The apparent magnitude of Jupiter varies to some extent because of its varying distance from the Earth, but this is not sufficient to explain all the observed changes. Since 1862, the mean opposition magnitude has ranged over 0.45 magnitudes so that, at the bright­est opposition, Jupiter was reflecting 50 per cent more light than at the faintest. This means that, as the patterns of the clouds of Jupiter vary, the albedo, which is the fraction of the incident sunlight which they reflect, also varies. The albedo also depends on the wavelength of the light or other radiation being measured, but in the visual range it is typically between 30 and 50 per cent.

The solar energy that is not reflected by Jupiter’s clouds is absorbed by the atmosphere. If just this amount of absorbed energy were re-emitted then Jupiter would have an effective temperature of 105 K, i.e. it would radiate the same amount of energy as a black body of this temperature and of the same size. Most of the energy is radiated at infrared wavelengths. Measure­ments show that the effective temperature is actually 125K, so that Jupiter is radiating twice as much energy as it absorbs from the Sun. There is therefore an internal energy source of equal power to the absorption of solar energy. This energy source has important effects on theories both of the internal structure of Jupiter and on the structure of its atmosphere. Spacecraft mea­surements show that the day and night sides of Jupiter have the same temperature. This implies that the absorbed solar energy is carried to the night side of Jupiter by the planet’s rotation much more quickly than energy is lost from the atmosphere, in agree­ment with theoretical calculations.

The belts, zones and other features of Jupiter’s atmosphere show that its structure varies from point to point and from time to time. We can however first consider the average structure of the atmosphere and ignore the local variations (weather) and the details of the dynamical processes. Calculations of a standard model atmosphere can then be made.

About 45 per cent of the incident energy is reflected and more than 10 per cent is absorbed fairly high in the atmosphere so that slightly more than 40 per cent must be absorbed by the clouds in the main body of the atmosphere, which is known as the tropo­sphere. A similar amount of energy comes from the internal source and enters the troposphere at its base. In the upper tropo­sphere the energy is transported by radiation, whereas in the deep atmosphere the main transport mechanism is convection.

The theoretical models of the atmospheric structure are subject to many uncertainties because the physical processes involved are not fully understood and the chemical composition is not com­pletely known.  The way in which temperature, pressure and density vary with depth according to one set of calculations. Despite the uncertainties involved, these results probably give a good idea of the overall atmospheric structure.

As mentioned above, the visible surface of Jupiter consists of two cloud layers. The upper layer of ammonia cirrus has its base where the temperature is about 150K and extends upwards to a temperature of 106 K – the point of zero depth . The position of the base of the lower cloud layer is less certain; some­where in the layers where the temperature lies between 200 and 225 K seems likely.

The two-dimensional structure of belts and zones strongly sug­gests that the thickness of the upper cloud layer varies consider­ably over the disc. The zones are known to reach about 20km higher in the atmosphere than the belts. Jupiter’s atmosphere is driven by the internal energy source rather than by the incident solar energy. This heat enters the atmosphere from below and sets up convection currents in the atmospheric material. As a result of the Pioneer missions, astronomers believe that the rising, heated material reaches the visible surface in the bright zones, where it cools, and then descends in one of the adjacent dark belts. Because of the planet’s rapid rotation the regions of ascending and descend­ing material form into regular belts and zones of constant latitude rather than into an irregular pattern of small cells, as happens on the Sun for example. An internally driven, rapidly rotating atmosphere such as Jupiter’s is inherently more stable than a more slowly rotating atmosphere that is driven by the incident sunlight such as is the case for the Earth’s atmosphere. There is also no variation in temperature during the day and night to disturb the atmosphere. As a result, large-scale changes in structure will take much longer on Jupiter than on the Earth.

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