Mercury has a very thin atmosphere. In 1974, Mariner 10 detected traces of oxygen, sodium, helium, potassium and hydrogen vapours. More recently, Earth-based telescopes have detected gaseous sodium, potassium and calcium. The hydrogen and helium may have originated from the solar wind, while the sodium may have come from surface rocks bombarded by the wind or meteorites. Astronomers have observed clouds of sodium vapour occasionally rising from the surface of Mercury.
The atmospheric pressure is only a million-billionth that of Earth's - as low as many vacuums created in Earth laboratories. Mercury has very little atmosphere because its gravity is too weak to retain any significant gas particles, and it is so hot that gases quickly escape into outer space. The quantities of these gases have been poorly determined and vary depending on the position of Mercury in its orbit. The Messenger probe should greatly increase our knowledge of Mercury's atmospheric composition.
The atmospheric gases are much denser on the cold night side of Mercury than on the hot day side.
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