So far, observations of Mercury from ground-based observers and the Mariner 10 spacecraft have not shown evidence of a significant atmosphere compared to other planets. Mariner 10 did observe a tiny amount of helium 1000 km above the surface, but this is probably produced by capturing particles from the solar wind and radioactive decay of Mercury's crust. Other elements observed in Mercury's extremely tenuous atmosphere (which exerts a pressure one million billionths that of Earth) include sodium, potassium and oxygen. When these gases are ionized by solar radiation, they are lost to the atmosphere through the interaction of the planet's magnetic field with the solar wind.

Temperatures at the surface range between -300 and 800^oF (hot enough for lead to melt). This large range in surface temperature is possible because Mercury is so close to the Sun (a year is only 88 Earth days long) and does not have sufficient atmosphere present to moderate the range in surface temperature.