Mercury is the nearest planet to the sun. It has the most extreme contrast in temperature between day (430°C) and night (-180°C) in the solar system. Daytime temperatures are high enough to melt zinc and tin. Mercury has virtually no atmosphere, meaning life as we know it is impossible.

Mercury was the Roman god of trade and commerce, in the same vein as Hermes of the Greeks, the messenger.

Mercury's orbit is highly eccentric, meaning it's path is elliptical. Also, mercury rotates three times for every two orbits of the sun. Mercury has no satellites.

Only one spacecraft has been to mercury, Mariner 10, passing three times in 1974-75. However NASA's Messenger is on the way, launched in August 2004, and will fly by three times and then enter mercury orbit in March 2011. Then in 2012, ESA/ISAS's BepiColombo will be launched, also into mercurian orbit. Because of mercury's proximity to the sun, it cannot safely be photographed by the Hubble Space Telescope.

Mercurian atmosphere is thin enough to be described as an exosphere, meaning the constituent atoms never collide. The identified elements in the exosphere are sodium, potassium, hydrogen, oxygen and neon. Ions and high frequency electromagnetic radiation from the sun are responsible for dislodging the atoms in the exosphere.

The surface of mercury is splattered with craters, the majority being formed in the early days of the solar system when rocky bodies moving at high speeds around the sun collided with the planet. The nonexistence of atmosphere meant that there was nothing to cushion the incoming bodies. Flat plains suggest possible volcanic activity. Earth based radar observations hint at possible water ice in the pole craters, possible only because of permanent shadow. The Caloris Basin is a notable landmark, a giant crater having a 650 km radius and is similar to the lunar basins.

Mercury is the second most dense planet after earth. The earth has more mass and so part of this density is attributable to gravitational compression. We can only speculate at the core composition of mercury, but it has a very high density for its radius so it is likely to have a large metallic core accounting for most of its mass. In this, mercury differs from the other terrestrial planets. Possible explanations are that mercury formed in an iron rich part of the early solar system, the sun's rays have reduced the metal oxides to solid metals which are denser, collisions or energy from the sun may have removed part of a less dense crust.

Mercury's point of perihelion was noticed to rotate the sun faster than predicted by Newton's laws of gravitation and proved to be strong evidence for Einstein's gravitational model; general relativity.