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Mercury is a small planet, with an orbital diameter about a third of our own, located approximately two and three quarters light minutes from the Sun. It is roughly 1/20th the Earth's mass and volume, with a surface temperature of -292 to 806 °F (−180 to 430 °C), with the highs at Mercurian high noon and the lows in craters near the poles. It is made up of 70% metallic material and 30% silicate material, with a relatively large core made up of molten iron.
Despite its small size, Mercury has been observed from Earth since ancient times due to its illumination by the Sun. With the dismissal of Pluto as a planet, Mercury is the new smallest planet in the solar system, although two moons, Ganymede and Titan, are larger, but still less massive.
The biggest mystery about Mercury is its large iron core, about 42% of its volume relative to Earth's 17%. There are various theories used to explain this. The first is an impact-based theory, which states that early on in the solar system's history, a large planetesimal slammed into the planet and stripped off most of its crust and mantle. The second is that, as the Sun was initially forming out of a primordial gas cloud, it bathed Mercury in a super-hot atmospheric envelope, searing off the surface material and expelling it outwards as solar wind. The third theory is that drag from a protosolar nebula prevented lighter particles from accreting into the planet Mercury, leaving it with mostly heavy elements. Future space missions will observe Mercury closely and look into evidence to confirm or refute these theories, which make different predictions about its surface composition.
Because Mercury is too small and hot to contain much of an atmosphere, it got a severe beating in the early days of the solar system, when there were much more stray rocks than today. This event is called the late heavy bombardment, and covered Mercury's surface in craters. Early volcanism contributed to create some small maria, or smooth plains, as well, like those seen on the Moon today.
Most of the details about Mercury's surface we know thanks to Mariner 10, a robotic probe sent to orbit the planet in 1974. After a few days of orbiting it ran out of fuel and our scientists stopped communicating with it. Mariner 10 is thought to still orbit the Sun, passing Mercury every few months.
In 2008, the MESSENGER spacecraft will reach Mercury, giving us more information about this very small, very hot planet.
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