Wildt had already suggested the basic gases in the atmosphere of Jupiter: primarily hydrogen and helium, with much smaller quantities of ammonia and methane. Undetected but possibly also present were nitrogen, neon, argon, and water vapor. The abundance of helium was particularly a problem; although it was presumably the second-ranking gas after hydrogen, it has no spectral features in visible light and its presence remained only a hypothesis, unconfirmed by observation.
Although the presence of a gas can usually be inferred from spectroscopy, solids or liquids cannot normally be detected in this way. Thus the composition of Jupiter’s clouds could not be determined directly. However, the presence of ammonia gas provided an important clue. At the temperatures expected in the upper atmosphere of the planet, ammonia gas must freeze to form tiny crystals of ammonia ice, just as water vapor in the Earth’s atmosphere freezes to form cirrus clouds. Most investigators agreed that the high clouds covering much of Jupiter must be ammonia cirrus. But ammonia crystals are white, so the presence of this material provides no explanation for the many colors seen on Jupiter. Additional materials must be present—perhaps colored organic compounds, produced in small amounts by the action of sunlight on the atmosphere.
Because Jupiter is five times farther from the Sun than is the Earth, a given area on Jupiter receives only about four percent as much solar heating as does a comparable area on Earth. Thus Jupiter is colder than Earth; even though it may be warm deep below its blanket of clouds, Jupiter presents a frigid face.
These blue filter photographs of Jupiter were taken at Mauna Kea Observatory, Hawaii. They show changes on Jupiter’s surface between 1973 and 1978, with the dates of the observations.
July 25, 1973
October 5, 1974
October 2, 1975