Observations of the chemical and thermal response of ‘ring rain’ on Saturn’s ionosphere
journal contributionposted on 31.01.2019, 13:39 by J O'Donoghue, L Moore, J Connerney, H Melin, TS Stallard, S Miller, K Baines
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 H+3, 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 H+3 density, rather than in column-averaged H+3temperatures, as a local reduction in electron density (due to charge exchange with water) lengthens the lifetime of H+ 3. However, what has been missing until now is a direct derivation of the H +3 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 H+3 parameters for the first time in the non-auroral regions of Saturn, using two H+3 lines, Q(1,0−) and R(2,2). We confirm that H + 3 density is enhanced near the expected ‘ring rain’ planetocentric latitudes near 45◦N and 39◦S. A low H+3 density near 31◦S, an expected prodigious source of water, may indicate that the rings are ‘overflowing’ material into the planet such that H+ 3 destruction by charge-exchange with incoming neutrals outweighs its lengthened lifetime due to the aforementioned reduction in electron density. Derived H+ 3 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 H+ 3 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 H+ 3 densities. When considering this mechanism alone, Saturn will lose its rings in 292+818 −124 million years.