The innermost planet is rarely seen because of the Sun's glare. With less than half Earth's gravity, Mercury retains only a wisp of an atmosphere (presumed to be helium). The lack of a significant atmosphere allows temperatures to fluctuate from 750 degrees Fahrenheit during the day to minus 320 Fahrenheit at night.

Like the other terrestrial planets - Venus, Earth and Mars - Mercury is made mostly of rock and metal. This small world is scarred by craters and looks somewhat like our Moon.

Mercury has been known since ancient times: the name Hermes, given by the Greeks, later translated to Mercurius by the Romans.