Mercury

The planet Mercury is the closest planet to the Sun. With a distance only one-third that of Earth's, Mercury's sunward face one of the hottest planetary surfaces in the Solar System. Other than Earth-bound telescopic observations, the only close analysis of Mercury came from NASA's Mariner 10 robotic probe which photographed about 40% of the surface in the 1970s. It was confirmed then that Mercury is as dry and lifeless as the Earth's Moon.

Like Earth's Moon, Mercury has no atmosphere, and the topography consists of impact craters, mountains, and scarps. Most of these features probably formed several billion years ago and have changed little since. Mercury may have once been volcanic, but there is no evidence of active volcanism today.

The existence of Mercury has been known since prehistoric times, occasionally appearing as a pinkish or coppery star in the twilight sky immediately before or after sunset.