Everything about Jupiter is big, very big. It is the first of the gas giants and next to the Sun, it is the largest object in the solar system with a diameter of just under 88,850 miles. If you were to lump all the other planets together, Jupiter would still be over 2.5 times bigger than their combined size.
In some ways Jupiter could perhaps be better compared to the Sun rather than some of the planets. This is because of the gaseous nature of its composition and its huge size. Jupiter is made up of about 90% hydrogen and 9% helium. Other gases such as ammonia and methane are also present in trace quantities.
However, it is the hydrogen we need to look at in conjunction with Jupiter’s mass. If we think back to the article on the Sun, we said that the internal pressure inside the Sun (due to its mass) was enough to generate the temperatures and pressures needed to fuse hydrogen into helium with the release of a large amount of energy. The Sun of course is much larger than Jupiter but if it contained about 50 times more hydrogen, then Jupiter could become a star in its own right. But it falls short and remains a planet. However, the temperature near its core is very high at approximately 30,000ºC.
It is believed that Jupiter possesses a small rock and metal core surrounded by a thick layer of metallic hydrogen. This in turn is surrounded by an outer layer composed of liquid hydrogen and helium. The outermost layer is the only part we can see using normal telescopes. It is the top of the atmosphere and consists of hydrogen and helium in gaseous form. So if we were able to pass down through the different layers we would encounter gas first, then liquid, then molten metal before reaching the solid core. Not the kind of place to take a holiday.
Let’s now take a closer look at the visible atmospheric surface of the planet. The most striking feature is the coloured bands around the planet. Careful observation over time, show these bands to be in motion. They do in fact, indicate the presence of storms. You see, Jupiter has a relatively fast rotation period of just less than 10 Earth hours. Combine this with the heat rising up from its core and it becomes a very good recipe for creating storms. And, like everything else about Jupiter, these storms are massive. They rage across the whole planet with hurricane force winds. And what’s worse, storms on Jupiter can last for hundreds of years. The different colours also give us valuable information about the storms. The bands which are lighter in colour or whitish, indicate that here the gases are rising. The darker or reddish-brown bands tell us that the gases are descending.
The most famous and distinctive feature on the surface of Jupiter is the Great Red Spot. This is a huge storm measuring over 3 times the diameter of the Earth. To our knowledge, we believe it has been raging over 400 years. Although its name is a good description, it is continually altering its size, colour and shape. But it is still easily seen through a good telescope.
Many of you will have heard of the Aurorae Borealis or Northern Lights that can be seen from time to time in the northern regions of our planet. There is also a version which can be viewed in the southern hemisphere. They are caused by the Earth’s magnetic field interacting with particles contained in the solar wind. Jupiter also has such aurorae but because its magnetic field is 20,000 times stronger than the Earth’s; the effects produced are amazing. And like the Earth, they are centred on each of the poles.
When it comes to moons, it is no surprise to find that Jupiter has the greatest number at 63. It probably has many more but they will be small and perhaps too insignificant to count as such. However, there are four moons that are very significant. They are called the Galilean Moons and as their name suggests, they were discovered by Galileo. He constructed his own telescope not long after they had been invented and he immediately pointed his up at the night sky.
In 1610 he turned his telescope on Jupiter and observed these four moons in orbit. They are called Ganymede, Callisto, Io and Europa. Only Europa is smaller than our own Moon. Ganymede is the largest at 3,270 miles across. Unlike the two moons of Mars which are little more than smallish, dead lumps of rock, the Galilean moons each have something of interest for us.
Ganymede is bigger than the planet Mercury. It is believed to have a rocky core. The crust we think is made of ice. But the amazing thing is that we believe that Ganymede, Callisto and Europa may contain oceans of liquid water beneath the surface. This may sound strange as they are so far away from the Sun. However, the giant mass of Jupiter may provide the answer. The gravitational forces exerted by the planet could produce tidal movements so strong as to be able to generate heat which keeps the water in a liquid state.
Callisto is the second largest of Jupiter’s moons. It has a smooth surface pitted with impact craters. Rock and ice are the main constituents of its interior.
Io is the innermost of the Galilean moons and orbits the planet every 42.5 hours. Io is different from the others and has a claim to fame. It is the most volcanic body in the solar system. Whereas the other Galilean moons are composed of rock and ice, Io is a dynamic world. It is covered with hundreds of active volcanos. Molten lava continually erupts from these volcanos and through vents on the surface.
The last of the Galilean moons is Europa and it is special. The reason for this is that we suspect that out of all of the bodies in the solar system except the Earth, Europa may be home to life. Its outer surface is scarred but below, we think there is an ocean of liquid water which may contain bacterial life. But why should Europa’s ocean stand a better chance of supporting life than those on Ganymede and Callisto? The answer is that the ocean on Europa is close to the surface. There is only a thin crust of ice above it whereas the other two have a thick layer of rock on top.
So it’s time to leave Jupiter and its moons and the chance of finding life and move out to the next planet. It is also a gas giant but there is something which makes it the most recognisable planet in the solar system. It is Saturn and its amazing system of rings. That will be the subject in the next article in this series.
John Pullen 