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 caused by the solar wind and the breakdown of Mercury's crust. Sodium, potassium and oxygen have also been detected in Mercury's very weak atmosphere, but these elements can be lost when they react with the Sun and Mercury's magnetic field.

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.