JOVIAN SYSTEM DATA ANALYSIS PROGRAM PROPOSAL SUMMARY

The revelation of a magnetosphere, oxygen emission, and a hydrogen corona of Ganymede changed the perception of Ganymede from one of a barren icy moon to one in which there are very intriquing possibilities for the satellite surface/atmosphere/magnetosphere interaction, perhaps some which have not been seen before. Speculation about the sources of the exosphere include sublimation of H2O directly from the surface [Ip et al., 1997], sputtering of H2O, O2, H, and H2 from the surface [Frank et al., 1997, Barth et al., 1997], sole sputtering of H+ from the surface, and diffusion of water group molecules from mini-atmospheres created in voids within the regolith [Noll et al., 1996]. An oxygen emission which implies the creation of a background presence of O2 was recently reported by Hall et al. [1998]. Dissociation and recombination of suspected molecular sources must occur in such a way that H+ alone flows away at supersonic speeds as reported by Frank et al. [1997]. The principal objective of this study is to explore the viability of some of these source mechanisms, particularly sublimation of water vapor from the surface. To meet this objective, a determination of the surface temperature field is essential, since many of the mechanisms above are critically dependent upon the temperature and abundance of the ice component of the surface. The surface of Ganymede appears to one in which the ice abundance varies locally at small spatial scales. We will use correlated data from the Near Infrared Mapping Spectrometer (NIMS) and multi-spectral photometry of the Solid State Imager (SSI) at high resolution to find the energy absorbtion rates from the surface. The technique we will use will allow for the identification of a number of surface unit types and the absolute ice abundances within each unit type at a spatial scale which approaches the surface-segration scale lengths proposed by Spencer [1987]. A thermal model will use this input to calculate the rate of sublimation from the surface over a diurnal cycle. For self-consistency in the calculation of both the upward and downward fluxes, consideration will be made of the sputtered flux, chemical reactions which change the concentrations of the atmospheric constituents in time, lateral transport of particles including collisionless transport, and energetics of the atmosphere such as heating by particles of magnetospheric origin. Preliminary results from the model suggest the importance of both sublimation and sputtering, mechanisms which contribute to the formation of the exosphere in differing spatial andtemporal regimes. The calculations will contribute toward increased understanding of thin molecular envelopes around icy moons and planets such as Mercury. Findings in this study may also prove to be relevant to Cassini?s exploration of the icy moons of Saturn. An Education/ Public Outreach Proposal is also attached to this study. The E/PO proposes participation in the Windows to the Universe project; an Internet-based public science learning tool. The proposed level of participation will be the development of a section about the physics of the formation of atmospheres around moons and small planets, and an interactive internet game for all ages.