Callisto (moon)
Callisto is the outermost of Jupiter's four largest moons, often referred to as one of the Galilean satellites, a title it earned following its discovery by Galileo Galilei in 1610. With a diameter of 4,800 kilometers (2,985 miles), it is one of the largest moons in the solar system, nearly the size of Mercury. Callisto is unique among its Jovian siblings due to its heavily cratered surface, which is the most pockmarked of any moon in the solar system, suggesting a long history of impacts. Unlike the geologically active moon Io, Callisto exhibits little geological activity, leading scientists to believe its surface has remained largely unchanged since its formation.
Recent studies indicate that beneath its icy crust, there may be a subsurface ocean, raising intriguing questions about the moon's potential for harboring life. Callisto has a thin atmosphere primarily composed of carbon dioxide, believed to have formed recently due to processes involving ultraviolet radiation and magnetic field interactions. As researchers continue to study Callisto through missions like the Galileo spacecraft, it becomes an important subject for understanding the diversity of celestial bodies in our solar system and the history of planetary formation. Furthermore, Callisto is being considered for future human exploration missions, given its resources and relatively low radiation environment.
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Callisto (moon)
The study of Callisto, Jupiter’s outermost natural satellite, has led to insights into the solar system's formation, the possibilities for extraterrestrial life, and the protection from comet impacts that Jupiter gives to the solar system's inner planets.
Overview
Callisto is the outermost of the four major satellites of the “gas giant” planet Jupiter. Galileo Galilei discovered the planet with one of the earliest telescopes 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 about 4,800 kilometers (2,985 miles), it is nearly the size of Mercury. Callisto is also tidally locked to Jupiter, meaning 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.
Callisto would be a frail old man if the Galilean satellites had personalities. Unlike the young and vibrant Io, Callisto has neither volcanoes nor large mountains anywhere on its surface. 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 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 the primordial Jovian system and 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 leaking in at some point. Callisto’s surface is uniformly covered in craters and is considered the most cratered satellite in the solar system. These impacts are the primary force that has shaped the planet, and sometimes, great rings around the impact craters appear. The two most prominent features, Valhalla and Asgard, are 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 1990s showed some minor erosion. This erosion is considered carbon dioxide sublimating through cracks in the surface ice.
Along with these large impact craters are numerous crater chains or catenae. After the 1979 Voyager flybys, the catenae were thought to result from debris from asteroid impacts. This idea was called into question after the spectacular impact of Comet Shoemaker-Levy 9 into Jupiter’s atmosphere in 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 a series of fragments traveling in formation. This event gave credibility to the idea of comets colliding with planets and satellites and helped explain Callisto’s pockmarked surface.
While the surface has given scientists relatively overt information about the satellite’s past, Callisto’s interior remains in mystery and conjecture. With a density of 1.86 grams/centimeter3, Callisto’s density is the smallest of the major Jovian satellites. Scientists at the National Aeronautics and Space Administration (NASA) believe that Callisto comprises 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 or differentiate due to density. For instance, Earth has 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 occurred in Callisto. Further data from subsequent flybys do not directly contradict this hypothesis but have made planetary scientists less confident. Other evidence for an undifferentiated interior comes from data showing that Callisto lacks its 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 oceans could explain the readings. More recent data has led scientists to conclude that if such an ocean exists, it is located deeper beneath the surface than previously believed.
Callisto also has a thin atmosphere composed primarily of carbon dioxide. With a pressure millions of times lower than Earth’s, the atmosphere appeared to have formed relatively recently, based on data from the Galileo flybys of 1998-1999. These data led scientists to believe that the atmosphere was no more than four years old due to a combination of processes known as photoionization and magnetospheric sweeping. Photoionization occurs 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 how 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, sweeping these ionized particles away from Callisto. This process would eventually cause Callisto’s atmosphere to fade if left unchecked.
The carbon dioxide gas must be replenished continually if the atmosphere is not transient. The apparent source of CO2 gas is Callisto’s icy surface. This ice would have to be located in a permanently shadowed region, away from direct light and protected from ionization. It has also been suggested that much of the carbon dioxide on the satellite’s surface and this tenuous atmosphere comes from the comet impacts that Callisto has sustained.
Knowledge Gained
Most Callisto data comes from the Voyager flybys of the late 1970s and the multiple flybys of the Galileo spacecraft during the late 1990s. 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 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 important because it is essential for life. Water was thought to exist on nearby Europa, and great efforts were made to ensure Galileo would not contaminate the surface. This included deliberately driving the probe into Jupiter’s atmosphere after the mission. Water on Callisto was a much bigger surprise. Could Callisto now be added to our solar system's small but growing list of potentially fertile worlds?
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 fluctuations in its intensity. The local magnetic environment around Callisto is similar to an electromagnet. Electromagnets have magnetic fields 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 by increased conductivity from the satellite. 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 and the conduction of current due to dissolved salts. This hypothesis is supported by similar data 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 phenomenon 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 undifferentiated. Data from multiple passes by Galileo showed a moment of just 0.38, within one standard deviation of theoretical uniformity. This debate will likely 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 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 astronomers understand 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 Jupiter 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 proof that Jupiter is the solar system's vacuum cleaner, keeping the inner planets safe from dangerous debris.
Researchers continue to work to gain more information on the moon. Recent simulations in a lab of Callisto's icy surface indicate that the moon may have an ozone. An ozone is important for protection from the sun in the formation of molecules necessary for life. While the Hubble Space Telescope has hinted at sulphur dioxide ice on Callisto, the simulation mimicking the environment found that the sulphur dioxide molecules were broken down by the irradiation, releasing oxygen. This revelation strongly supports the idea of an ozone layer. Scientists hope to glean further information from Europe's Space Agency's 2023 launch of Jupiter Icy Moons Explorer (JUICE). The mission will perform 21 close flybys of Callisto.
Studying Callisto may reveal much about the future of 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 would include a crewed mission to Jupiter, with 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 water source, 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 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 took a look around. Missions like Galileo, which observed the Jovian system from late 1995 to 2003, and Cassini, which observed Saturn from 2004 to 2017, offer a chance to understand the distant gas giant planets and their rocky satellites. Data from Galileo have pointed to the possibility of water on Callisto and have produced debates over its internal structure and trace of atmosphere—all from a previously thought dead world. 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 understanding our fellow travelers around the Sun.
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