Philips Atlas of the Universe

Jupiter

ATLAS OF THE UNIVERSE

J

upiter, 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 Mars.

Its 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 characteris-

tics 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 com-

pounds 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 for-

mation, 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.

▼ 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.

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