Why Does it Have an Atmosphere?
Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

 
Mercury is so small, scientists long assumed it had no atmosphere. But Mariner 10 surprised experts by revealing a tenuous net of gas around the planet. Mercury's thin atmosphere constantly escapes the weak gravity of the planet, but somehow, hydrogen and helium are constantly replenished. Scientists suspect the solar wind draws the gases back to the planet, and hope MESSENGER measurements can provide further insight.
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