Jupiter Is The Giant Planet

Press, Volume CV, Issue 30973, 1 February 1966, Page 12

Jupiter Is The Giant Planet

Jupiter contains more material than all the other planets combined. It has a diameter 11 times that of the earth. Even a small telescope reveals an obvious flattening at the poles. This is caused by the differences in the speed with which the planet rotates at different latitudes. At the equator Jupiter rotates in nine hours 50 minutes, whereas over the most of the planet the rotation is five minutes slower. This difference corresponds to a difference in zonal speed of about 200 miles an hour. As a result its equatorial diameter is 89,000 miles, which is some 6000 miles longer than its polar diameter. When Jupiter is viewed through a telescope its apparent surface is crossed by several parallel bands, some dark, others bright. The dark bands are called belts; the bright bands are known as zones. Within the belts especially, bright and dark spots can be seen together with structural details. If the planet is closely watched for half an hour objects move quite noticeably because of the rapid rotation of the planet. But, unlike the craters on the moon, these are not permanent features of the surface. They are changing continually in both shape and size. It is possible, however, to determine the speed at which each spot or other structural feature rotates. This is done by timing successive transits of the object across the central meridian of Jupiter. These measurements show that violent currents are present with sharp lines of discontinuity as between one region and that adjoining it. What we are really looking at is not a real surface, but a vast mantle of clouds. Great Red Spot Some features last for a much longer period than most. Of these the best known is the Great Red Spot. This is shaped like an oval, extending over some 25,000 miles in latitude. That is three times the diameter of the earth. It was first recorded by Cassini in 1665. At times its colour is a very intense red. At other times it is extremely pale without any definite colour. Its period of rotation is not constant but changes from time to time. Until very recent years almost all of the knowledge obtained of the currents in Jupiter’s atmosphere and the changes taking place came from observations made by amateur astronomers throughout the world. The planet was neglected professionally for more interesting researches. New Zealand astronomers have been well to the fore in following the planet and contributing much useful knowledge. Ronald Mclntosh, of Auckland, in particular, has made a long series of valuable observations, especially of the periodic outbursts of unusual’ activity that occur in the southern hemisphere of the planet. Now the results from amateurs are supplemented, but now superseded "by photography. Certain selected objects, such as the Great Red Spot, are photographed in different wave lengths as frequently as the skies permit. Rotation Changes

During a visit to the New Mexico State University in Las Cruces, the writer was able to discuss the results achieved there under a team lead by Clyde Tombaugh, the discoverer of Pluto. It has been possible to detect small, but periodic changes in the rotation period of the Great Red Spot because of the greater accuracy of photographic methods compared with visual estimates. Why the Spot should behave in this manner is uncertain but it does appear that it is really floating on a liquid sea.

Astronomers call the distance from the sun to the earth (roughly 93 million miles) one astronomical unit. This saves having to write a long row of figures when distances between the various bodies of the solar system are stated. Thus Jupiter’s mean distance from the sun is five astronomical units. As a result the planet receives only 1/27 as much heat as the earth. In 1964, a team of astronomers from the California Institute of Technology used the

200-inch telescope at Mount Palomar to determine the temperature of Jupiter’s atmosphere. The result was minus 230 degrees Fahrenheit in sunlight. When, however, the scientists scanned the region darkened by the shadow cast by the moon, Ganymede, on the planet’s atmosphere they obtained a surprising result. Within 15 minutes the temperature increased by more than 100 degrees. The result was verified by measurements wflen the moon Europa was crossing the disk. It is not definite just why there should be this sudden temperature increase. One explanation is that the atmosphere is photochemically stable and the shadow removes this stability. This may cause the atmosphere to became mare transparent so that the measurements are made of conditions at a deeper depth. Alternatively, it is thought that the breakdown in the shaded atmosphere may cause an upwelling of the deeper, warmer regions. It is known that both pressure and temperature increase downwards.

Jupiter is covered with a layer of dense clouds in an atmosphere that is very rich in hydrogen but containing other gases such as nitrogen, ammonia and methane. In relation to the size of the planet the outer atmospheric layer is comparatively thin. Below this, to a depth of about 600 miles, the planet possibly is covered by an ocean of liquid hydrogen. Gigantic Iceberg If this is correct then the Great Red Spot can be compared to a gigantic iceberg floating in a sea of hydrogen. On the other hand it may be floating in a gaseous atmosphere. In that case it seems reasonablly certain that this is very close to being in a state of liquefaction. One of the puzzles of Jupiter is how its atmosphere maintains its observed state of turbulence. Formerly, when it was believed that Jupiter was a hot planet, this problem was easily solved. After it was proved that it was cold it was suggested that possibly Jupiter is contracting very, very slowly. The observed turbulence ,could then be accounted for by the energy liberated in this slow contraction.

However it appears plausible to think that some of the energy is provided by the plant’s rotation. This would act roughly like the tidal forces on the earth, but we are then left to show how the energy becomes available to cause the observed effects. Jupiter has many unsolved riddles.