THE ATMOSPHERES OF THE OTHER PLANETS 395 
Jupiter were heated by the sun only and his atmosphere 
were transparent to radiation of all wave lengths. There- 
fore two possibilities suggest themselves: either Jupiter 
is heated from the inside as well as from the outside, or 
Jupiter has an appreciable greenhouse effect which is 
relatively much more pronounced than the Harth’s. 
The other giant planets also are considerably warmer 
than would be expected from their distances from the 
sun. This can be seen from the tremendous absorption 
by methane. If Uranus and Neptune were black bodies 
heated by solar radiation only with no greenhouse ef- 
fect, their temperatures would be 70K and 50K re- 
spectively, low enough to “‘freeze out” methane. Since 
methane is not frozen out the actual surface tempera- 
tures must be considerably higher [8, p. 89]. 
In summary, the outer atmosphere of all the major 
planets consists mostly of hydrogen and helium, with 
methane being the next most common gas, at least on 
Jupiter and Saturn. In the atmospheres of Uranus and 
Neptune, inert gases, such as neon, which have lower 
boiling points, may be more abundant than methane. 
The relative brightness of Jupiter’s limb as compared 
to the central portion, the rate of disappearance of a 
satellite behind the planet’s disc, and other optical 
phenomena indicate that comparatively little atmos- 
phere can exist above the ‘‘visible” surface of Jupiter, 
at least near the equator. The pressures at the surfaces 
of the major planets can be estimated from the amount 
of methane observed spectroscopically and an added 
amount of hydrogen [7]. 
The largest portion of the atmospheres of Jupiter 
and Saturn must lie below the visible surface. The varia- 
ble speed of the ‘‘surface”’ features on these planets 
shows that the surface is not part of any solid core. 
The exact thickness of the gaseous portion of the at- 
mospheres is difficult to estimate because both the 
lapse rate and the exact composition are unknown. 
Tt is possible to show from the oblateness, the moment 
of inertia, and the mean density of these planets that 
the density of their outer 10 per cent must be consid- 
erably less than 0.50 g cm [10]. Even gaseous hy- 
drogen will reach densities greater than that at depths 
of the order of 1000 km [8]. For this reason, the gaseous 
portion of the atmospheres is estimated to extend down 
to a depth of several hundred kilometers. Apparently, 
then, the visible features in the atmospheres of Jupiter 
and Saturn are high-level phenomena; in contrast to 
conditions on Mars, little is known from observations 
concerning the solid core of these planets. 
The appearance of Jupiter and Saturn in the tele- 
scope is characterized by a system of belts (dark or 
reddish color) and zones (light color), parallel to the 
latitude circles. Belts have also occasionally been seen 
on Uranus. Neptune is too far from us for any such 
detail to be recognized. Jupiter and Saturn differ from 
each other in that Jupiter’s belts are more pronounced 
near the equator, whereas Saturn’s are distributed more 
uniformly. Jupiter shows a ‘‘zone”’ centered at the 
equator (equatorial zone), generally bordered by the 
two equatorial belts. These are followed by the “‘tropi- 
cal” zones and further by a series of “temperate” 
belts and zones. North of 45°N and south of 45°S, one 
finds generally greyish regions, which, due to the fore- 
shortening, have been called the ‘‘polar” regions. The 
aspect of these zones and belts, in color and width, may 
change considerably from year to year. At irregular 
intervals a belt disappears altogether, apparently cov- 
ered by a white haze. Other zones and belts change their 
latitudinal boundaries and get narrower or wider as 
time progresses. 
Superimposed on these general markings is a wealth 
of detailed structure. White and dark spots may form 
almost anywhere, lasting normally from a week to 
several months; rifts appear in the belts, or wavelike 
patterns form along the boundaries between belts and 
zones. These patterns may have the peaked appearance 
of unstable waves yet persist for months. Only the 
Great Red Spot, an oval 22,000 miles long zonally and 
7000 miles wide meridionally, seems to show a high 
degree of permanence, having been observed in the 
south tropical zone of Jupiter with substantially the 
same shape for at least seventy years, possibly for 
several centuries. But even the aspect of the Red Spot 
changes from year to year; for a few years it may be 
very red, then turn grey or white and be almost in- 
visible. Its shape, also, undergoes changes; one or the 
other extremity may develop a peak, or the whole 
spot may become a little smaller or larger. There is no 
indication, however, that the Red Spot shows any 
systematic increase in its dimensions, which would be 
observed if it were composed of volcanic dust, a hy- 
pothesis accepted by many. Another marking, the South 
Tropical Disturbance, which has persisted smce 1901, 
spread almost all around the planet and disappeared 
entirely in 1949. This marking was apparently de- 
stroyed by diffusion. 
Spots on Saturn are relatively rare occurrences. Sev- 
eral times, large white spots have been observed for a 
number of weeks. No details of this type have been seen 
directly on the discs of Uranus or Neptune; however, 
Uranus occasionally shows small changes in brightness 
of the same period as the period of rotation. These 
changes are of rather an ephemeral nature and soon 
disappear, making it likely that they are due to fairly 
short-lived spots. 
The inclination of Jupiter’s orbit to its equator is 
only three degrees, so that seasonal effects, if any, 
should be small. Jupiter’s “year” is 12 Harth-years, 
and color changes in the belts with that period have 
been reported [11]. However, this result seems to be 
based on a good deal of uncertain interpretation. 
Saturn’s seasonal effects should be considerable be- 
cause of the 27-degree inclination between its orbit 
and equator; however, as seen from Saturn, an ob- 
server on Earth is always in nearly the same direction 
as the sun; hence only the summer hemisphere of Sat- 
urn can be studied in detail. The same is true to an 
even larger extent for Uranus, the equator of which is 
almost at right angles to its orbit. 
Circulation of the Atmospheres of the Major Planets 
All four major planets rotate about their axes rap- 
idly, with periods between 9 and 15 hours. The super- 
