Mercury is the closest planet to the Sun in our solar system. It is a small planet with an equatorial diameter of only 4880 km (3032 miles). Mercury's density is similar to that of Earth's, and its surface gravity is considerably less than Earth's. Scientists believe that Mercury formed at about the same time as Earth did, about 4.6 billion years ago.
Mercury doesn't have much of an atmosphere. Because of this, Mercury has wide temperature swings. During the day Mercury's temperature can reach as high as 800.6 °F (427 °C). During the night Mercury's temperature drops as low as -280 °F (-173 °C).
Mercury rotates on its axis every 59 Earth days and makes one complete orbit around the Sun every 88 days (compared to Earth's 365 days). Because of Mercury's fast orbit around the Sun and the very slow rotation on its axis, one day on Mercury lasts the equivalent of 176 Earth days.
Because Mercury is so close to the Sun it is hard for us to view it from Earth. However, unmanned space probes (e.g., Mariner 10 and the Messenger spacecraft) have been able to map out part of Mercury's surface.
Mercury's surface is heavily cratered due to objects, such as meteors, impacting its surface. With no atmosphere to speak of most meteors manage to make it to Mercury's surface. In fact, conditions on Mercury resemble those of Earth's moon, much more so than on Earth.
One difference between the Moon and Mercury's surface is that Mercury has large cliffs on its surface called scarps. Scientists believe that as Mercury's interior cooled the entire planet shrank, forming the scarps. Scientists are currently uncertain whether scarps are present on the entire surface of Mercury.
Mariner 10 detected a small magnetic field on Mercury, which suggests that Mercury may have an iron core.
There is some evidence of past volcanic activity, but there doesn't appear to be any activity there now.
Mercury has no moon.
Images of Mercury: Courtesy of NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington.