Bigger Than Mercury

StarDate: December 16, 2009

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The planet Mercury is just peeking into view in the southwest shortly after sunset this week. It's fairly bright, but it's quite low as the sky begins to darken, so any trees or buildings along the horizon will block it from view.

Mercury is the closest planet to the Sun. It's also the smallest of the Sun's planets. It's just 3,032 miles across -- less than half the diameter of Earth. Mercury is so small, in fact, that it's outranked by two moons.

One is Ganymede -- the largest moon in the solar system. It orbits the planet Jupiter, and it's almost 3300 miles across -- about 240 miles bigger than Mercury.

The other moon that surpasses Mercury is Titan, the largest moon of Saturn. Titan is 3200 miles across -- about 170 miles bigger than Mercury. Titan is unique among moons because it has a dense atmosphere -- even thicker than Earth's. Mercury, on the other hand, has no atmosphere at all -- just a few lonely particles captured from the solar wind.

Despite their size, Ganymede and Titan aren't considered planets because they don't orbit the Sun. Instead, they're satellites of other planets.

Still, you'll need to keep these moons in mind if you ever rank the solar system's largest worlds. The top seven are Jupiter, Saturn, Uranus, Neptune, Earth, Venus, and Mars. But Mercury does not rank number eight. Instead, the little planet is number 10 -- behind Ganymede and Titan.

Script by Ken Croswell, Copyright 2009

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