An unexpected and intense period of Titan atmospheric loss

The Saturnian system provides an example of a complex environment that includes a gas giant, numerous satellites and rings encompasses by a large magnetosphere. During its 13-year mission at Saturn from 2004-2017, the NASA provided numerous transformative observations and accumulated so much data that discoveries are still occurring from further analysis. These results have painted an unexpected picture of the Saturnian system as a very complex and dynamic system.

Perhaps one of the most intriguing results was that the large moon, Titan appeared to be having a much smaller impact on the Saturnian system despite it being extremely large (the 2^nd largest in the solar system and large than the planet Mercury) with an unprotected atmosphere that is denser than the Earth’s. Cassini observations revealed that Saturn’s magnetosphere material is dominated by cryogenic plumes from the much smaller icy moon Enceladus. Although the Enceladus plumes are somewhat variable, on average, this moon provides ~285 kg/s of water vapor to Saturn’s magnetosphere (Smith et al. 2021) while Titan’s atmosphere likely only provides <40 kg/s of N₂ & CH4 (Johnson et al. 2009).

Thus, the magnetosphere is dominated by water gas from Enceladus rather than nitrogen and hydrocarbons from Titan’s atmosphere. These particles are ejected into the magnetosphere as neutral charged atoms & molecules but unfortunately, such particles are generally much more difficult to detect (with the exception of certain species with specific spectral qualities. Fortunately, charged particles are generally much easier to detect at lower densities. Thus, charged particle observations of ion produced by ionizations of the source neutral particles are used as a proxy for the observing neutral particles. Throughout most of Saturn’s magnetosphere, water-group ions (H3O+, H2O+, OH+ & O+, also referred to as “W+” collectively) originating from the Enceladus plumes’ water vapor generally account for up to 90% of all ions (Thomsen et al. 2010) with hydrogen accounting for the next largest population followed by nitrogen ions presumably from Titan and Enceladus (Smith et al. 2007). Carbon ions exist as a very minor species Saturn’s magnetosphere with global relative abundances remaining pretty steady at ~1% relative to W+. These ions likely originate from the Enceladus plumes as well as hydrocarbons from Titan’s atmosphere. This, abundances and locations of minor/trace species in Saturn’s magnetosphere provide significant insight into source composition and activity.

Tidal forces cause Enceladus plume source rate to vary on the its orbital timescale which is much shorter than particle interactions rates. Thus, the amount of global magnetospheric neutral water particles remains relatively stable over time as shown using UV observations of total oxygen content (Melin et al. 2009). This allows for a stable comparison to examine the spatial and temporal distribution of minor/trace species to provide unique insight into composition and activity of sources of these particles. More specifically, we examine the relative abundances of significant trace species to water-group particles to explore unusual magnetospheric activity. The observations were collected by the CHarge Energy Mass Spectrometer (CHEMS) of the Cassini Magnetosphere Imaging Instrument (MIMI) (Krimigis et al. 2004) which measures mass and mass/charge of ~3-220 keV ions. We conducted a detailed analysis of the relative abundance of C+ to W+ over the entire 13-year mission (2004-2017) and surprising discovered an abrupt feature in the data. The relative abundance of C+ remains fairly constant at ~1% until early in 2014. At this point the relative abundance of C+ rather abruptly increases by half an order of magnitude (to ~5%) and then appears to exponentially fall off and has almost completely recovered by the end of the mission.      

In this presentation, we show that this global magnetospheric ion composition modification was observed resulting from an abrupt increase in Titan atmospheric loss most likely from a collisional event, surface activity, solar wind exposure or a interruption in the methane precipitation cycle. Titan was not thought to exhibit such enhanced atmospheric loss but this evidence indicates that such activity can impact the entire magnetosphere and opens up the possibility for similar additional atmospheric loss events on Titan as well as on other bodies.