Observations of the chemical and thermal response of 'ring rain' on Saturn's ionosphere

In this study we performed a new analysis of ground-based observations that were taken on 17 April 2011 using the 10-metre Keck telescope on Mauna Kea, Hawaii. Emissions from H₃⁺, a major ion in Saturn's ionosphere, were previously analyzed from these observations, indicating that peaks in emission at specific latitudes were consistent with an influx of charged water products from the rings known as 'ring rain'. Subsequent modeling showed that these peaks in emission are best explained by an increase in H₃⁺ density, rather than in column-averaged H₃⁺ temperatures, as a local reduction in electron density (due to charge exchange with water) lengthens the lifetime of H₃⁺. However, what has been missing until now is a direct derivation of the H₃⁺ parameters temperature, density and radiative cooling rates, which are required to confirm and expand on existing models and theory. Here we present measurements of these H₃⁺ parameters for the first time in the non-auroral regions of Saturn, using two H₃⁺ lines, Q(1,0^-) and R(2,2). We confirm that H₃⁺ density is enhanced near the expected 'ring rain' planetocentric latitudes near 45°N and 39°S. A low H₃⁺ density near 31°S, an expected prodigious source of water, may indicate that the rings are 'overflowing' material into the planet such that H₃⁺ destruction by charge-exchange with incoming neutrals outweighs its lengthened lifetime due to the aforementioned reduction in electron density. Derived H₃⁺ temperatures were low while the density was high at 39°S, potentially indicating that the ionosphere is most affected by ring rain in the deep ionosphere. Saturn's moon Enceladus, a known water source, is connected with a dense region of H₃⁺ centered on 62°S, perhaps indicating that charged water from Enceladus is draining into Saturn's southern mid-latitudes. We estimated the water product influx using previous modeling results, finding that 432 - 2870 kg s-¹ of water delivered to Saturn's mid-latitudes is sufficient to explain the observed H₃⁺ densities. Assuming that our Saturn northern Spring measurement represents all seasons, and that the rings are able to reorganize over time, the ring rain mechanism alone will drain Saturn's rings to the planet in 292_-124⁺ 818 million years.