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M. H. Burger, N. M. Schneider (Univ. Colorado/LASP)
The atmosphere of Jupiter's moon Io is escaping at a rate of ~ 1 ton/sec, but Io's bright disk hinders most studies of escape in Io's immediate vicinity. However, in November 1996 two high resolution images were taken of Io's sodium cloud with the Galileo Solid State Imager (SSI). Although light scattered off the Prometheus volcanic plume dominates the observed intensity, a sodium directional feature can be idenitfied through both the clear and the green SSI filters. Although this feature has been seen before by ground based observers, these images provide the highest spatial resolution ever obtained and the first detection within 10 RIo of the satellite.
The detection of this feature is a direct observation of mass loss from Io's atmosphere. The observed intensity is due to the resonant scattering of sunlight by fast moving (velocities > 10 km/s) neutral sodium atoms which have been ejected from the atmosphere. This observation is consistent with the pickup ion neutralization mechanism which has been used to explain the fast sodium observed at greater distances from Io (Wilson, Ph.D Thesis, University of Colorado, 1996). These observations can be used to characterize for the first time the spatial extent of the sodium source and the variations with radial distance from Io of the emitted intensity and, therefore, the density of sodium atoms. The implications on atmospheric and ionospheric losses are discussed.
This work is supported by the NASA Planetary Astronomy Program.