Next to Mars, going outward from the sun, is Jupiter. Between Mars and
Jupiter, however, there are more than three hundred million miles of
space, and the older astronomers wondered why this was not occupied by a
planet. We now know that it contains about nine hundred "planetoids," or
small globes of from five to five hundred miles in diameter. It was at
one time thought that a planet might have burst into these fragments (a
theory which is not mathematically satisfactory), or it may be that the
material which is scattered in them was prevented by the nearness of the
great bulk of Jupiter from uniting into one globe.
For Jupiter is a giant planet, and its gravitational influence must
extend far over space. It is 1,300 times as large as the earth, and has
nine moons, four of which are large, in attendance on it. It is
interesting to note that the outermost moons of Jupiter and Saturn
revolve round these planets in a direction contrary to the usual
direction taken by moons round planets, and by planets round the sun.
But there is no life on Jupiter.
The surface which we see in photographs (Fig. 12) is a mass of cloud or
steam which always envelops the body of the planet. It is apparently
red-hot. A red tinge is seen sometimes at the edges of its cloud-belts,
and a large red region (the "red spot"), 23,000 miles in length, has
been visible on it for half a century. There may be a liquid or solid
core to the planet, but as a whole it is a mass of seething vapours
whirling round on its axis once in every ten hours. As in the case of
the sun, however, different latitudes appear to rotate at different
rates. The interior of Jupiter is very hot, but the planet is not
self-luminous. The planets Venus and Jupiter shine very brightly, but
they have no light of their own; they reflect the sunlight.
Saturn is in the same interesting condition. The surface in the
photograph (Fig. 13) is steam, and Saturn is so far away from the sun
that the vaporisation of its oceans must necessarily be due to its own
internal heat. It is too hot for water to settle on its surface. Like
Jupiter, the great globe turns on its axis once in ten hours--a
prodigious speed--and must be a swirling, seething mass of metallic
vapours and gases. It is instructive to compare Jupiter and Saturn in
this respect with the sun. They are smaller globes and have cooled down
more than the central fire.
Saturn is a beautiful object in the telescope because it has ten moons
(to include one which is disputed) and a wonderful system of "rings"
round it. The so-called rings are a mighty swarm of meteorites--pieces
of iron and stone of all sorts and sizes, which reflect the light of the
sun to us. This ocean of matter is some miles deep, and stretches from a
few thousand miles from the surface of the planet to 172,000 miles out
in space. Some astronomers think that this is volcanic material which
has been shot out of the planet. Others regard it as stuff which would
have combined to form an eleventh moon but was prevented by the nearness
of Saturn itself. There is no evidence of life on Saturn.