Atmosphere of Mercury
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.
General information
Column density cm−2; Surface density cm−3[1]
~ 3 × 109; ~ 250
< 3 × 1015; < 1.4 × 107
< 3 × 1011; ~ 6 × 103
< 3 × 1011; ~ 4 × 104
< 9 × 1014; < 2.5 × 107
~ 2 × 1011; 1.7–3.8 × 104
~ 2 × 109; ~ 4000
~ 1.1 × 108; ~ 3000
~ 4 × 1010; ~ 7.5 × 103
~ 1.3 × 109; < 6.6 × 106
< 1 × 1012; < 1.5 × 107
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.
Observation difficulties[edit]
Mercury is the least explored planet of the inner Solar System due to the many difficulties of observation. The position of Mercury as seen from Earth is always very close to the Sun, which causes challenges when trying to observe it. The Hubble Space Telescope and other Earth-based space imaging systems have highly sensitive sensors so they can observe deep space objects. They must not be directed toward the Sun, lest its powerful radiation destroy the sensors.[16]
Instead, flyby and orbital missions to Mercury can study the planet and receive accurate data. Even though Mercury is closer to Earth than Pluto is, the transfer orbit from Earth to Mercury requires more energy. Mercury being so close to the Sun, space probes going there are accelerating as they approach, due to the Sun's gravitational pull. This requires the use of retrorockets, which use fuel that the probe must carry instead of better instruments.[20]