Fundamental Astronomy

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7. The Solar System
face, most likely a salty ocean that could even be 100 km
deep. At the centre, there is a solid silicate core.
Ganymede is the largest moon in the solar system. Its
diameter is 5300 km; it is larger than the planet Mercury.
The density of craters on the surface varies, indicating
that there are areas of different ages. Ganymede’s
surface is partly very old, highly cratered dark regions,
and somewhat younger but still ancient lighter
regions marked with an extensive array of grooves
and ridges. They have a tectonic origin, but the details
of the formations are unknown. About 50% of the
mass of the moon is water or ice, the other half being
silicates (rocks). Contrary to Callisto, Ganymede
is differentiated: a small iron or iron/sulphur core surrounded
by a rocky silicate mantle with an icy (or liquid
water) shell on top. Ganymede has a weak magnetic
field.
Callisto is the outermost of the large moons. It is
dark; its geometric albedo is less than 0.2. Callisto
seems to be undifferentiated, with only a slight increase
of rock toward the centre. About 40% of Callisto is ice
and 60% rock/iron. The ancient surface is peppered by
meteorite craters; no signs of tectonic activity are visible.
However, there have been some later processes,
because small craters have mostly been obliterated and
ancient craters have collapsed.
The currently known moons can be divided into
two wide groups: regular moons containing the small
moons inside the orbits of the Galilean satellites, and
the Galilean satellites, and irregular moons outside the
orbit of the Galilean satellites. The orbits of the inner
group are inclined less than one degree to the equator
of Jupiter. Most of the outermost moons are in eccentric
and/or retrograde orbits. It is possible that many of
these are small asteroids captured by Jupiter.
7.15 Saturn
Saturn is the second largest planet. Its diameter is about
120,000 km, ten times the diameter of the Earth, and the
mass, 95 Earth masses. The density is only 700 kg m −3 ,
less than the density of water. The rotation axis is tilted
about 27 ◦ with respect to the orbital plane, so every
15 years, the northern or the southern pole is well
observable.
Fig. 7.43. Saturn and its rings. Three satellites (Tethys, Dione,
and Rhea) are seen to the left of Saturn, and the shadows
of Mimas and Tethys are visible on Saturn’s cloud tops.
(NASA/JPL)
The rotation period is 10 h 39.4 min, determined
from the periodic variation of the magnetic field by the
Voyager spacecraft in 1981. However, Cassini spacecraft
observed in 2004 the period of 10 h 45 min. The
reason for the change is unknown. Due to the rapid rotation,
Saturn is flattened; the flattening is 1/10, which
can be easily seen even with a small telescope.
The internal structure of Saturn resembles that of
Jupiter. Due to its smaller size, the metallic hydrogen
layer is not so thick as on Jupiter. The thermal radiation
of Saturn is 2.8 times that of the incoming solar
flux. The heat excess originates from the differentiation
of helium. The helium atoms are gradually sinking
inward and the released potential energy is radiated
out as a thermal radiation. The abundance of helium