Notes

Notes

The Galilean satellites are immersed in Jupiter's large and dynamic magnetosphere, which contains ions derived variously from the solar wind, the Jovian atmosphere, and the surfaces of the satellites themselves. The major heavy ions (Z > 2) are oxygen and sulfur ions derived from the surface of Io. Outside the Io torus, the plasma contains a thermalized population with a kT of 100's of eV, detected by the Voyager PLS experiment (Bagenal and Sullivan, 1981), and a much hotter component with comparable number density and an equivalent kT of 10's of keV, measured by the LECP instrument (McNutt et al, 1981, Krimigis et al, 1981). There is also a high-energy tail with energies in the MeV range.
The hot and cold populations differ in composition, satellite impact geometry, and sputtering mechanism, and must be considered separately. Tables VIII and IX summarize current knowledge of the important characteristics of each component, and estimate the ice erosion rates on the surfaces of the icy Galilean satellites due to each. An ice density of 0.92 is assumed, to allow comparison with the sublimation rate calculations in Chapter 8 that make the same assumption. There are numerous uncertainties inherent in these estimates, including the following:
    1) The spatial and temporal variability of the plasma is poorly known, as Voyager only provided localized `snapshots'. The tabulated densities and fluxes refer to the `plasma sheet' near the Jovian equatorial plane, and densities outside this sheet are reduced by about an order of magnitude (e.g. McNutt et al, 1981). Callisto, and possibly Ganymede, spend some of their time outside the sheet.
    2) The energy range between a few keV and several 10's of keV per ion was not measured by Voyager and ion densities and compositions in this range are not known. The sputtering from ions in this range is not included in the numbers given. Even the width of the unmeasured energy range is not known, as the lower energy limit of the LECP instrument depends on the (unknown) ion composition (see point 3 below).
     3) The composition of the ions in the 100 keV range, which may be the most important sputterers, is essentially unknown. Table IX therefore sets limits by calculating rates assuming the plasma is composed entirely of H, O, or S ions. The analysis is complicated by the fact that the Voyager LECP instrument, which measures the ions of this energy, responds differently to ions of different composition (Krimigis et al, 1981), so that both number densities and energies are a function of assumed composition. It can be seen from Table IX that the sputtering rate depends drastically on the assumed plasma composition.
For both the high and low energy ions, fluxes due to both the thermal motions of the ions and their bulk co-rotation with Jupiter are tabulated, in order to indicate the anisotropy of the flux on the satellite surfaces. For the low energy ions the corotational flux is much greater than the thermal flux, indicating that bulk motion dominates and almost all the plasma impacts the trailing hemispheres of the satellites. The tabulated sputtering rates thus refer to the center of the trailing hemisphere.
In the case of the high energy ions the co-rotational flux, though always less than the `thermal' flux, is comparable if the plasma is composed mostly of heavy ions, indicating considerable concentration of the flux of even the energetic ions on the trailing hemispheres. Calculation of net fluxes as a function of position on satellite surfaces under these conditions is complex, and is not attempted here. The tabulated sputtering rate is that due to the `thermal' flux only, and actual rates will be greater in the center of the trailing hemisphere and less in the center of the leading hemisphere.
It appears from comparison of Tables VIII and IX that the sputtering rates due to the two plasma components are comparable, depending on the composition of the high energy component. The values for Europa are in broad agreement with those given in Seiveka and Johnson (1982).