Atmosphere of Mercury

Atmosphere of Mercury

Mercury's surface, with the atmosphere too thin to be visible.
General information
Chemical species	Column density cm−2; Surface density cm−3[1]
Composition
Hydrogen	~ 3 × 109; ~ 250
Molecular hydrogen	< 3 × 1015; < 1.4 × 107
Helium	< 3 × 1011; ~ 6 × 103
Oxygen	< 3 × 1011; ~ 4 × 104
Molecular oxygen	< 9 × 1014; < 2.5 × 107
Sodium	~ 2 × 1011; 1.7–3.8 × 104
Potassium	~ 2 × 109; ~ 4000
Calcium	~ 1.1 × 108; ~ 3000
Magnesium	~ 4 × 1010; ~ 7.5 × 103
Argon	~ 1.3 × 109; < 6.6 × 106
Water	< 1 × 1012; < 1.5 × 107

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).^[2] The exospheric species originate either from the Solar wind or from the planetary crust. Solar light pushes the atmospheric gases away from the Sun, creating a comet-like tail behind the planet.

The existence of a Mercurian atmosphere was contentious until 1974, although by that time a consensus had formed that Mercury, like the Moon, lacked any substantial atmosphere. This conclusion was confirmed in 1974 when the unmanned Mariner 10 spaceprobe discovered only a tenuous exosphere. Later, in 2008, improved measurements were obtained by the MESSENGER spacecraft, which discovered magnesium in the Mercurian exosphere.