Jupiter

Jupiter

Jupiter is the largest planet in the solar system and is easily visible in the night sky. Jupiter's mass (1.9 × 10²⁷kilograms [4.2 × 10²⁷ pounds]) is nearly two and a half times the mass of the rest of the solar system's planets combined. Jupiter's volume, filled mostly with gas, is 1,316 times that of Earth. The fifth planet from the Sun, Jupiter's year is 11.86 Earth years but its day is short, only nine hours and fifty-five minutes. Jupiter resembles a small star: its composition, like the Sun's, is mostly hydrogen and helium. It emits about twice the energy that it receives from the Sun and puts out over 100 times more heat than Earth. If Jupiter had been about 50 to 100 times larger, it might have evolved into a star rather than a planet.

Historic Observations of Jupiter

Jupiter has intrigued humans since antiquity. It is named for the king of the Roman gods, and most of its twenty-eight moons are named after the god's many lovers. In 1609 and 1610, Italian mathematician and astronomer Galileo Galilei and German astronomer Simon Marius began telescopic studies of Jupiter and its system. Galileo is credited with the discovery of Jupiter's four largest moons: Io, Europa, Ganymede, and Callisto, now called the Galilean satellites in his honor. These moons had an impact on the thinking of those times. It was believed then that Earth was the center of the universe and that all the planets and moons revolved around Earth. Galileo's observations showed that the four moons revolved around Jupiter, not Earth. This discovery contributed to Galileo's doom. He was condemned by the Catholic Church, forced to recant his discovery, and only in 1992 did Pope John Paul II agree that Galileo was right to support Copernicanism.

As telescopes improved, other astronomers continued to observe Jupiter and to study its colorful bands and the long-lived storm known as the Great Red Spot. Twenty-four other smaller satellites have been discovered, from Amalthea in 1892 to Leda in 1974 to twelve new moons in 2001. Observations from Earth showed that Jupiter has a massive magnetosphere and that the planet emits radiation at radio wavelengths . From this, astronomers deduced that Jupiter is surrounded by radiation belts , similar to Earth's Van Allen radiation belts , and that the planet must have a strong magnetic field.

Spacecraft Explorations

Space missions allowed scientists to make great leaps forward in the exploration of Jupiter and its moons. The first spacecraft to fly by Jupiter were Pioneer 10 (in 1973) and Pioneer 11 (in 1974). They passed as close as 43,000 kilometers (26,660 miles) from Jupiter. Their suite of instruments made important observations of the atmosphere, magnetosphere, and space environment around the planet. In 1979 the spacecraft Voyager 1 and Voyager 2 passed close to Jupiter and its moons, making startling discoveries that included auroras on Jupiter, a ring system surrounding the planet, and active volcanoes on the moon Io.

In 1995, the Galileo spacecraft became the first to orbit Jupiter. It dropped a probe into the planet that survived for 57.6 minutes, until it was crushed by Jupiter's enormous pressure. The probe's instruments sent back valuable information on the temperature, pressure, composition, and density of the upper atmosphere.

The Galileo probe provided scientists with their first glimpse inside the top layers of the atmosphere. One surprising discovery was that Jupiter has thunderstorms that are many times larger than those on Earth. The cause of the thunderstorms is the vertical circulation of water vapor in the top layers of Jupiter's atmosphere.

The main Galileo spacecraft has been making observations of Jupiter, its moons, and its environment since 1995, and these were slated to continue until 2002. Scientific observations continue to be made using Earth-based telescopes and the Hubble Space Telescope, which is in orbit around Earth. The combination of many sets of observations over time is extremely valuable for understanding Jupiter and its system.

The Atmosphere and Interior of Jupiter

Jupiter's atmosphere has alternating patterns of dark and light belts and zones. Within these belts and zones are gigantic storm systems such as the Great Red Spot. The locations and sizes of the belts and zones change gradually over time, and many of them can be seen through a telescope. The Great Red Spot has lasted for at least 100 years, and probably as long as 300 years. It rotates counterclockwise every six days, and this direction, plus its location in the southern hemisphere, indicates that it is a high-pressure zone. This differs from the cyclones that occur on Earth, which are low-pressure zones. The red color of the spot is something of a mystery. Several chemicals, including phosphorus, have been suggested as the cause of the red color but, on the whole, the reasons for Jupiter's different colors are not yet understood.

The atmosphere of Jupiter consists of about 81 percent hydrogen and 18 percent helium, with small amounts of methane, ammonia, phosphorus, water vapor, and various hydrocarbons. Observations by Galileo showed a cloud of fresh ammonia ice downstream from the Great Red Spot. Jupiter's atmosphere has strong winds, but the mechanisms that drive them are not well understood. There are at least twelve different streams of prevailing winds, and they can reach velocities of up to 150 meters per second (492 feet per second) at the equator. On Earth, winds are driven by large differences in temperature, differences that do not exist, at least not on the top part of Jupiter's atmosphere, where the temperature at the poles is about the same as that at the equator (-130°C [-202°F]).

The cloud layer, which is thought to be only about 50 kilometers (31 miles) thick, comprises only a small part of the planet. What is the interior of Jupiter like? The pressure inside Jupiter, which increases with depth, is enormous—it may reach about 100 million times the pressure on Earth's surface. Although we cannot directly observe Jupiter's interior, theory plus observations of the atmosphere and the surrounding environment suggest that below the cloud layer there is a 21,000-kilometer-thick (13,000-mile-thick) layer of hydrogen and helium. This layer gradually changes from gas to liquid as the pressure increases. Beneath this layer is a sea of liquid metallic hydrogen about 40,000 kilometers (24,800 miles) deep. Metallic hydrogen does not form on Earth, because our planet lacks the extreme pressures necessary to break up the hydrogen molecules and pack them so tightly that they break up and become electrically conductive. This electrically conductive metallic hydrogen is what drives Jupiter's strong magnetic field. Deeper still in Jupiter's interior is the core, which may be solid and rocky. It is estimated that the core is about one and a half times Earth's diameter, but ten to thirty times more massive. It is also very hot: about 30,000°C (54,000°F). This heat comes up through the layers and is detected at "hot spots" in the atmosphere, which are cloud-free holes.

Magnetic Field and Rings

Jupiter's sea of metallic hydrogen causes it to have the strongest magnetic field of any planet in the solar system. The field is inverted relative to Earth's, that is, a compass there would always point south. The region around the planet that is dominated by the magnetic field is called the magnetosphere. The stream of charged particles sent by the solar wind causes Jupiter's magnetosphere to be shaped like a teardrop, pointing directly away from the Sun. Inside the magnetosphere is a swarm of ions, protons , and electrons , which are called plasma. The plasma rotates along with Jupiter's magnetic field, blasting off charged particles. Some of them impact on the surfaces of the moons. On Io, volcanoes eject material into space, and the particles get caught up in Jupiter's magnetosphere. This creates a doughnut-shaped region of charged particles at about the distance from Jupiter of Io's orbit. This is called the Io plasma torus. It was first observed by the Pioneer spacecraft.

The Voyager missions showed that Jupiter is surrounded by faint rings. Unlike Saturn's rings, which are made up of icy particles, Jupiter's rings are made up of small dust particles. Two small satellites, Adrastea and Metis, are embedded within the rings. Observations by Galileo spacecraft showed that the dust comes from meteoroids impacting the satellites closest to Jupiter.

The Galilean Satellites

The Galilean satellites are all different from one another. Io and Europa have greater densities than Ganymede and Callisto, suggesting that the two inner moons (Io and Europa) contain more rock, and the outer moons more water ice.

Io.

Io is the most volcanically active body in the solar system. It is the only place outside Earth where eruptions of hot magma have been observed. Other planets and moons in the solar system have been volcanically active in the distant past. Io is about the same size as Earth's Moon and, had it not been for its peculiar orbit, it too would have cooled down and volcanism would have ceased. Tidal stresses are produced within Io as a result of the gravitational pull of Jupiter, Europa, and Ganymede. These stresses cause the interior of Io to heat up, leading to active volcanism. About 100 active volcanoes have been seen so far on Io, many of which were discovered from their thermal signature in infrared observations made by the Galileo spacecraft. Some of the active volcanoes have plumes that can reach 300 kilometers (186 miles) high. Io's surface is very young as a result of many continuous volcanic eruptions, and no impact craters have been seen. The colors of the surface—vivid reds, yellows, greens, and black—are different from those seen on other solid bodies in the solar system. These colors are a result of sulfur and silicates on the surface. Io's lavas are hotter than those seen on Earth today, reaching temperatures of 1,500°C (2,700°F). They may be similar in composition to ultramafic lavas on Earth, which erupted millions of years ago.

Europa.

Europa is particularly intriguing because of the possibility that it might harbor life. Observations by Galileo spacecraft showed that Europa's cracked surface resembles the ice floes seen in Earth's polar regions. High-resolution images show that some of the broken pieces of the ice crust have shifted away from one another, but that they fit together like a jigsaw puzzle. This suggests that the crust has been, or still is, lubricated from underneath by warm ice or liquid water. The two most basic ingredients for life are water and heat. Like Io, Europa is subject to tidal stresses because of

THE GALILEAN SATELLITES
Name	Radius	Distance from Jupiter	Density
Io	1,821 km	421,600 km	3.53 gm/cm-3
Europa	1,565 km	670,900 km	2.97 gm/cm-3
Ganymede	2,634 km	1,070,000 km	1.94 gm/cm-3
Callisto	2,403 km	1,883,000 km	1.85 gm/cm-3

Jupiter and Ganymede's gravitational pull. While Europa has no evidence of current active silicate volcanism, the tidal stresses may cause heating of the interior, providing the other key ingredient for life. Europa's surface does show evidence of ice volcanism. There are places where material appears to have come up from underneath as slushy ice and flowed on the surface. Europa has very few impact craters, indicating that its surface is young. Slushy ice flowing over the surface probably erased many impact craters. Europa's surface composition is dominated by water, but Galileo detected other compounds, including hydrogen peroxide (H₂O₂) on the surface and a thin oxygen atmosphere. The behavior of Jupiter's magnetic field around Europa implies that there may be ions circulating globally beneath the icy surface.

Ganymede.

Larger than the planets Mercury and Pluto, Ganymede was the first moon known to have a magnetic field, one of the earliest discoveries made by the Galileo mission. The field is stronger than that of Mercury. Ganymede has a core made up of metallic iron or iron sulfides. If the core is molten and moving, it would produce the strong magnetic field observed by Galileo. Ganymede's surface shows a complex geologic history. The surface is characterized by large dark areas and by bright grooved terrains. The grooves are thought to have formed when the crust separated along lines of weakness. Other images showed hillcrests and crater rims capped by ice, and old terrain cut by furrows and marked by impact craters. Observations in the ultraviolet made from the Hubble Space Telescope showed the presence of oxygen on Ganymede, and Galileo observations detected hydrogen escaping from Ganymede into space. These results indicate that Ganymede has a thin oxygen atmosphere. Astronomers believe that the atmosphere is produced when charged particles trapped in Jupiter's magnetic field come down to Ganymede's surface. The charged particles penetrate the icy surface, disrupting the water ice. The hydrogen escapes into space, whereas the heavier oxygen atoms are left behind.

Callisto.

About the same size as the planet Mercury, Callisto is Jupiter's second largest moon. Its surface is heavily cratered, implying that it is extremely old, probably dating from about 4 billion years ago, which is close to the time when the solar system formed. Callisto's surface is icy and has some large impact craters and basins surrounded by concentric rings. The largest impact basin is called Valhalla, and it has a bright central region 600 kilometers (372 miles) in diameter, with rings extending to 3,000 kilometers (1,860 miles) in diameter. Galileo observations showed that Callisto has a magnetic field. Underneath its icy crust, Callisto may have a liquid ocean, which, if it is as salty as Earth's oceans, could carry enough electrical currents to produce the magnetic field. A major discovery made by the Galileo mission is that Callisto has a thin atmosphere of carbon dioxide.

see also Exploration Programs (volume 2); Galilei, Galileo (volume 2); NASA (volume 3); Planetary Exploration, Future of (volume 2); Robotic Exploration of Space (volume 2); Shoemaker, Eugene (volume 2); Small Bodies (volume 2).

Rosaly M. C. Lopes

Bibliography

Beatty, J. Kelly, Carolyn Colins Petersen, and Andrew Chaikin, eds. The New Solar System, 4th ed. Cambridge, UK: Sky Publishing Corporation and Cambridge University Press, 1999.

Shirley, James. H., and Rhodes. W. Fairbridge, eds. Encyclopedia of Planetary Sciences. London: Chapman & Hall, 1997.

Internet Resources

NASA Jet Propulsion Laboratory, California Institute of Technology. <http://jpl.nasa.gov/>.