Jupiter is the largest of the planets in our Solar System and is the first of the gas giants. It’s the fifth planet from the Sun. The planet is named after the Roman god Jupiter, who was the god of the sky and thunder.

View of Jupiter taken by the Hubble Space Telescope in June 2019. Credit: NASA

Jupiter is a truly huge planet. It’s mass is approximately 318 times the mass of Earth, and it is 2.5 times as massive as all the other planets in the Solar System combined. Jupiter is comprised mainly of hydrogen and helium. It was thought that the core of the planet was solid and that there would be a hard transition between the core and the huge atmosphere surrounding it, kind of like the mud and water on the bottom of a lake. It turns out, however, that the there is not a sudden transition but that the core and atmosphere mix gradually going from solid to gas. It is though that there was a massive collision with another object and the core was shattered into pieces. After that the hydrogen and helium surrounded the shattered core.

The part of the atmosphere visible to the eye is dived into bands at different latitudes. There colour differences in the clouds are caused by plumes of gas containing phosphorous and sulphur. The planet rotates very fast, once every 10 hours and this causes strong jet streams which splits the gas into various bands. Jupiter has a very fast moving and dynamic atmosphere, and storms regularly form and disappear. The biggest storm on Jupiter is called the Great Red Spot and is the largest and longest lived storm in the Solar System. It is twice the width of the Earth and has been raging on Jupiter for as long as we have been able to see it. That’s over 300 years!

The Great Red Spot as seen by Voyager 1. Credit : NASA

Jupiter has 53 confirmed moons and 29 provisional moons. Most of these are small but the four most famous are Io, Europa, Callisto and Ganymede. These are known as the Galilean moons as they were discovered by Galileo when he first pointed the telescope at Jupiter. He noticed that there were four points of light moving around Jupiter. The Galilean moons were the first objects observed to orbit an object other than the Earth. This was was some of the first evidence that the the Earth was not at the center of the universe. These four moons are some of the biggest in the Solar System and are very interesting places.

Ganymede is the largest moon in the Solar System and is even bigger than the planet Mercury. Io is the most volcanically active in the Solar System with hundreds of active volcanoes. Interestingly the volcanic activity is caused by tidal forces on the moon. As the moon moves around Jupiter it is pulled by the strong gravity of the planet and also by the moons Ganymede and Europa. You can imagine this as two children fighting over a toy. This push and pull causes the moon to flex and stretch, heating the core by friction. Callisto is very heavily cratered and the Galileo spacecraft found evidence to indicate there could be a salty ocean beneath the rocky surface. Europa is smaller than Earth but scientists have strong evidence that there is a sub surface ocean on Europa that contains more water than all the Earth’s oceans combined. Europa is thought to be one of the most promising places to find look for microbial life in the solar System.

This composite includes the four Galilean moons. Shown from left to right are Io,Europa, Ganymede, and Callisto. Credit: NASA

Jupiter has been explored by a number of spacecraft over the years. The first to fly by the planet and get close enough to take pictures was Pioneer 10 in 1973. In 1979 Voyager 1 and 2 flew past Jupiter on their grand tour of the Solar System. On these flybys they discovered many moons, volcanoes on Io and also that Jupiter has faint rings! The first probe to orbit the planet was the Galileo spacecraft. It also carried a probe which detached from the spacecraft and plunged into Jupiter’s atmosphere, sending back data as long as it could. Over the years many other spacecraft, such as Cassini and New Horizons have flown past Jupiter on their way to the outer Solar System. The most recent mission to go to Jupiter is Juno. It reached the planet in 2016 and it is scheduled to be in orbit until July 2021. The purpose of Juno is to improve our understanding of the structure of Jupiter, the composition of the planet, its gravity and magnetic field.

The next planned missions to Jupiter are ESA’s JUpiter ICy Moons Explorer (JUICE) and NASA’s Europa Clipper. These are scheduled to launch in 2022 and 2024 respectively. JUICE will explore the moons Ganymede, Callisto and Europa, and will be the first spacecraft to orbit a moon other than the Earths moon, when it orbits Ganymede. The Europa Clipper is designed to explore Europa and it’s potential to be habitable for life.

Through a small telescope it is possible to see bands of colour and sometimes the Great Red Spot, when it is in view. It is also possible to see the four Galilean moons easily with a small telescope or binoculars. From night to night you can watch them move around the planet. Now is a great time to watch Jupiter. It can be seen in the south east from around midnight on. When you look at the tiny little points of light moving around the planet, remember that you are looking at exotic worlds in the distant Solar System that have subsurface oceans, volcanoes and may even be habitable.

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