THE LARGEST OBJECT IN THE SOLAR SYSTEM AFTER THE SUN, JUPITER IS A BLOATED BALL OF GAS STREAKED WITH MULTICOLORED CLOUDS. THIS RAPIDLY SPINNING PLANET IS CIRCLED CEASELESSLY BY WINDS AND STORMS.
The first of the giant planets beyond the asteroid belt, Jupiter is nearly five times farther away from the Sun than Earth is. Composed of gas at increasingly high pressure towards the core, almost like a miniature star, it has a gravitational pull strong enough to have captured a large family of moons. Even with the naked eye, Jupiter is easily identifiable as one of the brightest objects in the night sky. Rotating on its axis in just under ten hours, Jupiter has the shortest day of all the planets in the solar system. The planet spins so fast that its equator is forced outward in a noticeable bulge. The zones of high and low atmospheric pressure wrapped around the planet, identifiable by the different colors of clouds found within them, are stretched out by the rapid rotation. Nonstop winds race in both directions, stirring up giant storms large enough to engulf Earth. The Great Red Spot, Jupiter’s most prominent feature, is a storm that has been raging for more than 300 years.
JUPITER STRUCTURE GIGANTIC THOUGH JUPITER IS, THE MATERIALS THAT FORM THE PLANET ARE COMPARATIVELY LIGHT. DESPITE THIS, FORCES OF GRAVITATIONAL CONTRACTION DEEP INSIDE JUPITER TURN THE PLANET’S INTERIOR INTO A POWERHOUSE OF ENERGY.
While Jupiter’s interior is almost entirely pure hydrogen, the planet’s upper layers are enriched with more complex gases that form the well-defined striped atmosphere. Around 600 miles (1,000 km) below this apparent “surface,” pressures are high enough to transform hydrogen gas into liquid. Some 12,500 miles (20,000 km) farther inward, pressure is so intense—many millions of times the atmospheric pressure on Earth—that it tears the hydrogen atoms apart, freeing their hold over electrons and causing the hydrogen to behave like liquid metal. Within the planet, denser materials sink downward, while the lighter materials rise up. The power this generates allows Jupiter to pump out more energy than it receives from the Sun, mostly in the form of heat and radio waves. Huge electrical currents in the metallic hydrogen layer create the most powerful magnetic field of any planet in the solar system
ALTHOUGH JUPITER HAS NO SOLID SURFACE, THE TURBULENT CLOUDS THAT COVER ITS FACE ARE PACKED WITH DETAIL, AND INDIVIDUAL WEATHER SYSTEMS CAN PERSIST FOR YEARS OR EVEN CENTURIES IN THE SWIRLING ATMOSPHERE.
Jupiter’s most conspicuous features are the bands of cloud that encircle the planet parallel to its equator. Astronomers classify them as either light-colored zones or dark-colored belts. Zones are high-pressure areas where clouds pile up at high altitude, while belts are low-pressure clearings in which sinking, cloud-free air provides a window through to darker clouds below. Storms, such as the Great Red Spot, are areas of high pressure where the clouds tower high above everything else. The giant planet’s weather is created by the interaction of a number of different factors, including heat rising from Jupiter’s deep interior, the differential rotation that causes equatorial regions to move faster than polar latitudes, and convection in the upper atmosphere, which redistributes heat between Jupiter’s warm equator and its colder poles. The complex boundaries between belts and zones are shaped by powerful jet-stream winds that blow in opposite directions around the planet. These winds cause the zones to flow in an eastward direction (with the planet’s rotation) and the belts, in contrast, to flow in a westward (or retrograde) direction. The general system of belts and zones appears to be stable over long periods of time, although the width of specific bands can vary significantly, as can the hue and intensity of the clouds in the belts.
FITTINGLY FOR THE LARGEST PLANET IN THE SOLAR SYSTEM, JUPITER ALSO HAS THE BIGGEST FAMILY OF SATELLITES—AT LEAST 67 ARE KNOWN AT PRESENT. HOWEVER, JUST FOUR OF THESE ARE PLANET-SIZED AND DOMINATE THE JUPITER SYSTEM.
Jupiter’s satellites are divided into three major groups: four small inner satellites, sometimes called the Amalthea group; the four huge Galilean moons (discovered in 1610 by Italian astronomer Galileo Galilei); and 59 or more small outer moons, most of which are just a few miles across, though some are much larger. The Amalthea group and the Galilean moons are together referred to as regular satellites, which means they orbit in the same direction as Jupiter’s rotation and are all on roughly the same plane. The outer, irregular satellites are small bodies captured by Jupiter’s gravity throughout its life.
MISSIONS TO JUPITER
MOST OF THE SPACECRAFT THAT HAVE VISITED JUPITER HAVE MADE ONLY A BRIEF FLYBY DURING A GRAVITY-ASSIST MANEUVER ON THE WAY TO ANOTHER PLANET. ONLY ONE SPACECRAFT HAS GONE INTO ORBIT AROUND JUPITER ITSELF.
The first spacecraft to journey beyond the inner solar system were Pioneers 10 and 11. After proving that the asteroid belt could be safely crossed, they sent back the first close-ups of Jupiter in 1973 and 1974. They were followed by the more sophisticated Voyagers 1 and 2, which returned breathtaking images of Jupiter’s moons. The Galileo spacecraft entered orbit in 1995 and spent eight years surveying the Jupiter system in great detail, before self-destructing. Cassini and New Horizons, bound for Saturn and Pluto, respectively, followed later.