Stardate 09:01:98
Have you seen it yet?
If you have been watching Jupiter through
the last few weeks, perhaps you have gone out viewing after about 10 p.m.
If you have, you may have noticed another bright point of light in the
sky, not quite as bright as Jupiter, further towards the east, and yellow
in color. If your curiosity drove you to point a telescope at this object,
you surely know what it is. This is the second largest planet in our solar
system, Saturn.
Containing about 1/3 of the mass of Jupiter,, Saturn is,
however, nearly 85% as wide as the king of the planets. This creates a
situation where Saturn is a lot less dense than Jupiter. In fact, the density
of Saturn is less than the density of water. Whereas the density of water
is I gram per cubic centimeter, the density of Saturn is about .69 grams
per cubic centimeter, making Saturn over 30% less dense than pure water.
What this means is that if you could find a bathtub large enough, Saturn
would float on water.
Like Jupiter, Saturn also appears to have a core of liquid
metallic hydrogen. Due to the fact that the pressures inside of Saturn
are much less than the pressures inside of Jupiter, there is significantly
less liquid hydrogen on Saturn than there is on Jupiter.
It is because of this that the magnetic field of Saturn
is much weaker than the magnetic field of Jupiter. This is due to the fact
that the magnetic field of Jupiter is caused by the swirling of the metallic
hydrogen in it's journey around the planet. The magnetic field of Saturn
is caused by a similar process, but to a lesser degree. In fact, the magnetic
field of Saturn is only 5% as strong as the magnetic field of Jupiter.
The magnetic field of Earth, incidentally, is caused by the swirling of
molten nickel and iron towards the core of the Earth.
The best estimates of the rotational rate of Saturn come
from measurements of the rotational rate of Saturn's magnetic field. This
gives us a measurement of 10 hours, 39 minutes, and 25 seconds for the
length of one Saturn day. If you were to photograph the cloud tops of Saturn
(which is all you can see anyway), and were to carefully measure how fast
they appeared to be rotating, you would find that the clouds are rotating
faster than the length measured from observing Saturn's magnetic field.
Also, like Jupiter, Saturn radiates more energy than it
receives from the Sun. Saturn actually radiates twice as much energy as
it receives from the Sun. This is greater than the amount of energy which
should still- be radiating into space from the contraction of the planet
caused by gravity. The source of this extra energy appears to be heat caused
by the falling of droplets of liquid helium falling inwards through the
less dense liquid hydrogen surrounding it.
The winds of Saturn blow (at the equator) at the remarkable
speed of 500 meters per second. That is I 100 miles per hour.
Another interesting aspect of Saturn's atmosphere is the
strange white clouds which seen to appear from time to time when it is
more sunny in Saturn's northern hemisphere, and winter in it's southern
hemisphere. These strange clouds seem to be compose o ammonia rising from
deeper down in Saturn's atmosphere.
Saturn is also surrounded by a methane haze, blurring
out many features which would appear as dramatic color bands similar to
Jupiter. Also the fact that Saturn is twice as far from the Sun as.Jupiter
means that it receives only 1/4 as much heat as does it's larger sibling.
This causes the atmosphere of Saturn to be more stable than the atmosphere
of Jupiter. This means there is less stirring of the atmosphere, giving
less chance for gases to be present at different zones in the cloud tops
of Saturn.
Surely the most recognizable feature of Saturn is its
mighty ring system. Once believed to be the only planet to have rings,
we now know that all four of the Jovian planets (Jupiter, Saturn, Uranus,
and Neptune) have ring systems. The ring system of Jupiter, however, is
only 1/1000 as bright as the rings of Saturn. The rings of Saturn were
first seen by Galileo in 1609, but the poor quality of his optics caused
him to believe that the rings were two separate objects, one on each side
of the planet, looking much like ears. We now know that the rings of Saturn
are composed of thousands of tiny little ringlets, orbiting around Saturn,
looking much like the grooves on a phonograph record. These ringlets appear
to be held in place by many little shepherd moons, tiny satellites which
herd the tiny dust and pebble size pieces of Saturn's rings into neat little
ringlets.
The astronomer Christian Huygens discovered
in 1659 that the rings of Saturn did not touch the planet at any point.
In 1675, Giovanni Cassini discovered the largest gap in the rings, known
since that time as the Cassini Division. This is easily visible in any
good quality back yard telescope, from dark skies on a steady night.
The contributions of these two great astronomers are now
being honored by having their names attached to the latest Saturn space
probe. This mission, the last of NASA's big ticket missions, is entitled
- the Cassini spacecraft. This mission will have a probe penetrate the
atmosphere of Saturn's largest moon Titan. This Titan probe is known as
Huygens.
Titan, the largest moon in the Solar system, is larger
than the planets Mercury and Pluto. It is easily visible using any backyard
telescope, and has an atmosphere thicker than that of the Earth. Most astronomy
magazines will give a chart for the positions of Saturn's moons when the
planet is visible. See if you can manage to spot this largest moon in the
solar system.