Beyond Mercury, Venus, Earth, and Mars — the four terrestrial planets of our solar system– there are four giant planets that are remarkably different from the first. These Jovian planets have intrigued humans for many hundreds of years, their very names rooted in Roman mythology. This knol will explore the characteristics of these large planets and how our knowledge of them sheds light on the universe in its entirety.
The Jovian planets, in the order one would pass them if they were leaving the solar system, are Jupiter, Saturn, Uranus, and Neptune. All named after gods in Roman mythology, the Jovian planets were once regarded as rulers among the heavens. Jupiter is named after the god Jupiter, or Jove– the king of gods.  The name is fitting since Jupiter is the largest planet in the solar system. The word Jovian is derived from the Roman god Jove, and also connected to the Roman emperor Jovian who ruled from 363-364 B.C.  Saturn is Jupiter’s father, while Uranus is lord of the sky and Neptune rules the sea.  Galileo studied the Jovian planets in the early 1600′s, of which then there were only two– Jupiter and Saturn. In 1781 Uranus was discovered, and then in 1846 the discovery of Neptune completed the modern list of Jovian planets.
The Jovian planets have been studied to a much greater extent during the last half century thanks to the development of satellites and other spacecraft. In the 1970′s Pioneer 10 and Pioneer 11 flew past Jupiter and Saturn, taking pictures and collecting various data.  Less than a decade later the Voyager spacecraft were launched.  These have been followed by other spacecraft that have all been sent up to deliver data about the Jovian planets humans find so fascinating.
Size and Composition
All of the Jovian planets are very large. Jupiter’s volume is 1,400 times larger than Earth’s, Saturn’s is 769 times larger, Uranus is 67 times greater, and Neptune’s (the smallest of the Jovian planets) is 50 times larger than Earth.  Interestingly enough, Jupiter only has 318 times more mass than Earth despite being so big. Saturn’s mass is 95 times Earth’s, Uranus’ is 14 times, and Neptune’s is 17 times. Jovian planets can be large but have a low relative mass when compared to the terrestrial planets because of their differences in composition. The Jovian planets resemble the Sun more than they resemble the terrestrial planets in that they are made largely of gas. To be more specific, they are mostly made of hydrogen and helium with atmospheres containing hydrogen based compounds like methane, ammonia, and water.  At the center of the planets there is a relatively small and dense core which the gas surrounds. The distance to the core of Jupiter is believed to be approximately 60,000 kilometers. 
Though the Jovian planets resemble the Sun, they are not stars because they do not undergo nuclear fusion in their cores. Jupiter’s mass would have to multiplied 80 times over before it would have enough gravity to undergo nuclear fusion.
Shapes and Rotation
The Jovian planets are shaped a little differently than the terrestrial planets. The rapid rotation they have makes their equators bulge out, giving them a slightly squashed appearance. Saturn has the most pronounced bulge with an equator that is 10% wider than it’s poles.  The size of the equatorial bulge depends on the interaction between the planet’s gravity and the rate of rotation. Since the Jovian planets have fast rates of rotation, equatorial bulges are large. These equatorial bulges help give the planet an extra gravitational pull that keeps its moons and rings lined up with the equator. 
Measuring how fast a Jovian planet rotates is harder than it is with terrestrial planets because the observable clouds of gas that surround it are being influenced by winds as well as planetary rotation. Also, clouds rotate at different speeds near the equator than they do near the poles.  To work around this problem scientists measure the interior speed of the Jovian by tracking emissions from charged particles that are caught in the magnetosphere.  By finding out the rotation speed of the magnetosphere we can calculate the rotation speed of the entire planet. A full rotation for Jupiter and Saturn each takes about 10 hours while Uranus and Neptune take 16 to 17 hours. 
The terrestrial planets do not have many moons or other orbiting objects surrounding them. The few exceptions are Earth, which has one moon, and Mars, which has two moons.  In stark contrast, the Jovian planets have more than 100 moons orbiting them. Some of these moons have atmospheres and geological features comparative to the terrestrial planets.
Moons of the Jovian planets are categorized in three different sizes:small, medium, and large. Since the small moons are so tiny they are of little significance, especially for our purposes. Instead, we will focus on medium and large moons.
The most commonly known moons of Jupiter are Io, Europa, Ganymede, and Callisto. Altogether, however, Jupiter has at least 62 moons.  Ganymede is one of the largest moons in the solar system, surpassing the size of Mercury. Io is a moon with a great deal of geological activity including lava flows and volcanoes.  All this activity makes it the most volcanically active world in the solar system.  Europa’s surface is made mostly of ice and water, Ganymede is composed of silicate rock and water ice, and Callisto has various rocks and ices. Io, Europa, and Callisto are all larger than Pluto.
The medium and large size moons orbiting Saturn are Mimas, Enceladus, Telhys, Dione, Rhea, Titan, and Iapetus. Titan is comparable to Ganymede in size since it is bigger than the planet Mercury. Titan is the only moon to have an atmosphere as thick as it does, and is also the only other object in the solar system which has been proven to have large bodies of water.  Titan’s surface is mostly ice water and rock.
The main moons of Uranus are Miranda, Ariel, Umbriel, Titania, and Oberon.
Neptune’s two most popular moons are Triton and Nereid. Triton is a large moon with a retrograde orbit and an icy surface made of metal and rock.  Nereid has an extremely strange orbit that causes the moon to vary in distance from Neptune by quite a wide range.  This orbit makes it the most eccentric satellite in the universe.
All of the Jovian planets have rings orbiting them, with Saturn’s rings
so large they can easily be seen from Earth. These rings are made of
many tiny pieces of rock and ice, or particles, which have congregated
together and orbit around the planet. 
The rings appear flat because the particles that make them all orbit in
relatively the same plane. These rings also appear solid because there
are many millions or billions of particles.
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