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We investigate the role of secondary electron and ion emission from impact ofgas molecules on the Cassini Langmuir Probe (RPWS-LP, or LP) measurements inthe ionosphere of Saturn. We add a model of the emission currents, based onlaboratory measurements and data from comet 1P/Halley, to the equations used toderive plasma parameters from LP bias voltage sweeps. Reanalysing severalhundred sweeps from the Cassini Grand Finale orbits, we find reasonableexplanations for three open conundrums from previous LP studies of the Saturnionosphere. We find an explanation for the observed positive charging of theCassini spacecraft, the possibly overestimated ionospheric electrontemperatures, and the excess ion current reported. For the sweeps analysed indetail, we do not find (indirect or direct) evidence of dust having asignificant charge-carrying role in Saturn's ionosphere. We also produce anestimate of H2O number density from the last six revolutions of Cassini throughSaturn's ionosphere in higher detail than reported by the Ion and Neutral MassSpectrometer (INMS). Our analysis reveals an ionosphere that is highlystructured in latitude across all six final revolutions, with mixing ratiosvarying with two orders of magnitude in latitude and one order of magnitudebetween revolutions and altitude. The result is generally consistent with anempirical photochemistry model balancing the production of H+ ions with the H+loss through charge transfer with e.g., H2O, CH4 and CO2, for which watervapour appears as the likeliest dominant source of the signal in terms of yieldand concentration.

Implications from secondary emission from neutral impact on Cassini plasma and dust measurements