Mercury, being the closest to the Sun, with a weak magnetic field and the smallest mass of the recognized terrestrial planets, has a very tenuous and highly variable atmosphere (surface-bound exosphere) containing hydrogen, helium, oxygen, sodium, calcium, potassium and water vapor, with a combined pressure level of about 10−14 bar (1 nPa).

Solar light pushes the atmospheric gases away from the Sun, creating a comet-like tail behind the planet.

The Mercurian exosphere consists of a variety of species originating either from the Solar wind or from the planetary crust.

[3] In 2008 the MESSENGER probe confirmed the presence of atomic hydrogen, although its concentration appeared higher than the 1974 estimate.

[4] Mercury's exospheric hydrogen and helium are believed to come from the Solar wind, while the oxygen is likely to be of crustal origin.

It was discovered in 1985 by Drew Potter and Tom Morgan, who observed its Fraunhofer emission lines at 589 and 589.6 nm.

[7] Some research has claimed a correlation of the sodium abundance with certain surface features such as Caloris or radio bright spots;[5] however these results remain controversial.

[10] Mariner 10's ultraviolet observations have established an upper bound on the exospheric surface density at about 105 particles per cubic centimeter.

[15] Some observations show that Mercury is surrounded by a hot corona of calcium atoms with temperature between 12,000 and 20,000 K.[9] In the early 2000s, a simulation of Mercury's Na exosphere and its temporal variation was conducted to identify the source process that supplied crustal species to the exosphere.