Heavy Positive Ion Groups in Titan’s Ionosphere from Cassini Plasma Spectrometer IBS Observations

Richard P Haythornthwaite,Panayotis Lavvas,Geraint H Jones,Anne Wellbrock,Andrew J Coates,J Hunter Waite,Véronique Vuitton

doi:10.3847/psj/abd404

Abstract

Abstract Titan’s ionosphere contains a plethora of hydrocarbons and nitrile cations and anions as measured by the Ion Neutral Mass Spectrometer and Cassini Plasma Spectrometer (CAPS) on board the Cassini spacecraft. Data from the CAPS Ion Beam Spectrometer (IBS) sensor have been examined for five close encounters of Titan during 2009. The high relative velocity of Cassini with respect to the cold ions in Titan’s ionosphere allows CAPS IBS to function as a mass spectrometer. Positive ion masses between 170 and 310 u/q are examined with ion mass groups identified between 170 and 275 u/q containing between 14 and 21 heavy (carbon/nitrogen/oxygen) atoms. These groups are the heaviest positive ion groups reported so far from the available in situ ion data at Titan. The ion group peaks are found to be consistent with masses associated with polycyclic aromatic compounds (PAC), including polycyclic aromatic hydrocarbon (PAH) and nitrogen-bearing polycyclic aromatic molecular ions. The ion group peak identifications are compared with previously proposed neutral PAHs and are found to be at similar masses, supporting a PAH interpretation. The spacing between the ion group peaks is also investigated, finding a spacing of 12 or 13 u/q indicating the addition of C or CH. Lastly, the occurrence of several ion groups is seen to vary across the five flybys studied, possibly relating to the varying solar radiation conditions observed across the flybys. These findings further the understanding between the low-mass ions and the high-mass negative ions, as well as with aerosol formation in Titan’s atmosphere.

Highlights

Titan is the largest moon of Saturn and the only known moon in the solar system with a dense atmosphere
The ion group peaks are found to be consistent with masses associated with Polycyclic Aromatic Compounds (PAC), including Polycyclic Aromatic Hydrocarbon (PAH) and nitrogen-bearing polycyclic aromatic molecular ions
Titan flybys identified possible heavy PAH ions such as naphthalene, anthracene derivatives and an anthracene dimer at 130, 170 and 335 u/q respectively (Waite et al 2007) and these are thought to be formed through ion-molecule reactions in the upper atmosphere (Westlake et al 2014)

Summary

Introduction

Titan is the largest moon of Saturn and the only known moon in the solar system with a dense atmosphere. A plethora of trace species exist including hydrocarbons, nitriles and oxygen-bearing molecules such as carbon dioxide and carbon monoxide (Coustenis et al 2007; Waite et al 2005). The detection of benzene from both remote sensing (Coustenis et al 2003) and in situ (Waite et al 2007) measurements supported this interpretation, benzene being an aromatic precursor to larger PAHs. Early Titan flybys identified possible heavy PAH ions such as naphthalene, anthracene derivatives and an anthracene dimer at 130, 170 and 335 u/q respectively (Waite et al 2007) and these are thought to be formed through ion-molecule reactions in the upper atmosphere (Westlake et al 2014). Remote sensing infrared measurements have suggested the presence of PAHs/PANHS up to several hundred amu in the atmosphere (Lopez-Puertas et al 2013)

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Discussion

Conclusion