Mars
Your next stop is Mars, a planet that shines in the sky with a reddish color and is sometimes called the Red Planet. The orbit of Mars is more oval-shaped than that of the Earth. So the distance between Mars and the sun may be as little as 128.4 million miles (206.7 million kilometers) or as much as 154.8 million miles (249.2 million kilometers). The average distance between Mars and the sun is about 142 million miles (228 million kilometers). It takes Mars 687 Earth days to travel once around the sun. It rotates once on its axis in 24 hours and 37 minutes.
Mars is a small, rocky world that is very dry. Its diameter is 4,200 miles (6,760 kilometers), only a little more than half that of the Earth. Its gravitational pull is weaker than the Earth's.
Mars has two small satellites. Phobos, the larger one, measures about 16 miles (26 kilometers) in its longest dimension. It circles its planet at a distance of 2,462 miles (3,964 kilometers). Phobos completes its orbit in about 71/2 hours, which is less time than it takes Mars to turn once on its axis. Therefore, to a visitor on Mars, Phobos would seem to rise in the west and set in the east. Deimos is smaller than Phobos, measuring about 10 miles (16 kilometers) long. It circles Mars at a distance of 14,700 miles (23,600 kilometers).
Early space probes provided a great deal of information about Mars. They learned that Mars has no magnetic field and that there is very little atmosphere surrounding it. The atmospheric pressure on its surface is 1/600 of the atmospheric pressure on the Earth's surface. Its thin atmosphere is made up almost entirely of carbon dioxide. Temperatures at the planet's equator may occasionally reach 60°F (16°C). But nighttime polar temperatures can plunge to nearly −202°F (−130°C).
The surface of Mars has a variety of features. These include craters, giant extinct volcanoes, and a canyon system almost as long as the width of the United States. In 2004, the robotic rovers Opportunity and Spirit touched down on the surface of Mars. They revealed many details about the planet's geology and topography. They also found signs that liquid water once flowed on the planet, perhaps enough to have supported life. For more information, see the article Mars.
Jupiter
After passing through a belt of asteroids beyond the orbit of Mars, the next stop on your planetary journey is Jupiter. This planet is more than five times farther from the sun than Earth. Jupiter travels around the sun at an average distance of 483 million miles (778 million kilometers). It takes 11.9 Earth years to complete one orbit.
Jupiter is the largest planet. It is one of the four gas giants in our solar system. Its diameter is 89,000 miles (143,000 kilometers). This is more than eleven times greater than that of Earth. Jupiter takes less than ten hours to rotate once on its axis.
What appears to be Jupiter's surface is really the top of a thick, cloudy atmosphere. Nearly 90 percent of it is hydrogen, and most of the remaining 10 percent is helium. There are also very small amounts of methane, ammonia, and some other substances that give Jupiter's clouds their beautiful colors.
The Pioneer and Voyager space probes flew by the planet in the 1970's. They confirmed that at the top of Jupiter's atmosphere it is very cold, about −186°F (−121°C). Deeper down in the clouds, however, temperatures climb to thousands of degrees. The tremendous heat rising through the atmosphere combines with Jupiter's rapid rotation to produce large and violent weather systems. Cloud belts on Jupiter are driven by powerful winds that move at speeds of up to 350 miles (560 kilometers) per hour. The most amazing feature in Jupiter's clouds is an enormous orange-red area known as the Great Red Spot. The Great Red Spot is more than twice the size of our planet.
Deep below the clouds there is no solid surface. Instead, astronomers think there is probably a planet-wide ocean that consists mostly of hydrogen compressed under such great pressure that it forms a liquid with metallic properties. At the center of Jupiter there is probably a core, consisting of iron and silicates, about twenty times the total mass of Earth. The molten metallic materials in the core create a strong magnetic field that extends far out into space. At the top of Jupiter's clouds, this magnetic field is about 14 times stronger than that of the Earth. Jupiter's gravitational pull is about two and one-half times stronger than the Earth's.
Jupiter has 79 known satellites. The largest, Ganymede, is 3,270 miles (5,270 kilometers) in diameter. Callisto is slightly smaller, with a diameter of 3,000 miles (4,800 kilometers). Each of these satellites has an icy surface covered with craters. Another satellite, Europa, which is 1,900 miles (3,100 kilometers) across, is also covered with a layer of ice. Io has a diameter of about 2,200 miles (3,500 kilometers). Its surface is covered with volcanoes and lava flows. The other known satellites range from less than a mile to more than a hundred miles in diameter. Before the Voyager probes, little was known about Jupiter's satellites because they appear only as dots when seen through even the largest telescopes. The later Galileo probe found indications of liquid salt water beneath the ice of Europa, Ganymede, and Callisto. Voyager 1 also discovered a thin, delicate ring of fine particles circling Jupiter. For more information, see the article Jupiter.
Saturn
The next stop on your journey is Saturn, another gas giant. It orbits the sun at an average distance of 885.5 million miles (1.4 billion kilometers). It takes nearly 29.5 Earth years to make one trip around the sun.
Saturn is not quite as large as Jupiter. It has a diameter of 74,500 miles (120,000 kilometers). Like Jupiter, Saturn rotates very quickly; it takes 10 hours and 40 minutes for it to turn once on its axis.
Saturn has a very deep atmosphere, which is made up mainly of hydrogen and helium, with some methane and ammonia. At the top of its atmosphere, the temperature is about −300°F (−185°C). This low temperature causes ammonia in the outer atmosphere to freeze, forming high-altitude haze. This gives Saturn a softer appearance than Jupiter. There are also spots similar to Jupiter's Great Red Spot in Saturn's atmosphere, but they are smaller. The largest is about the size of Earth.
The largest of Saturn's 62 known satellites is Titan. The diameter of Titan is 3,200 miles (5,150 kilometers). This is the only satellite in the solar system that has a thick atmosphere. This atmosphere consists mostly of nitrogen. The pressure at Titan's surface is about one and a half times greater than at the surface of the Earth.
Saturn is one of the most beautiful objects in the sky because of the rings that circle the planet at its equator. The entire ring system is more than 170,000 miles (274,000 kilometers) in diameter. But it is less than 1 mile (1.6 kilometers) thick. The rings may have formed at the same time as the solar system, or they may be the remains of a nearby satellite that was broken up by Saturn's gravitational pull, which is almost the same as that of the Earth. For more information, see the article Saturn.
Uranus
Leaving Saturn and traveling even farther from the sun, you come to the planet Uranus. Uranus is 32,000 miles (51,500 kilometers) in diameter. It orbits the sun at an average distance of 1.8 billion miles (2.9 billion kilometers). It takes about 84 Earth years to complete one orbit. Uranus rotates once on its axis in an east-to-west direction in approximately 17 hours. This is also the rate at which its magnetic field, which is anchored in the planet's interior, and its interior rotate. Uranus' magnetic field is only about two-thirds as strong as that of the Earth.
The blue-green color of Uranus is caused by methane in its thick atmosphere. The temperature at the tops of its clouds is −355°F (−215°C). Scientists think that beneath the atmosphere is a vast ocean of water, methane, ammonia, and other elements. This ocean may surround a core consisting mostly of molten rock and metal. At the tops of Uranus' clouds, the gravitational pull is somewhat weaker than that of the Earth. A system of thin, very dark rings encircles the planet.
Most of the planets in our solar system spin in a more or less straight up-and-down position as they orbit the sun, although some are tilted a little more than others. If the solar system could be placed on a gigantic table, the planets would look like spinning tops as they moved around the sun. Uranus, however, tilts at the extreme angle of 97.8 degrees. You could almost say the planet lies on its side as it orbits the sun. This means that the north and south polar regions of Uranus experience alternate periods of day and night (and summer and winter), each of which is up to 42 Earth years long. If Uranus were much closer to the sun, this could result in very extreme seasons. Because of the planet's distance from the sun and the odd wind patterns in its atmosphere, Uranus has rather even global temperatures. The north and south polar regions are actually a little warmer than the regions around its equator.
Uranus has 27 known satellites. The largest is Titania. It has a diameter of about 1,000 miles (1,600 kilometers). The smaller satellites range in size from about 12 to 96 miles (20 to 155 kilometers) in diameter. For more information, see Uranus.