Beyond the Asteroid Belt lies the realm of the giant planets. These behemoths of the outer Solar System are chiefly made up of gas. Percentage of these gases vary from planet to planet and mostly contain large amounts of hydrogen and helium. There are two classes of the Giant Planets, the Gas Giants and the Ice Giants. In this post we will talk about the Gas Giants, Jupiter and Saturn
Jupiter
Jupiter from Hubble Space Telescope 2019
The first planet we meet is Jupiter. The first of the gas giants.This planet was known from antiquity and is the third brightest object in our night sky after the Moon and Venus.
Jupiter orbits the Sun at 5.2 AU (Astronomical Units) and takes 12 years to make one revolution of the Sun. It’s day is 10 hours long. This speed causes Jupiter’s poles to appear flattened into a oblate spheroid shape.
The appearance through a telescope shows Jupiter has latitudinal bands across its face. Giant storms exist within these bands, the most well known is The Great Red Spot, a storm that was first noted in 1664 by Robert Hooke and was possibly observed even earlier.
Jupiter was first observed by Galileo in 1610. He noted that Jupiter was accompanied by three stars. Over subsequent nights he observed that these stars appeared to ‘dance’ with Jupiter and was sometimes was joined by a fourth star. After contemplating these observations he came to the conclusion that these stars revolved around and were, in fact, moons of Jupiter. These four moons became known as the Galilean Moons in his honour. The four four moons observed by Galileo were named, in order from the planet, Io, Europa, Ganymede and Calisto. Due to its closeness to Jupiter the gravity asserts tidal forces that heat up Io‘s core causing volcanos of Sulphur on the surface. About 40 volcanos have so far been mapped on Io‘s surface. Europa, Ganymede and Calisto all have surfaces of rock and ice with all of them possibly harbouring a water ocean beneath their surfaces.
NASA‘s Pioneer 10 and Pioneer 11 flew past Jupiter in December 1973 and December 1974 respectively. They returned the first close up pictures of the atmosphere of Jupiter. They showed that the bands in Jupiter’s atmosphere were far more complex than first thought. They also returned data on Jupiter’s magnetic and radiation fields which showed that these fields were far stronger than expected. Pioneer 10 and Pioneer 11 also discovered new moons orbiting Jupiter. Both spacecraft also photographed the Great Red Spot and showed it to be made up of huge storms in the atmosphere of Jupiter. Pioneer 10 and Pioneer 11 also discovered a ring system around Jupiter.
Saturn
Saturn from Cassini orbiter 2006
Saturn lies next in our journey to the outer planets. Sixth planet from Sun Saturn and the second of the gas giants. It is also well known for its wonderful ring system. First observed by Galileo in 1610, his telescope only showed a planet with what appeared to be two stars beside it. Later Galileo observed Saturn with its rings open. Seeing this confused Galileo and he was never able to fully understand what he was seeing through his telescope. It was left to a later astronomer, Christiaan Huygens who used a more powerful telescope to observe Saturn and determine that Saturn was actually surrounded by rings.
Saturn orbits the Sun every 29.5 years and like Jupiter, has a fast rotation rate of 10.5 hours. This again give Saturn a oblate spheroid shape.
Saturn has 146 known moons with 63 formally named. It also has 100’s of smaller moons of around 3 km. There are also hundreds of moonlets orbiting within Saturn’s rings.
The largest moon is Titan, discovered in 1610 by Christiaan Huygens, and is the most interesting.
Titan is larger than all the other material orbiting Saturn combined. But what makes Titan so special is that it has a thick atmosphere which obscures the surface.
In 1979 Pioneer 11 flew past Saturn but unfortunately it sent back low resolution pictures due to a problem with it’s main onboard camera. Pioneer 11 did however measure the temperature of the surface of Titan and found it is too cold for life as we know it.
Voyager 1 flew past Saturn in 1980 sending back high resolution pictures of Saturn’s rings and moons. A close flyby of Titan revealed more information about Titan and its atmosphere. This information showed that Titan’s atmosphere was impenetrable by visible light.
In August 1981 the probe Voyager 2 flew by Saturn sending back more high definition pictures of Saturn and its ring system. It also discovered moonlets orbiting within Saturn’s rings. This discovery proved the theory that the rings of Saturn were divided into smaller rings due to the moonlets orbit around Saturn.
The last probe to visit Saturn was the successful Cassini-Huygens mission. Made up of two probes. Cassini was to go into orbit around Saturn while Huygens would attempt to land on Titan.
One of Cassini’s first discoveries was that Saturn‘s moon Enceladus had a water ocean about 175 miles below its surface. This has led to Enceladus to become one of the most likely places to look for life elsewhere in the Solar System.
Cassini also measured Titan’s atmosphere, finding large quantities of hydrocarbons at its north pole.
In 2008 NASA announced that a tenuous ring system had been discovered around the moon Rhea. In 2010 they announced the discovery of a tenuous atmosphere.
In January 2017 the probe Huygens parachuted through the atmosphere of Titan sending back images and videos of its decent.
The atmosphere of Titan was shown to consist mainly of Nitrogen and other minor elements. These other elements combine to make Methane and Ethane which form clouds in Titan’s atmosphere. These clouds produce Methane rain and along with the wind form surface features like dunes, rivers, lakes, seas and deltas similar to those on Earth. There are also seasonal weather patterns. Titan’s Methane cycle is very similar to Earth’s water cycle. Titan has been described as the most Earth-like body in our Solar System.
Both Jupiter and Saturn are huge planets compared with the inner planets but they serve a purpose in keeping the Earth safe from large asteroids that could pass close to even hit us. This does not mean the Earth is completely safe from large asteroids but more would be hit more frequently if Jupiter, particularly Jupiter, and Saturn were not there. Life on Earth would have found it very hard to get going on if the Earth was continuously being bombarded by large asteroids.
One last thing. Our attempts at finding planets around other stars, exoplanets as they are called, have found that large gas giants, like Jupiter and Saturn, are present in a lot of stars that have planets. Some are much bigger than Jupiter. Some are much closer to their sun orbiting in a matter of only a few days.