The planet Jupiter is a hot gas giant in fifth place out from the sun. It is the largest of the sun’s satellites, being over 300 times larger than Earth. Easily visible from UK, it appears at night as a yellowish star, the 4th brightest in the sky. It was visible to Galileo in 1610. A few years later, Robert Hooke observed the Great Red Spot through the first reflecting telescope and watched as it appeared to move across the face of the planet. He therefore concluded that Jupiter revolved on its axis.
Jupiter has a very small rock core covered with liquid metallic hydrogen, an electrical conductor responsible for the planet’s tremendous magnetic field which extends outwards further than the orbit of Saturn. This magnetic field traps radioactive particles, forming a dangerous radioactive belt extending well beyond the planet.
The atmosphere of Jupiter is composed of approximately 90% hydrogen with just under 10% helium, small amounts of methane, traces of rock dust, water and oxygen, which makes it, in fact, very like the consistency of the original “star matter” from which the solar system evolved. It appears solid because its sheer size compresses the central matter, leaving a cloud-like surface through which tremendous storms rage. It is much cooler than the sun, and surprisingly contains less water, but is much hotter than Neptune.
Because little is visible through this cloud layer, we do not know much about the planet, despite close visits since 1973 by Pioneer 10, Voyager1 and 2, and Ulysses. Another space craft, appropriately named Galileo, spent eight years orbiting and observing Jupiter. Today, the gas giant is closely monitored by the Hubble Space telescope and great hopes are entertained that the massive new Space Web Telescope will be able to penetrate its secrets.
The cloud layer is hot and winds of up to 500 mph, generated by internal heat, tear through alternate layers of clouds, revolving in opposite directions, giving the appearance of light and dark alternating rings. The winds are thought to extend thousands of kilometres below the cloud layer. Amongst the coloured rings are darker spots of varying sizes of which the Great Red Spot is by far the largest. This is thought to be a storm vortex, a high-pressure anticyclone, like a hurricane three times the size of Earth, which has raged, amazingly, for over 400 years. No-one knows why it remains so stable. By comparison, the storms on cooler Neptune last only a few years, or even months.
It is believed that larger storms absorb smaller ones and since there is no immediate underlying land mass, they do not get dissipated. It was also believed that their colouring is due to different minerals, possibly iron, phosphorus and sulphur, in the clouds at different levels, which the storm tears apart to allow light to penetrate. This hypothesis has since been modified.
Due to Jupiter’s strong magnetic field, once air has started to move under the pressure caused by changes in temperature, Coriolis forces deflect it as the planetary body rotates. These forces are much stronger on Jupiter than on Earth, where they are the cause of tropical cyclones and hurricanes. The clouds around the Great Red Spot are in constant motion, but not always as circular or oval vortices. In 1979, unusual and very complex cloud patterns were seen to one side of the red spot, like tidal ripples breaking against each other.
In 2006, the Hubble Space Telescope’s Wide Field camera picked up a second red spot, actually photographing its formation from smaller, whitish, oval storms. A third spot appeared, also the size of earth, but drifting towards the Great Red Spot. This new spot activity is thought to be due to climate change, as the planet seems to be getting hotter towards the Equator. The spots are shown on film, extending above the cloud layer, giving rise to recent speculation that their red colouration may be due to the action of ultra violet light on trace amounts of chemicals, mainly phosphorous, in material dragged up from lower layers.
Now, the Great red Spot appears to be growing smaller. From analysis of wind velocity data from 1979 to 2006, it seems that that its widest diameter has shrunk over the period by 15% . Xylar Asay-Davis, from the University of California, says it is shrinking, but not slowing down. When there are more, smaller storms, heat is moved more efficiently from the Equator to the poles, evening out the climate and diminishing the larger spots.
This process can be reversed again, but on Jupiter, nothing happens in a hurry. The Great Red Spot may be around for a few hundred years yet.