Mark Shappirio
GSFC
Ion Mass Spectrometry at Saturn: Results, discoveries, and lessons learned
For the last year and a half, Cassini has been orbiting Saturn, passing over, around and through the rings, and traveling past Saturn’s many moons. The Cassini Plasma suite (CAPS) has been sampling the magnetospheres population of ions, investigating its interaction with Saturn, the rings and moons, and the solar wind. Previous to the arrival of Cassini at Saturn, models existed for a variety of sources for the different species of ions within the magnetosphere, including the rings, the inner moons and Titan. Since the arrival of Cassini, we have been trying to confirm which models are the most accurate.
The Ion Mass Spectrometer (IMS) in the CAPS suite produces a great deal of information about the magnetospheric ions, but it is also a complicated instrument that we are still trying to understand. IMS uses thin carbon foils to generate secondary electrons for a start signal and the plasma ions, now neutralized, for a stop signal, a standard time of flight technique, to obtain a high count spectrum with a mass resolution M/ΔM ~ 8 amu. At the same time the TOF region of the instrument has a linear electric field so that the plasma ions which retain a positive charge when exiting the carbon foils (typically < 10% of the incident flux) are focused onto a separate detector which gives a low count, high mass resolution (M/ΔM ~ 50) spectra. The TOF of these positive ions in a perfectly linear electric field would be independent of energy. But in order to be able to distinguish atomic species from the same species in a molecule (say atomic H from H from a water ion) the electric field was slight “detuned”, giving the high resolution spectra a slight, non-linear, energy dependence. In addition, some species pick up a negative charge from the carbon foils, namely H, C and O. These negative ions are then accelerated and focused by the linear electric field and get incorporated into the low mass resolution high count spectra, in some cases interfering with neutral particle TOFs.
Despite, or in some cases because, of the complications of the IMS instrument a .number of discoveries have been made. IMS has found atomic and possibly molecular nitrogen in Saturns magnetosphere and localized its source to the e-ring/Enceladus (Young et al. 2005; Smith et al. 2005a). Water group ions (covering O+, OH+, H2O+ and H3O+) have also been measured also with a source in the inner region of the magnetosphere, either the rings or the icy satellites or both. And molecular oxygen from O2+ has been measured, with a potential source somewhere in the region of Dione. Finally there is evidence of small amounts of methane or methane fragments (CHx) whose source has not yet been localized.