Sputtering of sodium on the planet Mercury

Abstract

The discovery of a column density of neutral sodium vapour (∼10¹¹–10¹² atoms Na cm⁻²) associated with the solar-facing side of the planet Mercury¹ has rekindled interest in the possible presence of atmospheric gases on that planet. Recent observations² confirm the presence of Na emission, with its centroid shifted to the sunward limb. The sodium fluorescence is observed in the visible range. Previous ultraviolet observations by Mariner 10 provided evidence for the presence of H and He in the atmosphere (∼10¹¹ and 10¹² atoms cm⁻² respectively³) and gave upper limits on other possible species (such as O, Ne, Ar and C). Whereas the H and He can be explained primarily by capture from the solar wind⁴, we show here that this is not so for Na by evaluating the depletion rates. Photon and ion bombardment can release Na from its normal bonding state, producing adsorbed and ejected Na in ballistic trajectories. We show that ion sputtering cannot account for the observed column density but is an important loss mechanism for Na. Photons are likely to be the dominant stimulus, both directly through photodesorption and indirectly through thermal desorption of absorbed Na. We therefore conclude that the atmosphere produced is characterized by the planet's surface temperature, with the ion-sputtered Na contributing to a lesser, but more extended, component of the atmosphere.