to note that polarization. Scattering models are evithe two deviant points occur at almost the dently needed to test this idea and, if same wavelength equally below (7365 ~,) successful, may provide new constraints and above (7375 A) the q'(h) c u r v e : i t may on our knowledge of the Jovian atmosphere. be worth searching for a narrow dispersion These models would need to explain not feature centered at 7370 A in future ob- only the polarization profile but also the servations. At longer wavelengths q shows ratio of polarization to absorption depth a steady increase from - 0 . 0 1 % at 7700 A across the band. The relation between the to +0.03% at 8100 A (see Fig. 4), but profiles of polarization and intensity i s because of the large errors no definite shown in Fig. 7. Both the observed instatement can be made about the shape tensity profile (solid line) and the theoof the q(~) curve. retical intensity profile (dotted line) are shown: the theoretical profile, calculated without pressure broadening, was obtained INTERPRETATION using the laboratory values of the methane The principal phenomena to be explained absorption coefficient determined by Fink are the relatively symmetric increase of et al. (1977) and fits the observed profile the degree of linear polarization across the quite well. When the peaks of the intensity 7250-A methane band and the dispersion- and polarization profiles are made to like behavior of the degree of circular coincide, as in Fig. 7, we find that the

164

WOLSTENCROFT AND SMITH

I

0.2~

I Jupiter

/

i-r,

0

I • •

°0

• o

•

Tz

:o

~0.I uJN

•

• o

•

•

0.5

Z ILl

t.LJn"

/,..., ,,,, p.~. 0

........~ " - J ~ I

7000

z

"..~%

10

"

~

I

i

7500

I

8000 WAVELENGTH ,&,

FIo. 7. The degree of linear polarization (filled circles) and intensity between 6800 and 8200 ~.. The solid line shows the observed intensity across the 7250-), band (uncorrected for atmospheric transmission or instrument response), and the dotted curve represents the theoretical intensity profi*le for methane using the data of Fink et al. (1977).

polarization profile is broader than the intensity profile, which is a significant point to be explained by the models. The theoretical profile for methane only is also shown between 7600 and 8200 A: the observed polarization is much greater than that attributable to the methane band, namely, about 0.05% at 7900A, and is most logically attributable to a similar effect associated with the 7920-A ammonia band. However, at the central depth of 0.70 of the observed intensity profile (methane plus ammonia) centered on 7900 A, we expect a value of p (from Fig. 7) of 0.075%, assuming the continuum polarization and the efficiency of the polarization mechanism to be the same at 7250 and 7900 A. This is much less than the observed peak value of p of close to 0.2% at 7900 A and may indicate that the depolarization mechanism is more efficient at 7900 than at 7250 ,~. In interpreting the circular polarization curve q'(X) we assume that the circular polarization is produced in the following two-step process (Wolstencroft, 1976; Kawata and Hansen, 1976; Kawata, 1978): (1) the incident unpolarized sunlight is partially linearly polarized by scattering

from a molecule or aerosol; and (2) this light is scattered a second time by an aerosol, which produces a phase difference between the orthogonal components of linear polarization and hence a circularly polarized component. Total internal reflection inside the aerosols is the most likely source of the phase difference. The wavelength dependence of q' is thus a product of two terms, the first involving the increasing linear polarization toward the band center and the second the variation of the phase difference and hence of n, the real part of the refractive index across the band. The change of sign of q' at the band center relative to the continuum values thus implies that n changes sign in the manner of the anomalous dispersion curve for a solid with its absorption centered at 7250 A. On the basis of spectroscopic arguments this solid is most likely to be in the form of methane aerosols rather than ammonia aerosols: although laboratory spectra of solid methane and ammonia have not yet been obtained to our knowledge, theoretical considerations lead to the conclusion that for solid methane the wavelength of the band center should differ by

SPECTROPOLARIMETRY OF JUPITER a t m o s t a few tens of a n g s t r o m s f r o m 7250 A, t h e gaseous a b s o r p t i o n w a v e l e n g t h . H o w e v e r , this conclusion conflicts with the general belief t h a t J o v i a n t e m p e r a t u r e s are too high to p e r m i t either solid or liquid m e t h a n e aerosols to form. N e i t h e r is it likely t h a t a m m o n i a crystals with a m a n t l e of solid m e t h a n e are responsible for the circular polarization because of the widely different v a p o r pressures of m e t h a n e a n d a m m o n i a . I t seems therefore either t h a t t h e o b s e r v e d circular polarization is p r o d u c e d b y aerosols o t h e r t h a n m e t h a n e , w h i c h nevertheless h a v e a n a b s o r p t i o n at 7250/~, or t h a t a m e c h a n i s m o t h e r t h a n t h a t proposed is p r o d u c i n g the o b s e r v e d circular polarization. A n y theoretical i n t e r p r e t a t i o n of t h e o b s e r v a t i o n s r e p o r t e d here m u s t t a k e a c c o u n t of the c o m p l i c a t i o n t h a t only whole-planet m e a s u r e m e n t s were obtained. W e plan to m a k e f u r t h e r o b s e r v a t i o n s a t r e p r e s e n t a t i v e points across the p l a n e t a r y disk w i t h a n a p e r t u r e m u c h smaller t h a n the p l a n e t a r y d i a m e t e r : this will h a v e t h e twofold a d v a n t a g e of relieving the theoretician of one stage of i n t e g r a t i o n a n d of yielding larger o b s e r v e d polarizations t h a t can be m e a s u r e d with g r e a t e r relative precision. ACKNOWLEDGMENTS We thank W. Cormaek, A. Pickup, and Miss C. J. Lonsdale for assistance in making these observations, and Dr. G. Coyne for very considerable help in many aspects of this observing program. We are grateful to the University of Arizona for the award of telescope time and to Drs. M. J. S. Belton,

165

G. Hunt, M. G. Tomasko, and L. V. Wallace for useful discussions. One of us (R.J.S.) is supported by an SRC Research Studentship. REFERENCES BUGAENKO,O. I., GALKIN,L. S., ANDMOROZHENKO, A. V. (1971). Polarimetric observations of the major planets. I. Distribution of polarization over the disk of Saturn. Soviet Astron. J. 15, 290-295. COFFEEN, D. L., AND HANSEN,J. E. (1974). Polarization studies of planetary atmospheres. In Planets, Stars and Nebulae Studied with Photopolarimetry (T. Gehrels, Ed.), pp. 518-581. Univ.

of Arizona Press, Tucson. DICK, K. A., ANDFINK, U. (1977). Photoelectric absorption spectra of methane, methane and hydrogen mixtures, and ethane. J. Quant. Spectrosc. Radiat. Transfer 18, 433-446. FINK, V., BENNER, D. C., AND DICK, K. A. (1977). Band model analysis of laboratory methane absorption spectra from 4500 to 10 500 ~. J. Quant. Spectrosc. Radiat. Transfer 18, 447-457. GEHRELS,T., HERMAN,B. M., ANDOWEN, W. (1969). Wavelength dependence of polarization. XIV. Atmosphere of Jupiter. Astron. J. 74, 190-199. KAWATA, Y. (1978). Circular polarization of sunlight reflected by planetary atmospheres. Icarus 33, 217-232. KAWATA, Y., AND HANSF~N,J. E. (1976). Circular polarization of sunlight reflected by Jupiter. In Jupiter (T. Gehrels, Ed.), pp. 516-530. Univ. of Arizona Press, Tucson. McBRIDE, J. 0. P., AND NICHObLS, R. W. (1972). The vibration-rotation spectrum of ammonia gas. J. Phys. B 5, 408-417. MOROZHENKO, A. V., A.ND YANOVITSKII, E. G. (1973). The optical properties of Venus and the Jovian planets. I. The atmosphere of Jupiter according to polarimetrie observations. Icarus 18, 583-592. TOMASKO, M. G. (1976). Photometry and polarimetry of Jupiter. In Jupiter (T. Gehrels, Ed.), pp. 486-515. Univ. of Arizona Press, Tucson. WOLSTENCROFT, R. D. (1976). The circular polarization of the light from Jupiter. Icarus 29, 235-243.