The planets
1920[19] Tethy and Titan seen from Cassini (NASA)
[20] A crescent Enceladus appears with Saturn's rings in this Cassini spacecraft view of the
moon. The famed jets of water ice emanating from the south polar region of the moon are
faintly visible as a small white blur below the dark south pole. (NASA/JPL-Caltech/Space
Science Institute.)
to its activity. Certainly it has increased our knowledge of Saturn by a factor
of two.
The globe of Saturn shows belts similar to Jupiter but less prominent.
There are occasional spots. In 1933 one was discovered by W.T. Hay,
remembered as Will Hay, the stage and screen comedian. This was a white
spot which PM remembers seeing very clearly in a 3-inch refractor. Other
white spots have been seen since and may be commoner than we first
originally thought.
But of course the glory of Saturn lies in its rings. The other giant planets
also have rings, but they are dark whereas Saturn’s rings are icy and bright.
They are in fact made up of particles of ordinary water ice, all spinning round
the planet in the manner of dwarf moons. The individual particles are so
small and remote that the rings have the appearance of being solid, but a
moment’s consideration shows this to be untenable. No solid ring could
form and in any case a ring of that kind would promptly be torn to pieces
by Saturn’s powerful pull of gravity. The idea that the rings are made of icy
particles came from James Clerk Maxwell in the 1870s, and was confirmed
by measurements showing the inner rings move round the planet more
quickly than the outer rings. In fact they follow Kepler’s laws (Kepler was
the German 17th-century astronomer who drew up the three famous laws
of planetary motion. The closer to the main planet, the quicker they move.
This is exactly how a ring made up of discrete particles would be expected
to behave, but the rings are strange in another way. The overall diameter of