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Jupiter is the fifth and largest planet of the Solar System. It is over 300 times more massive than the Earth and has over 1300 times the Earth's volume. The mass of Jupiter is more than twice that of all the Other planets combined; no wonder it was named after the king of the gods. Jupiter orbirs the Sun at adistance of about 5.2 AU, completing an orbit every 11.9 Earth years. Surprisingly, despite Us size, Jupiter has the shortest 'day' of any planet, just under ten hours.

Considering the size of Jupiter, iis mass is actually quite low. The average density of Jupiter is only 1.3 times that of water (compared to Earth, whose density is about 3 times that of water). The reason for lhis is that Jupiter is composed mainly of gas. Together with Saturn, Uranus and Neptune it is one of the 'gas giants'. Like the Sun, most of Jupiter is hydrogen and helium, both very light elements, hence its low density.

Jupiter's surface appears to have alternate light and dark bands of colour parallel to its equator. These are known as 'zones' and 'belts'. Observations in the infrared show that the zones are cooler than the belts, which means they must be higher up in the atmosphere. The zones are points where high-pressure regions in the lower atmosphere rise up, and the belts are low-pressure regions where the gas falls back. This is a convection effect, taking heat from the planet's interior and passing it out into space, the banded Structure being a result of Jupiter's very fast rotation. The bands are different colors due to the different elements and molecules present in them and their different temperatures.

This rapid rotation also encourages massive Storms to form in the upper atmosphere. The most famous of these is the Great Red Spot, just south of the Jovian equator. The Great Red Spot was first seen in 1831 and has persisted ever since. It measures 15000 X 20000 km - large enough to swallow the Earth ten times. Observations show that it is slightly cooler than, and about 8 km higher than, the surrounding douds.

This picture was taken by WF/PC2 on 13 February 1995, when Jupiter was 961 million km away. The image clearly shows the Great Red Spot on the right, with three white Storms to its south-west. These storms have been observed for about 50 years, during which time they have evolved considerably They have been getting closer together while also moving further west from the Great Red Spot. The white colour of the clouds is caused by ammonia high in the atmosphere, which has been dragged from lower in the atmosphere by the storms. As the ammonia gets higher it cools until eventually it freezes and forms white ice particles.