Cloud Composition

The clouds that are seen through Earth-based telescopes and recorded in pictures of Jupiter are formed at different altitudes in the planet's atmosphere. Except for the top of the Great Red Spot, the white clouds are the highest, with cloud-top temperatures of about 120 kelvins (K; -240 °F, or -150 °C). These white clouds consist of frozen ammonia crystals and are thus analogous to the water-ice cirrus clouds in Earth's atmosphere. The tawny clouds that are widely distributed over the planet occur at lower levels. They appear to form at a temperature of about 200 K (-100 °F, -70 °C), which suggests that they probably consist of condensed ammonium hydrosulfide and that their colour may be caused by other ammonia-sulfur compounds such as ammonium polysulfides. Sulfur compounds are invoked as the likely colouring agents because sulfur is relatively abundant in the cosmos and hydrogen sulfide is notably absent from Jupiter's atmosphere above the clouds.

Jupiter is composed primarily of hydrogen and helium. Under equilibrium conditions—allowing all the elements present to react with one another at an average temperature for the visible part of the Jovian atmosphere—the abundant chemically active elements are all expected to combine with hydrogen. Thus it was surmised that methane, ammonia, water, and hydrogen sulfide would be present. Except for hydrogen sulfide, all these compounds have been found by spectroscopic observations from Earth. The apparent absence of hydrogen sulfide can be understood if it combines with ammonia to produce the postulated ammonium hydrosulfide clouds. Indeed, hydrogen sulfide was detected at lower levels in the atmosphere by the Galileo probe. The absence of detectable hydrogen sulfide above the clouds, however, suggests that the chemistry that forms coloured sulfur compounds (if indeed there are any) must be driven by local lightning discharges rather than by ultraviolet radiation from the Sun. In fact, the causes of the colours on Jupiter remain undetermined, although investigators have developed several viable hypotheses.

Sulfur compounds have also been proposed to explain the dark brown coloration of the ammonia clouds detected at still lower levels, where the measured temperature is 260 K (8 °F, -13 °C). These clouds are seen through what are apparently holes in the otherwise ubiquitous tawny clouds. They appear bright in pictures of Jupiter that are made from its thermal radiation detected at a wavelength of 5 micrometres (0.0002 inch), consistent with their higher temperatures.

The colour of the Great Red Spot has been attributed to the presence of complex organic molecules, red phosphorus, or yet another sulfur compound. Laboratory experiments support these ideas, but there are counterarguments in each case. Dark regions occur near the heads of white plume clouds near the planet's equator, where temperatures as high as 300 K (80 °F, 27 °C) have been measured. Despite their blue-gray appearance, these so-called hot spots have a reddish tint. They appear to be cloud-free regions—hence the ability to "see" into them to great depths and measure high temperatures—that exhibit a blue colour (from Rayleigh scattering of sunlight) overlain with a thin haze of reddish material. That these so-called hot spots occur only near the equator, the elliptical dark brown clouds only near latitude 18° N, and the most prominent red colour on the planet only in the Great Red Spot implies a localization of cloud chemistry that is puzzling in such a dynamically active atmosphere.

At still lower depths in the atmosphere, astronomers expect to find water-ice clouds and water-droplet clouds, both consisting of dilute solutions of ammonium hydroxide. Nevertheless, when the probe from the Galileo spacecraft entered Jupiter's atmosphere on Dec. 7, 1995, it failed to find these water clouds, even though it survived to a pressure level of 22 bars—nearly 22 times sea-level pressure on Earth—where the temperature was more than 400 K (260 °F, 130 °C). In fact, the probe also did not sense the upper cloud layers of ammonia and ammonium hydrosulfide. Unfortunately for studies of Jovian cloud physics, the probe had entered the atmosphere over a hot spot, where clouds were absent, presumably caused by a large-scale meteorological phenomenon related to the downdrafts observed in some storms on Earth.

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