Saturnus

Physical features

Saturn is an oblate spheroid, meaning that it is flattened at the poles, and it swells out around its equator. The planet's equatorial diameter is 120,536 km (74,898 mi), while its polar diameter (the distance from the north pole to the south pole) is 108,728 km (67,560 mi); a 9% difference. Saturn has a flattened shape due to its very fast rotation, once every 10.8 hours. Saturn is the only planet in the Solar System that is less dense than water. Even though the planet's core is very dense, it has a gaseous atmosphere, so the average specific density of the planet is 0.69 g/cm3. This means if Saturn could be placed in a large pool of water, it would float.

Storms and spots

Saturn's atmosphere is also known to form oval shaped clouds, similar to the clearer spots seen in Jupiter. These oval spots are cyclonic storms, the same as cyclones seen on Earth. In 1990, the Hubble Space Telescope found a very large white cloud near Saturn's equator. Storms like the one in 1990 were known as Great White Spots. These unique storms only exist for a short time and only happen about every 30 Earth years, at the time of the summer solstice in the Northern Hemisphere. Great White Spots were also found in 1876, 1903, 1933, and 1960. If this cycle continues, another storm will form in about 2020.

The Voyager 1 spacecraft found a hexagonal cloud pattern near Saturn's north pole at about 78°N. The Cassini−Huygens probe later confirmed it in 2006. Unlike the north pole, the south pole does not show any hexagonal cloud feature. The probe also discovered a hurricane-like storm locked to the south pole that clearly showed an eyewall. Until this discovery, eyewalls had only been seen on Earth.

Atmosphere

The outer part of Saturn's atmosphere is made up of about 96% hydrogen, 3% helium, 0.4% methane and 0.01% ammonia. There are also very small amounts of acetylene, ethane and phosphine. The hexagonal cloud The north polar hexagonal cloud first found by Voyager 1 and later by Cassini Saturn's clouds show a banded pattern, like the cloud bands seen on Jupiter. Saturn's clouds are much fainter and the bands are wider at the equator. Saturn's lowest cloud layer is made up of water ice, and is about 10 km (6 mi) thick. The temperature here is quite low, at 250 K (-10°F, -23°C). However scientists do not agree about this. The layer above, about 77 km (48 mi) thick, is made up of ammonium hydrosulfide ice, and above that is a layer of ammonia ice clouds 80 km (50 mi) thick. The highest layer is made up of hydrogen and helium gases, which extends between 200 km (124 mi) and 270 km (168 mi) above the water cloud tops. Auroras are also known to form in Saturn in the mesosphere. The temperature at Saturn's cloud tops is extremely low, at 98 K (-283 °F, -175 °C). The temperatures in the inner layers are much higher than the outside layers because of the heat produced by Saturn's interior. Saturn's winds are some of the fastest in the Solar System, reaching 1,800 km/h (1,118 mph), ten times faster than winds on Earth.

Interior

Saturn's interior is similar to Jupiter's interior. It has a small rocky core about the size of the Earth at its center. It is very hot; its temperature reaches 15,000 K (26,540 °F (14,727 °C)). Saturn is so hot that it gives out more heat energy into space than it receives from the Sun. Above it is a thicker layer of metallic hydrogen, about 30,000 km (18,641 mi) deep. Above that layer is a region of liquid hydrogen and helium. The core is heavy, with about 9 to 22 times more mass than the Earth's core.

Magnetic Field

Saturn has a natural magnetic field that is weaker than Jupiter's. Like the Earth's, Saturn's field is a magnetic dipole. Saturn's field is unique in that it is perfectly symmetrical, unlike any other known planet. This means the field is exactly in line with the planet's axis. Saturn generates radio waves, but they are too weak to be detected from Earth. The moon Titan orbits in the outer part of Saturn's magnetic field and gives out plasma to the field from the ionised particles in Titan's atmosphere.

Spokes

The Voyager space probe discovered features shaped like rays, called spokes. These were also seen later by the Hubble telescope. The Cassini probe photographed the spokes in 2005. They are seen as dark when under sunlight, and appear light when against the unlit side. At first it was thought the spokes were made of microscopic dust particles but new evidence shows that they are made of ice. They rotate at the same time with the planet's magnetosphere, therefore, it is believed that they have a connection with electromagnetism. However, what causes the spokes to form is still unknown. They appear to be seasonal, disappearing during solstice and appearing again during equinox.

History

The rings were first discovered by Galileo Galilei in 1610, using his telescope. They did not look like rings to Galileo, so he called them "handles". He thought that Saturn was three separate planets that almost touched one another. In 1612, when the rings were facing edge on with the Earth, the rings disappeared, then reappeared again in 1613, further confusing Galileo. In 1655, Christiaan Huygens was the first person to recognise Saturn was surrounded by rings. Using a much more powerful telescope than Galilei's, he noted Saturn "is surrounded by a thin, flat, ring, nowhere touching...". In 1675, Giovanni Domenico Cassini discovered that the planet's rings were in fact made of smaller ringlets with gaps. The largest ring gap was later named the Cassini Division. In 1859, James Clerk Maxwell showed that the rings cannot be solid, but are made of small particles, each orbiting Saturn on their own, otherwise, it would become unstable or break apart. James Keeler studied the rings using a spectroscope in 1895 which proved Maxwell's theory.

Exploration

Saturn was first explored by the Pioneer 11 spacecraft in September 1979. It flew as close as 20,000 km (12,427 mi) above the planet's cloud tops. It took photographs of the planet and a few of its moons, but were low in resolution. It discovered a new, thin ring called the F ring. It also discovered that the dark ring gaps appear bright when viewed towards the Sun, which shows the gaps are not empty of material. The spacecraft measured the temperature of the moon Titan.

In November 1980, Voyager 1 visited Saturn and took higher resolution photographs of the planet, rings, and moons. These photos were able to show the surface features of the moons. Voyager 1 went close to Titan and gained much information about its atmosphere. In August 1981, Voyager 2 continued to study the planet. Photos taken by the space probe showed that changes were happening to the rings and atmosphere. The Voyager spacecraft discovered a number of moons orbiting close to Saturn's rings, as well as discovering new ring gaps.

Drawing of Cassini in orbit around Saturn On July 1, 2004, the Cassini−Huygens probe entered into orbit around Saturn. Before then, it flew close to Phoebe, taking very high-resolution photos of its surface and collecting data. On December 25, 2004, the Huygens probe separated from the Cassini probe before moving down towards Titan's surface and landed there on January 14, 2005. It landed on a dry surface, but it found that large bodies of liquid exist on the moon. The Cassini probe continued to collect data from Titan and a number of the icy moons. It found evidence that the moon Enceladus had water erupting from its geysers. Cassini also proved, in July 2006, that Titan had hydrocarbon lakes, located near its north pole. In March 2007, it discovered a large hydrocarbon lake the size of the Caspian Sea near its north pole.

Cassini observed lightning occurring in Saturn since early 2005. The power of the lightning was measured to be 1,000 times more powerful than lightning on Earth. Astronomers believe that the lightning observed in Saturn is the strongest ever seen.