Astronomers measure distances within the solar system in astronomical units (AU). One astronomical unit is the average distance between Earth and the sun, which is about 93 million miles (150 million kilometers). The inner planets have orbits whose diameters are 0.4, 0.7, 1.0, and 1.5 AU, respectively. The orbits of the gas giants are much larger: 5, 10, 20, and 30 AU, respectively. Because of their different distances from the sun, the temperature, surface features, and other conditions on the planets vary widely. Mercury, the innermost planet, has no moon and almost no atmosphere. It orbits so close to the sun that temperatures on its surface can climb as high as 800 degrees F (430 degrees C). But some regions near the planet's poles may be always in shadow, and astronomers speculate that water or ice may remain there. No spacecraft has visited Mercury since the 1970's, when Mariner 10 photographed about half the planet's surface at close range. The Messenger spacecraft, launched in 2004, was scheduled to fly by Mercury three times before going into orbit around the planet in 2011. Thick clouds of sulfuric acid cover Venus. Image credit: NASA Venus is known as Earth's twin because it resembles Earth in size and mass, though it has no moon. Venus has a dense atmosphere that consists primarily of carbon dioxide. The pressure of the atmosphere on Venus's surface is 90 times that of Earth's atmosphere. Venus's thick atmosphere traps energy from the sun, raising the surface temperature on Venus to about 870 degrees F (465 degrees C), hot enough to melt lead. This trapping of heat is known as the greenhouse effect. Scientists have warned that a similar process on Earth is causing permanent global warming. Several spacecraft have orbited or landed on Venus. In the 1990's, the Magellan spacecraft used radar -- radio waves bounced off the planet -- to map Venus in detail. Earth, our home planet, has oceans of liquid water, and continents that rise above sea level. Image credit: NASA/Goddard Space Flight Center Earth, our home planet, has an atmosphere that is mostly nitrogen with some oxygen. Earth has oceans of liquid water and continents that rise above sea level. Many measuring devices on the surface and in space monitor conditions on our planet. In 1998, the National Aeronautics and Space Administration (NASA) launched the first of a series of satellites called the Earth Observing System (EOS). The EOS satellites will carry remote-sensing instruments to measure climate changes and other conditions on Earth's surface. The planet Mars has clouds in its atmosphere and a deposit of ice at its north pole. Image credit: NASA/JPL/Malin Space Science Systems Mars is known as the red planet because of its reddish-brown appearance, caused by rusty dust on the Martian surface. Mars is a cold, dry world with a thin atmosphere. The atmospheric pressure (pressure exerted by the weight of the gases in the atmosphere) on the Martian surface is less than 1 percent the atmospheric pressure on Earth. This low surface pressure has enabled most of the water that Mars may once have had to escape into space. The surface of Mars has giant volcanoes, a huge system of canyons, and stream beds that look as if water flowed through them in the past. Mars has two tiny moons, Phobos and Deimos. Many spacecraft have landed on or orbited Mars. The layers of dense clouds around Jupiter appear in a photograph of the planet taken by the Voyager 1 space probe. Image credit: JPL Jupiter, the largest planet in our solar system, has more mass than the other planets combined. Like the other Jovian planets, it has gaseous outer layers and may have a rocky core. A huge storm system called the Great Red Spot in Jupiter's atmosphere is larger than Earth and has raged for hundreds of years. Jupiter's four largest moons -- Io, Europa, Ganymede, and Callisto -- are larger than Pluto, and Ganymede is also bigger than Mercury. Circling Jupiter's equator are three thin rings, consisting mostly of dust particles. A pair of Voyager spacecraft flew by Jupiter in 1979 and sent back close-up pictures. In 1995, the Galileo spacecraft dropped a probe into Jupiter's atmosphere. Galileo orbited Jupiter from 1995 to 2003. Saturn is encircled by seven major rings. Image credit: NASA/JPL/Space Science Institute Saturn, another giant planet, has a magnificent set of gleaming rings. Its gaseous atmosphere is not as colorful as Jupiter's, however. One reason Saturn is relatively drab is that its hazy upper atmosphere makes the cloud patterns below difficult to see. Another reason is that Saturn is farther than Jupiter from the sun. Because of the difference in distance, Saturn is colder than Jupiter. Due to the temperature difference, the kinds of chemical reactions that color Jupiter's atmosphere occur too slowly to do the same on Saturn. Saturn's moon Titan is larger than Pluto and Mercury. Titan has a thick atmosphere of nitrogen and methane. In 1980 and 1981, the Voyager 2 spacecraft sent back close-up views of Saturn and its rings and moons. The Cassini spacecraft began orbiting Saturn in 2004. It carried a small probe that was designed to be dropped into Titan's atmosphere. Uranus appears in true colors, left, and false colors, right, in images produced by combining numerous pictures taken by the Voyager 2 spacecraft. Image credit: JPL Uranus was the first planet discovered with a telescope. German-born English astronomer William Herschel found it in 1781. He at first thought he had discovered a comet. Almost 200 years later, scientists detected 10 narrow rings around Uranus when the planet moved in front of a star and the rings became visible. Voyager 2 studied Uranus and its rings and moons close-up in 1986. The blue clouds of Neptune are mostly frozen methane. The other object shown is Neptune's moon Triton. Image credit: NASA/JPL Neptune was first observed in 1846 by German astronomer Johann G. Galle after other astronomers predicted its position by studying how it affected Uranus's orbit. In 1989, Voyager 2 found that Neptune had a storm system called the Great Dark Spot, similar to Jupiter's Great Red Spot. But five years later, in 1994, the Hubble Space Telescope found that the Great Dark Spot had vanished. Neptune has four narrow rings, one of which has clumps of matter. Neptune's moon Triton is one of the largest in the solar system and has volcanoes that emit plumes of frozen nitrogen.