JUPITER

JUPITER

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Jupiter, first of the giant planets, lies well beyond the main asteroid zone. It is the senior member of the Sun’s family; indeed, it has been said that the Solar System is made up of the Sun, Jupiter and various minor bodies.

Though it has only 1/1047 of the mass of the Sun, it is more massive than all the other planets combined. Despite its distance, it shines more brightly than any other planet apart from Venus and, very occasionally, Mars.

A casual look at Jupiter through a telescope is enough to show that it is quite unlike the Earth or MarsIts surface is made up of gas; it is yellow, and is crossed by dark streaks which are always called cloud belts. The disk is obviously flattened, because of the rapid rotation. Jupiter’s ‘year’ is almost 12 times ours, but the ‘day’ amounts to less than ten hours, and this makes the equator bulge out; the polar diameter is over 10,000 kilometres (over 6200 miles) shorter than the diameter measured through the equator.

With Earth, the difference is a mere 42 kilometres (26 miles). Jupiter is almost ‘upright’; the axial tilt is only just over 3 degrees to the perpendicular. Until less than a century ago it was believed that the giant planets were miniature suns, warming their satellite systems.

In fact the outer clouds are very cold indeed. According to the latest theoretical models, Jupiter has a silicate central core about 15 times as massive as the Earth, and this is admittedly hot; the temperature is rather uncertain, but 30,000 degrees C may be reasonably near the truth.

Around the core there is a thick shell of liquid hydrogen, so compressed that it takes on the characteristics of a metal. Further away from the centre there is a shell of liquid molecular hydrogen, and above this comes the gaseous atmosphere, which is of the order of 1000 kilometres (over 600 miles) deep, and is made up of well over 80 per cent hydrogen; most of the rest is helium, with traces of other elements. Spectroscopic analysis shows evidence of uninviting hydrogen compounds such as ammonia and methane.

It is no surprise to find that Jupiter consists mainly of hydrogen, which is, after all, much the most abundant element in the universe. In its make-up Jupiter is not very unlike the Sun, but it would be misleading to describe it as a ‘failed star’. For stellar nuclear reactions to be triggered, the temperature must reach 10 million degrees C.

It has been found that Jupiter sends out 1.7 times as much energy as it would do if it depended entirely upon what it receives from the Sun. This is probably because it has not had time to lose all the heat built up during its formation, between four and five thousand million years ago – though it has also been suggested that the excess may be gravitational energy, produced because Jupiter is slowly contracting at a rate of less than a millimetre per year.



The Jovian atmosphere is in constant turmoil. It seems that there are several cloud layers, of which one, at a considerable depth, may be made up of water droplets – with a giant planet it is not easy to define just where the ‘atmosphere’ ends and the real body of the planet begins! Higher up there are cloud layers of ice crystals, ammonia crystals and ammonium hydrosulphide crystals. Jupiter is a powerful source of radio waves; this was discovered in 1955 by American researchers (it must be admitted that the discovery was accidental).

The main emissions are concentrated in wavelengths of tens of metres (decametric) and tenths of metres (decimetric), and from their variations it seems that the rotation period of the Jovian core is 9 hours 55.5 minutes. It was also found, very unexpectedly, that the decametric radiation is affected by the position in orbit of Io, Jupiter’s innermost large satellite – for reasons which did not become clear until the space missions of the 1970s showed that Io is a violently volcanic world.

IMPORTANT READ

▼ Three views of Jupiter: photographs taken from the Cassini vehicle in October 2000. The effects of the planet’s rotation are very evident. The Great Red Spot can be seen towards the east (right) in the first frame, but has moved out of view on to the night side by the next frame. Ammonia clouds are responsible for the white colour of the equatorial zone. Conjunction of Venus and Jupiter, June 1991. The two planets are seen close together low in the sky; the bright red glow is an inconvenient light from a neighbouring house! The picture was taken from Selsey, in Sussex.

Planetary conjunctions are not uncommon, but the actual occultation of one planet by another is a very rare event. Jupiter, as imaged from the Cassini vehicle on 7 December 2000. At this time the Great Red Spot was very much in evidence; to the left and below the Spot there is s series of white ovals. The black circle towards the left is the shadow cast by Europa on to the planet. This photograph was generated in a computer by combining four separate images.


▼ The south-east quadrant of Jupiter: 18 November 2001, seen from the Cassini vehicle. The Great Red Spot, to the right, is about to be carried out of view by virtue of Jupiter’s rotation. Ganymede, the largest of Jupiter’s moons, is visible at bottom right. The image is a colour composite, produced by combining images taken with different filters.