A mosaic of one hemisphere of Mercury taken by the Mariner 10 on March 29, 1974. Image from NASA.
Mercury, only slightly larger than Earth's moon, is the second smallest planet in the solar system. Mercury's surface is covered with craters. Temperatures range from -300 to 800 degrees F. The interior structure of Mercury is thought to include a metallic core, an intermediate rocky layer, and a thin brittle crust. The composition of Mercury is probably high in iron. Surface features indicate that volcanic activity once existed at the surface, though there is little evidence of surface activity now.
Mercury's orbit is so close to the Sun that it is difficult to see from Earth. Because of the glare of the Sun, it can only be seen in twilight. In the 1960's scientists determined the rotation rate of the planet to be 59 Earth days. The Mariner 10 space mission passed by the planet three times in 1974. Images taken by Mariner 10 are the only close up images we have of the planet's surface.
Mercury does not have a significant atmosphere. Mariner 10 observed traces of helium 1000 km above the surface, probably formed by captured particles from the solar wind and radioactive decay of Mercury's crust. Other elements observed in Mercury's tenuous atmosphere include sodium, potassium and oxygen. When these gases are ionized by solar radiation, they are lost to the atmosphere through the interaction of the planet's magnetic field with the solar wind.
Mercury is the only terrestrial planet other than the Earth that has a significant magnetic field. This field, along with the planet's high density and small size relative to the Earth, indicates that it probably has a molten iron core. The magnetic is approximately dipolar and is tilted less than 10 degrees from Mercury's rotation axis. Mariner 10 observed a shock wave called a "bow shock" in front of the planet, where the planet's magnetic field meets the magnetic field carried by the oncoming solar wind.