again. So, the physical thickness and height of each layer is determined entirely by temperature." (Simon-Miller personal communication.)

Like Earth, the clouds that we see occur in the troposphere. In the lower troposphere, convection is the main method of moving the heat upward. In the upper troposphere, thermal radiation also plays a part. At the tropopause, convection almost stops [90]. Clouds generally cannot exist above the tropopause, except for the case in which strong convection causes the clouds to overshoot the tropopause a little and protrude into the stratosphere. The clouds in the troposphere are optically thick. We see the tops of the clouds illuminated by sunlight but we cannot see into the clouds. The cloud tops of the zones are generally higher than the cloud tops of the belts (Simon-Miller personal communication).

The stratosphere lies above the tropopause. In the stratosphere, heat is carried upward by thermal radiation, and there is no convection [91]. Temperature rises in the stratosphere with altitude. While the troposphere has distinct clouds, by contrast the stratosphere is composed of thin gases, sometimes interspersed or overcast with thin hazes or aerosols. Aerosols are small particles suspended in a gas. These gases are practically transparent, or optically thin. In the stratosphere, heat is absorbed from the Sun and from the radiation of Jupiter's own heat below. The stratosphere attempts to reradiate this heat upwards. However, the gases here are so thin, they cannot radiate heat away as rapidly as can be done at lower altitudes [92]. Thus, above the tropopause, temperature again rises with altitude, instead of falling as it does from 'sea level' up to the tropopause. At zero (0) altitude or 1 bar, the atmospheric temperature is ~165 K. The temperature at the tropopause is 105 K at a level of 100-160 mbar. The temperature at the upper stratosphere reaches about 170 K at 1 mbar. Here the temperature stabilizes until the thermosphere is reached at which point the temperature literally takes off. Beyond the stratosphere, the temperature in the thermosphere can reach between 850 and 1,300 K [93].

The ionosphere is defined as the upper layer of the atmosphere where there are many free ions and electrons. Hydrogen molecules are disassociated and ionized by solar ultraviolet radiation, and by electrons and ions raining down from the magnetosphere. The ionosphere extends several thousand kilometers above the molecular atmosphere. Here electrons can reach temperatures of 1,000-1,300 K. The thermosphere is defined as the upper layer of the atmosphere that is very hot. Here the neutral gas, 1,500-2,000 km above the cloud tops, can reach temperatures of 1,100 (±200) K. The thermosphere overlaps with the ionosphere [94].

We might suspect that color depends upon chemical composition; however, this is not necessarily the case. We refer to any agents that affect the shape of the reflection spectrum at continuum wavelengths in the extended visible spectrum from about 0.4 to 1 nm as chromophores [95]. To the question of color, spectra have not given any answers. Most potential colored substances (chromophores) do not produce distinct spectral lines, just very broad bands. Spectra of visible light show hardly any difference between the belts and zones [96]. All the major compounds predicted to form clouds in Jupiter's atmosphere are chemically simple and would be white. According to Rogers (1995), the main candidates for colored substances are alkali metals, ammonium hydrosulphide, sulphur, phosphorus, and organic polymers [97]. However, the constituent that causes the variations in the color of Jupiter's clouds is unknown [98].

Simon-Miller et al., (2001) performed radiative transfer analysis using data taken by the Galileo spacecraft Solid State Imager (SSI) during its nominal mission (December 1995 to December 1997). The objective was to use the methane band at 727 nm and 889 nm, and color sensitivities at 410 and 756 nm to identify the vertical position of cloud absorption that leads to coloration. For this analysis, Simon-Miller et al., (2001) selected areas in the EZ, NEB, the GRS, a cyclonic and anti-cyclonic oval, and a bluish-gray, 5-|im equatorial hotspot.

It is believed that Jupiter's visible cloud structure is dominated by hazes in the stable stratified upper troposphere and stratosphere, and by condensate clouds of ammonia, ammonium hydrosulfide, and water at deeper levels (Fig. 4.2) [100]. West et al. (1986), have presented an interpretation of their observations. Historically, belts and zones have been thought to be regions of downwelling and upwelling, respectively, and therefore are expected to have cloud decks of differing densities and elevations (Fig. 4.3). This idea was supported somewhat by the visible differences in coloration, with belts being reddish and generally darker. Color differences could arise either because (a) the clouds in belts are older and covered by a reddening agent (e.g., photochemical smog that has rained down or sunlight processing of the cloud particles), with less overturning than is seen in the zones, or (b) because they are deeper in the atmosphere, allowing sublimation of the overlying ammonia ice rime on a core of redder material [103]. Neither of these speculations has yet been verified, because of a lack of detailed information about vertical motions and about the distribution of cloud and coloring with height and latitude [104].

Altitude Pressure Temperature (km) (bars) (°K)

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