MERCURY: A LITTLE HELIUM AND HYDROGEN
The atmosphere on Mercury is very tenuous and is approximately a million billion times less dense than ours. Mercury's atmosphere seems to be supplied and constantly replenished by the solar wind. Helium and a couple of percent of atomic hydrogen have been identified as its principal constituents. The abundance of other atomic or molecular species, if they are present, is insignificant. No evidence has been found for atmospheric modification of any landform.
Just after Mercury formed some 4.6 billion years ago, gases like carbon dioxide escaping from the interior of the planet may have temporari Iy created an atmosphere of some extent. But soon after forming it would have escaped into space and vanished. This is because as mentioned above, the planet is not massive enough to hold much of an atmosphere at such a small distance from the sun, and probably was endowed with less gaseous material during its formation than the other terrestrial planets.
