CHAPTER 23 | COMPARATIVE PLANETOLOGY OF JUPITER AND SATURN 511
in hydrogen and helium (see Table 23-1), and models predict that it
is mostly liquid hydrogen with a core of heavy elements.
Infrared observations show that Saturn is radiating 1.8 times as
much energy as it receives from the sun, showing that heat is fl owing
out of its interior. It must be very hot inside Saturn, as is also true for
Jupiter. In fact, Saturn’s interior is too hot. It should have lost more
heat since it formed. Astronomers have calculated models indicating
that helium in the liquid hydrogen interior is condensing into drop-
lets and falling inward. Th e falling droplets, releasing energy as they
pick up speed, heat the planet. Th is heating is similar to the heating
produced when a star contracts and may also occur to some extent
in the atmospheres of Jupiter, Uranus, and Neptune.
Just as for Jupiter, you can learn more about Saturn’s interior
from its magnetic fi eld. Spacecraft have found that Saturn’s mag-
netic fi eld is about 20 times weaker than Jupiter’s. It also has cor-
respondingly weaker radiation belts. Models comparing Saturn
with Jupiter predict that the lower pressure inside Saturn produces
a smaller mass of liquid metallic hydrogen. Heat fl owing outward
causes convection in this conducting layer, and the rapid rotation
drives a dynamo eff ect that produces the magnetic fi eld. Unlike
most magnetic fi elds, Saturn’s is not inclined to its axis of rotation,
something you can see in ultraviolet images that show rings of
auroras around Saturn’s poles (■ Figure 23-14). Th is perfect align-
ment between Saturn’s magnetic axis and the axis of rotation is
peculiar, isn’t seen in any other planet, and isn’t understood.
Saturn’s atmosphere blends gradually into its liquid interior. The size of its core
is uncertain. (NASA/STScI)Celestial Profi le 8: Saturn
Motion:
Average distance from the sun 9.54 AU (1.43 109 km)
Eccentricity of orbit 0.056
Inclination of orbit to ecliptic 2.5°
Average orbital velocity 9.64 km/s
Orbital period 29.46 y
Period of rotation 10.66 h
Inclination of equator to orbit 26.4°Characteristics:
Equatorial diameter 1.21 105 km (9.42 D⊕)
Mass 5.69 1026 kg (95.1 M⊕)
Average density 0.69 g/cm^3
Gravity at base of clouds 1.16 Earth gravities
Escape velocity 35.6 km/s (3.2 V⊕)
Temperature at cloud tops 180°C (290°F)
Albedo 0.61
Oblateness 0.102Personality Point:
The Greek god Cronus was forced to fl ee when his son Zeus took power.
Cronus fl ed to Italy, where the Romans called him Saturn, protector of
the sowing of seed. He was celebrated in a weeklong wild party called the
Saturnalia at the time of the winter solstice. Early Christians took over
the holiday to celebrate Christmas.Visual + ultraviolet Auroras vary day today because of changes
in the solar wind.■ Figure 23-14
Auroras on Saturn occur in rings around the planet’s magnetic poles. Because
the magnetic fi eld is not inclined very much to the axis of rotation, the rings
occur nearly at the planet’s geometrical poles. (Compare with Figure 23-4.)
(NASA, ESA, J. Clarke, Boston Univ. and X. Levay, STScI)