The Outer Planets: Jupiter, Saturn, Uranus, and Neptune
THE OUTER PLANETS JUPITER SATURN URANTS AND NEPTUNE Jupiter is huge it would have to be about 80 times more massive to generate nuclear reactions and become a star Hot glowing lava erupts from a volcano on Jupiter's moon lo in an image captured by the Galileo orbiter It is a mosaic of several images including enhanced color images taken in visible light The bright white yellow and orange at left are false colors added to images taken at infrared wave lengths to indicate temperature White is the hottest and orange the coolest NASA JPL California Institute of Technology Nevertheless Jupiter reigns at the center of a system of dozens of moons like a minia ture solar system Four of the moons are quite large and would probably be considered plan ets themselves if they did not orbit a planet They are remarkably dissimilar worlds Io is the most volcanically active body in the solar system Frequent eruptions renew its surface and cover over any craters that form Europa s surface is also fairly young but its features are smoothed not by fiery lava but by water ice 12
CHARACTERISTICS OF UPITER which possibly flows up from beneath the sur face and freezes Europa may harbor an ocean of warm salty water just under its icy crust Callisto has an ancient crater scarred surface that seems to have been largely undisturbed by geologic activity for some 4 billion years Ganymede the solar system s largest moon is bigger than the planet Mercury and has a plan etlike magnetic field An outer planet Jupiter is much farther from the Sun than Earth and the other inner planets are Its orbit lies between the main asteroid belt and Saturn's orbit Its inner planetary neighbor Mars orbits between the Jupiter's Galilean moons from left to right Io Europa Ganymede and Callisto appear in a montage created with images from the Galileo orbiter The images are scaled to show the moons sizes relative to one another NASA JPL Caltech 13
THE OUTER PLANETS JUPITER SATURN Uranus AND NEPTUNE asteroid belt and Earth Like the other outer planets Saturn Uranus and Neptune Jupiter is much larger and less dense than Earth and the other rocky inner planets SIZE Mass AND DENSITY Jupiter is named for the ruler of the ancient Roman gods the equivalent of the ancient Greek god Zeus The ancient Romans did not know how large the planet is but the name turned out to be fitting Jupiter encom passes more matter than all the other planets in the solar system combined Its diameter at the equator is some 88 846 miles 142 984 kilometers It is about 320 times as massive as Earth Jupiter s large mass produces strong gravitational effects on other members of the solar system It forms gaps for instance in the distribution of the asteroids in the main belt and changes the trajectory of comets The planet s density is very low only about 1 3 times that of water By comparison Earth is more than 55 times denser than water ORBIT AND SPIN Jupiter like all the other planets travels around the Sun in a slightly elliptical or 14
CHARACTERISTICS OF UPITER oval shaped orbit It completes one orbit in about 1 86 Earth years which is the length of a year on Jupiter Its average distance from the Sun is about 483 million miles mil lion kilometers which is more than five times greater than Earth's Jupiter spins quite quickly on its axis faster than the other seven planets It completes one rotation in about 9 hours and 55 min utes which is the length of a day on Jupiter The atmosphere spins at slightly different rates with the clouds near the equator com pleting a rotation a few minutes faster than the clouds at higher latitudes The force of a planet s rotation causes it to bulge slightly at the equator and to flatten slightly at the poles Jupiter's rapid spin accentuates this so it is less perfectly spherical than most other plan ets Jupiter s spin axis is tilted only about 3 degrees For this reason it does not have sea sons like Earth Mars and other planets with tilted axes RING SrSTEM Jupiter s thin ring system was discovered only by spacecraft by Voyager in 1979 The rings are composed of tiny dust particles that orbit the planet The main ring is about 4 000 15
