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Mercury's primordial atmosphere dissipated shortly after the planet's formation because of both the low level of gravity on the planet, the high temperature, and the effects of the solar wind. However, nowadays there are traces of a very tenuous atmosphere containing hydrogen, helium, oxygen, sodium, calcium and potassium with a combined pressure level of 10-15 bar (which can be considered negligible).
[edit] Atmospheric composition
This atmosphere is not stable—atoms are continuously lost and replenished from a variety of sources. Hydrogen and helium atoms probably come from the solar wind, diffusing into Mercury’s magnetosphere before later escaping back into space. Radioactive decay of elements within Mercury’s crust is another source of helium, as well as sodium and potassium. Water vapor is probably present, being brought to Mercury by comets impacting on its surface.[2]
[edit] Temperature
Temperature variations in bodies without atmospheres (or with extremely weak atmospheres such as Mercury) are very pronounced. For example, during the day Mercury's surface reaches a temperature of 420°C, while at night it dips to –180°C. Due to these huge changes in the temperature, significant surface effects may be related to the thermal stresses experienced by the materials.
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