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Saturn's Moons

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.

Comets

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.

Earth’s Moon

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.

Asteroids

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 almost 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.

Neptune's Triton

Triton, the largest satellite of Neptune, orbits in the opposite direction from most moons, suggesting that Neptune captured it in the distant past. Millions of years from now, Triton will move so close to Neptune that tidal forces will rip Triton apart, forming bright new rings around the giant planet.

Most of what we know about Triton came from Voyager 2, which photographed a landscape that is tinted subtle shades of pink, brown, and blue. Much of it resembles a cantaloupe, with ridges thousands of feet tall. Flowing ice or vaporizing gas may have carved this wrinkly terrain.

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