The Outer Planets: Jupiter, Saturn, Uranus, and Neptune
he most distant and perhaps most intriguing planets in our solar sys tem are known as the outer planets Jupiter Saturn Uranus and Neptune Of these bodies ancient astronomers could see only bright Jupiter and Saturn in the night sky Thanks to deep space probes and con stantly improving Earth based observational technology scientists today are steadily unraveling mysteries about the planets their moons and their rings This book takes you on a journey to the most remote planets in our solar system Our first destination is Jupiter the larg est planet in the solar system It is so big that more than 1 300 Earths could fit inside it It is the fifth planet from the Sun but bright enough that we can see its multicol ored stripes with only a small telescope Also visible is a red oval called the Great Red Spot which is a storm in Jupiter's atmo sphere that has been going on for more than 300 years Jupiter s surface and interior are composed mainly of hydrogen and helium The planet has 63 known moons and a nar row system of rings Its orbit is elliptical oval and one orbit is equal to about 1 86 Earth years IO
INTRODUCTION Next we encounter Saturn the second largest planet in our solar system Saturn was the most distant planet ancient astronomers could see Like Jupiter Saturn has a gaseous surface mainly made up of hydrogen and is surrounded by an atmosphere with complex weather patterns Interior hydrogen is a liq uid and the core is liquid metallic hydrogen Saturn has a typical elliptical orbit pattern Its magnificent rings consist largely of water ice and dust particles Although the rings are thin the diameter of the main rings extends for some 170 000 miles 270 000 kilome ters and the fainter outer rings extend even farther Saturn also has more than 60 moons Uranus is the third largest planet we encounter on our journey It is seventh from the Sun A small amount of methane in its atmosphere gives it a blue green hue but otherwise Uranus appears nearly featureless in visible light Ancient astronomers could not see Uranus and even 21st century tele scopes are not powerful enough to show great detail Like all the planets Uranus has an oval shaped orbit but its rotation is unusual Instead of rotating almost perpendicular to its orbit like most planets Uranus's rota tion is nearly parallel to its orbit Uranus has
THE OUTER PLANETS JUPITER SATURN URANus AND NEPTUNE Volcanoes erupt on lo one of Jupiter s Galilean satellites in a mosaic of images captured by the Galileo spacecraft One plume visible on lo at the far left is about 90 miles 40 kilometers high Another plume at the center of the moon is revealed by its shadow which extends to the right Photo NASA JPL Caltech NASA photo PIAo1081
INTRODUCTION about a dozen narrow rings which are not as bright as Saturn's and 27 known moons Like Jupiter and Saturn this planet is not solid It is made up of hydrogen helium water and other compounds Finally we arrive at the farthest planet from the Sun Neptune At more than 2 5 bil lion miles 4 billion kilometers from Earth Neptune was not discovered until the mid 1800s Although Neptune is the smallest of the four outer planets it is the densest Neptune is a gas giant too and is primar ily made up of hydrogen and some helium Pressures in the planet s interior are likely high enough to render it liquid Neptune spins so quickly that a day one rotation lasts only 16 hours Neptune has a series of six rings and four of the planet s 13 moons orbit within the rings Curiously one of Neptune s moons Titron revolves around the planet in a direction opposite the planet s rotation For many years Jupiter Saturn Uranus and Neptune were shrouded in mystery Astronomers have much to learn about this quartet of gas giants Probes such as Voyager 1 and 2 have gathered enough data and pho tographs to present a broader picture of the planets rings and moons that you will learn about in these pages 9
