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

TL;DR

This study analyzes Titan's ionosphere using Cassini CAPS IBS data, identifying heavy positive ion groups likely related to polycyclic aromatic compounds, and explores their variations and implications for atmospheric chemistry.

Contribution

It reports the heaviest positive ion groups observed in Titan's ionosphere and links them to polycyclic aromatic hydrocarbons, advancing understanding of Titan's complex atmospheric chemistry.

Findings

Heavy positive ion groups between 170 and 310 u/q identified.

Ion group peaks consistent with polycyclic aromatic compounds.

Variation in ion groups correlates with solar radiation conditions.

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) onboard 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.