The only spacecraft to explore Mercury was Mariner 10 in 1974-75 that imaged only about half of the planet, leaving the remainder unexplored.

Mercury rotates exactly three times around its axis for every two orbits around the Sun. That means that one solar day on Mercury (sunrise to sunrise), lasts two Mercurian years (176 Earth days).

The surface temperature varies from 427° C (on the sunny side) to -183° C (on the night side) which is the greatest temperature range of any planet or satellite in the solar system. There is some evidence of water ice in the protected shadows (where the sun never shines) of some craters near Merucy's north pole.

The planet's density is about the same as Earth's but Mercury lacks the degree of gravitational compression of the Earth. This implies that Mercury has an enormous iron core composing about 75 percent of Mercury's diameter. A rocky mantle and crust only about 600 km thick surround the core. One explanation for the large iron core is that it is the result of the collision between Mercury and a large planetesimal.

The Caloris Basin (1300 km in diameter) was caused by a large impact, The Shock wave of this impact traveled through the planet to instantaneously form the hilly, odd terrain on the exact opposite side of the planet.