When we look at Jupiter, whether it be through a telescope, or from spacecraft images. shows not the surface of the planet, but the atmosphere. The atmosphere appears as alternating bands of light regions, called zones, and dark regions called belts, that run parallel to the equator. The zones are higher in altitude than the belts, and are correspondingly lower in temperature. It is believed that the belts represent descending areas of low pressure. Jupiter radiates heat energy out to space by way of convection. The zones carry energy to the surface and then cool, and sink again.
It was the markings in Jupiter's clouds that first allowed astronomers to measure the giant planets' rate of rotation. As it turns out, the rotation rate varies with latitude. Near the equator the rotation rate is 9 hours 50 minutes. At the poles, the planet rotates in 9 hours 55 minutes. This varied rate is known as differential rotation. The Earth is solid and all parts rotate at the same rate. Jupiter is not solid, and such a fast rotation, with speeds at the equator of 43,000 km/hr, causes the planet to flatten at the poles.
It is this same high rate of rotation that powers the atmosphere, and causes it to stretch into the bands we see. Jet streams form between the boundaries of the belts and zones which create disturbances. These jet streams are very fast, over 3 times the speed of the fastest jet stream on Earth. These disturbances may be short lived, or they might last for many hundreds of years.
The most famous and longest lived of these disturbances is Jupiter's Great Red Spot. It was first observed by the English astronomer Robert Hooke in 1630. The spot changes some in size, but it is an average of about 14,000 km wide and around 40,000 km long. It is a rising area of high pressure and is higher in altitude than the zones and is also a few degrees cooler. The spot rotates counterclockwise, once every 7 days. Behind the spot is a region of turbulence form the atmosphere forced to flow around the spot.
What is the atmosphere composed of? More of less, Jupiter's composition is nearly an exact copy of the Sun. There is about 82 % hydrogen, 18 % helium and traces of nearly all other elements. Most of this is in the form of molecular compounds, ammonia, methane, molecular hydrogen and water. The upper areas of the zones are believed to be ammonia ice crystals. Liquid ammonia probably floats below that.
entire atmospheric structure is about 1000 km thick, but there does not
appear to be any distinct boundary between the atmosphere and what lies
below. Apparently it just gets denser until it reaches a total liquid state.
Copyright © 1997 Kathy A. Miles and Charles F. Peters II