Abstract
ABSTRACT A significant but unexpected result of the Cassini mission was the discovery of heavy organic negative ions in Titan’s ionosphere at altitudes between about 950 and 1400 km by the CAPS Electron Spectrometer (ELS). The heaviest ions were observed during the T16 fly-by with masses over 13 000 u/q. This is significantly higher than the maximum masses observed during other fly-bys. We study T16 CAPS-ELS observations and examine the evolution of mass spectra at different altitudes. We also study maximum mass trends using a large data set from all available CAPS-ELS observations of the Cassini mission in order to investigate the conditions necessary to allow negative ions to grow to the highest masses. For the first time, we are able to investigate the relationship between the highest mass particles and seasonal effects. We find that the combination of high latitude and winter conditions, resulting in long-term restricted solar flux, create an environment in which ion growth can reach the highest masses, as observed during T16. Restricting solar flux long term, and hence photodestruction reactions such as photodetachment, appears to be essential for negative ions to grow beyond 10 000 u/q.
Highlights
Titan orbits Saturn at a distance of 20RS and is the second largest moon in our Solar system with a radius of 2576 km (Waite et al 2005; Horst 2017; Nixon et al 2018)
We presented a detailed study of negative ions observed during the Cassini polar Titan fly-by T16 using CAPS-Electron Spectrometer (ELS)
We observed that the peak of group 7 in the mass spectra moves to higher masses at lower altitudes, which agrees with modelling results by Lavvas et al (2013)
Summary
Titan orbits Saturn at a distance of 20RS (where 1RS is Saturn’s radius, 60 268 km) and is the second largest moon in our Solar system with a radius of 2576 km (Waite et al 2005; Horst 2017; Nixon et al 2018) It has a very dense and extended nitrogen and methane-based atmosphere with an exobase at around 1500 km (Cui, Yelle & Volk 2008). Negative ions at Titan were first detected by Cassini’s CAPS Electron Spectrometer (ELS) during the first Titan fly-bys (Coates et al 2007; Waite et al 2007) They have subsequently been observed by CAPS-ELS in every sampled part of Titan’s ionosphere, provided the altitude was below approximately 1400 km and CAPS pointed in the ram direction (see Section 3 for a more detailed description). Densities are observed to peak at a range of altitudes depending on mass; the heaviest densities tend to peak near 1000 km whereas lighter ion
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