Mercury is a small planet with a large iron core and a surface temperature of -292 to 806°F. It has been observed from Earth since ancient times and is the smallest planet in the solar system. Theories about its core include an impact-based theory, a super-hot atmospheric envelope, and the drag of a protosolar nebula. The late heavy bombardment event covered the surface with craters, and Mariner 10 provided information about its surface in 1974. MESSENGER will provide more information in 2008.
Mercury is a small planet, with an orbital diameter about one-third ours, located about two and three-quarters light-minutes from the Sun. It is about 1/20 the mass and volume of Earth, with a surface temperature of -292 at 806°F (−180 to 430°C), with maxima at noon on Mercury and minima in craters near the poles. It is composed of 70% metallic material and 30% silicate material, with a relatively large core made 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, albeit 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 compared to Earth’s 17%. There are various theories used to explain it. The first is an impact-based theory, which claims that early in the solar system’s history, a large planetesimal slammed into the planet and tore off most of its crust and mantle. The second is that, since the Sun was initially forming from a primordial gas cloud, it plunged Mercury into a super-hot atmospheric envelope, scorching the surface material and ejecting it outward as solar wind. The third theory is that the drag of a protosolar nebula prevented lighter particles from accumulating in the planet Mercury, leaving it with mostly heavier elements. Future space missions will take a close look at Mercury and examine evidence to confirm or disprove these theories, which make different predictions about its surface composition.
Because Mercury is too small and too hot to hold much of the atmosphere, it took a major hit in the early days of the solar system, when there were many more roaming rocks than today. This event is called the late heavy bombardment and it covered the surface of Mercury with craters. Early volcanism also helped create some small maria, or smooth plains, like those seen on the Moon today.
We know most of the details about Mercury’s surface thanks to Mariner 10, a robotic probe sent into orbit around the planet in 1974. After a few days in orbit 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, providing us with more information about this very small and very hot planet.
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