"Ed" wrote in message
ups.com...

Mercury does have a slight Atmosphere composed of helium.


The 'atmosphere' of Mercury is almost entirely a short term effect from
outgassing from surface rocks. I had a quick browse online and read
somewhere that it is so tenuous that the constituents of the atmosphere are
more likely to collide with the surface than other atoms, i.e. most
atomic/molecular paths are ballistic rather than 'random walk'.

A general description of atmospheres would include a requirement that one be
significantly held by the parent planet - but this is not the case for
Mercury. Were it not for outgassing there would not be an atmosphere.

Owen