Salem Press

Solar System
Callisto

Category: Natural Planetary Satellites

Study of Callisto, Jupiter's outermost natural satellite, has led to insights into the formation of the solar system, the possibilities for extraterrestrial life, and the protection from comet impacts that Jupiter gives to the inner planets of the solar system.

Overview
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 system.

Callisto's surface is twice as bright as Earth's Moon but still much darker than the surfaces of its Jovian siblings. The first few kilometers of the surface layer is primarily ice, with a darker material having leaked in at some point. Callisto's surface is uniformly covered in craters and is thought to be the most cratered satellite in the solar system. These impacts are the primary force that has shaped the planet, and sometimes great rings appear around the impact craters. The two largest features, Valhalla and Asgard, are respectively 3,000 kilometers (1,865 miles) and almost 1,600 kilometers (1,000 miles) in diameter. While impacts have been the primary force in shaping Callisto's surface, data from the Galileo space probe in the late 1990's showed that some minor erosion has occurred. This erosion is thought to be carbon dioxide sublimating through cracks in the surface ice.

Along with these large impact craters, there are numerous crater chains, or catenae. After the 1979 Voyager flybys, the catenae were thought to be the result of debris from asteroid impacts. This idea was called into question after the spectacular impact of Comet Shoemaker-Levy 9 into Jupiter's atmosphere during late May, 1994. This comet had come within a special distance from Jupiter, known as the Roche limit, and been broken up by the force of gravity. What was once one large comet was now a series of fragments traveling in formation. This event gave credibility to the idea of comets colliding with planets and satellites and has helped to explained Callisto's pockmarked surface.

While the surface has given scientists relatively overt information about the satellite's past, Callisto's interior remains shrowded in mystery and conjecture. With a density of 1.86 gram/centimeter³, Callisto's density is the smallest of the major Jovian satellites. Scientists at the National Aeronautics and Space Administration (NASA) believe that Callisto is made up of roughly equal parts rock and ice, but the exact internal structure is unclear. Early observations led Galileo scientists to believe that Callisto is undifferentiated, meaning it has the same composition throughout.

Most rocky bodies in the solar system, such as Earth, have multiple layers that form during their creation. Molten materials tend to separate out, or differentiate, due to density. Within Earth, for instance, there is a dense core of iron and some nickel. Moving away from the core are different layers of decreasing density. Initial readings from Galileo showed that this process had not taken place in Callisto. Newer data, from subsequent flybys, do not directly contradict this hypothesis but have made planetary scientists less certain. Further evidence for an undifferentiated interior comes from data showing that Callisto also lacks its own magnetic field, suggesting a lack of a metallic core.

Curiously enough, however, Callisto does alter Jupiter's magnetic field within its vicinity. Because this perturbation in the field arises from increased conductivity within the planet, scientists speculate that a subsurface ocean may exist. Only an ocean with the salinity similar to Earth's own oceans could explain the readings.

Callisto also has an extremely thin atmosphere composed primarily of carbon dioxide. With a pressure millions of times lower than Earth's, the atmosphere appeared, based on data from the Galileo flybys of 1998-1999, to have formed relatively recently. These data led scientists to believe that the atmosphere was no more than four years old and due to a combination of processes known as photoionization and magnetospheric sweeping. Photoionization takes place when ultraviolet rays (the same rays that cause sunburns) come in contact with individual carbon dioxide (CO₂) molecules; each CO₂ molecule ejects an electron, similar to the way a solar calculator generates current. Removal of an electron causes the molecule to become charged. Since charges interact with magnetic fields, Jupiter's enormous magnetic field acts like a giant broom and sweeps these ionized particles away from Callisto. Left unchecked, this process would eventually cause Callisto's atmosphere to fade away.

If the atmosphere is not transient, the carbon dioxide gas must be replenished on a continual basis. The obvious source of CO₂ gas is Callisto's icy surface. This ice would have to be located in a region that is permanently shadowed, away from direct light and protected from ionization. It has also been suggested that much of the carbon dioxide that exists on the satellite's surface, as well as this tenuous atmosphere, comes from the comet impacts that Callisto has sustained.

Knowledge Gained
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, magentometric 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 small, but growing, list of potentially fertile worlds within our solar system?

The possibility of a subsurface ocean arises from data on the local magnetic field around Callisto. Callisto does not possess an interior magnetic field but orbits well within the boundary of Jupiter's magnetic field. During multiple flybys, Galileo measured this magnetic field and detected fluxuations in its intensity. The local magnetic environment around Callisto is similar to an electromagnet. Whereas electromagnets have magnetic fields that are induced by the flow of electrons through a looped wire, Jupiter's magnetosphere does the opposite, capturing charged particles from the solar wind and creating electric currents in space. Galileo's instruments showed that this magnetic field was altered due to increased conductivity from the satellite itself. While surface ice would not have any effect, the phenomenon could be explained by a subsurface ocean with a salinity level similar to Earth's oceans, conduction of current due to the presence of dissolved salts. This hypothesis is supported by the fact that similar data were taken at Europa, where planetary scientists are more confident that water exists below the surface.

More controversial is the continuing debate over Callisto's differentiation, or lack thereof. This controversy arose from data regarding Callisto's moment of inertia, a measurement of mass that indirectly comes from a body's rotation. This is the phenomenon that controls an ice skater's rotation, increasing it if the arms are brought close to the body and decreasing it when the arms are extended outward. Planetary scientists take this information one step further to determine the composition of a planet or satellite. A moment of inertia of 0.40 would mean that Callisto is totally undifferentiated. Data from multiple passes by Galileo showed a moment of just 0.38, within one standard deviation of theoretical uniformity. This debate is likely to continue for many years, until another spacecraft is sent. Regardless of the answer, the idea that Callisto is not as differentiated as Ganymede, a satellite similar in size and in distance from Jupiter, hints at an interesting beginning of the Jovian system. Answering the question of Callisto's interior will give scientists insight into planet and satellite formation.

While its innards will remain a mystery, Callisto's surface has helped astromoners understand more about comets, comet impacts, and Jupiter's role as protector of the solar system's inner planets (those between it and the Sun). Before the discovery of Comet Shoemaker-Levy 9, the idea of comets impacting planets was not universally accepted. Watching the comet slam into giant Jupiter, and the subsequent "bruises" it temporarily left behind, made the idea of cometary impacts more acceptable. Scientists also learned that it was Jupiter that caused the comet to split into fragments in the first place, leading many to believe that the gas giant has done this in the past. The fact that crater chains exist on the Jupiter-facing hemisphere of Callisto is evidence of past impacts and further evidence that Jupiter is the vacuum cleaner of the solar system, keeping the inner planets safe from dangerous debris.

Finally, studying Callisto may reveal much about the future humankind, specifically the possibilities of colonizing the solar system. Project HOPE, or Human Outer Planet Exploration, is a futuristic concept mission put forth by NASA. Part of this exploration is a crewed mission to Jupiter, including a landing on Callisto. Callisto is an optimal choice for a human landing for two reasons. The first is its icy surface, which would provide both a source of water, allowing astronauts to "live off the land," and an opportunity for a first-rate study of impact geology. Second, Callisto's orbit places it in a region of low radiation from Jupiter. This remote, icy outpost would make an excellent location from which to study the Jovian system's past, present, and future.

Context
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 take 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.

Michael P. Fitzgerald

Further Reading
Bagenal, Fran, Timothy E. Dowling, and William B. McKinnon, eds. Jupiter: The Planet, Satellites, and Magnetosphere. New York: Cambridge University Press, 2007. A series 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, pp. 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 Books, 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 of the Galileo mission, what is missing can easily be found on numerous NASA websites.

Khurana, K. K., M. G. Kivelson, D. J. Stevenson, G. Schlbert, C. T. Russell, R. J. Walker, and C. Polanskey. "Induced Magnetic Fields as Evidence for Subsurface Oceans in Europe and Callisto." Nature 395 (October 22, 1998). This article is the resource for all discussions of the possible subsurface ocean on Callisto.

Leutwyler, Kristin, and John R. Casani. The Moons of Jupiter. New York: W. W. Norton, 2003. Casani was the original Galileo program manager, and this book offers a heavily illustrated discussion of the Galilean satellites as well as a number of the lesser known Jovian satellites. The authors attempt to accompany their scientific findings with an artful text, which may please the tastes of some readers more than others.

McKinnon, William B. "Mystery of Callisto: Is It Undifferentiated?" ICARUS 130 (1997): 540-543. This article explains why the question of Callisto's differentiation does not have a definitive answer.

Melosh, H. J., and P. Schenk. "Split Comets and the Origin of Crater Chains on Ganymede and Callisto." Nature 365 (October 21, 1993). Discusses the hypothesis that crater chains on Callisto come from previous comets similar to Shoemaker-Levy 9.

Showman, Adam P., and Renu Malhotra. "Galilean Satellites." Science 286 (October 1, 1999). An excellent overview of Jupiter's four largest satellites.

See Also
Europa; Ganymede; Io; Jupiter's Satellites; Planetary Satellites.