Jupiter orbits the Sun in 11.9 years and lies at 5.2 AU from the Sun (one AU = 150 million kilometres). Its interior is believed to harbour a small rocky core, about the size of the Earth, surrounded by a mantle of liquid metallic hydrogen. "Metallic" in this case means that it is under so much pressure that the electrons dissociate from the nuclei and move freely in the liquid, allowing it to conduct electricity. Above this layer is "normal" liquid hydrogen, and finally the picturesque, but violent, atmosphere, about 1000 km thick.
This planetary heavyweight measures 142,800 km in equatorial diameter, more than 11 times the diameter of the Earth, yet it whips around in only 9 hours 55 minutes. On Earth, low- and high-pressure weather systems form cyclonic or anti-cyclonic spiral patterns, and dissipate quickly over land where friction and surface features absorb their energy. On Jupiter, however, the rapid spin drags these features out to the extent that individual low- and high-pressure systems wrap around the whole planet, forming the familiar belts and zones we are able to discern with our telescopes. The famous "Great Red Spot" (GRS) is a long-lived storm feature which has been observed continuously since its discovery in 1666. The GRS measures about 25,000 by 12,000 km.
Jupiter's polar diameter is only 133,50 km, accounting for its oblate appearance, which is evident, even in small telescopes. The planet has an axial tilt of only 3°.
The atmosphere contains hydrogen, helium and traces of water, ammonia, methane and other organic compounds. The bluish clouds are the warmest and lie at the lowest altitude. The browns, whites and reds are higher, in that order. Wind speeds are high. The Galileo atmospheric probe observed speeds of 360 km per hour at the cloud-tops, and over 600 km per hour through the next 35 km of atmosphere. The atmosphere exhibits aurorae, lightning and continuous cyclonic storms along the margins between the belts and zones. Here we see delicate swirling patterns, and many "white spots", which are temporary features similar to, but smaller than the GRS.
Jupiter has a strong magnetic field, generated by the metallic hydrogen interior. Its field is 4000 times stronger than Earth's. The magnetosphere extends 20 Jovian radii into space, and interacts with the inner moon, Io, causing emissions from the latter's volcanoes to form into a plasma torus (a donut-shaped ring) of ionized sulphur and oxygen around Jupiter.
Jupiter has 16 known moons in all. The four brightest are visible in binoculars and are dubbed the "Galilean Moons" after their discoverer. These are, in order from Jupiter, Io, Europa, Ganymede and Callisto. (More on Jupiter's moons in the next issue.)
Like all the gas giants, Jupiter has a ring system, although it is not visible from Earth. The first ring was discovered by Voyager 2 in 1979 after the spacecraft travelled beyond the planet and looked back towards the Sun. A second, very tenuous ring was revealed through subsequent processing of the Voyager 2 data, and announced in 1986. These ring particles, about the size of particles in cigarette smoke, orbit retrograde around Jupiter (in the opposite direction to the moons' orbits). The rings are made mainly from dark non-reflective dust, unlike Saturn's magnificent system which is made from chunks of ice. Whilst the rings are possibly permanent features, their individual particles are constantly replenished. They are believed to arise from the surfaces of moonlets embedded within the ring, which are bombarded by high-speed micrometeorites. After a brief stay, they spiral into the planet 50,000 km below.
Logic tells us that Jupiter should be a cold world, being so distant from the Sun. And liquid hydrogen, in our experience, is only formed in very cold temperatures. In Jupiter's case, however, it is simply pressure from the vast mass of gas above which forces the hydrogen into a liquid. If you find this hard to imagine, just think of an LP gas cylinder, which contains "gas" kept in the liquid state by pressure. At the same time, Jupiter continuously radiates twice as much heat as it receives from the Sun. This is the residual heat from the original gravitational collapse of gas which formed the planet.
Many readers will recall the dramatic impact of Comet Shoemaker-Levy 9 onto Jupiter in 1994. This event was witnessed by amateurs all over the world with effects still obvious after two weeks. On the night of the first impact, I happened to be working with Koolang Observatory at a camp on the northern Central Coast. I was showing Jupiter to a queue of children when the first black blotch came into view. I remember my tremendous excitement, and my frustration at not being able to share that moment with my colleagues (busy with telescopes on other objects, and inside giving talks), and in particular, with my husband, who was "keeping the home fires burning" (as astronomers' spouses tend to do!). The moment I got home that night, though, I set up the telescope to give him a glimpse, and he told me all about how they had shown it on the TV news!!
Few people are aware, however, that an earlier impact was recorded. A Japanese researcher, Isshi Tabe, found a drawing by the Italian astronomer Giovanni Cassini, which has been interpreted as a comet impact in 1690. It shows the position of the disturbance on the planet, and its changing appearance over 18 days, from 5th to 23rd December.
To the Chinese, Jupiter is called Sui xing, the "year star". Its complete journey through the constellation takes approximately 12 years, and is the inspiration for the Chinese chronology based on twelve years, one for each of the animals: Rat, Ox, Tiger, Rabbit, Dragon, Snake, Horse, Sheep, Monkey, Cock, Dog and Pig. Other Asian cultures also have similar patterns of time-keeping based on a 12-year cycle.