Missions to Saturn

Saturn

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Four 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, producing the generally hazy appearance. There are none of the vivid colors so striking on Jupiter.

 The main belts are usually obvious enough, though there are long periods when a large part of one or the other hemisphere is hidden by the rings. Spots are usually inconspicuous, but there are major outbreaks now and then. 

Bright white spots were seen in 1876, 1903, 1933 (discovered 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 291 ⁄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 equatorial 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 hemisphere did appear to be somewhat colored (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 hemisphere 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.

IMPORTANT READ

The southern hemisphere from Cassini, 14 December 2004. Note the wave-like patterns in the cloud bands.

Saturn’s turbulent atmosphere from Cassini, 23 January 2005. Two vortices appear to be interacting.

A storm on Saturn, from Cassini, 6 February 2005. The atmosphere is in constant turmoil, with turbulence occurring where two air masses moving at different speeds border one another.

The southern hemisphere from Cassini, 14 December 2004. Note the wave-like patterns in the cloud bands. Saturn’s turbulent atmosphere from Cassini, 23 January 2005. Two vortices appear to be interacting.

The southern hemisphere from Cassini, 6 December 2004. The dark spot towards the top of the image is a storm. The dark band below is a region of apparent stability, contrasting with the turbulence seen towards the bottom of the image.

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 cloud-top picture.

Source: - Atlas of the Universe by Patrick Moore.