Mercury, the planet closest to the Sun is a small rocky ball. Mercury has been known since ancient times because of its brightness, it shines by reflecting sunlight. Mercury is the planet the orbits closest to the Sun. Mercury is only 3,031 miles (4,878 km) in diameter. It is the smallest of the four terrestrial planets and smaller than Jupiter's moon Ganymede or Saturn's moon Titan. It is 36 million miles (57.9 million km) from the Sun or about .4 Astronomical Units. Mercury travels around the sun faster than any other object in the Solar System. It completes one orbit every 88 days. Mercury spins on its axis every 59 days (exactly two-thirds of its 88-day year. Every two Mercurian years the planet spins three times.
In the spring of 1974 a small spacecraft named Mariner 10 gave us our first close-up view of Mercury. Photos showed that the planet has many craters, mountains, and valleys, like those on the Moon. Because Mercury is so close to the Sun, the planet is very hard to study through telescopes. While Mercury generally resembles the Moon, it does not have "seas" of lava found on the lunar surface. Instead, its gently rolling lava plains feature many raised ridges, known as scarps, which tend to run north-south. Scientists think that as the planet slowly cooled over many millions of years, the planet may have shrunk a bit and wrinkled, forming the scarps.
Mercury can best be seen with the naked eye when it is as far from the Sun in the sky as it can be, at its greatest eastern and western elongation. For a few days at these times Mercury can be viewed in the eastern sky rising before the Sun and is known as the "morning star" and in the western sky, low over the horizon as the "evening star" for a short time after sunset.
Mercury has a rock crust made of the same kind of rock found on Earth. Beneath the crust is a thicker mantle layer of the same material. Mercury has an iron core about the size of the Moon.