Missions to Saturn
ATLAS OF THE UNIVERSE
hemisphere is hidden by the rings. Spots are usually in-
conspicuous, but there are major outbreaks now and then.
Bright white spots were seen in 1876, 1903, 1933 (dis-
covered by W. T. Hay – perhaps better remembered by
most people as Will Hay, the actor), 1960 and 1990. The
most prominent of these have been the spots of 1933,
which persisted for some weeks, and of 1990, which were
well imaged by the Hubble Space Telescope and were
clearly due to an uprush of material from below. The time
intervals between these white spots have been 27, 30, 27
and 30 years respectively. This is close to Saturn’s orbital
period of 29^1 ⁄ 2 years, which may or may not be significant;
at any rate, observers will be watching out for a new white
spot around the year 2020. The spots are important because
they tell us a good deal about conditions below the visible
surface, and also help in measuring rotation periods.
The Voyager missions confirmed that Saturn, again
like Jupiter, has a surface which is in constant turmoil
(even if not in the way that Proctor had supposed in 1882)
and that windspeeds are very high. There is a wide equa-
torial jet-stream, 80,000 kilometres (50,000 miles) broad
and stretching from about latitude 35°N to 35°S, where
the winds reach 1800 kilometres per hour (1120 miles per
hour), much faster than any on Jupiter. A major surprise
was that the wind zones do not follow the light and dark
bands, but instead are symmetrical with the equator. One
prominent ‘ribbon’ at latitude 47°N was taken to be a
wave pattern in a particularly unstable jet-stream.
A careful search was made for spots. There is nothing
remotely comparable with the Great Red Spot on Jupiter,
but one relatively large oval feature in the southern hemi-
sphere did appear to be somewhat coloured (it was first
noted by Anne Bunker, and became known as Anne’s
Spot), and there were other, smaller markings of the same
kind, some of which were noted by Voyager 1 and were
still present when Voyager 2 made its fly-by – though it is
not likely that any of them are really long-lived.
Saturn’s seasons are very long, and this means that
there are measurable temperature differences between the
two hemispheres. The Sun crossed into the northern hemi-
sphere of the planet in 1980, but there is a definite ‘lag’
effect, and during the Voyager encounters the northern
hemisphere was still the colder of the two; the difference
between the two poles amounted to 10 degrees C.F
our spacecraft have passed by Saturn. The first encounter,
by Pioneer 11 in September 1979, was in the nature of a
brief preliminary reconnaissance; Pioneer had not originally
been planned to go on to Saturn after its rendezvous with
Jupiter, but when it became clear that this was a possibility
full advantage was taken of it. Pioneer did indeed send back
useful information, but the main results have come from the
Voyagers (1980 and 1981) and Cassini-Huygens (2005).
Voyager 1 was scheduled to survey not only Saturn
itself, but also Titan, the largest of the satellites, which
was known to have an atmosphere and to be a world of
exceptional interest. Had Voyager 1 failed, then Voyager 2
would have had to study Titan – and this would have
meant that it would have been unable to continue on to
Uranus and Neptune. Therefore, there was great relief
when Voyager 1 proved to be a success.
Saturn is a much blander world than Jupiter. The cloud
structure is of the same type, but the lower temperature
means that ammonia crystals form at higher levels, pro-
ducing the generally hazy appearance. There are none of
the vivid colours so striking on Jupiter.
The main belts are usually obvious enough, though
there are long periods when a large part of one or the otherSaturn from Cassini.
This detailed mosaic of
126 images was obtained
over two hours on 6 October- The spacecraft was
about 6.3 million kilometres
(3.9 million miles) from the
planet, and the smallest
features visible are
38 kilometres (24 miles)
across.
Computer-generated
image of Saturn’s rings,
from data obtained by the
first Saturn probe, Pioneer
11, which bypassed the
planet at 21,400 km (13,300
miles) on 1 September 1979.
This is not a photograph,
but shows ring data taken
at 6° above the plane of the
rings as though it were seen
from 90° above the rings (or
directly over Saturn’s north
pole). The graphic shows the
rings as though the observer
were 1 million km (620,000
miles) above the north pole.
The resolution of the ring
bands is 500 km (300 miles);
the area occupied by the
planet has been filled by
a selected portion of a
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