In 1995, after 6 years of travel to Jupiter, the atmospheric entry probe on the Galileo spacecraft descended into the atmosphere of Jupiter. Near the top of Jupiter’s troposphere at a temperature of about –140°C (about 130 K), it found that ammonia had condensed. Next it found a layer of ammonium hydrosulfide clouds at a temperature of about –80°C (about 190 K). Soon after, information from the probe ended. (While descending slowly via parachute to an atmospheric pressure of 22 bars and a temperature of about 100°C [373 K], the Galileo probe failed, presumably because its transmitter got too hot.) What lies below this level in Jupiter’s atmosphere must for now be left to theories and atmospheric models.
Why are some clouds so colorful, especially Jupiter’s? In their purest form, the ices that make up the clouds of the giant planets are all white, similar to snow on Earth. The colorful tints and hues must come from impurities in the ice crystals, just as syrups color snow cones. These impurities are probably elemental sulfur and phosphorus, as well as various organic materials produced by the photochemical action of sunlight on atmospheric hydrocarbons (carbon chains with hydrogen atoms attached). Ultraviolet photons from the Sun have enough energy to break apart molecules of hydrocarbons such as methane, acetylene, ethane, and others. The molecular fragments can then recombine to form complex organic compounds that condense into solid particles, many of which are quite colorful. Photochemical reactions also occur in Earth’s atmosphere. Some of the photochemical products produced close to the ground are called smog.
Uranus and Neptune are not bluish green because of clouds. The upper tropospheres of Uranus and Neptune are relatively clear, with only a few white clouds—probably composed of methane ice crystals—appearing here and there. Methane gas is much more abundant in the atmospheres of Uranus and Neptune than in those of Jupiter and Saturn. Like water, methane gas tends to selectively absorb the longer wavelengths of light—yellow, orange, and red. Absorption of the longer wavelengths leaves only the shorter wavelengths—green and blue—to be scattered from the relatively cloud-free atmospheres of Uranus and Neptune. This gives them a characteristic bluish green color. Molecular scattering contributes to the bluish color and is so strong in the clear Uranus atmosphere that it completely hides the thick ammonia and water cloud layers that lie far below.