 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.
- ^ Killen 2007, p. 456, Table 5
- ^ "NASA—Mercury". Archived from the original on 2005-01-05. Retrieved 2009-09-26.