Mariner 10's ultraviolet observations have established an upper bound on the exospheric surface density at about 105 particles per cubic centimeter. This corresponds to a surface pressure of less than 10−14 bar (1 nPa).[19]
The temperature of the Hermian exosphere depends on species as well as geographical location. For exospheric atomic hydrogen, the temperature appears to be about 420 K, a value obtained by both Mariner 10 and MESSENGER.[10] The temperature for sodium is much higher, reaching 750–1500 K on the equator and 1,500–3,500 K at the poles.[20] Some observations show that Mercury is surrounded by a hot corona of calcium atoms with temperature between 12,000 and 20,000 K.[15]
Tail
Because of Mercury's proximity to the Sun, the pressure of Solar light is much stronger than at Earth's location. Solar radiation pushes neutral atoms away from the Sun, creating a comet-like tail behind Mercury.[21] The main component in the tail is sodium, which has been detected as far as 56,000 km (about 23 RM) from the planet.[21] This sodium tail expands rapidly to a diameter of about 20,000 km at a distance of 17,500 km.[22] In 2009 MESSENGER also detected calcium and magnesium in the tail, although these elements were only observed at distances less than 8 RM from the planet
The temperature of the Hermian exosphere depends on species as well as geographical location. For exospheric atomic hydrogen, the temperature appears to be about 420 K, a value obtained by both Mariner 10 and MESSENGER.[10] The temperature for sodium is much higher, reaching 750–1500 K on the equator and 1,500–3,500 K at the poles.[20] Some observations show that Mercury is surrounded by a hot corona of calcium atoms with temperature between 12,000 and 20,000 K.[15]
Tail
Because of Mercury's proximity to the Sun, the pressure of Solar light is much stronger than at Earth's location. Solar radiation pushes neutral atoms away from the Sun, creating a comet-like tail behind Mercury.[21] The main component in the tail is sodium, which has been detected as far as 56,000 km (about 23 RM) from the planet.[21] This sodium tail expands rapidly to a diameter of about 20,000 km at a distance of 17,500 km.[22] In 2009 MESSENGER also detected calcium and magnesium in the tail, although these elements were only observed at distances less than 8 RM from the planet
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