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, ρ_A the density of its atmosphere and A the surface area, then a simple computation indicates^1,2 that the rate of mass loss from this atmosphere is 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 ρ_w and a velocity V_w, then it adds matter to Mercury at the rate and the stead state situation is given when Inserting the appropriate numerical values, β≈10⁻¹¹, V_E²≈ 2 × 10¹⁰, and V_w≈ 10⁸ (all in c.g.s. units gives) 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.