Nitrogen-containing Anions and Tholin Growth in Titan's Ionosphere: Implications for Cassini CAPS-ELS Observations

CAPS consisted of three sensors, one being ELS. The ELS was a top-hat electrostatic analyzer initially designed to measure differential electron velocity distributions. Measurements of negative ions in Titan's cold ionosphere were obtained while the spacecraft was in the ram direction. With Cassini at supersonic velocities (∼6 km s⁻¹), negative ions appear as sharp beam features seen by ELS (Coates et al. 2007). The CAPS actuator therefore swept across the field of view in the spacecraft's ram direction, resulting in multiple negative ion spectra during flybys. The observed ram energy-per-charge can be converted into mass-per-charge knowing the relative spacecraft speed and taking into account a spacecraft potential correction, assuming singly charged anions. This conversion theoretically enables the identification of particles up to 150,000 u/q (Coates et al. 2007), although to date 10⁴ u/q represents the highest anion mass observed at Titan. The analyzer had an energy bandwidth ΔE/E = 16.7%, corresponding to the energy resolution of the sensor. This roughly corresponds to a mass resolution of ∼6. By contrast, the ion mass spectrometer used in this study has a mass resolution of 100 at 100 u/q, enabling us to distinguish more species in the laboratory.

Figure 1 shows a comparison between the experimental spectrum and CAPS-ELS measurements taken during the T18 encounter at ∼1235 km. This flyby occurred on 2006 September 23 over Titan's north polar region, entering on the nightside and exiting on the dayside, with a closest approach near the terminator. Background electron counts are removed by subtracting averaged counts from non-ram pointing anodes (Coates et al. 2007; Wellbrock et al. 2013; Desai et al. 2017).

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The experimental spectrum was taken in an N₂:CH₄ 95:5% plasma (black, bottom panel) with the major predicted anions. Approximately eight groupings up to 120 u/q in the experimental spectrum are found (Table 1), with main peaks alternatively separated by 15 and 9 u from 26 to 74 u/q. Although few ions are detected between 120 and 150 u/q, a secondary increase is observed starting at 155 u/q up to the mass detection limit 200 u/q. The most intense ion at 26 u/q is attributed to CN⁻ (Vuitton et al. 2009). The other cyano group anions, C₃N⁻ and C₅N⁻, are detected at 50 and 74 u/q, respectively. Early work by Vuitton et al. (2009) suggested that CN⁻ was mainly formed via DEA of HCN resulting from supra-thermal electron impact. More recently, however, Mukundan & Bhardwaj (2018) and Vuitton et al. (2018) used updated DEA cross sections of HCN, resulting in proton transfer between H⁻ and HCN (Reaction 1) as the major pathway to CN⁻ formation. H⁻ is mainly produced through DEA of CH₄ (Mukundan & Bhardwaj 2018).

$$\begin{eqnarray}&&{{\rm{H}}}^{-}+\mathrm{HCN}\longrightarrow {\mathrm{CN}}^{-}+{{\rm{H}}}_{2}.\end{eqnarray} \tag{ 1 }$$

The main pathways for heavier species (Biennier et al. 2014; Dobrijevic et al. 2016; Mukundan & Bhardwaj 2018; Vuitton et al. 2018) are given below. Mukundan & Bhardwaj (2018) found REA to be the main production pathway to C₃N⁻ due to differences in the neutral profiles used in both models.

$$\begin{eqnarray}&&{\mathrm{CN}}^{-}+{\mathrm{HC}}_{3}{\rm{N}}\longrightarrow {{\rm{C}}}_{3}{{\rm{N}}}^{-}+\mathrm{HCN},\end{eqnarray} \tag{ 2 }$$

$$\begin{eqnarray}&&{\mathrm{CN}}^{-}+{\mathrm{HC}}_{5}{\rm{N}}\longrightarrow {{\rm{C}}}_{5}{{\rm{N}}}^{-}+\mathrm{HCN}.\end{eqnarray} \tag{ 3 }$$

These mass patterns are consistent with the ELS distributions >20 u/q (Figure 1, bottom panel) notably at 26 u/q. At 35–40 u/q, we note an intermediate peak not reported previously in the ELS data, matching our mass group 3 surprisingly well. This further peak is only discernable intermittently at high Titan altitudes and is not observed during all encounters (e.g., it is absent in the T40 spectrum taken at 1244 km in Desai et al. 2017). The 50 u/q peak is also consistent with the 45–70 u/q group present in the ELS spectrum. For larger mass species, it is noteworthy to point out that anions could be present at nearly every mass-per-unit charge of the spectrum given the continuous distribution of ELS over this mass range. At higher masses (>70 u/q), the laboratory spectrum further seems to match the ELS data at 70–90 and 110–120 u/q, which were characterized by Desai et al. (2017). A further increase in intensity starting at 140–150 u/q is seen in both spectra, although the low ion signal currently renders the interpretation difficult.

Figure 2 shows the same experimental spectrum as in Figure 1 (bottom panel) with attributions of species <100 u/q. Unexpected intense peaks stand out at 41, 65, and 66 u/q. Horvath et al. (2010a), who used a point-to-plane corona discharge at higher pressure, putatively assigned the peaks at 41 and 65 u/q to the diazirinyl CHNN⁻ and dicyanomethanide C₃HNN⁻ anions, respectively. Other species that could correspond to these peaks according to the NIST are C₂H₃N⁻, ${{\rm{C}}}_{3}{{\rm{H}}}_{5}^{-}$, and ${{\rm{C}}}_{5}{{\rm{H}}}_{5}^{-}$. Comparisons with tholin signatures and gas phase pathways are detailed hereafter, and confirm the attributions by Horvath et al. (2010a). Note the presence of a peak at 19 u/q, which likely corresponds to a fluorine F⁻ contamination. The presence of F⁻ is presumably due to polytetrafluoroethylene (PTFE) from the temperature probe sheaths located near the polarized electrode in the reactor. This signal appears later over the accumulation, indicating that its contamination does not affect the overall signal of the spectrum. Also in this mass group, NH3 is inferred by the attribution of ${\mathrm{NH}}_{2}^{-}$ at 16 u/q (Horvath et al. 2010b, 2011).

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CHNN⁻ can either be in a cyclic four-π electron conformer or as an open biradical isomer (Gordon & Kass 1995). These diazo isomer compounds are stable and can contain nitrogen–nitrogen double bonds (Kroeker & Kass 1990; Gordon & Kass 1995).

Current photochemical models largely account for nitrile incorporation of products containing only one nitrogen atom. Diazo and triazo species however, are not taken into account and their implications on the gas phase chemistry are poorly understood with respect to the nitrogen-rich tholins. Two previous studies by Gautier et al. (2011) and Cunha De Miranda et al. (2016) on the gas phase and solid tholin material produced in the PAMPRE reactor can be clues to this conundrum. Gautier et al. (2011) first identified C₄H₃N₅ (Tetrazolo[1,5-b]pyridazine) as the main aromatic gas phase product using gas chromatography. This heteroaromatic compound contains multiple single- or double-bonded nitrogen atoms. Furthermore, Cunha De Miranda et al. (2016) revealed the presence of heteroaromatic structures present in tholins such as triazole rings (C₂H₃N₃). This compound, along with the cyclic diazirine c-CH₂N₂ or cyanamide H₂N–CN, were shown to be strong candidates for the observed N = N patterns. These isomers may indeed contain doubly bonded nitrogen atoms, which may account for the direct incorporation of N₂ into tholins. Such N-rich patterns can be directly linked with the anions assigned here. The diazo and triazo radicals attributed in Figure 2 could indeed serve as important intermediates for the heterogeneous chemistry observed in tholins. Nitrogen incorporation into tholins may thus be facilitated by the potential presence of these nitrogen-rich (diazo and triazo) compounds in the gas phase.

Furthermore, the putative presence of intermediate species such as CHNN⁻ (41 u/q) could contribute more or less significantly to the overall 35–60 u/q mass grouping to which Desai et al. (2017) found a nominal mass center of 49 u/q, within the 2σ uncertainty, near closest approach. Non-attributed intermediate compounds formed seen here at high altitudes could potentially still be present in the ELS continuum at lower altitudes. Therefore, such a mass-over-charge shift to slightly lower values could indicate the presence of these intermediate compounds falling within the fitted distribution.

In spite of currently limited knowledge of diazo anion chemistry, we propose here mechanisms describing the putative presence of CNN⁻ and CHNN⁻ (40 and 41 u/q) or their possible isomers NHCN⁻ and NCN⁻. CH₂CN⁻ could also be a candidate at 40 u/q.

A candidate molecule within this mass range that is potentially available for the initiation of further pathways is CH₂N₂ (e.g., He & Smith 2014; Cunha De Miranda et al. 2016), for which three isomeric forms are considered here: diazomethane CH₂N₂, cyclic diazirine c-CH₂N₂, and cyanamide H₂N–CN. CH₂N₂ was previously excluded from the composition of tholins by Cunha De Miranda et al. (2016), while both cyclic diazirine and cyanamide have been identified among several nitrogenous prebiotic organic compounds present in tholins (He & Smith 2013, 2014; Cunha De Miranda et al. 2016). Formation of linear CH₂N₂ (diazomethane) following $1\,{\mathrm{CH}}_{2}+{{\rm{N}}}_{2}\longrightarrow {\mathrm{CH}}_{2}{{\rm{N}}}_{2}$ was shown by Braun et al. (1970) to be significant only for high pressures that are neither relevant to our experimental setup nor to Titan's upper atmosphere conditions. HCN and NH₃ alone were suggested to contribute to the formation of cyanamide H₂N–CN (and its dimer dicyanodiamide C₂H₄N₄) according to Reaction 4 (He & Smith 2013). The cyanamide isomer can also go through a number of low-energy EA or DEA reactions (Reactions 5, 6 and Tanzer et al. 2015).

$$\begin{eqnarray}&&\mathrm{HCN}+{\mathrm{NH}}_{3}\longrightarrow {{\rm{H}}}_{2}{\rm{N}}\mbox{--}\mathrm{CN}+{{\rm{H}}}_{2},\end{eqnarray} \tag{ 4 }$$

$$\begin{eqnarray}&&{{\rm{H}}}_{2}{\rm{N}}\mbox{--}\mathrm{CN}+{{\rm{e}}}^{-}\longrightarrow {\mathrm{NHCN}}^{-}+{\rm{H}}\ (41\,u/q),\end{eqnarray} \tag{ 5 }$$

$$\begin{eqnarray}&&{{\rm{H}}}_{2}{\rm{N}}\mbox{--}\mathrm{CN}+{{\rm{e}}}^{-}\longrightarrow {\mathrm{CNN}}^{-}+{{\rm{H}}}_{2}\ (40\,u/q).\end{eqnarray} \tag{ 6 }$$

Similar routes from the parent neutral diazirine c-CH₂N₂ may also occur (Reactions 7 and 8).

$$\begin{eqnarray}&&{\rm{c}} \mbox{-} {\mathrm{CH}}_{2}{{\rm{N}}}_{2}+{{\rm{e}}}^{-}\longrightarrow {\rm{c}} \mbox{-} {\mathrm{CH}}_{2}{{\rm{N}}}_{2}^{-}\ (42\,u/q),\end{eqnarray} \tag{ 7 }$$

$$\begin{eqnarray}&&{\rm{c}} \mbox{-} {\mathrm{CH}}_{2}{{\rm{N}}}_{2}+{{\rm{e}}}^{-}\longrightarrow {\mathrm{CNN}}^{-}+{{\rm{H}}}_{2}\ (40\,u/q).\end{eqnarray} \tag{ 8 }$$

In group 5, the other diazo anion C₃HNN⁻ (dicyanomethanide) attributed to the peak at 65 u/q, following Horvath et al. (2010a), has an intensity of an order of magnitude greater than C₅N⁻. It is accompanied by a second intense peak at 66 u/q. This mass can correspond to two anions, C₄H₄N⁻ and ${{\rm{C}}}_{2}{{\rm{N}}}_{3}^{-}$ (the pyrrolide anion and dicyanamide, respectively). C₄H₄N⁻ was tentatively attributed by Horvath et al. (2010a), whose parent neutral is pyrrole C₄H₅N. Given the previous detections of pyrrole and other heterocyclic aromatic compounds in this and other similar plasma discharge experiments, as well as in tholin material (e.g., McGuigan et al. 2006; Gautier et al. 2011; He & Smith 2014; Mahjoub et al. 2016), such an ion can be a candidate for 66 u/q. However, Carrasco et al. (2009) carried out analyses of the soluble fraction of tholins and measured charged ionic species by collision-induced dissociation (see also Somogyi et al. 2012). They found recurrent ionic fragments at 66 u/q which, given its fragmentation patterns observed in negative ion mass spectrometry, revealed dicyanamide ${{\rm{C}}}_{2}{{\rm{N}}}_{3}^{-}$ to be a better candidate at this mass-to-charge ratio. Therefore, we attribute the gas phase N-containing carbanion precursor ${{\rm{C}}}_{2}{{\rm{N}}}_{3}^{-}$ as a likely compound at the 66 u/q peak. Interestingly, this compound has a v-shaped bent conformation (C_2V) with a permanent dipole moment, and was found to be unreactive and extremely stable, potentially existing in cold regions of the ISM (Yang et al. 2011; Nichols et al. 2016). Furthermore, it was shown by Carrasco et al. (2009) and Somogyi et al. (2012) that ${{\rm{C}}}_{2}{{\rm{N}}}_{3}^{-}$ was part of a fragmentation pattern involving HCN, C₂H₂, and NH₂CN formal additions. The attribution here of ${{\rm{C}}}_{2}{{\rm{N}}}_{3}^{-}$ and its relative intensity in the gas phase sustains its important role as a precursor "seed" as proposed in the aforementioned studies. This N-containing carbanion may indeed play an important role in the polymerization growth of tholins and large gas phase N-rich precursors (Carrasco et al. 2009; Somogyi et al. 2012).

Finally, ${{\rm{C}}}_{2}{{\rm{N}}}_{3}^{-}$ (along with ${{\rm{C}}}_{4}{{\rm{N}}}_{3}^{-}$) has no substantial reactivity with protons, and its stability in hydrogen and N-containing carbanion-rich environments may make it a viable candidate as a gas phase precursor (Bierbaum 2011). Given the bent and polar structure of ${{\rm{C}}}_{2}{{\rm{N}}}_{3}^{-}$, future rotational spectrum analyses of this compound should help in its potential detection in planetary atmospheres and the ISM.