A Hubble Space Telescope image of Saturn in true color.
Saturn is the most distant of the five planets known to ancient
stargazers. In 1610, Italian Galileo Galilei was the first astronomer to
gaze at Saturn through a telescope. To his surprise, he saw a pair of
objects on either side of the planet, which he later drew as "cup
handles" attached to the planet on each side. In 1659, Dutch astronomer
Christiaan Huygens announced that this was a ring encircling the planet.
In 1675, Italian-born astronomer Jean Dominique Cassini discovered a gap
between what are now called the A and B rings.
, Saturn is a gas giant. It is
made mostly of hydrogen and helium. Its volume is 755 times greater than
Earth's. Winds in the upper atmosphere reach 500 meters per second in
the equatorial region. (In contrast, the strongest hurricane-force winds
on Earth top out at about 110 meters per second.) These super-fast
winds, combined with heat rising from within the planet's interior,
cause the yellow and gold bands visible in its atmosphere.
Saturn's ring system
the most extensive and complex in our solar system; it extends hundreds
of thousands of kilometers from the planet. In fact, Saturn and its
rings would just fit in the distance between Earth and the Moon. In the
early 1980s, NASA's two Voyager
spacecraft revealed that Saturn's rings are made mostly of water ice,
and they found "braided" rings, ringlets, and "spokes" - dark features
in the rings that seem to circle the planet at a different rate from
that of the surrounding ring material. Some of the small moons orbit
within the ring system as well. Material in the rings ranges in size
from a few micrometers to several tens of meters.
Saturn has 46 known natural satellites (moons) and there are probably many more waiting to be discovered. The largest,
, is a bit bigger than the
. Titan is
shrouded in a thick nitrogen-rich atmosphere that might be similar to
what Earth's was like long ago. Further study of this moon promises to
reveal much about planetary formation and, perhaps, about the early days
of Earth as well.
In addition to Titan, Saturn has many smaller icy satellites. From
, which shows
evidence of surface changes, to Iapetus
, with one hemisphere
darker than asphalt and the other as bright as snow, each of Saturn's
satellites is unique.
Saturn, the rings, and many of the satellites lie totally within
Saturn's enormous magnetosphere, the region of space in which the
behavior of electrically charged particles is influenced more by
Saturn's magnetic field than by the solar wind. Images taken by NASA's
Hubble Space Telescope show that Saturn's polar regions have aurorae
similar to Earth's Northern and Southern Lights. Aurorae occur when
charged particles spiral into a planet's atmosphere along magnetic field
The next chapter in our knowledge of Saturn is already under way. The Cassini-
spacecraft arrived at Saturn on July 1, 2004 and immediately began sending
back surprising new information about the planet, rings and moons. The Cassini
orbiter is carrying the European Space Agency's Huygens probe, which will
descend through Titan's atmosphere in January 2005 and collect data on the
atmosphere and surface of the moon. Cassini will orbit continue to orbit
Saturn during a four-year primary mission.