Atmosphere of Mercury
.jpg) Atmosphere of Mercury |
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| Species | CD, cm−2 | SD, cm−3 |
| Hydrogen (H) | ~ 3 × 109 | ~ 250 |
| Molecular hydrogen | < 3 × 1015 | < 1.4 × 107 |
| Helium | < 3 × 1011 | ~ 6 × 103 |
| Atomic 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 |
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neon, silicon, sulfur, argon, iron, carbon dioxide, etc. |
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Mercury 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 10−14bar (1 nPa). 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 had been contentious before 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.
The Mercurian exosphere consists of a variety of species originating either from the Solar wind or from the planetary crust. The first constituents discovered were atomic hydrogen (H), helium (He) and atomic oxygen (O), which were observed by the ultraviolet radiation photometer of the Mariner 10 spaceprobe in 1974. The near-surface concentrations of these elements were estimated to vary from 230 cm−3 for hydrogen to 44,000 cm−3 for oxygen, with an intermediate concentration of helium. In 2008 the MESSENGER probe confirmed the presence of atomic hydrogen, although its concentration appeared higher than the 1974 estimate. Mercury's exospheric hydrogen and helium are believed to come from the Solar wind, while the oxygen is likely to be of crustal origin.
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