Topical Tens: 5th July: Jupiter

A year ago, in 2016, NASA's Juno spacecraft reached Jupiter and entered into its orbit after a five-year flight. Here are a few things we know about Jupiter.

Jupiter is big. Its equatorial diameter is 142,984 km and more than 1300 Earths could fit inside it. In terms of mass, it's 318 times bigger than Earth. Not only is Jupiter the biggest planet in the Solar System and the fourth brightest object in the sky after the Sun, Moon and Venus, it is actually twice as massive as all the other planets put together. It is so big that it actually emits more energy than it gets from the sun.
It also has the strongest gravity. A rocket would have to travel at 135,000 mph in order to escape its pull. Its magnetic field is also the biggest in the solar system - 14 times that of Earth.
Another record Jupiter holds is for the number of moons. It has 67 confirmed satellites and could have as many as 200. Most of them are probably asteroids captured by Jupiter's gravity. Almost all of them are less than 10 kilometers in diameter, and were only discovered when the Pioneer 10 arrived at Jupiter. The inner moons of Jupiter are called Metis, Adrastea, Amalthea, and Thebe. The largest four moons are called the Galilean moons because they were discovered by Galileo Galilei in 1610. These are Io, Europa, Ganymede and Callisto. His discovery provided the first proof that planets other than Earth could have moons. Ganymede is the largest moon in the solar system and is bigger than the planet Mercury.
Jupiter is almost a mini solar system in itself. In the sequel to Stanley Kubrick's classic film 2001: A Space Odyssey, 2010: Odyssey Two, Jupiter is converted into a star. This idea was picked up as well by the series Stargate SG-1, where the Aschen intend to turn Jupiter into a second sun. Could it be done, though? Theorists have speculated that it could if Jupiter had got 80 times more massive, because then, nuclear fusion would have occurred in its core. It's highly unlikely to happen. Theorists have also calculated that if Jupiter's mass increased by four times, it would actually get smaller because it would start to pull in on itself.
One of Jupiter's features is the Great Red Spot. Italian astronomer Giovanni Cassini first spotted that in 1665. We now know it's a huge storm, about three times as big as the Earth, which could have been raging for 350 years. Or perhaps not. The red spot Cassini saw was twice as big as the one we observe today. This may mean that the storm is gradually waning, or that as soon as one storm blows itself out, another will appear, and we're looking at a different one. The red spot circulates anticlockwise and takes six (earth) days to rotate completely. We don't know what makes it red, possibly it contains organic compounds.
It may be the biggest planet, but it has the shortest day. It turns on its axis once every 9 hours and 55 minutes. It spins so fast it flattens slightly at the poles. This, combined with its massive gravity, means Jupiter acts like a slingshot to any space debris that comes near it, or indeed, spacecraft which have carefully calculated how to use Jupiter's gravity to fling them out into deep space rather then using fuel.
At the centre of Jupiter is a rocky core, slightly bigger than Earth but weighing about 20 times more; probably not an Earth sized diamond as Arthur C Clarke postulated. The temperature near the planet's core may be hotter than the surface of the sun. This is covered by an ocean of hydrogen which is so compressed it has turned to liquid. This layer is about 1,000 kilometres deep and is the reason Jupiter's magnetic field is so strong. Above that is the atmosphere which is mostly hydrogen (90%). The clouds are made of ammonia crystals, sulphur, and mixtures of these two compounds. Finally, it has a ring system - four rings made of dust particles presumably captured after comets and asteroids collided with the moons.
Jupiter takes 4,333 days (11.9 years) to orbit the sun.
Jupiter was first observed by the ancient Babylonians. At least, they were the first to write down what they saw, around the 7th or 8th century BC. They saw Jupiter as the god Marduk, patron of the city of Babylon. To the Greeks, it represented Zeus and to Germanic and Norse people it represented Thor. Jupiter is named after the king of the Roman gods. Jupiter is associated with Thursday. In Romance languages, the word for Thursday is often derived from Jupiter (e.g., joi in Romanian, jeudi in French, jueves in Spanish, and giovedì in Italian).
Astrologically speaking, Jupiter rules Sagittarius and Pisces. It governs higher learning, philosophy, religion, geometry, exploring ideas, long distance travel, optimism, ethics, sport, gambling, success, prosperity, animals, humour, goodwill, mercy, the liver and pituitary gland. On the downside, it's associated with the disposition of fats, overindulgence and laziness (so blame Jupiter if you get fat!) It's also seen as having a protective role (since its slingshot effect gets rid of comets and Asteroids that might otherwise have hit us) and that of encouraging us to grow and expand.