Title: Sources of Nitrogen in Saturn's Inner Magnetosphere
Abstract: The Cassini Plasma Spectrometer (CAPS) detected low energy N during 15 orbits when Cassini passed through Saturn’s inner magnetosphere (3.5 < L < 13.5), providing the first evidence for the presence of nitrogen that is locally ionized in this region (Smith et al. 2005). Voyager’s detection of unresolved mass/charge 14-16 amu/e ions in this region caused much debate over the possible presence of N in Saturn’s magnetosphere. However CAPS can distinguish between N and water group ions and we have confirmed the presence of nitrogen ions as a minor species. Two principal nitrogen sources have been suggested: neutrals from Titan’s atmosphere ionized in the inner magnetosphere (Smith et al. 2004) and nitrogen compounds trapped in the icy satellite and ring particle surfaces (Sittler et al 2005). Based on CAPS data we reported earlier that the spatial distribution of the nitrogen ions is consistent with an Enceladus source. We examine the neutral nitrogen and water tori and the ion source distributions using 3-D Monte Carlo model. This model examines the escape of water and nitrogen molecules from Enceladus and nitrogen atoms and molecules from Titan, accounting for the recycling of neutrals by low energy charge exchange collisions (Johnson et al 2006). Our results show the dominant N source is the small icy moon Enceladus and is consistent with the detection of out-gassing water vapor containing N2 (Waite et al. 2006) from the recently discovered Enceladus south polar jets, although other parent nitrogen molecules can not yet be ruled out. We also examine the relative abundance of nitrogen to the dominant water group ions. This analysis reveals a nitrogen spatial distribution resembling the observed neutral OH cloud topography. Interestingly, while Titan’s nitrogen-rich atmosphere does not appear to be a significant source of nitrogen ionized near its orbit, a Titan-generated neutral nitrogen torus could produce a fraction of the ions we observe at distances less than 13Rs. Here we present additional CAPS N observations and modeling showing that Enceladus is likely the principal nitrogen source in the inner magnetosphere but Titan can account for some of the observed nitrogen ions at the largest distances discussed.
Publication Year: 2006
Publication Date: 2006-01-01
Language: en
Type: article
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