Details of Saturn’s Rings
ATLAS OF THE UNIVERSE
W
hen the first missions to Saturn were planned, it was
quite naturally thought that stray ring particles might
present a serious hazard. The initial foray was made by
Pioneer 11, which passed within 21,000 kilometres
(13,000 miles) of the cloud-tops. Estimates of its survival
ranged from 99 per cent down to only 1 per cent, and it
was a relief when the probe emerged unscathed. The
Voyagers did not approach so closely to Saturn, 124,200
kilometres (77,000 miles) and 101,300 kilometres (63,000
miles) respectively, and they also were undamaged. The
scan platform of Voyager 2 jammed during the outward
journey from Saturn, and for a while it was thought that a
collision with a ring particle might have been responsible,
but the problem turned out to be one of insufficient lubri-
cation. During the Uranus and Neptune encounters the
scan platforms worked perfectly.
The main surprise was that the rings proved to be
made up of thousands of ringlets and narrow divisions;
there are even rings inside the Cassini and Encke gaps.
Some sort of wave effect may be involved, though it is
fair to say that even now we do not fully understand the
dynamics of the system.
The innermost or D region of the system is not a true
ring, as there is no sharp inner edge, and the particles may
spread down almost to the cloud-tops. The C or CrêpeRing particles seem on average to be about 2 metres
(7 feet) in diameter; in the B Ring the particle sizes range
from 10 centimetres to about a metre (4 to 40 inches),
with temperatures of 180°C in sunlight down to 200°C
in shadow. Here we find strange, darkish radial ‘spokes’;
they had been glimpsed earlier by Earth-based observers
such as Antoniadi, but the Voyagers gave the first clear
views of them. Logically they ought not to exist, because,
following Kepler’s Laws, the orbital speeds of the particles
decrease with increasing distance from the planet, and the
difference in period between the inner and outer edges
of Ring B is over three hours – yet the spokes persisted
for hours after emerging from the shadow of the globe,
and when they broke up they were replaced by new ones
coming from out of the shadow. Presumably they are due
to particles elevated away from the ring-plane by magnetic
or electrostatic forces. The spokes are confined entirely to
Ring B.
Ring A is made up of particles ranging from fine
‘dust’ to larger blocks up to about 10 metres (over 30
feet) across. The main division in it, Encke’s Division,
was found to contain some discontinuous, irregular ring-
lets along with a tiny satellite, now named Pan. Another
satellite, Atlas, moves close to the outer edge of Ring A,
and is responsible for its sharp border.Saturn’s rings. This image
was obtained from Voyager
2, from a range of 4 million
km (2.5 million miles).
‘Spokes’ can be seen in the
B ring. They are known to
move due to the rotation of
the ring system. The ‘Braided’ F Ring
as imaged from Voyager 1
from a range of 750,000 km
(470,000 miles). The complex
structure of the ring was
unexpected, and seems to
be due to the gravitational
effects of the small satellites
Prometheus and Pandora. False colour Saturn’s
rings,from Voyager 2 on
20 August 1981. Sunlight
is seen coming through
the Cassini Division. The
resolution is down to 56 m
(180 feet).108-151 Atl of Univ Phil'05 7/6/05 12:17 pm Page 112