Features of Mercury
Mercury was first visited by NASA's Mariner 10 space probe in 1974. In a series of three flybys, the probe managed to map most of the planet's surface. When the photos were received on Earth, the features might easily have been mistaken for the Moon. Mercury's surface, however, is almost completely covered with craters. The moon has more open areas known as maria, which are mostly crater-free. Mercury has only one major open area known as the Caloris Basin. These open areas are thought to have been formed by more recent volcanic flows, which may have covered the older craters. The surface is also marked by steep cliffs, also known as rupes, which rise as much as 6300 feet (1920 m) above the valley floors.
In 2008, a second spacecraft passed close to Mercury. This spacecraft was named MESSENGER, which stands for MErcury Surface, Space ENvironment, GEochemistry and Ranging. MESSENGER brought high resolution cameras to the planet and revealed many new features and new terrain not previously seen on the Mariner mission. MESSENGER is scheduled to enter orbit around Mercury in March 2011, where it will spend a full year mapping the planet's surface. New images from MESSENGER show that Mercury's surface is more homogeneous, ancient, and more heavily cratered, unlike the surfaces of the Moon and Mars.