The atmospheric abundance of SO₂ on Io

Near-ultraviolet spectra of Io have been obtained for the first time at high-resolution with the International Ultraviolet Explorer (IUE) satellite in order to determine the SO₂ abundance in the dayside atmosphere for both the leading (east) and trailing (west_ hemispheres. The improved resolution (∼0.2 A ̊), compared with previous low-resolution observations (∼8 A ̊), is adequate for determination of the spectral contrast of the SO₂ gas absorption. We have compared the derived geometric albedos with various model albedos that would result from proposed SO₂ atmospheres, as well as from localized atmospheres produced by sublimation (on the basis of recent estimates of the distribution of optically thick SO₂ frost and temperature) and by direct volcanic output. Alternatively with a model atmosphere composed of a homogeneous layer, we have placed an upper limit of 2 × 10¹⁷ cm^-2 (or 0.0074 cm-atm, corresponding to 4 × 10^-9 bar surface pressure and 2 × 10¹¹ cm^-3 surface density for a 10-km scale height) on the average SO₂ column density for both hemispheres. This upper limit implies that a collisionally thick SO₂ atmosphere of intermediate density may have been present on Io's dayside at the time of our observations. In addition, small atmospheric regions of relatively large density (about an order of magnitude higher than the average upper limit) associated with volcanic plumes are also compatible with the observations. Some intermediate atmospheres may contain sufficiently large SO₂ densities near the evening terminator to reproduce the evening ionospheric profile measured by Pioneer 10 on the leading hemisphere (away from any known volcanic plumes).