The ionosphere of Ganymede
Introduction
The icy Galilean satellites of Jupiter have atmospheres. The atmosphere of Europa has been modeled numerically in detail in 3-D by Saur et al. (1998) and their results are consistent with observation. Ganymede is significantly larger than the other satellites and, more importantly, has an intrinsic magnetic field as discovered by the Galileo spacecraft (Kivelson et al., 1996; Gurnett et al., 1996).
The earliest indication of a possible atmosphere at Ganymede was a stellar occultation observation by Carlson et al. (1973), who found an atmospheric pressure of the order of a microbar. Observations by the UVS (Ultraviolet Science) telescope of Voyager 1 placed an upper limit on the pressure about five orders of magnitude lower (Broadfoot et al., 1981). Hubble space telescope (HST) observations by Hall et al. (1998) are consistent with the lower density, which is in contradiction to the early ionosphere/atmosphere model of Yung and McElroy (1977). Kumar and Hunten (1982) showed that the atmosphere can have two possible states that differ in pressure by a factor of 105. The uncertainties in the H2O vapor pressure extrapolations are, according to Kumar and Hunten (1982), sufficient to leave indeterminate the differentiation between the high and low density states of the system. The latter is a regime in which the generation rate of the constituents of the atmosphere is easily accommodated by thermal escape. As we shall see below, the HST observations fit the low-density state model of Kumar and Hunten (1982).
In this paper, we shall attempt to evaluate the distribution of plasma density in the exospheric ionosphere in both the polar cap and the equatorial regions of Ganymede. We also present new Galileo plasma wave science (PWS) measurements that have implications for the nature of the ionosphere. A simple steady-state analytic model is developed for Ganymede, which is consistent with the findings of PWS and HST but raises severe questions with respect to published interpretations of Plasma Science (PLS) data. We find the latter to be inconsistent with the physical processes that must be taking place at and near the surface of Ganymede.
Section snippets
The observations
In this section, we shall first present the PWS data obtained during the Ganymede 1 and Ganymede 2 (hereafter G1 and G2, respectively) encounters and then discuss the published Galileo Ultraviolet Science (UVS) and PLS data.
We also quote data and inferences from the energetic particle detector (EPD), radio occultation studies, ground based observations and HST findings. The implications of the combined data sets will be shown to be consistent with our model. The horizontal and vertical
Composition and sources
The existence of an indigenous atmosphere entails the presence of an ionosphere as well. Sputtering, even in the absence of major sublimation, can deliver molecules to the space above the surface and, if conditions are favorable, support a significant atmosphere. As mentioned above, the existence of a molecular oxygen atmosphere on both Europa and Ganymede has been established (Hall 1995, Hall 1998). In this section, we explore the sources and the physical mechanisms that lead to the
Ion densities
We consider two separate regions on the surface of Ganymede, the polar cap region, for which the latitude λ>45° and the lower latitude regions equatorward of this limit. Temperature maps published by Orton et al. (1996) show a range of temperatures from about in the subsolar equatorial zone to below near the poles and in the pre-dawn sector. For the low temperatures in the polar cap region, all sputtering products can be expected to recondense immediately with the exception of
The closed field line region
During encounter G8, the spacecraft entered a region of closed field lines as indicated by the detection of trapped energetic electrons (Williams et al., 1997) and the magnetometer measurements by Kivelson et al. (1998). In this region, the energetic particle spectra change drastically (Eviatar et al., 2000). The magnetospheric thermal plasma density can be expected to be very low in this region, because of the inaccessibility of closed drift paths to injected Jovian particles (Eviatar et al.,
Conclusions
We have presented observations of the radial profile of the electron density in the near-Ganymede polar cap region from the G1 and G2 flybys. The n/B ratio computed from these profiles is indicative of a polar cap outflow. We have developed a simple rate equation model that predicts an ionosphere on Ganymede comprised of oxygen ions, with molecular oxygen dominant in the polar region and atomic oxygen in the equatorial zone. The predicted neutral composition in both regions is similar. Our
Acknowledgements
We gratefully acknowledge helpful discussions with W.S. Kurth, M.A. McGrath, R.E. Johnson, P.D. Feldman, D.F. Strobel, L.J. Paxton and M.E. Brown. We are grateful to M.G. Kivelson for her gracious permission to use the magnetometer data upon which Fig. 4 is based. The work at Tel Aviv University and at the Max-Planck-Institut für Aeronomie was sponsored in part by the German–Israel Foundation for Basic Research (GIF) under grant I-562-242.07/97. The research at the University of Iowa was
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