Overview (The Solar System)
Callisto is the outermost of the four major satellites of the “gas giant” planet Jupiter. It was discovered with one of the earliest telescopes by Galileo Galilei in 1610. Hence, it is often referred to as one of the Galilean satellites. Callisto is one of the largest satellites in the solar system, ranking third behind Jupiter’s Ganymede and Saturn’s Titan. With a diameter of 4,800 kilometers (2,985 miles), it is nearly the size of the planet Mercury. Callisto is also tidally locked to Jupiter, meaning that its “day” is the same length as its month, 16.82 Earth days. As a result, the same side of the Satellite always faces Jupiter, just as the Moon always presents the same face toward Earth.
If the Galilean satellites had personalities, Callisto would be a frail old man. Unlike the young and vibrant Io, Callisto has neither volcanoes nor large mountains anywhere on its surface. In fact, its total lack of geological activity, both above and below the surface, means that its surface most likely resembles what the satellite looked like during its formation. This is at least partly due to the lack of tidal forces from nearby Jupiter. The lack of squeezing and pulling from Jupiter’s gravity reduced the heat and energy within the satellite, leading to a relatively tranquil geology. This unique surface gives astronomers and geologists a glimpse of not only the primordial Jovian system but also the primordial solar...
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Knowledge Gained (The Solar System)
The vast majority of Callisto data comes from the Voyager flybys of the late 1970’s and the multiple flybys of the Galileo Spacecraft during the late 1990’s. Before that, the satellite was, at best, a foggy image in ground-based professional telescopes and a minuscule, but predictable, pinprick of light in backyard telescopes. Even Hubble Space telescope images taken in October of 1995 showed a blurry surface. Only uncrewed space probes would produce the information needed to gain further understanding.
Both Voyagers 1 and 2, which took images on their way to the outer solar system, revealed a relatively dead world, battered by impact craters. Two decades later, Galileo returned to focus purely on the Jovian system. Its more sophisticated instruments offered higher-resolution imagery, magnetometric information, and spectroscopic information.
Galileo’s most significant discovery about Callisto was the possibility of an underground ocean, similar to Earth’s oceans. The discovery of water in the Solar system is always a major event because it is thought to be an essential ingredient for life. Water was already thought to exist on nearby Europa, and great efforts were made to ensure that Galileo would not contaminate the surface. This included deliberately driving the probe into Jupiter’s atmosphere at the conclusion of the mission. Water on Callisto was a much bigger surprise. Could Callisto now be added to the...
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Context (The Solar System)
Callisto is a wonderful example of how taking a second look leads to a different perception. The Voyager images offered snapshots of Callisto while racing through the solar system’s highway. The Galileo probe effectively pulled over and took a look around. Missions like Galileo, which observed the Jovian system from late 1995 to 2003, and Cassini, which began observing Saturn in 2004, offer a chance to understand the distant gas giant planets along with their rocky satellites. Data from Galileo have pointed to the possibility of water on Callisto and have produced debates over its internal structure and its trace of atmosphere—all from a world previously thought dead. Callisto has shown that every object in the solar system has a distinct and complicated personality, arising from a mysterious past, and that we have a long way to go when it comes to understanding our fellow travelers around the Sun.
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Further Reading (The Solar System)
Bagenal, Fran, Timothy E. Dowling, and William B. McKinnon, eds. Jupiter: The Planet, Satellites, and Magnetosphere. New York: Cambridge University Press, 2007. A collection of articles provided by recognized experts in their fields of study, this volume offers a comprehensive look at the biggest planet in the solar system. Excellent repository of photography, diagrams, and figures about the Jovian system and the various spacecraft missions that unveiled its secrets.
Carlson, Robert W. “A Tenuous Carbon Dioxide Atmosphere on Jupiter’s Moon Callisto.” Science (February 5, 1999): 283ff. A discussion of Galileo data regarding CO2 in Callisto’s atmosphere.
Cole, Michael D. Galileo Spacecraft: Mission to Jupiter. New York: Enslow, 1999. Provides a full description of the Galileo spacecraft, its mission objectives, and science returns through the primary mission. Particularly good at describing mission objectives and goals. Suitable for a younger audience.
Harland, David H. Jupiter Odyssey: The Story of NASA’s Galileo Mission. New York: Springer Praxis, 2000. Provides virtually all of NASA’s press releases and science updates during the first five years of the Galileo mission in a single volume, along with an enormous number of diagrams, tables, lists, and photographs. Also provides a preview of the Cassini mission. Although the book’s coverage ends before completion...
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Callisto (Myths and Legends of the World)
In Greek mythology, Callisto was one of many human women who were seduced or raped by . Daughter of the king of Arcadia, Callisto joined the followers of the goddess Artemisin Greek mythology, virgin goddess of the hunt; daughter of Zeus and Leto and twin sister of Apollo (identified with the Roman goddess Diana). As a member of this group, Callisto swore to remain a virgin. But Zeus was tempted by Callistohose name meant "most beautiful"nd tricked her into lovemaking.
After her encounter with Zeus, the unfortunate Callisto was turned into a bear by Zeus, his wife (Hera), or Artemis as punishment for breaking her vow of Still, Callisto bore Zeus a son named Arcas, whom Zeus protected. One version of Callisto's story says that Artemis shot the bear while hunting. Another says that Callisto was shot by her own son. Feeling sorry for her, Zeus placed Callisto in the heavens, where she became the constellation known as Ursa Major, the Great Bear.
See also ARCADIA; ARTEMIS; HERA; ZEUS.
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