CHAPTER 23 | COMPARATIVE PLANETOLOGY OF JUPITER AND SATURN 517
Saturn’s Moons
Saturn has over known satellites—far too many to exam-
ine individually—but these moons share characteristics common
to icy worlds. Most of them are small and dead, but one is big
enough to have an atmosphere and perhaps even oceans or
lakes—but not of water.
Titan
Saturn’s largest satellite is a giant ice moon with a thick atmo-
sphere and a mysterious surface. From Earth it is only a dot of
light, with no visible detail. Nevertheless, a few basic observa-
tions can tell you a great deal about this strange world.
Titan’s mass can be estimated from its infl uence on both
passing spacecraft and on other moons. Its mass divided by its
volume reveals that its density is 1.9 g/cm^3. Its uncompressed
density (Chapter 19) is only 1.2 g/cm^3. Although it must have a
rocky core, it must also contain a large amount of ice.
Titan is a bit larger than the planet Mercury and almost as
large as Jupiter’s moon Ganymede. Unlike those worlds, Titan
has a thick atmosphere. Its escape velocity is low, but it is so far
from the sun that it is very cold, and most gas atoms don’t move
fast enough to escape. (Look back to Figure 22-13.) Methane was
detected spectroscopically in 1944, and various hydrocarbons
were found beginning in about 1970 (■ Table 23-3), but most of
Titan’s air is nitrogen with only 1.6 percent methane.
When the Voyager 1 and Voyager 2 spacecraft fl ew past
Saturn in the early 1980s, their cameras could not penetrate
Titan’s hazy atmosphere (■ Figure 23-17). Measurements showed
that the surface temperature is about 94 K (290°F), and the
23-5
SCIENTIFIC ARGUMENT
Why do the belts and zones on Saturn look so faint?
One of the most powerful tools of critical thought is simple
comparing and contrasting. You can make that the theme of
this scientific argument by comparing and contrasting Saturn
with Jupiter. In the atmosphere of Jupiter, the dark belts form
in regions where gas sinks, and zones form where gas rises. The
rising gas cools and condenses to form icy crystals of ammonia,
which are visible as bright clouds. Clouds of ammonia hydrosulfi de
and water form deeper, below the ammonia clouds, and are not as
visible. Saturn is twice as far from the sun as Jupiter, so sunlight
is four times dimmer. The atmosphere is colder, and gas currents
do not have to rise as far to reach cold levels and form clouds.
That means the clouds are deeper in Saturn’s atmosphere than in
Jupiter’s atmosphere. Because the clouds are deeper, they are not
as brightly illuminated by sunlight and look dimmer. Also, a layer
of methane-ice-crystal haze high above the ammonia clouds makes
the clouds even less distinct.
Now build a new argument comparing the ring systems. How is
Saturn’s ring system similar to, and different from, Jupiter’s
ring system?■ Table 23-3 \ Some Organic Compounds
Detected on TitanC 2 H 6 Ethane
C 2 H 2 Acetylene
C 2 H 4 Ethylene
C 3 H 4 Methylacetylene
C 3 H 8 Propane
C 4 H 2 Diacetylene
HCN Hydrogen cyanide
HC 3 N Cyanocetylene
C 2 N 2 Cyanogensurface atmospheric pressure is 50 percent greater than on Earth.
Model calculations show that in the conditions on Titan, meth-
ane could condense from the atmosphere and fall as rain. Some
scientists hypothesized that Titan is covered by methane lakes or
seas.
Sunlight converts methane (CH 4 ) into the gas ethane (C 2 H 6 )
plus a collection of other organic molecules.* Some of these mol-
ecules produce the smoglike haze; and, as the smog particles
gradually settle, they are predicted to deposit smelly, organic goo
on the surface. Th is goo is important because similar organic
molecules may have been the precursors of life on Earth. (You
will examine this idea further in Chapter 26.)
Before you try to imagine fl oundering through this strange
landscape, you can consider the recent observations made by the
Cassini spacecraft, which began exploring Saturn and its moons
in 2004.
Infrared cameras and radar instruments on Cassini have
been able to see through the hazy atmosphere. Th e surface is not
a featureless ocean of methane, nor an icy plane covered with
goo. Rather, the surface consists of icy, irregular highlands and
smoother dark areas. Th ere are only a few craters, which suggests
geological activity is erasing craters almost as quickly as they are
formed.
Th e Cassini spacecraft released a probe named Huygens,
which parachuted down through the atmosphere of Titan and
eventually landed on the surface. Huygens radioed back images
of the surface as it descended under its parachute, and those
images show dark drainage networks that lead into dark smooth
areas (Figure 23-17). Th ose dark regions, which look a lot like
bodies of liquid, are actually dry or mostly dry. Precipitation may
have washed the black goo off the highlands into the stream
channels and lowlands, so that they look smooth and dark even*Organic molecules are common in living things on Earth but do not have to
be derived from living things. In other words, biological substances are
organic, but organic substances are not necessarily biological. One chemist
defi ned an organic molecule as “any molecule with a carbon backbone.”