Saturn as seen by the approaching Cassini spacecraft.
With the Sun behind Saturn, the Cassini probe was able to see two new rings and confirm the presence of two others.
Saturn is the sixth planet from the Sun. It is a gas giant, second only in size to Jupiter with a diameter more than 9 times that of Earth. It has a spectacular system of rings and a large collection of moons. Saturn also has the most flattened shape of any of the major planets. Its average density is so low that if placed in a big enough tub of water, it would easily float.
History of observations
Saturn is the most distant of the five planets known to ancient stargazers. At its brightest (magnitude -1.9), it outshines every star in the sky, including brilliant Sirius. In 1610, Galileo Galilei became 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, Christiaan Huygens announced that this was a ring encircling the planet. In 1675, Giovanni Cassini discovered a gap between what are now called the A and B rings.
Saturn's atmosphere consists of 96.3% hydrogen and 3.25% helium, with traces of methane, ammonia, ethane, ethylene, and phosphine. Because Saturn is colder than Jupiter, the more colorful chemicals sink lower in its atmosphere and can't seen. The result is markings that are much less dramatic than those on Jupiter, although bands and some small spots are still visible.
Alternate jet streams of east-west and west-east circulation, with speeds of up to about 1,800 kilometers per hour (1,100 mph), have been traced in the motion of the cloud tops and are responsible for the banded appearance of the planet. Electrical processes and heat from within Saturn enrich the layered chemical mixture of the atmosphere which probably transitions from superheated water near the core to ammonia ice at the observed cloud tops.
At Saturn's center is believed to lie a core of rocky material about the size of Earth, but more dense. Around this is a metallic hydrogen shell some 30,000 kilometers deep, surmounted, in turn, by a region composed of liquid hydrogen and helium with a gaseous atmosphere some 1,000 kilometers deep.
Saturn has a modest magnetic field similar in strength to Earth's. In 1997 the Hubble Space Telescope observed auroral displays caused by the interaction between the solar wind and the planetary magnetic field.
|mean distance from Sun||1,427 million km (886.9 million mi, 9.54 AU)|
|perihelion distance||1,349 million km (838.6 million mi)|
|aphelion distance||1,512 million km (940 million mi)|
|diameter: equatorial/polar||120,536 km (74,913 mi) / 108,728 km (67,574 mi)|
|equatorial diameter (Earth = 1)||9.449|
|mass (Earth = 1)||95.2|
|mean density||0.69 g/cm3|
|axial period||10.2 hours|
|orbital period||29.46 years|
|gravity at cloud tops (Earth = 1)||0.92|
|escape velocity||35.47 km/s (127,690 km/h, 79,360 mph)|
|atmospheric composition||96.3% hydrogen, 3.25% helium,
traces methane, ammonia, etc
|mean temperature (cloud tops)||-180 °C (-292 °F)|
|number of moons||23|