Recent Earth-Based Studies of Jupiter
In the past, a great deal of planetary research was basically descriptive, consisting of visual observations and photography. Beginning in the 1960s, a new generation of planetary scientists began to apply the techniques of modern astrophysics and geophysics to the study of the solar system. Inspired in part by the developing space programs of the United States and the Soviet Union, scientists began to ask more quantitative questions: What are the surfaces and atmospheres made of? What are the temperatures and wind speeds? Exactly what quantities of different elements and isotopes are present? And how can these new data be used to infer the origin and evolution of the planets?
The major features of Jupiter are shown in schematic form. The planet is a banded disk of turbulent clouds; all its stripes are parallel to the bulging equator. Large dusky gray regions surround each pole. Darker gray or brown stripes called belts intermingle with lighter, yellow-white stripes called zones. Many of the belts and zones are permanent features that have been named. One feature of particular note is the Great Red Spot, an enigmatic oval larger than the planet Earth, which was first seen more than three centuries ago. During the years the spot has changed in size and color, and it escaped detection entirely for nearly fifty years in the 1700s. However, since the mid-nineteenth century the Great Red Spot has been the most prominent feature on the face of Jupiter. [2935]
N North polar region North north north temperate belt North north temperate zone North north temperate belt North temperate zone North temperate belt North tropical zone North equatorial belt Equatorial zone equatorial band South equatorial belt N. component S. component South tropical zone Great Red Spot South temperate belt South temperate zone South south temperate belt South south temperate zone South polar region S
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
November 20, 1976.
January 28, 1978.
December 19, 1978.
The development of a new science, infrared astronomy, in the 1960s made it possible to measure these low temperatures directly. In the case of a cloudy planet like Jupiter, the infrared emission evident at various wavelengths originates at different depths in the atmosphere. It is a general property of any mixed, convecting atmosphere that the temperature varies with depth; the rate of variation depends only on the composition of the atmosphere, the gravity of the planet, and the presence or absence of condensible materials to form clouds. On Jupiter it is about 1.9° C warmer for each kilometer of descent through the atmosphere. Thus, although the ammonia clouds are very cold, a little above -173° C, if one goes deep enough one can reach temperatures that are quite comfortable. With a variation of 1.9° C per kilometer, terrestrial “room temperature” would be reached about 100 kilometers below the clouds.
To measure the total energy radiated by a planet, it is necessary to utilize infrared radiation at wavelengths more than one hundred times longer than the wavelengths of visible light. Even when detectors were developed that could measure such radiation, it was impossible to observe celestial sources such as Jupiter because of the opacity of the terrestrial atmosphere. Even a tiny amount of water vapor in our own atmosphere can block our view of long-wave infrared. To make the required measurements, it is necessary to carry a telescope to very high altitudes, above all but a fraction of a percent of the offending water vapor.
In the late 1960s a Lear-Jet airplane was equipped with a telescope and made available by NASA to astronomers to carry out long-wave infrared observations from above 99 percent of the terrestrial water vapor. In 1969 Frank Low of the University of Arizona and his colleagues used this system to make a remarkable discovery: Jupiter was radiating more heat than it received from the Sun! Repeated observations demonstrated that between two and three times as much energy emanated from the planet as was absorbed. Thus Jupiter must have an internal heat source; in effect, it shines by its own power as well as by reflected sunlight. Theoretical studies suggest that the heat is primordial, the remnant of an incandescent proto-Jupiter that formed four and one-half billion years ago.
Images of Jupiter in visible light (below) and five-micrometer infrared light show the planet’s characteristic belts and zones. The infrared image reveals areas that emit large amounts of thermal energy. The source of the energy is thought to be breaks in the Jovian cloud cover, which allow investigators a glimpse of the deep regions of the atmosphere. One of the mysteries of Jupiter concerns its heat balance: The planet appears to radiate more heat than it receives from the Sun. [P-20957]
At the same time that the internal heat source on Jupiter was being revealed with long-wave airborne infrared telescopes, a new discovery was being made from short-wave infrared observations. The clouds of Jupiter are too cold to emit any detectable thermal radiation at a wavelength of 5 micrometers (about ten times the wavelength of green light). Nevertheless, images of Jupiter at 5 micrometers revealed a few small spots where large amounts of thermal energy were being emitted. The sources of the energy appeared to be holes or breaks in the clouds, where it was possible to see deeper into hotter regions. The discovery of these hot spots opened the possibility of probing deep regions of the Jovian atmosphere that had previously been beyond the reach of direct investigation.