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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.

Jupiter's Moons

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.

Mars' Moons

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.

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.

Pluto

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.

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