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, at present there are traces of a very tenuous atmosphere containing hydrogen, helium, oxygen, sodium, calcium, potassium and water vapor, with a combined pressure level poorly known: NASA estimates have varied from 10-15 bar (0.1 nPa)[1][clarification needed] (which can be considered negligible), to "about 0.00000000003 pound per square inch",or approximately 200 nPa. This gives a rough mass estimate of 1.6*106 kg.

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