Jovian planets

Jupiter, Saturn, Uranus, and Neptune are called the Jovian planets. These gas giants have a mass 15-320 times greater than Earth, are of very low relative density, are mainly fluid (gas and liquid), and are composed of relatively light elements, such as hydrogen and helium. All of them are surrounded by ring systems, a host of diverse satellites, and complex magnetospheres.

Overview

The four Jovian, or gas giant, planets are different geologically and physically from the terrestrial planets Mercury, Venus, Earth, and Mars. Massive gaseous and liquid bodies composed primarily of hydrogen and helium, the Jovian planets are relatively rapid rotators. Each rotates about its axis in less than twenty-four hours. The atmospheres of the Jovian planets—the feature that dominates observational work done on these planets—are very similar in composition. Hydrogen represents about 90 percent of the atoms, with helium making up the bulk of the remaining atmospheric gases. Methane and ammonia are also present, although ammonia on the two colder planets, Uranus and Neptune, most likely precipitated out of the atmosphere. Weather systems that dominate these atmospheres, particularly in the case of Jupiter and Saturn, consist of rapidly rotating belts and zones visible from Earth. In the case of Jupiter, wind speeds on the order of 300 kilometers per hour are typical, while on Saturn, winds of two to three times that speed have been measured. Note that on Earth, hurricane-force winds rarely exceed 150 kilometers per hour. Ironically, wind speeds in the colder Uranus and Neptune are even higher than those seen on Jupiter and Saturn.

Both Jupiter and Saturn are much hotter than expected in view of their distance from the Sun. They have heat sources deep within their planetary interiors and, thus, can produce extensive thermal cells to drive high-speed winds. The nature of the heat within these planets is not entirely evident, and there is some evidence that even in the case of Uranus, there may also be a heat-driven weather system resulting from a much more modest heat source on the planet. Uranus displays little atmospheric structure in visible light, but there are some features in ultraviolet light. Images from Voyager 2 have shown that Neptune has an actively driven weather system. Dark storms and white streakers are seen to evolve in short time frames.

Although there are no measurements to indicate what lies below these turbulent atmospheres, there is indirect evidence that pressures become higher toward the center of a typical Jovian planet. A portion of the interior is liquid. A very unusual state of liquid metallic hydrogen exists toward the center of Saturn and Jupiter. This liquid metallic state would enhance the thermal and electrical conductivity of the planetary interiors and is undoubtedly primarily responsible for the strong magnetic fields associated with Jupiter and Saturn. Pressures necessary to create liquid metallic hydrogen are on the order of millions of times the atmospheric pressure at the surface of Earth. Although they can be re-created in tiny cells in the laboratory, no such pressures have been sustainable in large-scale systems for prolonged periods on Earth. Thus, the liquid metallic hydrogen layer in the interiors of Jupiter and Saturn has effects that are not yet fully described. Uranus and Neptune probably have no such layers, for their masses need to be better to produce such enormous interior pressures.

If one were proceeding inward toward a Jovian planet’s center, one would next approach the core of that planet. Theorists disagree as to what might exist there, but the dominant opinion is that the cores would be largely solid and would contain relatively heavy metals, such as iron, or they may be silicon in some high-pressure phase. In the case of Jupiter, such a solid core might have a mass twenty times greater than that of Earth, but this is a small fraction of the total mass of this planet, which is 320 times that of Earth. Uranus and Neptune might have solid cores on the order of several Earth masses, while that of Saturn would be about five to ten Earth masses.

Little or nothing has been learned experimentally about planetary interiors; this is true even for Earth and its fellow terrestrial planets. Modeling planetary interiors, particularly on the scale necessary in the cases of Jupiter and Saturn, requires knowledge of pressure effects on bulk matter at pressures of millions of atmospheres and at alleged temperatures of 50,000 kelvins or hotter. It is known, however, that all the Jovian planets have a very low density, that is, a low specific gravity. Specific gravity is a measure of relative density, using water as a unit of one gram per cubic centimeter or 1,000 kilograms per cubic meter. Saturn has a specific gravity or relative density of 0.7; this means it would float if one could find an ocean of water big enough to place it. It is by far the least dense of all the planets. Jupiter has an average density of 1.3 grams per cubic centimeter, while Uranus and Neptune have average densities of 1.2 and 1.7 grams per cubic centimeter, respectively. A typical terrestrial planet has a specific gravity of about five. Earth’s density is, on average, 5.5 grams per cubic centimeter. Overall, such relatively low-density measurements indicate the predominance in Jovian planetary structures of light elements, such as hydrogen and helium.

Jupiter has a sizable magnetosphere. Its strong magnetic field is about ten times as intense as Earth's. Jupiter’s magnetosphere, which consists of trapped charged particles in amounts that would be lethal to humans, is so large that Saturn, which is 9.5 astronomical units (AU) from the Sun, passes through it. Saturn is almost twice as far from the Sun as Jupiter (at about 5.2 AU), yet that of Jupiter very strongly influences its magnetosphere. Saturn itself has a magnetic field slightly larger than that of Earth.

All Jovian planets rotate about their axes rapidly compared to the terrestrial planets, which take at least twenty-four hours to make one rotation. (Earth is the fastest-rotating terrestrial planet.) All Jovian planets thus exhibit some degree of oblateness. Saturn has an oblateness of about 0.1, which means that its equatorial diameter is about 10 percent bigger than its polar diameter, and thus, it appears noticeably flattened at the poles. Saturn takes ten hours and thirteen minutes to make one complete rotation; Jupiter spins even faster, taking only nine hours and fifty-five minutes to complete a rotation. Uranus takes seventeen hours for one full rotation, while Neptune takes about sixteen hours.

These rapid rotations are surprising for such gaseous and liquid planets because an angular momentum principle of elementary physics would have bigger bodies rotate more slowly than smaller ones. Such rapid rotation rates are a mystery of the first magnitude in solar physics and geophysics. The correlation between magnetic fields and rotation rates is not very strong. Why some planets have powerful magnetic fields and others negligible ones is unknown. In general, if magnetic fields result from dynamo currents deep within planets, then rapid rotators should have strong magnetic fields. This is largely true in the case of both Jupiter and Saturn. Uranus has a magnetic field weaker than that of Saturn. Neptune has a magnetic field roughly comparable to that of Uranus.

Neptune and Uranus are smaller and colder versions of Saturn and Jupiter. Uranus has a pale blue, almost greenish-blue appearance, undoubtedly because of methane. Imaging from Voyager photographs has shown that Uranus has belts and zones, although they are less spectacular than those of Jupiter and Saturn. Ammonia within the Uranian atmosphere and that of Neptune is thought to have precipitated to the surface. In August 1989, Voyager 2 flew by Neptune and returned images of that planet and its atmosphere. Photographs of Neptunian surface features are difficult to captre from Earth’s surface because of the distance (thirty AU from the Sun). However, the Hubble Space Telescope and the James Webb Space Telescope have been used for studying the planets and avoiding this issue. Some of the most productive research was performed by astronomer Heidi Hammel. Voyager 2 showed Neptune is also a pale blue planet with irregular marked bands in its atmosphere. It has a gigantic Dark Spot, somewhat analogous to Jupiter’s Great Red Spot, which seems to cause a tremendous sinking and upswelling of its atmospheric winds. This spot has a diameter about the same as the diameter of Earth. Additionally, extremely high clouds, about fifty kilometers above the (normal) Neptunian atmosphere, make its atmosphere different from those of the other Jovian planets. Winds of 200 meters per second have been measured in the Neptunian atmosphere, with unique streams and band systems. In 2022, the James Webb Space Telescope captured some of the clearest imaged of Neptune ever, including several of it moons and its methane-ice clouds.

All Jovian planets possess numerous moons or satellites. By 2008, Neptune was discovered to have at least fourteen satellites. Uranus has at least twenty-seven detected moons, while Saturn has sixty-two and Jupiter has at least sixty-three. Surprisingly, these planets rotate so rapidly while, at the same time, each has such a large family of satellites. The presence of such satellites should have slowed the rotation rates if the satellites and the primary planets had common origins. Many satellites present the same face toward their primary planet and, thus, are tidally locked.

All Jovian moons also have rings, although they vary considerably in texture and content. The Voyager 1 spacecraft discovered a very thin ring around Jupiter in 1979. Since then, research has revealed that there are four rings and more structures in Jupiter’s ring system. It is referred to as the halo ring, the main ring, and the gossamer ring; the ring has two parts—the inner Amalthea gossamer ring and the outer Thebe gossamer ring. Jupiter’s rings are dark and composed of dust; therefore, they are not visible from Earth. Saturn’s ring system of mainly icy particles is incredibly dynamic, with several gaps due to gravitational resonances and shepherding by tiny, embedded moonlets. Uranus has at least nine complete rings, some considerably brighter than others; notably, five of Uranus’s rings were discovered in the late 1970s by ground-based observations, not by the Voyager 2 mission, although the Voyager probe provided the first intense study of the entire ring system. Hubble-based research found four additional dark rings long after the Voyager flyby. Neptune has five principal rings, which were first labeled as partial arcs. They are faint and dusty in appearance and, in that sense, resemble the rings of Jupiter. They are named after astronomers who conducted significant research on the planet: Galle, Urbain Le Verrier, Lassell, Arago, and Adams.

Methods of Study

Galileo’s 1610 discovery of Jupiter's four large natural satellites (Io, Europa, Ganymede, and Callisto, known as the Medician and later as the Galilean moons) launched the age of modern science. By the 1650s, Christiaan Huygens in Holland and other astronomers in Italy had conclusively established that Saturn had rings and at least one large satellite, Titan. The Great Red Spot on Jupiter was first observed in 1660, and the zones and belts on Jupiter and Saturn have been clearly detected by enterprising visual astronomers. For three centuries, astronomers around the globe have tracked the Great Red Spot and noted changes in the belts and zones of these two gigantic planets. Uranus, too, was viewed by many from the 1700s onward, but it was not clearly designated as a planet until the late eighteenth century.

With the advent of spectroscopy in the nineteenth century, helium was discovered first on the un and shortly thereafter on Jupiter and Saturn. In the early twentieth century, it was learned that methane and ammonia were present in the Jovian atmospheres as well. In 1955, radio astronomers detected radio signals coming from Jupiter’s magnetosphere.

It was not until the 1970s, however, that the greatest discoveries about the Jovian planets were made. Data from Voyager 1 revealed the unexpected existence of rings around Jupiter. Earth-based observations showed rings around Uranus and Neptune, and images returned from Voyager 2 in the 1980s revealed ten satellites circling Uranus and orbiting Neptune.

Radio astronomy probes the decimetric and decameter radio signals emitted from Jupiter, which signal the extent of its magnetosphere and the relationship between its halo and volcanic innermost satellite Io, respectively. Radio astronomy conducted by the Cassini spacecraft in orbit around Saturn provided insight into the nature of the magnetic field on one of its moons, Enceladus. Infrared astronomy also has been very helpful in determining some of the features of the cold Jovian planets Uranus and Neptune. Many experimental techniques have been used to determine the size and extent of the ring systems surrounding these planets, and still, there are many unanswered questions about these systems.

The atmospheres of Jupiter and Saturn have been probed with all sorts of sensitive spectrometers, but experimental information is valid only for a penetration depth of a few tens of kilometers. What lies below the turbulent, fast-moving atmosphere has not been experimentally detected; all that scientists can do is rely on the best theories and modeling techniques presently available.

The Pioneer 10 and 11 probes found that Jupiter is a tremendous source of electrons and generates several times as much heat as it receives from the Sun. The origin of these electrons and heat is far from clear to the most discriminating theorists in physics and geophysics. There are no comparable conditions on Earth or the nearby terrestrial planets to produce such effects. Voyager 2 passed Neptune in August 1989, and its use as a planetary probe effectively ceased. Its next primary objective was to characterize the approach to interstellar space. Hubble Space Telescope (HST) has produced much better images of Uranus and Neptune than previously available regularly.

The Galileo spacecraft arrived at Jupiter in 1995. A special probe released from Galileo entered Jupiter’s atmosphere on December 7, 1995, and its instruments detected a new radiation belt, fierce winds, lightning, and upper-atmosphere densities and temperatures much higher than expected. The Cassini spacecraft, launched in October 1997, began exploring the Saturn system from an orbital vantage point beginning in 2004. The Huygens probe that Cassini carried along on its journey from Earth to Saturn was released and sent down through the atmosphere of Saturn’s largest satellite, Titan, a moon with a thick atmosphere that obscures its surface. Cassini carried an imaging radar to map the satellite’s surface during repeated close flybys. Huygens survived its plunge through the atmosphere and landed on a mushy, cryogenic surface. Huygens sent its data on two redundant channels, but because of a software error, only one transmitted properly; fortunately, an alternative path recovered most of the data that otherwise could have been lost. Huygens and Cassini found evidence of complex hydrocarbons under cryogenic conditions on the surface of Titan. Cassini’s primary mission was completed in 2008, and the program received a fully funded two-year extension.

Context

Early exploration of the Jovian planets and their extensive satellite systems was expected to provide scientific clues as to how the solar system formed. Instead, new mysteries have appeared, spurring additional robotic exploration of the outer solar system.

For atmospheric physicists, the weather systems evident in the atmospheres of both Jupiter and Saturn have provided much material for study. The Great Red Spot and several of the lesser white spots on Saturn and Jupiter have proved to be cyclonic or anticyclonic storms that can maintain themselves for decades. The Great Red Spot has been observed for at least 350 years. Could such a massive storm system be maintained on Earth? What conditions on Jupiter contribute to the tremendous longevity of the Great Red Spot? In attempting to answer questions of this sort, scientists have modeled all manner of weather systems, which has proved helpful in deciphering Earth's meteorological patterns. Thus, Jupiter and Saturn have served as gigantic, high-pressure, turbulent laboratories for atmospheric modelers. Indeed, the greatest potential outcome of comparative planetology is a better understanding of complex geophysical and atmospheric physics processes right here on Earth. Such an understanding is fundamental to determining whether or not Earth is presently undergoing global warming of natural or human-made origin.

Even in the esoteric discipline of fluid mechanics, particularly in studying turbulent flow, data from Jupiter and Saturn have been unexpectedly helpful. These studies are critical in airframe design and, when coupled with modern computer modeling techniques, have proved to be very valuable in designing supersonic airframes and high-speed hydrofoils. Neptune’s Dark Spot should provide fodder for fluid mechanics and meteorology studies well into the twenty-first century. Many scientists believe that solar-system locations most likely to host life or organic chemistry necessary for life are either Jupiter’s ice-covered satellite Europa, or Saturn’s satellites Titan and Enceladus. Some life systems could be operating in either of these locations, for the energy and chemical conditions seem suitable. Should some sort of complex organic molecules or anaerobic bacteria be found on Jupiter or Europa, the perennial mystery of how life formed on Earth and why it exists at all could be addressed intelligently, perhaps for the first time. Saturn’s satellite Enceladus has cryogenic geysers in its south polar region. Neptune’s satellite Triton has also appeared to contain some cryogenic geyser activity. The unexpected detection of warm liquids in the outer solar system could drive biological networks. Titan has organic materials that are believed to indicate the primordial Earth, although the satellite is far colder than Earth was when life developed here. Thus, Titan might be a frozen example of what the early Earth might have been when life first arose.

In 1955, radio astronomers Bernard Burke and Kenneth Franklin, while studying the Crab Nebula, inadvertently discovered radio emissions from Jupiter. Decades later, Voyager 2 recorded the most significant electrical current ever measured as it passed near Jupiter. In the first half of the twentieth century, most scientists did not realize that Earth, with its reasonably strong magnetic field, produced a magnetosphere just as Jupiter did. It was not until early American spacecraft discovered the Van Allen radiation belts that radio engineers, astronomers, and plasma physicists realized Earth’s magnetosphere was a smaller version of Jupiter’s. The magnetospheres of Jupiter, Saturn, and even Earth are still not wholly understood; what influence they might have had on planetary origins and developments is unknown. Eventually, studies of Jupiter and Saturn might provide clues regarding the forces and mechanisms behind electrical storms, violent atmospheric electricity, and radio blackouts that can have pronounced effects on life on Earth.

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