New observational constraints on the temperature inversions of Uranus and Neptune

ICARUS 53, 141--146 (1983)

New Observational Constraints on the Temperature Inversions of Uranus and Neptune A. T. T O K U N A G A , *'l G. S. ORTON,I "'2 AND J. CALDWELL:I:'2 *Institute for Astronomy, University of Hawaii, 2680 Woodlawn Drive, Honolulu, Hawaii 96822, ~Jet Propulsion Laboratory, 4800 Oak Grove Drive, Pasadena, California 91109, and ¢Department of Earth and Space Sciences, State University of New York, Stony Brook, New York 11794 Received May 24, 1982; revised September 13, 1982 We present 20-p.m photometry of Uranus and Neptune which confirms the presence of a temperature inversion in the lower stratospheres in both planets. We find the brightness temperature difference between 17.8 and 19.6 ~m to be 0.8 _+ 0.5°K for Uranus and 1.8 -+ 0.6°K for Neptune. These results indicate that the temperature inversions on both planets are weaker than previously thought. Comparison to model atmospheres by J. Appleby [Ph.D. thesis, SUNY at Stony Brook (1980)] indicates that the temperature inversions can be understood as arising from heating by the absorption of sunlight by CI-h and aerosols. However, the stratospheric CH4 mixing ratio on Neptune must be higher than that at the temperature minimum.

summary of these results is given by Trafton (1981). Significant differences also occur in the shapes of the strong 1- to 3-1xm CH4 bands of Uranus and Neptune that indicate greater aerosol scattering on Neptune (Fink and Larson, 1979). The radiation at these wavelengths arises from reflected sunlight, and the temporal variations in the near-infrared flux from Neptune are probably caused by changes in the clouds or haze (Brown et al., 1981). The purpose of this paper is to present observations at 17.8 and 19.6 Ixm that better constrain models for the temperature inversion on Uranus and Neptune. The observations at these wavelengths provide a means of testing models of the temperature inversion since the flux at 17.8 ~m originates at higher levels in the atmosphere than at 19.6 I~m due to the broad pressureinduced absorption of the S(1) rotational line of H2 centered at 17 I~m (Courtin et al., 1978; Orton, 1981). We were particularly interested in understanding how the temperature inversion could be maintained on Neptune at a higher temperature than on Uranus, despite the greater distance of Neptune from the Sun. Our observations

INTRODUCTION

Uranus and Neptune have similar mass and radius and also similar appearance at ultraviolet and visible wavelengths, but recent thermal infrared observations show that the pressure-temperature profiles of the atmosphere of these planets are very different. First, emission from CH4 and C2H6 at 8 and 12 ixm, respectively, has been observed on Neptune, but not on Uranus (Macy and Sinton, 1977; Gillett and Rieke, 1977). This could be due to a hotter stratosphere on Neptune (consistent with stellar occultation results), or to differing stratospheric composition between the planets, or both. Second, far-infrared observations show that Neptune has a significant internal heat source, amounting to 1.6 times the amount of solar radiation absorbed, but Uranus does not (Loewenstein et al., 1977; Stier et al., 1978; Trafton, 1981). A detailed Staff Astronomer at the NASA Infrared Telescope Facility. 2 Visiting Astronomer at the Infrared Telescope Facility, which is operated by the University of Hawaii under contract from the National Aeronautics and Space Administration. 141

0019-1035/83 $3.00 Copyright © 1983 by Academic Press, Inc. All rights of reproduction in any form reserved.

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confirm the presence of a temperature inversion on both planets, although in each case the inversion was weaker than previous infrared observations had indicated. OBSERVATIONS

Uranus and Neptune were observed at 17.8 and 19.6 ixm using the bolometer system on the NASA Infrared Telescope Facility on three observing runs: July 1980, March 1981, and July 1981. These filters have small bandpasses compared to previous work (1.6 Ixm at 17.8 ~m and 2.7 txm at 19.6 txm). Their transmission profiles are shown in Fig. 1. The effective wavelength of the filters that we quote here takes into account the wavelength shift when the filter is cooled and the nominal telluric absorption with 1 mm precipitable H20. The major uncertainty in the effective wavelength arises from the wavelength shift of 1.5 _ 0.5% when the filter is cooled for use at liquid nitrogen temperature (based on information supplied by the manufacturer). By comparison, the effective wavelength is

15

WAVELENGTH (IJm) 20 25

z_ wO -J

shifted by only 0.3% for a wide range of telluric absorption. Corrections to the monochromatic flux were computed to take into account the very different temperatures of the standard star and that of Uranus and Neptune by using the standard method outlined by Morrison (1973) and Low and Rieke (1974). These corrections were less than 2% because of the relatively narrow bandpass of the filters, and they were applied to results presented here. Another correction must be made to our data to take into account the effects of seeing and diffraction. To minimize background noise we used a 6-arcsecdiameter aperture for all of our observations, even though Uranus had an apparent diameter of 4 arcsec. We calculate that approximately 20% of the flux from Uranus was lost, assuming 2 arcsec seeing, and therefore we multiplied the flux density of Uranus by 1.25. This correction is based only on average seeing conditions; however, the results are likely to be in greater error without it. No correction was necessary for Neptune, which had an apparent diameter of 2.3 arcsec. We used ~ Boo as a primary standard and ot Sco as a secondary standard in all of our observations. The absolute flux scale for Boo that we adopted is given in Table I; it is based on the absolute calibration of et Boo by Simon et al. (1972). The observations are summarized in Table II, with only the averaged surface brightness for each observing run being presented. The results at 19.6 Ixm were averaged by weighting by the inverse square of the standard deviation. Note that the TABLE I A B S O L U T E F L U X D E N S I T I E S FOR ~ B o o

0

600 400 FREQUENCY (cm-')

FIG. 1. Filter transmission for the 17.8- and 19.6-1xm filters used. The transmission was m e a s u r e d at room temperature, then shifted by 1.5% to shorter wavelengths to take into account the change in wavelength when the filter is used at liquid nitrogen temperature.

Effective wavelength (p.m)

FWHM bandpass (~m)

× 10 t7 W cm -2 ~ m 1

17.8 19.6

1.6 2.7

23.0 16.0

TEMPERATURE INVERSIONS ON URANUS AND NEPTUNE

143

TABLE 1I SURFACE BRIGHTNESS OF URANUS AND NEPTUNE a k (l~m)

x 10 -9 W cm 2 ixm-1 sr-~ July 1980

March 1981

Weighted a ve ra ge

Brightness t e m p e r a t u r e (°K)

3.70 - 0.25 7.13 ± 0.38

56.1 ± 0.3 55.3 ± 0.2

9.64 ± 0.63 14.0 ± 0.9

60.1 ± 0.3 58.3 ± 0.3

July 1981 Uranus

17.8 19.6

-7.15 ± 1.11

-5.70 ± 1.11

3.70 ± 0.25 7.34 ± 0.43 Neptune

17.8 19.6

11.4

-± 2.7

14.8

-± 2.2

9.64 ± 0.63 14.2 ± 1.1

A s s u m i n g an equatorial radius of 25,900 km for Uranus (Danielson et al., 1972) and 24,800 km for N e p t u n e (Freeman and Lyng~, 1970).

results for July 1981 were the most accurate and that the previous observations were consistent within the uncertainties. DISCUSSION

Our observations were aimed specifically at establishing the best possible constraint on the temperature inversions of Uranus and Neptune. In this way the amount of heating required in the stratosphere can be determined more precisely for comparison with the postulated sources of heating. The opacity at 20 Ixm is dominated by the pressure-induced dipole absorption of H2 which has maxima at 17 and 28 Ixm, as clearly seen in the Voyager infrared spectra of Jupiter and Saturn (Hanel et al., 1981). The optical depth at 17.8 and 19.6 Ixm reaches unity at a pressure level of approximately 80 and 100 mbar, respectively, in the atmospheres of Uranus and Neptune. Thus a higher brightness temperature at 17.8 Ixm than at 19.6 Ixm, as observed, is indicative of a temperature inversion in the atmospheres of Uranus and Neptune. From the results shown in Table II, we determine that the brightness temperature difference between 17.8 and 19.6 Ixm is 0.8 +_ 0.5°K for Uranus and 1.8 +__ 0.6°K for Neptune, where the uncertainty is the sum of the errors for the individual measurements at the two wavelengths. Although

the temperature difference for Uranus is small, we found that the brightness temperature at 17.8 Ixm was consistently higher than at 19.6 for the individual observations during the July 1981 observing run. Thus we have confidence that a temperature inversion exists at the 80-mbar level, albeit a very weak one. Stellar occultation results (Dunham et al., 1980; Sicardy et al., 1981) also show the existence of a temperature inversion at lower pressure levels (3 x 10 2-3 x 10 4 mbar). Our results have improved accuracy over previous 20-1xm observations by Rieke and Low (1974) and Gillett and Rieke (1977). The general conclusion by these authors that temperature inversions exist on Uranus and Neptune is unchanged. However, the magnitude of the temperature inversions is smaller than that reported by Gillett and Rieke (1977). T h e y found the brightness temperatures at 17.7 txm, with a filter having a similar bandpass as our 17.8tzm filter, t o b e 5 7 . 0 _+ 1.1 and 63.2 _ 1.1°K for Uranus and Neptune, respectively. This is 1.7°K higher than the brightness temperature we found for Uranus and 3. I°K higher for Neptune. Thus our observations indicate that the temperature inversion is cooler than previously thought, a result which makes it easier to understand the energetics of the temperature inversion, as

144

TOKUNAGA, ORTON, AND CALDWELL

discussed below. It may be that the stratospheres of Uranus and Neptune have recently become cooler, but the precision of the data does not permit quantitative assessment of this possibility. The observations reported here can be used in two ways. One of these is as a discriminator among existing models. For example, a radiative-convective equilibrium model for Uranus by Appleby (1980) closely fits our data. This model involves stratospheric heating by absorption of sunlight by methane gas and aerosols. CH4 is distributed vertically according to saturation equilibrium with a stratospheric mixing ratio equal to that at the temperature minimum. Some additional heating which is 15% of the incident solar flux is provided by aerosols distributed uniformly with pressure above the 540-mbar level, the top of the convective zone. The temperature structure corresponding to this model is shown in Fig. 2. Other published models for Uranus [see Fig. 7 of Gautier and Courtin

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FIG. 2. Temperature structure for Uranus derived in this work (short-dashed and solid lines) compared to a model by Appleby (1980; long-dashed line). The solidline curve represents a perturbation by us on Appleby's model to produce consistency with our new data. Our data cannot distinguish between the short-dashed and the solid-line models.

(1979)] do not fit the 17.8-p.m observations. For example, the Gautier and Courtin models " N " and " I " are respectively, too warm and too cool at 17.8 ~m. Other models by Danielson (1977) and Wallace (1975) also do not predict the proper 17.8~m brightness temperature. A more satisfactory use of the data, in principle, is to constrain the temperature structure directly without relying on a priori models, as in temperature sounding. Unfortunately the weighting functions at 17.8 and 19.6 Ixm are not sufficiently different to recover completely independent temperature at two pressure levels. The best we can do is to determine a mean temperature within an atmospheric scale height of the 63-mbar level (the center of the weighting functions) and to determine the lapse rate above this level providing a best fit to the data. A full discussion of this technique is given by Orton (1981). The short-dashed line in Fig. 2 represents the results of such a limited temperature sounding. Appleby's model is assumed to pressures greater than 250 mbar. The 63and 250-mbar levels are connected by a linear lapse rate in log pressure, and a linear lapse rate is assumed for all pressures less than 63 mbar. The fitting procedure consists of finding best values for the 63-mbar temperature and the overlying lapse rate. The lapse rate, extrapolated to the microbar region, is close to the stellar occultation temperature of 140°K at 3 × 10-z to 3 × 10-4 mbar (Sicardy et al., 1981). A similar temperature recovery technique has been used to perturb Appleby' s model in order to produce a temperature structure consistent with our data and the stellar occultation temperatures. In this case linear interpolation and extrapolation of corrections to the temperatures in Appleby's model were made. It is evident from Fig. 2 that only a small pertubation of Appleby's model is required in order to fit our data. It is also instructive to note that both the solid and short-dashed curves in Fig. 2 produce identical results at 17.8 and 19.6 i~m, illustrating

TEMPERATURE INVERSIONS ON URANUS AND NEPTUNE the limitations of the vertical resolution associated with the data. For Neptune, all of the models by Gautier and Courtin (1979) and by Appleby (1980) were constrained to fit the high 17.7I~m brightness temperature reported by Gillett and Rieke (1977), and therefore do not fit our data at 17.8 I~m. Interestingly, the variable flux model by Macy and Trafton (1975) for an effective temperature of 57°K is within 1.5°K of the observations, and most closely matches our data. Figure 3 shows the result of a limited temperature sounding of the lower stratosphere of Neptune, derived in a way identical to the Uranus results shown in Fig. 3. As in Fig. 2, the short-dashed curve shows the temperature at 63 mbar and the overlying lapse rate deduced from our Neptune data. We also show a model of Neptune from Appleby (1980) in which the CH4 is assumed to be in local saturation equilibrium at all atmospheric levels. We perturbed this model in a manner shown by the solid line to produce a temperature structure consistent with our data. We fixed the temperature at the 10-4-bar level in order to maintain consistency with the temperatures 10-'t

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FIG. 3. Temperature structure for Neptune derived in this work(short-dashedand solidlines)comparedto a modelby Appleby(1980; long-dashedline).

145

derived from stellar occultation results (Veverka et al., 1974). The results of our two temperature retrievals, between which our data do not discriminate, are obviously quite similar in the lower stratospheres. The Neptune model by Appleby requires that the stratospheric CH4 abundance be at local saturation to explain the observed stratospheric temperature and the CH4 emission at 8 p~m. This in turn requires that the CH4 be in some fashion transported into the stratosphere past the temperature minimum "cold trap" at 0.1 bar, through convective overshoot via strong localized convection, or transport of small CH4 particles into the warm stratosphere where they resublimate into the gas phase. SUMMARY Temperature inversions in the lower stratospheres of Uranus and Neptune were confirmed by narrow-band photometry at 20 ~m. The magnitude of the temperature inversions was found to be smaller than previously reported, thus reducing the amount of stratospheric CH4 that is apparently required to support the temperature inversions. Comparison of the data to atmospheric models for Uranus by Appleby (1980) indicates that Uranus' temperature inversion may be supported by both a continuum absorber and stratospheric CH4 in low abundance. The temperature inversion is stronger on Neptune, despite its greater distance from the Sun compared to Uranus. Model calculations by Appleby (1980) suggest that the stratospheric abundance of CH4 is near local saturation on Neptune. Such a situation could arise from the injection of CH4 through the temperature minimum to Neptune's stratosphere by strong atmospheric convective motions. Local saturation is intrinsically unstable and small temperature perturbations could then produce the high cloud deck and haze layer observed on Neptune by Joyce et al. (1977) and Fink and Larson (1979).

146

TOKUNAGA, ORTON, AND CALDWELL

Further work is necessary at thermal wavelengths, especially at 8 and 30 Ixm, to understand better the manner in which the temperature inversion is supported on Uranus and Neptune. It is particularly important to observe Uranus at 8 Ixm to determine how much CH4 is present in Uranus' stratosphere, and we have undertaken such a project. ACKNOWLEDGMENTS This work was supported by NASA Contract NASW 3159 (ATT) and NASA Grant NSG 7320 (JC). G.S.O. acknowledges the support of the Planetary Atmospheres Program of the NASA Office of Space Science and Applications for work carried out under NASA Contract NAS 7-100 to the Jet Propulsion Laboratory, California Institute of Technology. We are grateful to Dr. John Appleby for discussions on an earlier version of this paper. REFERENCES APPLEBY, J. (1980). Ph.D. thesis, SUNY at Stony Brook. BROWN, R., D. P. CRUIKSHANK, AND A. T. TOKUNAGA (1981). The rotation period of Neptune's upper atmosphere. Icarus 47, 159-165. COURTIN, R., D. GAUTIER, AND A. LACOMBE(1978). On the thermal structure of Uranus from infrared measurements. Astron. Astrophys. 63, 97-101. DANIELSON, R. E. (1977). The structure of the atmosphere of Uranus. Icarus 30, 462-478. DANIELSON, R. E., M. G. TOMASKO, AND B. D. SAVAGE (1972). High-resolution imagery of Uranus obtained by stratoscope II. Astrophys. J. 178, 887-900. DUNHAM, E., J. L. ELLIOT, AND P. J. GIERASCH (1980). The upper atmosphere of Uranus: Mean temperature and temperature variations. Astrophys. J. 235, 274-284. FINK, U., AND H. P. LARSON (1979). The infrared spectra of Uranus, Neptune, and Titan from 0.8 to 2.5 microns. Astrophys. J. 233, 1021-1040. FREEMAN, K. C., AND G. LYNGX (1970). Data for Neptune from occultation observations. Astrophys. J. 160, 767-780. GAUTIER, D., AND R. COURTIN (1979). Atmospheric thermal structures of the giant planets. Icarus 39, 28-45. GILLETT, F. C., AND G. H. RIEKE (1977). 5--20 micron observations of Uranus and Neptune. Astrophys. J. 218, L141-LI44.

HANEL, R., B. CONRATH, M. FLASAR,V. KUNDE, W. MAGUIRE, J. PEARL, J. PIRRAGLIA, R. SAMUELSON, D. HEARTH, M. AALLISON, D. CRUIKSHANK, D. GAUTIER, P. GIERASCH, L. HORN, R. KOPPANY, AND C. PONNAMPERUMA(1981). Infrared observations of the Saturnian system from Voyager 1. Science 212, 192-200. JOYCE, R. R., C. B. PIECHER, D. P. CRU1KSHANK, AND D. MORRISON (1977). Evidence for weather on Neptune, I. Astrophys. J. 214, 657-662. Low, F. J., AND G. H. RIEKE (1974). The instrumentation and techniques of infrared photometry. In Methods of Experimental Physics, Vol. 12 (N. Cadeton, Ed.), pp. 415-462. Academic Press, New York. LOEWENSTEIN, R. F., D. A. HARPER, AND H. MOSEELY (1977). The effective temperature of Neptune. Astrophys. J. 218, LI45-LI46. MACY, W., AND W. SINTON (1977). Detection of methane and ethane emission on Neptune but not on Uranus. Astrophys. J. 218, L79-L81. MACY, W., AND L. TRAETON (1975). Neptune's atmosphere: The source of the thermal inversion. Icarus 26, 428-436. MORRISON, D. (1973). Determination of radii of satellites and asteroids from radiometry and photometry. Icarus 19, 1-14. ORTON, G. (1981). Atmospheric structure of the outer planets from thermal emission data. In Infrared Astronomy (C. G. Wynn-Williams and D. P. Cruikshank, Eds.), pp. 35-53. Reidel, Dordrecht. RIEKE, G. H., AND F. J. L o w (1974). Infrared measurements of Uranus and Neptune. Astrophys. J. 193, LI47-L148. SICARDY, B., M. COMBES, A. BRAHIC, P. BOUCHER, C. PERRIER, AND R. COURTIN (1981). The 15 August 1980 occultation by the Uranian system: Structure of the rings and temperature of the upper atmosphere. Preprint. SIMON, T., D. MORRISON, AND O. P. CRUIKSHANK (1972). 20-Micron fluxes of bright stellar standards. Astrophys. J. 177, L17-L20. Stier, M. T., W. T. Traub, G. G. Fazio, E. L. Wright, and F. J. Low (1978). Far-infrared observations of Uranus, Neptune, and Ceres. Astrophys. J. 226, 347-349. TRAFTON, L. (1981). The atmospheres of the outer planets and satellites. Rev. Geophys. Space Sci. 19, 43-89. VEVERKA,J., K. WASSERMAN,AND C. SAGAN (1974). On the upper atmosphere of Neptune. Astrophys. J. 189, 569-575. WALLACE, L. (1975). On the thermal structure of Uranus. Icarus 25, 538-544.