Mercury is one of the four rocky planets in this solar system, along with Venus, Earth, and Mars. It has an extremely thin atmosphere, which has resulted in numerous craters from collisions with meteors and asteroids. Astronomers have learned quite a bit about Mercury from radio telescopes and space missions, but much of this planet still remains a mystery.
Scientists believe Mercury is mainly made of molten iron, and that most of this iron is contained in a very large core. In 1974, the spacecraft Mariner made a surprising discovery--that Mercury has a magnetic field, indicating the presence of liquid iron. Because small planets cool off soon after forming, scientists assumed that Mercury had a solid iron core and a silicate mantle. In 2007, radio telescopes showed definitive evidence that the core is indeed liquid.
For the core to still be liquid in this very small planet, impurities in the iron must exist. Astronomers speculate that large amounts of another element, probably sulfur, lowered the melting temperature. Small protoplanets may have left trails of sulfur behind, which Mercury picked up during its formation.
Mercury has an extremely thin atmosphere, and because of this, has undergone numerous collisions with meteors, asteroids, comets, and early protoplanets. Since Mercury consists of so much iron but the surface has very little, researchers theorize that collisions have stripped away much of the planet's crust. Most of Mercury's surface consists of loose and fragmented dark, dry soil, consisting of remains from micrometeorite impacts. This soil is called regolith, a technical name for what people call dirt, dust, sand, and gravel, the material which lies over solid bedrock. Earth's regolith is created by biological processes and weathering, while that of Earth's moon is probably more like Mercury's.
Scientists also originally believed that Mercury had no water because of its lack of atmosphere, but in 1991, radio telescopes indicated possible ice deposits in the polar regions, in crater bottoms where sunlight never reaches and the temperatures are always bitterly cold. This ice probably originated in comets or meteorites.
With spectroscopy, astronomers can study the light that Mercury reflects and determine the elements in its atmosphere, which include helium, hydrogen, calcium, oxygen, potassium and sodium. The elements are continually blasted away by high-energy particles of the solar wind, and it makes Mercury's atmosphere very unstable. Scientists do not know how the atmosphere is constantly replenished.
Mercury is the most dense object in the solar system except for the Earth. Mercury's core is larger than that of Earth, and probably takes up most of the planet. Mercury is smaller than two of Jupiter's moons, and is not much bigger than Earth's moon. NASA's Messenger mission, launched in 2004, came close enough to Mercury in 2008 to begin sending images and data back to Earth, and is scheduled to begin a year-long orbit of the planet in 2011. Astronomers are excited to receive further information about this mysterious planet.