Mercury
The Sun's nearest neighbour: small, fast, extreme
Summary
Mercury is the planet closest to the Sun and the smallest in the Solar System. A year on Mercury lasts only 88 Earth days: it circles the Sun faster than any other planet. It has almost no atmosphere, and its surface is covered with craters, much like the Moon's.
In depth
Mercury's radius is 2,439 km, about 1/3 of Earth's, making it smaller even than Jupiter's moon Ganymede. But its closeness to the Sun (58 million km on average, about 1/3 of Earth's distance) puts it through extreme conditions: +430°C by day, -180°C at night, the largest temperature range in the Solar System. It has practically no atmosphere (only a very thin layer called an exosphere). A Mercury day (from noon to noon) lasts 176 Earth days, twice as long as a Mercury year! This strange situation comes from its spin and orbit being locked in a 3:2 resonance. Its surface is heavily cratered, like the Moon's, but with a puzzling difference: Mercury's iron core makes up about 70% of the planet's mass (on Earth, 32%). There are several theories for this unusual density; the most popular is that a large collision stripped away its outer layers.
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