Atmosphere of Mercury

Abstract

THE recently released news concerning the possible atmosphere surrounding Mercury seems to have taken the astronomical community by surprise. It had generally been expected that because of the high temperatures (of the order of 450 K) existing there and also because of the low surface gravity, any atmosphere originally present, or any released from subsequent events on the surface would quickly evaporate since the random thermal speed of the atoms would be greater than the escape velocity. If V E is the escape velocity from Mercury, ρA the density of its atmosphere and A the surface area, then a simple computation indicates1,2 that the rate of mass loss from this atmosphere is "figure presented" where β = m/2 kT, k being Boltzmann's constant and m the atomic mass. All this ignores the solar wind which carries mass into the atmosphere. In reality the problem becomes a boundary layer problem in which the magnetic field of Mercury plays a part, but we can see simply that the main effect is to generate an atmosphere. If the wind has a density ρw and a velocity V w, then it adds matter to Mercury at the rate "figure presented" and the stead state situation is given when "figure presented" Inserting the appropriate numerical values, β≈10-11, V E2≈ 2 × 10 10, and V w≈ 108 (all in c.g.s. units gives) "figure presented" There should therefore be a very tenuous atmosphere, basically composed of hydrogen being maintained in this way. Any helium and argon released by radioactivity would become trapped in this atmosphere and should be detected. © 1974 Nature Publishing Group.