Saturn
North Polar Region
Lat. 90° to 55° approx.
The northernmost part
of the disk. Its colour
is variable: sometimes
bright, sometimes dusky.
North Temperate Zone
Lat. 70° to 40° approx.
Generally fairly bright,
but from Earth few details
can be seen.
North Temperate Belt
Lat. 40°
One of the more active
belts on the disk, and
usually easy to see
telescopically except when
covered by the rings.
North Tropical Zone
Lat. 40° to 20°
A generally fairly bright
zone between the two
dark belts.
North Equatorial Belt
Lat. 20°
A prominent belt, always
easy to see and generally
fairly dark. Activity within
it can sometimes be
observed from the Earth.
Equatorial Zone
Lat. 20° to 20°
The brightest part of the
planet. Details can be
observed in it, and there
are occasional white
spots. The most prominent
example of a white spot
in the 20th century was
in 1933.
South Equatorial Belt
Lat. 20°
A dark belt, usually about
the same intensity as the
corresponding belt in the
northern hemisphere.
South Tropical Zone
Lat. 20° to 40°
A generally bright zone.
Little detail to be seen
telescopically.
South Temperate Belt
Lat. 40°
Generally visible when not
covered by the rings.
South Temperate Zone
Lat. 40° to 70°
A brightish zone, with little
or no visible detail as seen
from Earth.
South Polar Region
Lat. 70° to 90° approx.
The southernmost part
of the disk. Like the north
polar region, somewhat
variable in its depth of
shading.ATLAS OF THE UNIVERSE
S
aturn, second of the giant planets, is almost twice
as remote as Jupiter, and has an orbital period of over
29 years, so that it is a slow mover across the sky – it was
natural for the ancients to name it in honour of the God
of Time. It can become brighter than any star apart from
Sirius and Canopus, and in size and mass it is inferior
only to Jupiter.
Telescopically, Saturn may lay claim to being the
most beautiful object in the entire sky. It has a yellowish,
obviously flattened disk crossed by belts which are much
less obvious than those of Jupiter. Around the planet is
the system of rings, which can be seen well with even a
small telescope except when the system lies edgewise on
to us (as in 1995). There are three main rings, two
bright and one semi-transparent; others have been
detected by the space probes which have flown past
Saturn from Earth: Pioneer 11 in 1979, Voyager 1 in 1980
and Voyager 2 in 1981.
Much of our detailed information about Saturn has
been drawn from these space missions, but it was already
known that in make-up the globe is not unlike that of
Jupiter, even though there are important differences in
detail – partly because of Saturn’s lower mass and
smaller size, and partly because of its much greater
distance from the Sun. The polar flattening is due to
the rapid rotation. The period at the equator is 10 hours
14 minutes, but the polar rotation is considerably longer.
Visually, the periods are much less easy to determine than
with those of Jupiter because of the lack of well-defined
surface markings.
The gaseous surface is made up chiefly of hydrogen,
together with helium and smaller quantities of other
gases. Below the clouds comes liquid hydrogen, at first
molecular and then, below a depth of 30,000 kilometres(19,000 miles), metallic. The rocky core is not a great deal
larger than the Earth, though it is much more massive; the
central temperature has been given as 15,000 degrees C,
though with considerable uncertainty.
One interesting point is that the overall density of the
globe of Saturn is less than that of water – it has even been
said that if the planet could be dropped into a vast ocean,
it would float! Though the mass is 95 times that of the
Earth, the surface gravity is only 1.16 times greater. All
the same, Saturn has a very powerful gravitational pull,
and has a strong perturbing effect upon wandering bodies
such as comets.
Saturn, like Jupiter, sends out more energy than it
would do if it relied entirely upon what it receives from
the Sun, but the cause may be different. Saturn has had
ample time to lose all the heat it must have acquired
during its formation stage, and there are suggestions that
the excess radiation may be gravitational, produced as
droplets of helium sink gradually downwards through the
lighter hydrogen. This would also explain why Saturn’s
uppermost clouds contain a lower percentage of helium
than in the case of Jupiter.
Saturn emits a radio pulse with a period of 10 hours
39.4 minutes, which is presumably the rotation period of
the inner core. The magnetosphere is somewhat variable
in extent, but stretches out to approximately the distance
of Titan, the largest of Saturn’s satellites. Radiation zones
exist, though they are much weaker than those of Jupiter.
The magnetic field itself is 1000 times stronger
than that of the Earth, and the magnetic axis is almost
coincident with the axis of rotation, though the centre of
the field is displaced northwards along the axis by about
2400 kilometres (1500 miles) and the field is stronger at
the north pole than at the south.D108- 151 UNIVERSE UK 2003CB 7/4/03 5:12 pm Page 108