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ICARUS 24, 292--298 (1975) The Atmosphere of Uranus PETER H. STONE Institute for Space Studies, Goddard Space Flight Center, N A S A New York, New York 10025 and Department of Meteorology, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 R e c e i v e d S e p t e m b e r 3, 1974 C u r r e n t k n o w l e d g e o f t h e a t m o s p h e r e of U r a n u s is r e v i e w e d a n d specific o b j e c t i v e s are s u g g e s t e d for s a t e l l i t e m i s s i o n s t o U r a n u s . T h e a n o m a l o u s comp o s i t i o n of U r a n u s m a k e s d e t e r m i n a t i o n s o f its a t m o s p h e r i c c o m p o s i t i o n p a r t i c u l a r l y v a l u a b l e for t e s t i n g t h e o r i e s of solar s y s t e m e v o l u t i o n . T h e w e a k n e s s o f its a t m o s p h e r i c h e a t i n g m a k e s t h e d e t e r m i n a t i o n o f its a t m o s p h e r i c s t r u c t u r e a n d d y n a m i c s p a r t i c u l a r l y v a l u a b l e for t e s t i n g t h e o r i e s of a t m o s p h e r i c b e h a v i o r . T h e large a x i a l i n c l i n a t i o n of U r a n u s implies a n a n o m a l o u s l a t i t u d i n a l v a r i a t i o n o f t e m p e r a t u r e a n d d y n a m i c s different f r o m t h a t of t h e o t h e r p l a n e t s . 1. INTRODUCTION The exploration of any new planetary atmosphere represents an opportunity to extend our knowledge and test our theories of atmospheric behavior. In this paper we will summarize current knowledge of the atmosphere of Uranus and suggest specific objectives for satellite missions to Uranus. Throughout we will focus our attention on those aspects of Uranus which make its atmosphere unique, and therefore particularly useful for furthering our understanding of atmospheric behavior. An extensive review of the literature on Uranus through 1971 has been given by Newburn and Gulkis (1973), so our summary will be brief. Section 4.1 of their paper should be read in conjunction with this paper. Measured values of parameters important for understanding the Uranus atmosphere are summarized in Table I. For purposes of comparison this table includes the values of these parameters for other atmospheres for which satellite missions have been planned. The values t h a t are particularly uncertain are preceded by a " ~ " . We will discuss the composition, structure, dynamics, and ionosphere of Copyright © 1975by AcademicPress, Inc. All rights of reproduction in any form reserved. Printed in Great Britain Uranus in Sections 2 to 5, respectively, pointing out the significance of the parameters listed in Table I were appropriate. Our suggested objectives for satellite measurements are given in Section 6. 2. COMPOSITION The mass and radius of Uranus clearly distinguish its composition from those of the other planets in Table I. This distinction can be illustrated by an appropriate form of the mass-radius diagram, such as t h a t shown in Fig. 1. Here we have plotted the planets' mean densities (M/Ro 3) vs the logarithm of their masses, normalized to the Earth's mass. As the mass of a planet increases, the central pressure and mean density increase. I f all the planets had the same composition, Fig. 1 would show a monotonic increase of density with mass. In fact such a relationship does not occur, implying differing compositions and differing molecular weights for the planets. Since planets with higher mean molecular weights must have either a higher mean density or a lower mass, the planets in the upper left portion of Fig. 1 have the highest mean molecular 292 THE ATMOSPHERE 293 OF URANUS TABLE I V A L U E S O:F P L A N E T A R Y Planet T Orbital period (years) i Inclination of equator to orbit (degrees) t Rotation period (hours) Venus ]~arth Mars Jupiter Saturn Uranus 0.62 1.0 1.9 11.9 29.5 84.0 ~ 180 23 24 3 27 98 2 830 24.0 24.6 ~ 9.8 ~ 10.2 ~ 10.8 Ro l~dius (kin) g Equatorial surface gravity (em/sec 2) 6 200 6 400 3 400 71 600 60 000 ~ 25 400 4 F (mass of the earth) Solar constant (erg cm-2see -1 ) a Bond albedo Effective temperature (K) Te 0.82 1 O.ll 318 95 15 2.7 × 106 1.4 × 106 6.0 × 105 5.1 × 104 1.5 × 104 3.8 × 103 0.77 0.30 0.15 0.45 0.61 ~ 0.35 228 256 218 ~ 130 ~ 100 ~ 55 3. STRUCTURE Our knowledge of the composition suggests t h a t a reasonable estimate for the mean molecular weight in the Uranus atmosphere is 3. This value together with the parameters given in Table 1 enable us to infer several other parameters important for the structure of the atmosphere. These parameters are listed in Table I I for the different planetary atmospheres. The un- ~ increasing © {g/crn 3) 5 Mass with any accuracy. The planet's mean density indicates t h a t other gases likely to be present are He, NI-I3, H20, and H2S. Prinn and Lewis (1973) considered what condensates might be present, and concluded t h a t cloud layers of CH 4, NH3, NH4SH, or H:O are possible. H2S, H20, and NH4SH will be primarily confined to the deeper layers, at pressures of 10 bars or more (Prinn and Lewis, 1973). C) Earth ©Mercury M 850 980 370 2 290 910 ~ 830 weights while those in the lower right portion have the lowest. Saturn and Jupiter approximate the solar composition, while Uranus and Neptune stand out as being significantly enriched in the heavier elements, but not as enriched as the terrestrial planets. Spectroscopically, two molecules have been identified in the Uranus atmosphere: H a (Herzberg, 1952; Spinrad, 1963; Giver and Spinrad, 1966) and CH 4 (Owen, 1967). Owen's spectroscopic measurements of CHt4 indicated t h a t its mixing ratio was about one order of magnitude larger than would be consistent with a solar composition in accord with the implication of Fig. 1. However, recent measurements with higher resolution indicate t h a t CH 4 may not be so enriched after all (Belton and Hayes, 1975). The uncertain structure of the atmosphere makes it impossible to deduce the absolute and relative abundances of H 2 and CH 4 M/R~ PARAMETERS molecu[or Venus © Mars ght 3 2 E) Neptune 1 0 Uronus Saturn 0 Jupiter 0 I -I " 0 l I I 0 +l +2 Iogja M/M e ~o. 1. M a s s - r a d i u s diagram for the planets. +.3 294 PETF.R H. STONE certainty in the value of/x for Uranus only curves shows t h a t t h e y are only consistent introduces an uncertainty in the values of with the presence of a cloud deck. Prinn R, Cp, /', H, and r of about 30%. The and Lewis' (1973) analysis of geometric bound on the internal heat source, q, is albedo's shows t h a t t h e y require the quite uncertain because T e has only been presence of a high altitude haze. Belton and measured for the 17.5 to 25 micron range Spinrad's (1973) analysis of H 2 lines shows which contains only 0.1% of the long wave t h a t the line structure is consistent with radiation (Low, 1966). q does appear to be the presence of a thin, high altitude haze, appreciably less for Uranus than for and/or a deep cloud deck. I f we compare Jupiter or Saturn. The value of the radia- these results with the thermodynamic tive time constant, ~, was calculated for calculations of Prinn and Lewis (1973), the levels where solar radiation is absorbed and if we assume t h a t the heavier elements (Stone, 1973). Uncertainty in the location are enriched by an order of magnitude of these levels makes r for Uranus un- compared to the solar composition, then certain by a factor of about 3, but it is the evidence points to the presence of a dearly much larger than for Jupiter or thin CH 4 haze at pressures ~0.2 to 0.7 bars, Saturn. and a thick NH 3 cloud at pressures ~4 to Measurements of temperatures in the 10 bars. radio wavelengths show temperatures as The above results can be combined to high as 200K on Uranus (Mayer and construct a tentative model for the mean McCullough, 1971). When compared with vertical structure of the Uranus atmosthe 20 micron temperature of 55K (Low, phere. This structure is shown in Fig. 2. 1966), these measurements indicate t h a t Because of the uncertainties in our knowlthe temperature increases with depth in edge, this model should only be viewed as a the atmosphere. The adiabatic lapse rate "best guess." The particularly large ungiven in Table I I represents a practical certainty in the temperature profile at upper bound on the rate at which the small depths (see Section 5) and large temperature can increase with depth. depths is indicated schematically by the Trafton's (1967) calculations show t h a t the thermal opacity of H2 alone is sufficient lO-~t to produce radiative equilibrium profiles / / I1 t h a t are statically unstable at pressures P(Bc~'s: near I bar and higher. Since the stabilizing effect of large scale dynamical fluxes is i0-I . likely to be small (Stone, 1973), nearadiabatic lapse rates are likely to occur in C~4 the levels where solar radiation is abHaze sorbed, i.e., in the vicinity of 1 to 5 bars pressure. The structure at deeper levels will depend on the strength of the internal heat source. Its apparent weakness raises the possibility of a subadiabatic layer NH3 below the levels where solar radiation is Cloud I0 absorbed. The brightness temperature at 1 cm is ~ 1 3 0 K (Pauliny-Toth and Kellermann, t~ 1970). Since NH 3 is a strong absorber near ]0 2 1 cm, and condenses near temperatures of ,b #o ,;o 4;0 ,doo 130 K, this brightness temperature suggests T (°K) the presence of an ammonia cloud layer near the 130K level. Recent work provides FIG. 2. Inferred temperature profile and cloud even stronger evidence of clouds. Danielson layers as a function of pressure in the Uranus et al.'s (1972) analysis of limb darkening atmosphere. THE ATMOSPHERE 295 OF URANUS TABLE II VALUES Planet Venus Earth Mars Jupiter Saturn Uranus OF ATMOSPHERIC PARAMETERS Mean molecular weight Ratio of specific heats R Gas constant (10~ergsK-1 g-D Up Specific heat (106ergsK-I g-D Adiabatic lapse rate (°K/kin) 44 29 44 ~ 2 ~ 2 ~ 3 1.29 1.41 1.29 1.5 1.5 1.6 1.9 2.9 1.9 ~ 40 ~ 40 ~ 30 8.5 10 7.7 ~ 130 ~ 130 ~ 80 10.0 9.8 4.9 ~ 1.8 ~ 0.7 ~ 1.0 broken curves in Fig. 2. I f CH 4 is not superabundant, the CH 4 haze would occur at slightly lower pressures, N0.1 to 0.4 bars (Priim and Lewis, 1973). The radiative time constant for Uranus (Table II) is about 600 Earth years, which is long compared to both its orbital and rotation periods (cf. Table I). Therefore diurnal and seasonal changes in atmospheric structure in the visible layers will be minimal and one would not anticipate any exceptional seasonal effects due to the unusually large inclination of Uranus' axis of rotation. However, latitude effects should be reversed from normal: the highest temperatures should occur in polar regions which receive more heat from the Sun than the equatorial regions in the course of one revolution around the Sun. At higher atmospheric levels the radiative time scale is considerably less; e.g., it becomes equal to Uranus' orbital period at about the 200mbar level (Stone, 1973). However, the deeper layers where most solar radiation is absorbed will have the same sort of moderating effect on the higher layers as the Earth's oceans have on the Earth's atmosphere, and seasonal effects will tend to be suppressed even in the higher layers. Jupiter 0.1 I Saturn Mars I0 q I~ternal heat source (ergsCm-Zsec-D H Scale height (kin) Ra£tive time constant (set) 0 0 0 5.1 7.6 11 ~20 ~40 ~ 20 109 5 x 106 3 x 105 N 2 X 10 a ~2xl0a ~ 2 × 1 0 lo ~ 9 × 103 N$×103 ~ 102 4. DYNAMICS Uranus is the first planet far enough from the E a r t h t h a t dynamical activity manifested by clouds cannot be observed with ground based observations. Consequently we have no direct information about motions in the atmosphere, although we can be sure motions will be present because of differential solar heating. The data given in Table 1 show t h a t the dynamical drives for the Uranus atmosphere are weaker than for any of the other planets tabulated. Specifically, the imposed heating from the Sun, F, and the interior, q, is weaker than for the other planets. In addition, the radiative time constant r, is longer than for the other planets, and thus the heating will be less efficient at driving motions than on the other planets. Stone (1973) has calculated the effect of dynamics on the layers where solar radiation is absorbed on Uranus. Although the calculation is speculative it does give a quantitative measure of the weakness of the dynamical drives on Uranus. Figure 3 shows the result in terms of the Richardson n umber, Ri, which is a measure of the overall dynamical stability of an atmosphere. Not only should the Uranus atmosphere be more stable than Earth I00 Uranus I000 Neptune I0000 Strong Weok Turbulence Turbulence F I Q . 3. I n f e r r e d D y n a m i c a l s t a t e o f t h e U r a n u s a t m o s p h e r e c o m p a r e d t o t h e o t h e r r o t ~ t i n g p l a n e t s . 296 PETER H. STONE any rotating atmosphere previously explored, but it is potentially the first to be explored which is more stable than the Earth's. I f the atmosphere were in radiative equilibrium, the mean latitudinal temperature gradient would be of the order of 1K/1000km (cf. T e and/~0), sufficient to drive thermal winds of the order of 10m/ sec. Stone (1973)estimated t h a t dynamical fluxes would reduce the mean gradient by an order of magnitude, giving typical gradients and velocities of 1K/10 000km and l m s e c -1. However, much higher velocities could occur in localized currents. Horizontal dynamical fluxes which are this efficient would reduce the relaxation time of the mean horizontal gradient by an order of magnitude below the radiative time constant (Stone, 1973), which would be sufficient to allow some seasonal effects to be present. The expected, anomalous latitudinal temperature gradient caused by the large axial inclination will make the dynamical regime distinct from t h a t of the other planets. For example, the winds will be predominantly easterly rather than westerly, and therefore the rotation rate of atmospheric features will generally be slower than t h a t of the planet. No matter what the external drives are, on a rotating planet there are certain natural horizontal space scales and time scales associated with the motions t h a t can occur (Stone, 1973). I f we adopt 1 msec -l as a typical velocity for Uranus, these space scales range from 20 to 1000kin. The smaller scales are associated with the most unstable dynamical modes, and the larger ones with the most stable modes. Given the weakness of the heating of Uranus' atmosphere, the dominant scales are likely to be near the upper end of this range. Similarly, the corresponding time scales range from a few hours to about 40 Earth days, with the longer time scales likely to dominate. sphere and other upper layers of Uranus' atmosphere (pressures much less than 1 bar). The structure of these upper layers will depend on the presence of any local absorption, on dynamical transports of heat from the lower layers, and on local values of eddy mixing coefficients. None of these quantities are known for Uranus, and therefore the structure of these upper layers is problematical. The best t h a t one can do is to adopt an approach like t h a t of McElroy (1973); i.e., construct a model of the upper layers which is based on the same principles and assumptions which have been found to work well for Jupiter and Saturn. Such a model indicates t h a t electron densities of the order of 10Scm -3 will be reached in Uranus' ionosphere, and t h a t protons will represent the dominant positive ion. 6. OBJECTIVES FOR SATELLITE MISSIONS Measurements which yield new information about any of the items discussed in Sections 2 t~ 5 would be useful for extending our understanding of atmospheric behavior. However, there are two problems of fundamental interest which current knowledge suggests Uranus will be particularly valuable in resolving. One is the problem of the evolution of the solar system, the planets, and their atmospheres. Current atmospheric compositions are an important test of different evolutionary theories (Cameron, 1973). Uranus' anomalous position in the mass-radius diagram (Fig. 1) makes its atmosphere a particularly valuable one for testing such theories. The second problem is the problem of how an atmosphere responds to changes in external heating. The weak heating of Uranus' atmosphere make it the first possible test of theories of how atmospheric structure and dynamics change when the heating is reduced and the stability increased (cf. Fig. 3). Such theories are crucial for developing models of climatic change for the earth. In determining the composition of 5. IONOSPHERE Uranus' atmosphere it would be valuable There are as yet no observations which to obtain accurate abundances for H 2, He, yield direct information about the iono- CH4, and Ntt3. These are likely to be the 297 THE ATMOSPHERE O:F URANUS most abundant gases in the accessible part of the atmosphere and their abundances would automatically yield useful measures of how enriched the atmosphere is in the heavier elements, and place important constraints on evolutionary theories and on models for the interior structure. These same abundances, together with measurements of the size, refractivity, location and density of cloud particles, would supply information for determining pressures, thermal opacities, and cloud compositions, all of which are necessary for theories of atmospheric structure and dynamics. In addition abundance determinations of deuterium and of the isotopes of He, Ar, Ne, C, and N are important for cosmological theories and for theories of solar system evolution (Cameron, 1973). In general it would be desirable to measure the abundances of all these constituents with an accuracy of 10-~ mole fractions, so as to determine accurately the enrichment of the heavy elements. To determine the structure and dynamics of the atmosphere it will be necessary to measure the temperature as a function of latitude, longitude, and pressure; to measure the motions, scales, and symmetries of visual cloud patterns; and to measure winds in the cloud layers. To put these measurements in a theoretical context, it will be necessary to determine the composition (see above), the absorption of solar radiation as a function of depth, and the thermal balance. The last can be found by measuring albedos and thermal emissions as a function of position, phase angle, and wavelength. Useful information about the ionosphere can be obtained by measuring electron densities. The discussion and numbers given in Sections 3 to 5 indicate t h a t measurements of these quantities will be quite useful if t h e y are able to resolve horizontal temperature gradients of 0.1K/10 000km, vertical gradients of 0 . 1 K k m -1, horizontal velocities of 10cmsec -~, cloud features with scales of 20km, Bond albedos of 0.01, total short and long wave fluxes at the surface of 10 erg cm -2 sec-l, and electron densities of 104cm -3. Knowledge of the atmospheric structure will yield the boundary condi- tions necessary for models of the interior structure. I n situ observations would have a distinct advantage over remote observations for several of the measurements listed above. For example, the low flux levels associated with Uranus' low temperatures will make it difficult to determine atmospheric composition by remote spectroscopy. A mass spectrometer on a probe would have no such difficulty. Also any remote observations of the deeper layers are likely to be blocked by clouds. For example, the temperature profile below the layers where solar radiation is absorbed will probably only be accessible to a probe. This profile can have a crucial influence on motions in the higher layers because of the effective boundary condition it places on them. On the other hand remote observations will be necessary for any measurements which require wide spatial or phaseangle coverage, for example, imaging, temperature mapping, and thermal balance measurements. The expected, anomalous latitudinal variation of temperature makes it particularly important to obtain measurements in both high and low latitudes. ~EFEREI~CES BELTON, !~. J. S., AND HAYES, S. (1975). An estimate of the temperature and abundance of CH 4 and other molecules in the atmosphere of Uranus. Icarus 24, 348-357. BELTON, M. J. S., AI~D SPI17RAD, H. (1973). H2 pressure-induced lines in the spectra of the major planets. Astrophys. J. 185,363. CA~ERO1% A. G. W. (1973). Elemental and isotopic abundances of the volatile elements in the outer planets. Space Sci. Rev. 14, 392. DANIELSOI~, ~ . , TOMASKO, 1~., AND SAVAGE, B. (1972). High-resolution imagery of Uranus obtained by the Stratoscope II. Astrophys. J. 178, 887. GIVER, L. P., X_~rDSPX~RAD,H. (1966). Molecular hydrogen feabures in the spectra of Saturn and Uranus. Icarus 5, 586-589. HE,BERG, G. (1952). Laboratory absorption spectra obtained with long paths. I n The Atmospheres of the Earth and Planets (G. P. Kuiper, Ed.) p. 406. Univ. of Chicago Press, Chicago. Low, F. J. (1966). The infrared brightness temperature of Uranus. Aatrophys. J. 146, 326. 298 P E T E R H. STONE MAY]~, G. H., ~ McCULLOUGH, T. P. (1971). Microwave radiation of Uranus and Neptune. Icarus 14, 187. McELRoy, iK. B. (1973). The ionospheres of the major planets. Space Sci. Rev. 14, 460. N E W B ~ , R. L., AND GULK~S, S. (1973). A survey of the outer planets Jupiter, Saturn, Uranus, Neptune, Pluto, and their satellites. SpaceSei. Rev. 14, 179. OWEN, T. (1967). Comparison of laboratory and planetary spect~ra: IV. The identification of the 7500-A bands in the spectra of Uranus and Neptune. Icarus 6, 108. P A U L ~ - T o T H , I. I. K., AND KELLERMANN,K. I. (1970). Millimeter-wavelength measurements of Uranus and Neptune. Astrophys. Left. 6, 185. P I ~ I ~ , R. G., A-~D LEWIS, J. S. (1973). Uranus atmosphere : structure and composition. Astrophys. J. 179, 333. SPINRAD, H. (1963). Pressure-induced dipole lines of molecular hydrogen in the spectra of Uranus and Neptune. Astrophys. J. 138, 1242. STONE, P. H. (1973). The dynamics of the atmospheres of the major planets. Space Sci. Rev. 14, 444. TRAFTO~,L. 1~. (1967). Model atmospheres of the major planets. Astrophys. J. 147, 765.