Impact of electron chemistry on the structure and composition of Io's atmosphere

Two-dimensional model calculations (altitude and solar zenith angle) are performed to investigate the impact of electron chemistry on the composition and structure of Io's atmosphere. The calculations are based upon the model of Wong and Smyth (2000, Icarus 146, 60-74) for Io's SO₂ sublimation atmosphere with the addition of new electron chemistry, where the interactions of the electrons and neutrals are treated in a simple fashion. The model calculations are presented for Io's atmosphere at western elongation (dusk ansa) for both a low-density case (subsolar temperature of 113 K) and a high-density case (subsolar temperature of 120 K). The impact of electron-neutral chemistry on the composition and structure of Io's atmosphere is confined primarily to an interaction layer. The penetration depth of the interaction layer is limited to high altitudes in the thicker dayside atmosphere but reaches the surface in the thinner dayside and/or nightside atmosphere at larger solar zenith angles. Within most of the thicker dayside atmosphere, the column density of SO₂ is not significantly altered by electrons, but in the interaction layer all number densities are significantly altered: SO₂ is reduced, O, SO, S, and O₂ are greatly enhanced, and O, SO, and S become comparable to SO₂ at high altitudes. For the thinner nightside atmosphere, the species number densities are dramatically altered: SO₂ is drastically reduced to the least abundant species of the SO₂ family, SO and O₂ are significantly reduced at all altitudes, and O and S are dramatically enhanced and become the dominant species at all altitudes except near the surface. The interaction layer also defines the location of the emission layer for neutrals excited by electron impact and hence determines the fraction of the total neutral column density that is visible in remote observation. Electron chemistry may also impact the ratio of the equatorial to polar SO₂ column density deduced from Lyman-α images and the north-south alternating and System III longitude-dependent asymmetry observed in polar O and S emissions.