Jupiter

Planet Profile

Mass (kg)............................................1.90 x 10^27
Diameter (km)........................................142,800
Mean density (kg/m^3) ...............................1314
Escape velocity (m/sec)..............................59500
Average distance from Sun (AU).......................5.203
Rotation period (length of day in Earth hours).......9.8
Revolution period (length of year in Earth years)....11.86
Obliquity (tilt of axis in degrees)..................3.08
Orbit inclination (degrees)..........................1.3
Orbit eccentricity (deviation from circular).........0.048
Mean surface temperature (K).........................120 (cloud tops)
Visual geometric albedo (reflectivity)...............0.44
Atmospheric components...............................90% hydrogen, 10% helium, .07% methane Rings................................................Faint ring. Infrared spectra imply dark rock fragments.

Jupiter Image Gallery

I INTRODUCTION

Jupiter (planet), fifth planet from the Sun and the largest planet in Earth's solar system. With the exception of the Sun, the Moon, and Venus, Jupiter is the brightest object in Earth's sky—more than three times brighter than Sirius, the brightest star. Due to its prominence in the sky, the ancient Romans named Jupiter for Jove, the chief god of Roman mythology.

Jupiter orbits the Sun at an average distance of 780 million km (484 million mi), which is about five times the distance from Earth to the Sun. Jupiter's year, or the time it takes to complete an orbit about the Sun, is 11.9 Earth years, and its day, or the time it takes to rotate on its axis, is about 9.9 hours, less than half an Earth day.

Unlike the rocky inner planets of the solar system (Mercury, Venus, Earth, and Mars), Jupiter is a dense ball of gas. It has a relatively small core of molten rock and iron, but Jupiter has no solid surfaces. Jupiter's mass is about 318 times the mass of Earth and its diameter is about 11.2 times the diameter of Earth. The force of gravity at the level of the highest clouds in Jupiter's atmosphere is about 2.5 times the force of gravity on Earth.

Because Jupiter has such a large diameter and high rate of rotation, material at the surface must travel quickly to circle the planet. This speed gives the material a great deal of momentum, or a strong tendency to fly away from the planet and continue traveling in a straight line through space. Material at the equator has the highest speed because, in a Jovian day, it must travel the greatest distance to circle the planet. Hence, material at the equator has the greatest momentum, and the strongest tendency to fly away from the planet. Because of Jupiter's weak, gaseous structure, the planet can not hold this material in as well as a more solid planet could, which results in Jupiter having the distorted shape of a flattened ball. The diameter of its equator is 143,000 km (89,000 mi), yet the diameter through its axis of rotation is only 133,700 km (83,000 mi).

II OBSERVATIONS FROM EARTH

Jupiter was first viewed through a telescope in 1610 by Italian philosopher and scientist Galileo Galilei. Until that time, the dominant world view, which was developed by 2nd-century Alexandrian astronomer Ptolemy, held that all of the stars and planets move in orbits around Earth. Galileo, however, observed four satellites, or moons, in orbit around Jupiter. This simple observation of astronomical objects in orbit about another astronomical object other than Earth touched off what is known as the Copernican revolution, named after Polish astronomer Nicolaus Copernicus. Copernicus had earlier developed a cosmology in which Earth orbits the Sun, which is now known as the Copernican System. The Copernican revolution was one of the key elements of the Renaissance and the Age of Enlightenment that continues to influence thinking to the present day. The moons that Galileo saw were collectively named the Galilean moons in honor of their discoverer.

When viewed through a modern telescope, the oblate (flattened) disk of Jupiter has a pearly color with bands of pastel browns and blues. Earth-based observers can best observe Jupiter when it is near solar opposition—that is, when Jupiter is on the side of Earth opposite the Sun, or when both planets are aligned with the Sun on the same side of the Sun. At opposition, the distance from Earth to Jupiter is at its annual minimum, and Jupiter appears as much as one and one-half times larger than it does at other times. Also at opposition, Jupiter rises at sunset and sets at sunrise, which means that it is visible all night long. Because Jupiter orbits the Sun in the same direction as Earth, Earth has to travel a little more than a full year to catch up to Jupiter from one opposition to the next. The time interval between exact oppositions is about 399 days.

In the mid-1950s radio astronomers discovered that Jupiter emits strong radio waves at many frequencies. The radio data indicates that Jupiter has a magnetic field similar to Earth's, but much stronger: At its upper atmosphere Jupiter's magnetic field is about ten times more intense than Earth's field at Earth's surface. Also like Earth's field, Jupiter's field is tipped about 10° relative to its axis of rotation. The interaction of Jupiter's magnetic field with charged particles ejected from the Sun creates radio noise near the poles and auroras similar to Earth's aurora borealis, or northern lights.

As Jupiter rotates its north and south magnetic poles are obscured to different extents, which makes the intensity of radio noise vary in a regular pattern. The pattern repeats at intervals of 9 hours 55.5 minutes, indicating that this is the rate of rotation of Jupiter's interior. With this rapid rotation, Jupiter's entire surface can be observed in two days during the long observing periods of opposition.

III COMPOSITION AND STRUCTURE OF JUPITER

By measuring the velocity of Jupiter's satellites, astronomers have been able to calculate the gravitational force that Jupiter exerts on them. Because the gravitational force exerted by a planet is proportional to its mass, astronomers have thus been able to calculate Jupiter's mass. Spacecraft flying by Jupiter have made possible more detailed studies of Jupiter's gravitational field, giving clues about its inner structure. These spacecraft have also relayed close-up television images and the results of chemical studies of the composition of Jupiter's outer layers. Putting all of this information together, astronomers have assembled a detailed picture of Jupiter.

A Composition of Jupiter

Jupiter's diameter is 11.2 times larger than Earth's, which means that its volume is more than 1300 times the volume of Earth. However, Jupiter's mass is only 318 times the mass of Earth. The density of Jupiter is therefore less than one-fourth of the density of Earth. Jupiter's low density (1.33 gm/cc vs. 5.52 gm/cc for Earth) indicates that the planet is composed primarily of the lightest elements—hydrogen and helium.

The Galileo spacecraft, a probe launched by the National Aeronautics and Space Administration (NASA), began orbiting Jupiter in 1995. It measured high winds and a puzzling lack of water molecules deep in Jupiter's atmosphere. It also found that there are about 6.4 hydrogen molecules, or not quite 13 hydrogen atoms, for each helium atom. This is similar to the ratio of these elements scientists have measured in the outer envelope of the Sun and supports the theory that Jupiter was formed from the same cloud of material as the Sun (see Planetary Science).

When spacecraft fly by a planet, the gravitational field of the planet causes them to accelerate, or change speed. Changes in the speed of the spacecraft are reflected by changes in the frequency of the radio signals that they send back to Earth (see Doppler Effect). Detailed analyses of radio signals sent back to Earth by several spacecraft that have flown near Jupiter indicate that Jupiter has a core made of material that is about as dense as the Earth's average density. Astronomers believe that this core material is rock and metal.

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