Mercury's exosphere: Origin of surface sputtering and implications

The existence of gaseous species H, He, O, Na and K, established by euv spectroscopic observations on Mariner 10 for the first three and ground- based observations for the last two species with column contents of the order of 10(12) cm(-2) or less, qualify the gaseous envelope of Mercury as an exosphere (column content per definition less than or equal to 10(14) cm(-2)). Whereas H and He seem to have its origin in the direct supply from the solar wind at the heavier constituents can arise from particle and photon interactions with surface materials. Some observations have suggested that Na extends to 700 km above the surface at the subsolar point, implying the existence of non-thermal atomic velocities of at least 2 km s(-1). Although Mercury has a small intrinsic magnetic field, solar wind protons have access to parts of its surface. Since particle sputtering liberates volatiles from the surface at energies greater than the mean thermal energy, the sputtered particle profile is governed by a scale height much larger than the barometric scale height. The surface densities of these sputtered volatiles then exhibit a lower exponential decay than would be expected from a barometric law using the exospheric (surface) temperature. Mercury's exosphere is a low attenuation medium for solar euv radiation which therefore can penetrate to its surface. Under these circumstances no ionospheric ''layer'' can be formed. The ionization of exospheric species leads to electron-ion pair formation which is balanced by chemical loss and transport processes. Since radiative recombination is an extremely slow loss process for atomic ions, plasma diffusion under gravity, which has Ba short time constant, controls the ionized species. Thus, the densities of the ionized exospheric component do not exceed a few electrons/ions cm(-3). This yields an extremely low height-integrated Pedersen conductivity, which must be taken into account in the understanding of convection processes in the magnetosphere. (C) 1997 Elsevier Science Ltd.