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The microwave spectrum of ammonia in Jupiter's atmosphere
ICARUS 29, 283--285 (1976)
The Microwave Spectrum of Ammonia in Jupiter's Atmosphere J O H N R. D I C K E L University of Illinois Observatory, Urbana, Illinois 61801 R e c e i v e d S e p t e m b e r 11, 1975 ; r e v i s e d J a n u a r y 5, 1976 H i g h - f r e q u e n c y - r e s o l u t i o n o b s e r v a t i o n s of t h e m i c r o w a v e i n v e r s i o n lines of a m m o n i a in J u p i t e r h a v e b e e n c o m p a r e d w i t h t h e m o d e l s of t h e t e m p e r a t u r e i n v e r s i o n in t h e s t r a t o s p h e r e of t h e p l a n e t t o d e d u c e t h a t m u c h of t h e a m m o n i a m u s t b e f r o z e n o u t in t h e c l o u d layer, l e a v i n g a s m a l l e r m i x i n g r a t i o a b o v e .
Recent empirical models of Jupiter's resolution spectroscopy program discussed upper atmosphere from Pioneer 10 and 11 below, some mean brightness temperature data (Orton and Ingersoll, 1976; Kliore measurements were made centered on the and Woiceshyn, 1976) suggest t h a t the 1,1 inversion transition at a frequency of planet has a temperature inversion in 23,694MHz in December 1974 using the its upper atmosphere above the strato- 36-m telescope at the Haystack Observasphere. Although the detailed structure of tory. This telescope is very effective for the inversion region is not yet available, I such measurements because, although have examined the crude models to see the atmospheric extinction is still important, effect of this high-temperature layer upon the telescope gain is not dependent upon the microwave spectrum of the ammonia ambient conditions and can be well inversion transitions near a wavelength 'calibrated as a function of position. The of 1.26cm. Two significant conclusions measurements were made near the time emerge from this quick look: (i) To get the of meridian transit when Jupiter was low brightness temperature observed in 30-40 ° above the horizon. The telescope the center of the ammonia band (about was equipped with a maser amplifier with 130K, which is also approximately the a noise temperature of 200K and a effective temperature of the planet) the nominal bandwidth of 20MHz. Beam total ammonia opacity in the temperature- switching was employed for both scans inversion region must be small, and (ii) for and on-off measurements of the planet on a n y reasonable pressure the individual 4 successive days. The two beams, with lines in the ammonia spectrum will be half-power beam widths of about 85 arcsec, smeared sufficiently by pressure broaden- were separated by nine beam widths in ing to show little fine structure in the azimuth. Because the emission from spectrum. These conclusions have been Jupiter at the frequency is virtually all confirmed with observations of mine and thermal radiation from the planetary disk, of others. we have corrected the observed fluxes for the smearing which would be caused by Mean Brightness Temperature emission from a uniform (slightly elliptical) Broadband observations of the micro- disk with the optical dimensions of the wave emission in the vicinity of the planet. (During the period of observation ammonia band have been made by a the polar semidiameter of Jupiter averaged number of workers over several years [see about 17.5 arcsec and the correction for the the reviews by Dickel et al. (1970), Wrixon size of the source was always less than et al. (1971), and Gnlkis et al. (1974)]. The 10%.) The resultant brightness temperamajor problem in obtaining an accurate ture was 139 d: 5 K (internal error only). brightness temperature remains t h a t of The absolute calibration was made by absolute calibration. As part of a high- comparison with the same kind of observaCopyright © 1976by Academic Press, Inc. All rights of reproduction in any form reserved. Printed in Great Britain
283
284
J O H N R. D I C K E L
tions of the compact H I I region DR21 when it was in the same altitude range as Jupiter. As discussed by Dent (1972) this source has a very well understood spectrum so t h a t its flux density at 23,694MHz of 19.04janskys ~ should be accurate to 3%. Thus the final absolute brightness temperature of J u p i t er at 23,694MHz is 1 3 9 ± 7K, which is in good agreement with the previous measurements. To investigate the implications of these data on parameters in the Jovian atmosphere, I have compared the observed brightness temperatures with the microwave spectra predicted by the original models of Jupiter's atmosphere calculated b y Goodman (1969) but with the added
170
160
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tJA n I'-"
~30
I
I
I
[,I 2,2 1203600
I
3,3 J
I
23800
FREQUENCY
24000
( MHz )
FIC~. 1. S p e c t r a o f t h e c e n t e r o f t h e 1 . 2 6 c m
ammonia band in Jupiter's atmosphere for three different models of the variation of the ammonia mixing ratio with height. (1) An ammonia mixing ratio of 2 x 10-4 throughout, (2) a mixing ratio of 2 x 10-4 below the temperature minimum and 2 x l0 -6 above, (3) a mixing ratio of 2 x 10-4 below the temperature minimum and zero above. The positions of the (1,1), (2,2), and (3,3) lines in the inversion spectrum of ammonia are also sketched. I One jansky = lO-26Wm-2I-Iz-1.
temperature-inversion layer similar to t hat from Orton and Ingersoll (1976). These models used an effective temperature for the planet of 130 K, a temperature minimum of 108K at the level with a pressure of 0.1 bar, then a maximum temperature in the upper inversion layer of 175 K. I t should be noted t h a t the effective temperature is the equilibrium temperature of the planet integrated over all wavelengths and not necessarily t h a t measured in the ammonia band. Figure 1 shows the spectrum in the center of the ammonia band for three such models. In models (1) and (2) the high temperature of the upper levels of the atmosphere causes the individual ammonia lines to appear in emission, but in model (3) the lines arise in the lower convective region, where the temperature decre~es outward, and so are seen in absorption. Changing the effective temperature of the planet, the details of the temperature inversion, or other parameters of the models will cause relatively minor changes in the spectrum [see, e.g., Goodman (1969); Gulkis et al. (1974)] but will not as significantly affect the mean temperature in the center of the ammonia band. In model (2) the bulk of the ammonia absorption occurs in the low convective region and the ammonia in the upper atmosphere provides only a small contribution in the line centers. I t is not possible to get a reasonable temperature in the line centers if the wings are formed anywhere in the upper temperatureinversion zone unless the temperature inversion reaches a value of only about 140K which is less than expected from the infrared results. From this analysis I conclude t h a t the ammonia in Jupiter's atmosphere cannot be in its vapor-phase equilibrium throughout the whole atmosphere; the reduced abundance in the region above the temperature minimum must be less than 1/100 of the mixing ratio found in the warm convective region below the clouds. A likely hypothesis is t hat the ammonia is almost entirely frozen out of the atmosphere in the cold stratosphere and above this region it maintains the low vaporphase mixing ratio found in the cold region below.
N H 3 MICROWAVE SPECTRUM OF JUPITER
Fine Structure in the Spectrum Figure 1 shows not only the mean brightness temperature of the ammonia band but also the fine structure of the individual lines, and it can give the reader a feel for the effects of the pressure upon the line shape. Because of the low mixing ratio of the ammonia in the upper levels the individual lines are not very strong and, for all the acceptable models, the lines near the band center should differ by less than about 3 K from the gaps between the lines where their wings overlap. I t might be noted t h a t the thermal broadening of ammonia at these temperatures is only about 50kHz and so can be ignored in our discussion. The small value of the fine structure has been roughly confirmed by the observations of Gulkis et al. (1974), who found variations over a total bandwidth of 200MHz of less than about 4 K and by some new higher resolution observations by me of the spectrum between the 1,1 line at 23,694MHz and the 2,2 line at 23,723MHz. These data were obtained with the Haystack antenna, a 100-channel autoeorrelation spectrometer, and the same maser amplifier with the total bandwidth of 20MHz described above, so t h a t some tuning was required to cover the full spectral interval. This somewhat limited both the continuity and the integration time available but the upper limit to any fine structure over the 40MHz scanned in the spectrum was less than 4% of the continuum level. Thus, the individual lines were less than 5 K in brightness temperature above the surrounding continuum, in agreement with the discussion above. More detailed analysis of the
ammonia cutoff must await sensitivity measurements.
285 higher-
ACKNOWLEDGMENTS I thank M. L. Meeks and G. C. Goodman for help and advice. The Haystack Observatory of the Northeast Radio Observatory Corporation is supported by the National Science Foundation under Grant MPS71-02109A07. This research was supported by NASA Grant N G R 14-005-176 to the University of Illinois. REFERENCES
DENT, W. A. (1972). A flux density scale for microwave frequencies. Astrophys. J. 177, 93-99. DICKEL, J. R., DEGIOANNI, J. J. C., AND GOODMAN, G. C. {1970). The radio spectrum of Jupiter. Radio Sci. 5, 517-527. GOODMAN, G. C. (1969). Models of J u p i t e r ' s atmosphere. Ph.D. Thesis, University of Illinois. GULKIS, S., KLEIN, M. J., AND POYNTER, R. L. (1974). Ju p i t er 's microwave spectrum: Implications for the upper atmosphere. I n Exploration of the Planetary System-I.A.U. Symposium No. 65 (A. Woszczyk and C. Iwaniszewski, Eds.), pp. 367-373. Reidel, Dordreeht. KLIORE, A. J., AND WOICESHYN, P. M. (1976). Structure of the atmosphere of J u p i t e r from Pioneer 10 and 11 radio occultation measurements. In Jupiter (T. Gehrels, Ed.). Univ. Arizona Press, Tucson. ORTON, G., AND INGERSOLL, A, P. (1976). Pioneer 10 and 11 and ground-based infrared data on J u p i t e r : The thermal structure and He/H2 ratio. I n Jupiter (T. Gehrels, Ed.). Univ. Arizona Press, Tucson. WRIXON, G. T., WELCH, W. J., AND THORNTON, D. D. (1971). The spectrum of J u p i t e r at millimeter wavelengths. Astrophys. J. 169, 171-183.
The Microwave Spectrum of Ammonia in Jupiter's Atmosphere J O H N R. D I C K E L University of Illinois Observatory, Urbana, Illinois 61801 R e c e i v e d S e p t e m b e r 11, 1975 ; r e v i s e d J a n u a r y 5, 1976 H i g h - f r e q u e n c y - r e s o l u t i o n o b s e r v a t i o n s of t h e m i c r o w a v e i n v e r s i o n lines of a m m o n i a in J u p i t e r h a v e b e e n c o m p a r e d w i t h t h e m o d e l s of t h e t e m p e r a t u r e i n v e r s i o n in t h e s t r a t o s p h e r e of t h e p l a n e t t o d e d u c e t h a t m u c h of t h e a m m o n i a m u s t b e f r o z e n o u t in t h e c l o u d layer, l e a v i n g a s m a l l e r m i x i n g r a t i o a b o v e .
Recent empirical models of Jupiter's resolution spectroscopy program discussed upper atmosphere from Pioneer 10 and 11 below, some mean brightness temperature data (Orton and Ingersoll, 1976; Kliore measurements were made centered on the and Woiceshyn, 1976) suggest t h a t the 1,1 inversion transition at a frequency of planet has a temperature inversion in 23,694MHz in December 1974 using the its upper atmosphere above the strato- 36-m telescope at the Haystack Observasphere. Although the detailed structure of tory. This telescope is very effective for the inversion region is not yet available, I such measurements because, although have examined the crude models to see the atmospheric extinction is still important, effect of this high-temperature layer upon the telescope gain is not dependent upon the microwave spectrum of the ammonia ambient conditions and can be well inversion transitions near a wavelength 'calibrated as a function of position. The of 1.26cm. Two significant conclusions measurements were made near the time emerge from this quick look: (i) To get the of meridian transit when Jupiter was low brightness temperature observed in 30-40 ° above the horizon. The telescope the center of the ammonia band (about was equipped with a maser amplifier with 130K, which is also approximately the a noise temperature of 200K and a effective temperature of the planet) the nominal bandwidth of 20MHz. Beam total ammonia opacity in the temperature- switching was employed for both scans inversion region must be small, and (ii) for and on-off measurements of the planet on a n y reasonable pressure the individual 4 successive days. The two beams, with lines in the ammonia spectrum will be half-power beam widths of about 85 arcsec, smeared sufficiently by pressure broaden- were separated by nine beam widths in ing to show little fine structure in the azimuth. Because the emission from spectrum. These conclusions have been Jupiter at the frequency is virtually all confirmed with observations of mine and thermal radiation from the planetary disk, of others. we have corrected the observed fluxes for the smearing which would be caused by Mean Brightness Temperature emission from a uniform (slightly elliptical) Broadband observations of the micro- disk with the optical dimensions of the wave emission in the vicinity of the planet. (During the period of observation ammonia band have been made by a the polar semidiameter of Jupiter averaged number of workers over several years [see about 17.5 arcsec and the correction for the the reviews by Dickel et al. (1970), Wrixon size of the source was always less than et al. (1971), and Gnlkis et al. (1974)]. The 10%.) The resultant brightness temperamajor problem in obtaining an accurate ture was 139 d: 5 K (internal error only). brightness temperature remains t h a t of The absolute calibration was made by absolute calibration. As part of a high- comparison with the same kind of observaCopyright © 1976by Academic Press, Inc. All rights of reproduction in any form reserved. Printed in Great Britain
283
284
J O H N R. D I C K E L
tions of the compact H I I region DR21 when it was in the same altitude range as Jupiter. As discussed by Dent (1972) this source has a very well understood spectrum so t h a t its flux density at 23,694MHz of 19.04janskys ~ should be accurate to 3%. Thus the final absolute brightness temperature of J u p i t er at 23,694MHz is 1 3 9 ± 7K, which is in good agreement with the previous measurements. To investigate the implications of these data on parameters in the Jovian atmosphere, I have compared the observed brightness temperatures with the microwave spectra predicted by the original models of Jupiter's atmosphere calculated b y Goodman (1969) but with the added
170
160
t40 uJ n.-
tJA n I'-"
~30
I
I
I
[,I 2,2 1203600
I
3,3 J
I
23800
FREQUENCY
24000
( MHz )
FIC~. 1. S p e c t r a o f t h e c e n t e r o f t h e 1 . 2 6 c m
ammonia band in Jupiter's atmosphere for three different models of the variation of the ammonia mixing ratio with height. (1) An ammonia mixing ratio of 2 x 10-4 throughout, (2) a mixing ratio of 2 x 10-4 below the temperature minimum and 2 x l0 -6 above, (3) a mixing ratio of 2 x 10-4 below the temperature minimum and zero above. The positions of the (1,1), (2,2), and (3,3) lines in the inversion spectrum of ammonia are also sketched. I One jansky = lO-26Wm-2I-Iz-1.
temperature-inversion layer similar to t hat from Orton and Ingersoll (1976). These models used an effective temperature for the planet of 130 K, a temperature minimum of 108K at the level with a pressure of 0.1 bar, then a maximum temperature in the upper inversion layer of 175 K. I t should be noted t h a t the effective temperature is the equilibrium temperature of the planet integrated over all wavelengths and not necessarily t h a t measured in the ammonia band. Figure 1 shows the spectrum in the center of the ammonia band for three such models. In models (1) and (2) the high temperature of the upper levels of the atmosphere causes the individual ammonia lines to appear in emission, but in model (3) the lines arise in the lower convective region, where the temperature decre~es outward, and so are seen in absorption. Changing the effective temperature of the planet, the details of the temperature inversion, or other parameters of the models will cause relatively minor changes in the spectrum [see, e.g., Goodman (1969); Gulkis et al. (1974)] but will not as significantly affect the mean temperature in the center of the ammonia band. In model (2) the bulk of the ammonia absorption occurs in the low convective region and the ammonia in the upper atmosphere provides only a small contribution in the line centers. I t is not possible to get a reasonable temperature in the line centers if the wings are formed anywhere in the upper temperatureinversion zone unless the temperature inversion reaches a value of only about 140K which is less than expected from the infrared results. From this analysis I conclude t h a t the ammonia in Jupiter's atmosphere cannot be in its vapor-phase equilibrium throughout the whole atmosphere; the reduced abundance in the region above the temperature minimum must be less than 1/100 of the mixing ratio found in the warm convective region below the clouds. A likely hypothesis is t hat the ammonia is almost entirely frozen out of the atmosphere in the cold stratosphere and above this region it maintains the low vaporphase mixing ratio found in the cold region below.
N H 3 MICROWAVE SPECTRUM OF JUPITER
Fine Structure in the Spectrum Figure 1 shows not only the mean brightness temperature of the ammonia band but also the fine structure of the individual lines, and it can give the reader a feel for the effects of the pressure upon the line shape. Because of the low mixing ratio of the ammonia in the upper levels the individual lines are not very strong and, for all the acceptable models, the lines near the band center should differ by less than about 3 K from the gaps between the lines where their wings overlap. I t might be noted t h a t the thermal broadening of ammonia at these temperatures is only about 50kHz and so can be ignored in our discussion. The small value of the fine structure has been roughly confirmed by the observations of Gulkis et al. (1974), who found variations over a total bandwidth of 200MHz of less than about 4 K and by some new higher resolution observations by me of the spectrum between the 1,1 line at 23,694MHz and the 2,2 line at 23,723MHz. These data were obtained with the Haystack antenna, a 100-channel autoeorrelation spectrometer, and the same maser amplifier with the total bandwidth of 20MHz described above, so t h a t some tuning was required to cover the full spectral interval. This somewhat limited both the continuity and the integration time available but the upper limit to any fine structure over the 40MHz scanned in the spectrum was less than 4% of the continuum level. Thus, the individual lines were less than 5 K in brightness temperature above the surrounding continuum, in agreement with the discussion above. More detailed analysis of the
ammonia cutoff must await sensitivity measurements.
285 higher-
ACKNOWLEDGMENTS I thank M. L. Meeks and G. C. Goodman for help and advice. The Haystack Observatory of the Northeast Radio Observatory Corporation is supported by the National Science Foundation under Grant MPS71-02109A07. This research was supported by NASA Grant N G R 14-005-176 to the University of Illinois. REFERENCES
DENT, W. A. (1972). A flux density scale for microwave frequencies. Astrophys. J. 177, 93-99. DICKEL, J. R., DEGIOANNI, J. J. C., AND GOODMAN, G. C. {1970). The radio spectrum of Jupiter. Radio Sci. 5, 517-527. GOODMAN, G. C. (1969). Models of J u p i t e r ' s atmosphere. Ph.D. Thesis, University of Illinois. GULKIS, S., KLEIN, M. J., AND POYNTER, R. L. (1974). Ju p i t er 's microwave spectrum: Implications for the upper atmosphere. I n Exploration of the Planetary System-I.A.U. Symposium No. 65 (A. Woszczyk and C. Iwaniszewski, Eds.), pp. 367-373. Reidel, Dordreeht. KLIORE, A. J., AND WOICESHYN, P. M. (1976). Structure of the atmosphere of J u p i t e r from Pioneer 10 and 11 radio occultation measurements. In Jupiter (T. Gehrels, Ed.). Univ. Arizona Press, Tucson. ORTON, G., AND INGERSOLL, A, P. (1976). Pioneer 10 and 11 and ground-based infrared data on J u p i t e r : The thermal structure and He/H2 ratio. I n Jupiter (T. Gehrels, Ed.). Univ. Arizona Press, Tucson. WRIXON, G. T., WELCH, W. J., AND THORNTON, D. D. (1971). The spectrum of J u p i t e r at millimeter wavelengths. Astrophys. J. 169, 171-183.