Jupiter's satellites

Jupiter has many natural satellites, among which are some of the largest and most intriguing in the solar system. Pioneer 10, Pioneer 11, Voyager 1, Voyager 2, and Galileo collected photographs and other vital information about several of these Jovian satellites, which have been used to describe the histories of these bodies and the solar system in general.

Overview

The four largest satellites of Jupiter—Io, Callisto, Europa, and Ganymede—were discovered in 1610 by Galileo Galilei and have been objects of curiosity for astronomers since that time. By 1990, astronomers had discovered sixteen moons around Jupiter—fourteen using Earth-based telescopes and two using photographs from Voyager. One of the fourteen was confirmed by the Voyager spacecraft spacecraft. After the arrival of the Galileo spacecraft in orbit about Jupiter, and in cooperation with space-based and Earth-based telescope observations, the discovery of Jovian satellites increased regularly. Jupiter has between eighty-five and ninety satellites, several of which are unnamed, provisional satellites. This number will continue to change as previously undetected satellites are discovered or as comets and asteroids enter Jupiter's orbit and become new satellites. The four largest satellites of Jupiter are larger or roughly equivalent to the size of Earth's Moon, while most of the other satellites are less than ten kilometers in diameter. Satellites of the outer planets are classified as regular if their orbits are nearly circular and are in or near the plane of the equator of the planet. Irregular satellites have highly elliptical orbits, an orbital plane tilted relative to the equator, or revolve in a retrograde direction.

The largest Jovian satellites, the often-called Galilean moons, are all regular satellites, as are the four small satellites whose orbits are nearer to Jupiter than Io’s—Metis, Adrastea, Amalthea, and Thebe. Most of Jupiter's outer satellites have irregular orbits and are thought to have been asteroids or comets that were pulled into Jupiter's orbit. Only the Galilean satellites and Amalthea have been examined closely by spacecraft. The other satellites are too small and too distant from the probes that have passed through the Jovian system to be effectively studied.

Many of the outer planets are known to have icy surfaces rather than the rocky surfaces that characterize the terrestrial planets (Mercury, Venus, Earth, and Mars) and their satellites. Ice behaves somewhat like a plastic unless its temperature is less than 133 kelvins—that is, it will flow and not retain a structure over long periods. This process is called plastic deformation. Rocky surfaces do not undergo plastic deformation. It is expected, therefore, that the terrestrial planets and satellites would retain structural features such as craters indefinitely, while an icy satellite might not be able to do the same unless the temperature is always below 133 kelvins.

Io

Io is the Galilean satellite nearest Jupiter. It is a rocky satellite with little or no remaining water. In fact, it may be the driest place in the outer solar system. Its density is similar to that of Earth’s Moon, so it is assumed that its bulk composition must also resemble that of the Moon. Its surface features, however, are dramatically different from those of Earth's Moon. The surface displays yellow, red, brown, black, and white materials. Io shows no cratering but contains layered lava plains, volcanic mountains, calderas, and other evidence of volcanism. Of the nine active volcanoes found by Voyager 1, seven were still active when Voyager 2 passed. The highest mountains were about nine kilometers tall. The polar areas are darker than the remainder of Io and show evidence of long periods of deposition, faulting, and erosion. Significant changes in Io’s volcanic activity were noted when the Galileo spacecraft arrived in 1996, fifteen years after Voyager 1 passed by.

Io’s volcanic activity takes on several forms. Plume activity is associated with the region 45° north and south of the equator. The material in the plumes is ejected at a velocity of 3,200 kilometers per hour. Much of this material is sulfur dioxide, which then crystallizes and falls back to the surface as a white solid. The material in the plumes is apparently carried to the surface along fissures in the surface rather than through pipe vents like those characteristic of volcanoes found on Earth. Some of Io’s volcanoes produce a lava flow rather than a plume. Hot spots exist on the surface where a large amount of energy is transferred from the interior of Io but where the quantity of volatile material available is insufficient to produce a plume. The location of such hot spots is not limited to the band in which the plume volcanoes are found. As a consequence of the extensive volcanic activity, the surface of Io changes rapidly. Significant details changed between the flyby of Voyager 1, the flyby of Voyager 2, and the arrival of the orbiting Galileo spacecraft. Galileo discovered that Io has an iron core.

Europa

Europa also provided surprises for the Voyager scientists. Density measurements imply that the moon is about 90 percent rocky core and 10 percent water ice as the crust. Because of its nearness to Jupiter, it was expected to have an extensively cratered surface. Voyager photographs of Europa, however, show one of the smoothest surfaces in the solar system. Few craters are visible. The surface is marked with long, narrow lines resembling an egg's cracking pattern, along with dark spots and mottling. Lines are regions where the crust cracked, and water from the interior was squeezed out onto the surface. A reflectivity of 64 percent is an indicator of relatively pure water ice on the surface of Europa. Since an old surface would be expected to collect dust and other space debris and produce a dirty surface with low reflectivity, the pure water ice implies some process that continually renews Europa’s surface. After Galileo's close flybys in 1996 and 1997, scientists concluded the ice crust is no thicker than 150 kilometers and floats on a liquid ocean.

Ganymede

Ganymede is the largest of the Jovian satellites, although it is not much larger than Callisto. Ganymede has a metallic core surrounded by a layer of ice and silicates, and its crust is probably thick water ice. The metallic core generates a magnetic field. About one-half of Ganymede’s surface has a dark, cratered terrain, while the remaining half is much lighter and has fewer craters.

Lighter areas of Ganymede have a series of parallel mountains and valleys reminiscent of the low Appalachian Mountains in North America. These may have developed from long cracks or faults that are separated by strips of land that have alternately been lifted or depressed. Depressed areas could have flooded with liquid water from the interior. This type of mountain building results from tension in the crust rather than the compression of the crust that is believed to have caused the Appalachian Mountains to be uplifted. The reflectivity of the lighter areas is about 40 percent, while in the more heavily cratered areas, it is only 25 percent. Bright ray craters are found in all parts of the satellite. (A ray crater is one in which the debris cast out of the crater at the time of impact is distributed along radial directions like rays or beams.) These craters are evidently the result of water from under the crust that has splashed out on the surface when some object collided with Ganymede. A dome 260 kilometers in diameter and 2.5 kilometers in height that is surrounded by several small craters may be evidence of water volcanism. Impacts that formed the small craters may have weakened the crust so that water flowed up through newly formed cracks and holes. There are very few large craters, but some ghost craters exist. These show the details of the crater, but actual physical features, such as walls, have disappeared because of the plastic deformation of Ganymede’s surface.

Callisto

Callisto is the outermost of the Galilean satellites and probably the least active. Its surface temperature ranges from 150 kelvins at noon to 100 kelvins before dawn. At temperatures as cold as these, a layer of ice only one meter thick would take 4.5 billion years to evaporate. The dark, heavily cratered surface has a reflectivity of only 18 percent, which implies that the surface ice has a low purity. Callisto’s landscape is almost exclusively the product of impacts. The crater features are subdued, and no large impact basins exist. This relative smoothness is probably the result of the plastic deformation of the crust. In the Valhalla basin, visible rings on the surface may be the remnant of a large impact basin. Few ray craters are visible.

Other Satellites

Amalthea is one of the innermost satellites of Jupiter, following Metis and Adrastea. It is small and elongated, with a length of 270 kilometers and an average diameter of 155 kilometers. Its long axis is pointed toward Jupiter throughout its orbit. The composition of its dark red surface and of its bulk is unknown. Four features were found by Voyager—two craters, Pan and Gaea, and two mountains, Mons Ida and Mons Lyctas. Nothing definite is known about these features. The scarred surface reflects a history of bombardment.

Beyond the Galilean moons are two main groups of additional satellites of Jupiter. First is a group in a prograde orbit, moving in the same direction as Jupiter's. Beyond these is the so-called retrograde group, which orbits in the opposite direction of Jupiter's orbit. When scientists announced the discovery of twelve new Jovian moons in 2018, one unusual satellite was noted for having a prograde orbit despite being located in the retrograde group. This tiny moon, named Valetudo, had a relatively high chance of colliding with another satellite, an event detectable from Earth.

Jupiter also has a ring system, though it is far less apparent than the famous rings of Saturn. Jupiter's rings were first detected by Voyager 1. The Galileo mission provided further evidence that the rings are made up of very fine dust particles, likely created by collisions between small moons and meteoroids.

Context

Although the Galilean moons have been known since 1610, because of their small size, astronomers initially found it difficult to identify details on the surface of any of them. Photographs taken by equipment carried high into Earth’s atmosphere showed Io to have dark polar regions. Careful brightness measurements indicated a variation as the position in orbit changed, suggesting that the surface on a given satellite did not have a uniform reflectivity. These data also led astronomers to conclude that the satellites were tidally locked to Jupiter so that their rotation periods were equal to their periods of revolution.

Another interesting feature of the orbits is that the periods of revolution of Io, Europa, and Ganymede are tied together by gravitational coupling. The period for Europa is twice that for Io, and the period for Ganymede is twice that for Europa. If any of these three orbits changed, the other two would also change to restore the existing ratios. One difficulty in the models that describe the origin of Jupiter is that they offer no account of a process through which these three satellites could move into such an orbital relationship.

Voyager data revealed four satellites that differed from expectations in many ways and had significant differences among themselves. Models have been constructed that attempt to describe processes that would lead to the formation of the moons and give rise to their conditions. The region where Io and Europa are believed to have been formed would have had a fairly high temperature, and volatile materials such as water would not have been present in significant quantities. The tidal heating that Io experiences has driven off any volatile materials, such as water and carbon dioxide, that may have been a part of the original body.

Europa retained more water at formation than did Io and has not lost it. There appears to be enough heat in the silicate core to keep much of the water in the liquid phase and allow it to flow through cracks on the moon’s surface. This flow process has essentially resurfaced Europa during its history.

Ganymede has a mass that is roughly 50 percent water. Much of this water is in liquid form. The core is assumed to be warm enough to transfer energy to the water to keep it liquid. The water’s liquid state renews parts of the icy surface.

Callisto’s icy surface is covered with significant quantities of silicate materials. Although there may be a layer of liquid water in the interior, the crust is thick enough to prevent any of the liquid from reaching the surface. The process of plastic deformation has eliminated the features of the larger impact basins, although some of the materials that would have been part of the basin are visible in the icy crust, giving the impression that the moon has a large bull’s-eye drawn on it.

The next stage in the investigation of Jupiter’s satellites would be to send to Jupiter a spacecraft capable of performing detailed investigations over prolonged periods in close orbital proximity to the satellites rather than just doing multiple flybys. Such a mission was proposed and initially funded. The Jupiter Icy Moons Orbiter (JIMO) would have used nuclear propulsion not only to enter the gravitational sphere of Jupiter but also to go from orbit around one satellite to another over a very prolonged mission lasting perhaps a decade. When the program met technical difficulties, and costs threatened to rise well above the allocated $1 billion, JIMO was canceled. In 2011, the National Aeronautics and Space Administration (NASA) launched the orbiter Juno to further study Jupiter. Juno entered into a polar orbit around the planet in 2016 and was planned to make approximately thirty-three orbits, providing new insights into the Jovian system and its satellites. This mission resulted in various scientific discoveries, including the observation of an asteroid crashing into Jupiter, a deeper knowledge of storms on Jupiter, and an understanding of the planet's mesosphere.

Knowledge Gained

The composition of the surface of Io has been reasonably well established. The white solid is sulfur dioxide condensed from plume activity. The remainder of the surface is largely elemental sulfur forms, which can have a variety of colors depending upon the process occurring during solidification. Sodium has been detected in the space around the satellite. The existence of active volcanoes on Io requires the presence of molten material under the surface. The tidal action of Jupiter and other Jovian satellites upon Io is probably the main source of heat energy that generates molten material. Although Jupiter exerts a very large attractive force on Io, the gravitational attractions of the other satellites cause the surface of Io to be pulled in competing directions. Friction resulting from the flexing of the surface causes the material beneath the crust to melt. The thickness of the crust is not precisely known, but it is thought to be minimal. A sulfur sea may exist below the crust. An additional source of heat is the electric current induced in the iron sulfide core of the satellite as it interacts with the strong magnetic field of Jupiter. A portion of the radio waves emitted by the Jovian system originate from Io’s interaction with Jupiter's magnetic field and the plasma disk (a disk of charged particles) that surrounds the planet.

Europa is another Jovian satellite that shows signs of activity, although how recent that activity may have been is not known. Some scientists believe that there may be an ocean of liquid water as much as 100 kilometers deep lying below the icy crust of Europa. Occasional cracks in the crust allow the water to flow onto the surface, erasing any evidence of past impacts when large flows occur or simply come up through the cracks to form the ridges that mark the surface. Evidence of water vapor plumes, similar to those observed on Saturn's moon Enceladus, was detected by the Hubble Space Telescope and through reanalysis of Galileo data. These findings boosted proponents of the theory that Europa might harbor extraterrestrial life.

Ganymede also shows evidence of having been active after the major cratering epochs ended within the solar system. Mountains formed from tension in the surface are evidently a consequence of a series of internal upheavals during the earlier life of the satellite. It is possible that these changes were initiated by changes in the crystal structure of the ice as the core of Ganymede slowly cooled. Resulting expansions and contractions could have cracked the surface and allowed the lower areas to be flooded with a lavalike flow of liquid water.

Callisto is believed to have been inactive since the initial formation of its crust. It is generally accepted that Ganymede and Callisto are differentiated objects. This means that initially, the objects were a uniform mix of water-ice and rocky materials. This mixture was heated from some source, perhaps radioactivity of the rocky material, and the denser, rocky material settled to the core, leaving the less dense water on the surface, where it eventually froze and formed the crust. For Callisto, it seems that this development brought an end to its self-generated geologic activity. Callisto’s other topographic features resulted from collisions with other space objects.

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