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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.
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