and nitrogen (N). The differences are 
due mainly to the mass of the planet 
and its proximity to the sun — in other 
words, its temperature. The giant plan- 
ets — Jupiter, Saturn, Uranus, and Nep- 
tune — retain all those four elements, 
including abundant amounts of hydro- 
gen, and so the other three elements ap- 
pear as methane (CH 4 ), water (H,0), 
and ammonia (NH,). The terrestrial 
planets and Titan, by contrast, are not 
massive enough to retain hydrogen, 
which is the lightest element and there- 
fore tends to escape. With oxygen avail- 
able in the form of water, the terrestrial 
planets converted carbon to carbon di- 
oxide (CO,). On Titan water is frozen 
and inert; therefore, carbon must have 
combined with itself to form hydrocar- 
bons. These condense to form the dark 
haze particles that fall to the surface. 
As on the terrestrial planets, nitrogen 
remains in the atmosphere as N 2 but no 
water or oxygen enters the picture. 
Methane gas could condense to form 
snow and rain on Titan, however, and 
liquid methane oceans could conceiv- 
ably exist at the surface. In this sense, 
methane is the “water” of Titan. 
Thus Titan followed an evolutionary 
path different from that of the earth, 
largely because of its much lower tem- 
peratures. Mars and Venus also had 
different evolutionary paths, again be- 
cause of temperature. Current belief is 
that the earth is probably the only ob- 
ject in the solar system capable of hold- 
ing liquid water, and therefore prob- 
ably the only object where life could 
have evolved. 
Hyperion and Iapetus complete the 
list of Saturn’s “regular” satellites, 
those in circular orbits all in the same 
plane. They seem to be icy objects, like 
the inner five, and are also smaller than 
massive Titan. Iapetus is curious be- 
cause of the fivefold difference in 
brightness between its bright, leading 
hemisphere and its dark, trailing hemi- 
sphere. Again, water ice is a candidate 
for the bright material, which has the 
reflectance properties characteristic of 
ice. Saturn and Jupiter also have small 
“irregular” satellites in huge, inclined, 
eccentric orbits. Theory suggests that 
these are captured objects, as distinct 
from the inner satellites in their flat, 
circular orbits. The former group are 
loosely bound to their parent planets, 
and their present orbits could have 
evolved after the formation of Saturn. 
The regular inner satellites must have 
formed with Saturn in nearly their pres- 
ent orbits. 
Saturn itself provided some nice data 
for the Voyager spacecraft. The colors 
in the atmosphere are more muted than 
those of Jupiter, and there are fewer 
large, oval structures such as the Great 
Red Spot. But the banded cloud pattern 
is prominent, and the associated winds 
are remarkable. Winds are measured 
relative to Saturn’s internal rotation, 
which we can sense by looking at the 
magnetic field. Winds at the equator 
flow eastward at more than 1,000 miles 
per hour. Jupiter’s equatorial winds are 
only one-quarter this speed. Other dif- 
ferences between the two planets are 
not so great, making the difference in 
wind speeds hard to explain. Both are 
fluid objects, with no solid surfaces for 
winds to rub against; hence it is easy to 
imagine high winds on both planets. 
There is currently a debate over how 
deep the winds go, that is, whether they 
extend through the interior to the core 
of the planets or are confined to the thin 
layer where sunlight is deposited and 
radiation is emitted. This is a debate be- 
tween astronomers and meteorologists 
over whether the giant planets resemble 
stars more than they resemble earth. 
A curious feature of the winds on Ju- 
piter is that turbulent eddies contribute 
energy to the east-west currents, not 
the reverse as one might expect. The ed- 
This photograph, taken as the 
Voyager spacecraft traveled past 
Saturn and away from the sun, shows 
the planet as a crescent, with its 
shadow falling on the rings. The 
dark, narrow band in the rings is 
called the Cassini Division. The 
lower part of the crescent is 
visible through this band, or gap. 
49 
