THE ATMOSPHERES OF THE OTHER PLANETS 
By S. L. HESS 
Lowell Observatory and Florida State University 
and H. A. PANOFSKY 
New York University 
Introduction 
The study of planetary atmospheres is a relatively 
new field of meteorology. Its main impetus comes from 
the likelihood that the behavior of the several planetary 
envelopes, with their varying masses, rotations, con- 
stituents, and other physical parameters, may yield im- 
portant evidence bearing on the general laws which 
govern our own atmosphere. Much of the information 
sought through a study of the other planets cannot be 
obtained by investigation of the terrestrial atmosphere 
simply because its various parameters of interest are 
fixed and the effects of their variation cannot be de- 
termined. 
Beyond the possibility of gaining information that 
may add to our understanding of Harth’s atmosphere 
lies the hitherto somewhat neglected fact that the 
behavior of planetary atmospheres, per se, is a proper 
field of study for meteorologists. One really needs no 
further justification than the knowledge that there exist 
several atmospheres, besides Earth’s, whose behavior 
may be observed and interpreted. 
In Table I various meteorological parameters are 
listed for all the planets known to possess atmospheres. 
They are divisible into two distinct groups. The first 
three planets listed are relatively small and have at- 
mospheric constituents of moderately high molecular 
weight. The last four are much larger and have pre- 
dominantly light-weight constituents. This difference in 
composition is a natural consequence of the greater 
masses and far lower temperatures of the outer planets, 
since these two factors enable them to retain the hght 
gases which the smaller, warmer planets have lost to 
space. 
The most interesting planets in this list are Mars and 
Jupiter. This is because both planets present, at times, 
sufficiently large discs to enable us to examine and 
photograph the atmospheric phenomena which are pres- 
ent. Venus, too, has a large enough disc for this purpose 
but it is characteristic of her that little visible atmos- 
pheric or surface detail exists. Thus our knowledge of 
Venus’ atmosphere is rather small. Saturn, Uranus, and 
Neptune have successively smaller discs and, in addi- 
tion, probably have progressively less detail. 
Venus 
The planet Venus is the closest in size and mass to 
Earth; she is also our nearest planetary neighbor. De- 
spite this we know relatively little about her atmosphere 
and surface. Little detail can be discerned in visible 
391 
light, which leads one to believe that we may be seeing 
not the actual solid surface but a continuous cloud sheet. 
This is consistent with the planet’s high albedo (0.59). 
The lack of surface markings prevents a direct deter- 
mination of the rotation period. Spectroscopic investi- 
gations of the rotation indicate that the day on Venus 
is probably more than three of our weeks [9, p. 317]. 
It may even be that Venus always presents the same 
face to the sun, in which case her day would be equal 
to her year (225 terrestrial days). In any event, we may 
be certain that the rotation is so slow that atmospheric 
motions are not subject to significant Coriolis deflec- 
tions. 
There is no question that Venus possesses an atmos- 
phere. This is demonstrated by various physical phe- 
nomena [9, p. 318] and by the positive, spectroscopic 
identification of carbon dioxide as one of its constitu- 
ents. The amount of COs is estimated to be some 500 
times that contained above a unit area on Harth [3, 
p. 351]. No appreciable amounts of oxygen or water 
vapor are present. Consequently the basic cloud layer 
cannot be aqueous; it may be dust. When Venus is 
photographed in ultraviolet light, large dark and light 
bands may appear. The nature of these markings is 
unknown, except that they are undoubtedly manifesta- 
tions of some sort of cloud because they vary in shape 
and size from day to day. 
Despite our lack of observational knowledge of the 
circulation of Venus’ atmosphere we may be confident, 
in view of the long period of rotation, that the major 
feature of the circulation is a rapid exchange of air be- 
tween the warm, sunlit hemisphere and the cool, dark 
hemisphere. This would be a direct circulation, and the 
direction of motion ought to be close to the direction of 
the pressure-gradient force. This circulation apparently 
succeeds in transferring considerable quantities of heat 
to the dark side of Venus because infrared radiation can 
be detected from the night side. Also important in 
keeping the dark side warm is the large greenhouse ef- 
fect supplied by such an enormous quantity of COs. 
On occasion the far infrared radiation (10 «) emitted 
by the warm CO, fails to appear [4]. This is certainly 
not due to a failure of the gas to emit such radiation, 
but must be due to a transient layer of high cloud which 
prevents the energy from escaping. The nature of this 
high cloud layer (as distinct from the lower cloud sur- 
face) is unknown, but it may well be condensed carbon 
dioxide itself. Even on such a warm planet as Venus the 
saturation vapor pressure of CO, can be reached in the 
