New model of Saturn's ionosphere with an influx of water from the rings

Abstract

Current theoretical models of Saturn's ionosphere are similar to those of Jupiter's because of the gross similarity of their upper atmospheres. In the case of Jupiter, the theoretical models can be fitted reasonably well to ionospheric electron density profiles^1,2 obtained from the Pioneer and Voyager radio occultation experiments^3–5. In contrast, the theoretical models of Saturn's ionosphere are inconsistent with both the ionospheric electron density profiles^6,7 obtained from the Pioneer and Voyager occultation observations^8–10 and the large diurnal variation of maximum ionospheric electron density deduced from studies of Saturn lightning discharges^11,12. We propose a radically different model of Saturn's ionosphere in which water plays a major role as a minor constituent present by downward diffusion from an external source. Our model Saturn ionosphere is a classical ‘F2’ type layer resulting from the photodissociative production of H⁺ from H₂ and rapid chemical loss by a series of charge exchange reactions with water. A planet-wide influx of ∼4 × 10⁷ molecules cm⁻² s⁻¹ of water from the rings is consistent with the observed ionospheric electron densities. An enhanced influx of water occurs at latitudes (−38°, +44°) connected magnetically to the inner edge of Saturn's B ring¹² where an electromagnetic erosion process¹³ takes place. Present-day influx at these latitudes may be as large as ∼2 × 10⁹ molecules cm⁻² s⁻¹.