Mercury's surface closely resembles the Moon's. It is covered by impact craters, ancient lava flows, and quake fault lines. Mile-high cliffs stretch for hundreds of miles across the planet's surface. The huge Caloris impact basin, 800 miles (1,300 km) wide, decorates one side of the planet.
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Why does the Moon have phases?
Because it is so far from the Sun, astronomers had a hard time measuring Pluto's size. They finally got it right in the 1980s, after James Christy discovered a companion object. By watching Pluto and the companion, named Charon, eclipse each other, they measured Pluto's diameter at about 1,400 miles -- about one-third less than the diameter of Earth's Moon.
Pluto is basically a ball of frozen nitrogen, methane, and carbon dioxide wrapped around a small core of rock. On average, it's farther from the Sun than any other of the major planets, so its surface is bitterly cold.
For astronomers, though, a comet's beauty is not just in its appearance, but in its content: A comet is an icy time capsule that contains some of the original material from the cloud of gas and dust that gave birth to Earth and the other planets.
Current theory says that the planets formed as small grains of ice and rock stuck together to form larger bodies, called planetesimals, which then merged to form the planets.
But the next year, astronomers found a second object in a similar orbit, then another, and another. Instead of a planet, they had found the first of the asteroids -- large chunks of rock that were left over from the formation of the solar system. Ceres is the largest, with a diameter of around 600 miles (1,000 km).
Astronomers have discovered hundreds of thousands of these boulders, and with automated searches they are discovering thousands more each year. Most orbit the Sun in a broad region known as the asteroid belt, which is between the orbits of Mars and Jupiter.
Earth and the Moon are more like a double planet than a planet and a moon. The Moon is quite large in comparison to Earth -- about one-quarter of Earth's diameter. The two gravitationally interact with each other, most famously causing Earth's ocean tides.
Deimos is farther away and moves slowly from east to west. Deimos would look like a small dot of light in the sky. Phobos is slowly moving closer to Mars. In another 50 to 100 million years, it will crash into Mars.
Phobos is small, dark, and airless. And it's one of the driest bodies in the solar system.
Io: Fire World
Robotic probes may someday provide close-up views of some of the most remarkable vistas in the solar system, from the canyons of Mars to the ice-geysers of Triton. For a true hot-spot, they might show us the surface of Io, one of the moons of Jupiter. It is an eerie landscape of active volcanoes, tall mountains, and plains covered with frozen sulfur.
Titan: Smog World
Surrounded by a thick atmosphere that's topped by orange smog, and with a landscape that in some ways resembles Earth, Titan is one of the solar system's most intriguing moons.
Titan's frigid atmosphere is about 60 percent denser than Earth's at the surface, and clouds float across the sky. A global haze of organic molecules tops the atmosphere, so it was almost impossible to study the surface until the Cassini spacecraft, which carries instruments that can peer through the haze, arrived at Saturn in 2004.