Mercury's thin atmosphere contains hydrogen, helium, and oxygen. It also has smaller amounts of sodium, potassium, calcium, and magnesium. This picture shows sodium near Mercury. Red and green areas have the most sodium.
Click on image for full size
Image courtesy of NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington.
Atmosphere of Mercury
Mercury has almost no atmosphere. The planet's small size means that its gravity is too weak to hold down a normal atmosphere. There is a very thin atmosphere around the planet. Mercury's thin atmosphere is constantly being "blown away" into space by the pressure of sunlight and by the solar wind. Gases are constantly being added to Mercury's atmosphere, too. That's why it still has any atmosphere at all - even though that atmosphere is really, really thin.
Mercury's atmosphere contains small amounts of hydrogen, helium, oxygen, sodium, potassium, calcium, and magnesium. Some of the gas particles come from the solar wind. Others are made by radioactive decay or when micrometeorites smash into the surface. All of these gases are soon carried away from Mercury by the solar wind and by Mercury's magnetic field. Atmospheric pressure at the planet's surface is less than one trillionth of Earth's (around one nanopascal or 10-14 bar).
Temperatures at the surface range between 100 and 700 kelvins (-280° F to 800° F or -173° C to 427° C). Lead melts at 600 kelvins! 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 enough atmosphere present to moderate the range in surface
temperature.
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