The White Eye of Saturn
9. November 1990
A very large, white spot has recently appeared on the giant planet Saturn . It is probably a great storm in the planet's atmosphere, which has been initiated by upwelling of clouds from the lower layers into the uppermost regions. The spot began as a small, white feature in Saturn's northern hemisphere and has since developed rapidly so that it now appears to completely encircle the planet's equatorial regions. "Great White Spots" have been seen on Saturn in 1876, 1903, 1933 and 1960 (see below), but the present one seems to be the biggest of them all.
At this moment Saturn is situated in the southern constellation of Sagittarius and is therefore best observed with southern telescopes. It has been monitored at the ESO La Silla Observatory since early October by Belgian astronomer Olivier Hainaut (on temporary assignment to ESO from Institut d'Astrophysique, Liège, Belgium), together with several other astronomers. Most of the observations have been made with the ESO New Technology Telescope (NTT) and ESO/MPI 2.2 m telescope.
Development of the spot
The new phenomenon was first reported on 25 September 1990 by astronomers at the Las Cruces Observatory in New Mexico, USA, as a white spot at northern latitude +12°. It was watched by many amateur astronomers in various countries as it slowly grew in size to about 20 000 km on October 2. Further observations determined the spot's rotation period to about 10 hrs 17 min, that is somewhat slower than the surrounding atmosphere.
During the next days the spot became longer and longer and by October 10, its length was approximately half of Saturn's visible diameter. After that it continued to expand and on exposures made at ESO from October 23 onwards it encircles the entire planet as a bright equatorial band. At the same time, several new, intensively bright spots have been sighted inside the larger feature; they are now being followed with great interest. There is no indication yet that the phenomenon has started to fade away.
There are less than two dozen reports about spots on Saturn during the past 200 years, but only four of these were "Great White Spots" which lasted more than a couple of weeks and none appears to have approached the enormous size of the present spot. We are therefore witnessing a very rare event.
The first known Great White Spot was detected in December 1876 by American astronomer Asaph Hall in Washington D.C. and the next one was found in June 1903 by E.E. Barnard with the 40-inch refractor at Yerkes Observatory, near Williams Bay, Wisconsin. The third and fourth were both found by eagle-eyed amateurs; in August 1933 by Will Hay in England, and in March 1960 by J.H. Botham in South Africa. All of these spots were seen in the northern hemisphere of Saturn: those in 1876 and 1933 at about the same latitude as the present one, while the two others were further north at +40° (1903) and +58° (1960).
What is a "Great White Spot"?
Detailed observations of the giant planets Jupiter and Saturn have been made since the invention of the astronomical telescope in the early 17th century. The "meteorological" studies of their atmospheres took a great stride forward during the flybys of the Pioneer and Voyager spacecraft, from which accurate measurements were made at close distance.
It has long been known that the "surface" of Jupiter shows many more bands and whirls than that of Saturn; this is now explained by the presence in the Saturnian atmosphere of a high layer of aerosols (small solid particles) and haze (liquid drops) which hide the view of the patterns of streams and turbulence below.
The five Great White Spots have appeared with amazing regularity, about once every thirty years, that is with the same period as the orbital revolution around the Sun. Moreover, these spots have all developed near the moment of Saturnian ``mid-summer" in the northern hemisphere, when the insolation (amount of solar energy received) is the greatest possible here. It is therefore obvious that there the emergence of large spots in the north must be triggered by some mechanism that is related to heating of the atmosphere.
Most planetary astronomers agree that the Great White Spots are upwellings from the lower atmosphere, whereby large clouds move upwards and become visible when they penetrate the uppermost, hazy layers. They resemble the towering cumulonimbus clouds often seen in the Earth's atmosphere. However, the lifting mechanism is not yet known; one possibility is that their upward motion is due to the release of heat by water condensation, perhaps in combination with strong updrafts from sublimating ammonia grains.
The spots become longer, as the clouds are carried along by strong winds in the upper atmosphere. Eddies and whirl patterns undoubtedly develop because of the different wind velocities at different latitudes, but due to their smaller size they are very difficult to observe from the Earth. This may imply that the spots, perhaps in particular those which have emerged more recently, are actually gigantic storm centres, just like the Giant Red Spot on Jupiter, that has now been visible for almost 400 years.
Since the Great White Spots on Saturn last much shorter, in the past cases at the most a few months, it will now be very interesting to follow the new one during some time to learn exactly how it disappears. Observations are therefore continuing at ESO as well as at other observatories.
This Press Release is accompanied by two B/W pictures. One shows the development of the 1990 Great White Spot over a two-week period (October 8 to 23); the other is a computer enhanced version of an NTT image, obtained in blue light on October 16; this image is also available on request in a false-colour version (in which the internal structure of the spot is better seen than on the B/W photo).
 Saturn is the fifth planet from the Sun and the outermost planet known before the advent of the telescope. It moves in a nearly circular orbit around the Sun at a mean distance of 1430 million km; one revolution lasts 29.5 years. Saturn's equatorial diameter is about 120 000 km (9.5 times larger than that of the Earth) and it is significantly flattened towards the poles. The rotation period depends on the latitude; it is more rapid at the equator (about 10 hrs 14 min) than near the poles. Its polar axis is inclined by , i.e. somewhat more than is the case for the Earth ( ), and it therefore has pronounced seasons. The surface we observe is not solid, but merely represents the upper layer of a very deep atmosphere, mostly consisting of hydrogen and helium; there are also small amounts of methane and ammonium and possibly other organic substances. Saturn may have a solid core of silicates and other minerals, possibly surrounded by a mantle of metallic hydrogen. The planet emits more energy than it receives from the Sun and must therefore have an unknown source of energy in its interior. It also has a strong magnetic field. Saturn has at least 17 satellites (moons), ranging in size from Titan (diameter 5150 km) to the small Calypso and Telesto ( km). However, Saturn is perhaps most famous because of its ring system which consists of small particles that orbit the planet in many hundreds of well defined rings, whose thickness is probably only a few tens of metres. The largest, dark gap in the rings is known as ``Cassini's division'', after the French astronomer who first saw it in 1675.
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