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

The Cassini spacecraft carried a full suite of scientific instruments including fields, waves, remote sensing, neutral, and ion particle instruments. One of these instruments was the CAPS instrument which studied ions and electrons using three electrostatic analyzers mounted on an actuating platform. CAPS was in operation up to 2011 and a short time during 2012. The three CAPS sensors were the Electron Spectrometer (ELS), Ion Mass Spectrometer (IMS) and the Ion Beam Spectrometer (IBS).

Only data from CAPS IBS were used for the present study. CAPS IBS is a curved-electrode electrostatic analyzer and measures ion flux as a function of kinetic energy and direction (Young et al. 2004). The ions enter the sensor through three entrance fans (Field of View, $150^\circ \times 1\buildrel{\circ}\over{.} 4$) which are tilted at 30^◦ to each other. The energy resolution of IBS is 0.014 ${\rm{\Delta }}E/E$ and during the Titan encounters, the sensors swept from 207 eV down to 3 eV through 255 adjacent energy bins (Young et al. 2004). As CAPS is mounted on an actuating platform, IBS can be rotated to increase its field of view but for the interest of this study we have focused on the Titan flybys where CAPS was at fixed actuation. This direction is approximately in the −X direction in the spacecraft frame and is aligned with the view direction of INMS. By using flybys where CAPS has a fixed actuation in the ram direction more data points can be obtained throughout the flyby.

The first stage of the analysis was running a peak finding algorithm on each IBS energy sweep over a flyby. An example of one energy sweep can be seen in Figure 1. A peak was defined as any bin with a count number above the noise level of the instrument as well as above the Poisson counting error, i.e., the square root of the count number, when compared with adjacent energy bins. This was repeated across a flyby, Figure 2 shows the peaks identified during the T59 flyby over plotted on a spectrogram.

Zoom In Zoom Out Reset image size

Zoom In Zoom Out Reset image size

During the Titan flybys, Cassini had a high velocity (6 km s⁻¹) relative to the low ion thermal (150 K) and wind velocities (<230 m s⁻¹) in the ionosphere (Crary et al. 2009). The combination of these factors means that the ions appear as a highly-directed cold supersonic beam in the spacecraft frame. The ions therefore appear at kinetic energies associated with the spacecraft velocity and ion mass and therefore the measured eV/q spectrum can be converted to a u/q spectrum. Under the assumption of singular charged ions, the u/q spectrum can be interpreted as an u spectrum.

In environments such as Titan's ionosphere where the ion temperatures are low, the expected peak flux for an ion species α, occurs at energy ${E}_{\alpha }$ as approximated by Equation (1) (Crary et al. 2009; Mandt et al. 2012; Westlake et al. 2014),

$$\begin{eqnarray}&&{E}_{\alpha }=\displaystyle \frac{1}{2}{m}_{\alpha }{\left({v}_{\mathrm{sc}}+{v}_{\mathrm{wind}}\right)}^{2}+e{{\rm{\Phi }}}_{\mathrm{sc}}+8{kT}.\end{eqnarray} \tag{ 1 }$$

Equation (1) takes into account various effects that alter the peak energy flux such as ion temperature (8 kT), spacecraft potential (e${{\rm{\Phi }}}_{\mathrm{sc}}$) and ion wind (v_wind). Through the inversion of Equation (1) we can find a mass associated with each peak in the energy spectra. The width of each beam is roughly equal to the thermal velocity of the ions, as described by Equation (2),

$$\begin{eqnarray}&&\displaystyle \frac{M}{{\rm{\Delta }}M}\approx \displaystyle \frac{{v}_{\mathrm{sc}}+{v}_{\mathrm{wind}}}{\sqrt{\tfrac{2{kT}}{{m}_{\alpha }}}}.\end{eqnarray} \tag{ 2 }$$

Spacecraft charging can affect the energy/charge of the particles measured by CAPS sensors. Spacecraft potential values are inferred from another Cassini instrument, the Radio and Plasma Wave Science (RPWS) Langmuir probe (Gurnett et al. 2004). A constant offset between the Langmuir probe and CAPS IBS of +0.2 ± 0.15 V has been found by previous studies (Crary et al. 2009; Westlake et al. 2011; Mandt et al. 2012). For this study interpolated Langmuir probe potentials were used with an applied offset of +0.2 V. Also, a fixed temperature of 150 K was applied during the analysis. This value is representative of the temperatures seen in Titan's ionosphere which are between 100 and 200 K (Crary et al. 2009).

Examining the magnitudes of the three terms in Equation (1) the dominant parameter is the ion wind. Taking the highest reported ion wind value of 260 ms⁻¹, this would represent a 9% shift in the ion kinetic energy, i.e. 2.8 eV shift for a 31.9 eV ion, which are the lightest ions examined in this study. The spacecraft potential term typically is between −0.5 and −2 eV (Crary et al. 2009). 2 eV represents a 6% energy shift for a 31.9 eV ion. Lastly, an ion temperature of 150K corresponds to a 0.1 eV shift, a 0.3% shift for a 31.9 eV ion. As seen in Equation (1), the ion temperature and spacecraft potential terms are not mass dependent, implying that for higher mass ions the energy shifts due to the ion temperature and spacecraft potential terms decrease as a proportion of the measured energy ${E}_{\alpha }$.

The ion wind calculation relies on previously identified major peaks. The determination for which peaks to use was influenced by the work of Mandt et al. (2012), who used the INMS instrument to calculate ion densities. The three major peaks used in this study were the 28, 39, and 91 u/q peaks, these ions were used as they were found to be the most abundant ions within their respective mass groups (Mandt et al. 2012).

These peaks can be used to estimate the ion winds that are present in Titan's upper atmosphere and ionosphere. The winds affect the detected energy of the ions by shifting the energy at which they peak in each spectrum as seen in Equation (1). To construct mass spectra from the ion data this effect must be accounted for. Using the previously stated abundant ions we can calculate the ion wind by using a "mass-dependent effective spacecraft potential" defined as (Crary et al. 2009),

$$\begin{eqnarray}&&{{\rm{\Phi }}}^{{\prime} }\approx {\rm{\Phi }}+\displaystyle \frac{{{mv}}_{\mathrm{sc}}}{e}{v}_{\mathrm{wind}},\end{eqnarray} \tag{ 3 }$$

and by differentiating we find

$$\begin{eqnarray}&&{v}_{\mathrm{wind}}=\displaystyle \frac{e}{{v}_{\mathrm{sc}}}\displaystyle \frac{d{{\rm{\Phi }}}^{{\prime} }}{{dm}},\end{eqnarray} \tag{ 4 }$$

where Φ is the spacecraft potential, ${{\rm{\Phi }}}^{{\prime} }$ is the effective spacecraft potential, v_sc is the spacecraft velocity and v_wind is the along track ion wind velocity. By finding the effective spacecraft potential corrections for the three major mass peaks to be at their observed energies and calculating the gradient, the ion wind can be found using Equation (4).

A full analysis of the ions would require convolutions of the ion distributions at separate masses with the IBS energy response and then a subsequent fit to the observed data. At the high masses studied here this becomes impractical as a result of the limited ability to resolve different masses due to the ${\rm{\Delta }}E/E$ energy resolution at FWHM of the IBS sensor. IBS was designed with a ${\rm{\Delta }}E/E$ of 0.014 (Young et al. 2004) and calibration tests measured a ${\rm{\Delta }}E/E$ of 0.02 (Vilppola et al. 2001). Here we use a ${\rm{\Delta }}E/E$ of 0.017, which was the operational ${\rm{\Delta }}E/E$ value (Young et al. 2004) and was used in Crary et al. (2009). A ${\rm{\Delta }}E/E$ of 0.017 results in the effective $M/{\rm{\Delta }}M$ mass resolution being limited to <60 by the instrument's energy response.

How the energy resolution acts as a limiting factor is shown in Figure 3. The top panel is representative of a low-mass ion beam as observed by IBS, while the middle and lower panels show high-mass ion beams observed by IBS. As can be seen in the top panel, the low-mass ion beam can be well resolved across multiple energy bins. The sequences of blue peaks in Figure 3 can be compared with the first peak in Figure 1, as both represent beams with similar ion masses. The peak in Figure 1 appears across more bins due to the logarithmic scaling.

Zoom In Zoom Out Reset image size

In contrast, the middle and lower panels show beams relating to ion masses of 214 and 217 u/q, both of which could cause a peak in the same IBS energy bin. From this it could be concluded that the same methodology can be applied to both low and high masses, but comparing with Figure 1 we can see this is not the case. At the high energies around 40 and 50 eV/q, the beams are not resolved over three or four bins, with some peaks displaying similar intensity on either side of the peak. This means that the fitting cannot be performed over several peaks and we are left to conclude that any ion mass that generates an ion beam with a peak energy within the FWHM of the energy bin is plausible. Returning to the middle and bottom panels of Figure 3, this means any mass between 214 and 217 u/q can generate a peak in the energy bin centered around 40.2 eV/q, thus creating the uncertainty in our mass resolution.

After the peaks in the energy spectra were found and converted to mass spectra, the masses were binned at a 1 u/q resolution, similar to that done by Crary et al. (2009). This process does not generate a mass spectrum where one can analyze fragmentation patterns but shows which cations are more abundant than cations of neighboring masses.

The total mass uncertainty is calculated by adding in quadrature the uncertainty from the energy resolution and the uncertainty resulting from the 1 u/q binning process. The energy resolution dominates the total uncertainty, being ±2 or ± 3 u/q compared with  ±0.5 u/q from the binning.

The five flybys studied all occurred in 2009, all with similar characteristics such as Solar Zenith Angle (SZA), latitude, and longitude as can be seen in Table 1. CAPS was at a fixed actuation during these five flybys, meaning that rammed ionospheric ions were measured across the entire flyby, resulting in a greater number of data points than the previous actuating flybys. The times when Cassini exited Titan's shadow can also be seen in the table. In total 448 energy sweeps were used covering altitudes from 955 to 1001 km. There is variation in the solar illumination conditions between the data used from the flybys. The data selected from T55 was almost entirely in Titan's shadow, while T59 was illuminated the entire time with the other flybys containing a mixture.

For the comparison with Crary et al. (2009), after the peaks were found in the energy spectra and converted to mass values, the mass values were binned at a 1 u/q resolution. Then the number of occurrences of a peak in each 1 u/q bin across the five flybys was summed and then divided by the total number of sweeps examined across the flybys. This generates a percent occurrence value, representing how often a peak occurred in the studied data and is shown by the black line in Figure 5. Eight mass groups were found in the 100–200 u/q mass range with each group being centered around a significant peak. For comparison, the percent occurrence found by Crary et al. (2009) is shown by the blue line.

Zoom In Zoom Out Reset image size

This comparison is necessary due to this study applying different methodology than that applied previously in Crary et al. (2009). The main difference is that the cross-calibration between INMS and IBS data was not performed in this study, but was applied in the previous Crary et al. (2009) study. The comparison helps to link the existing study to the higher mass resolution measurements of INMS. Other differences include the mass peaks found by Crary et al. (2009) being derived from interpolating the energy spectra to 1 u/q mass bins using fit determined parameters and then identifying peaks. Lastly, Crary et al. (2009) studied flybys up to 2008 January and this study examines flybys during 2009, therefore, a comparison at this mass range is useful to check for variation with time.

Although there are methodology differences this study reproduces the mass peaks identified by Crary et al. (2009) fairly accurately. Agreement between the new methodology and previous work gives us confidence that the cross-calibration is not necessary for this study and to extend the method to study high-mass ions above 200 u/q.

The high-mass ions and the mass group structure is displayed in Figure 6. The mass group numbering (C#) indicates the number of heavy atoms (carbon/nitrogen/oxygen) present. Clear structure can be seen for the C14 to C19 groups, corresponding to masses between 172 and 251 u/q. These groups have a major peak and well-defined troughs between the groups. Furthermore, below 251 u/q the spacing between peaks of 12–14 u/q agrees with the trend observed at lower masses.

Zoom In Zoom Out Reset image size

The C20 and C21 groups have major peaks but also display minor peaks and the troughs between groups are not as evident. At these masses, the 12–14 u/q spacing trend breaks for C20, with a 16 u/q gap between C19 and C20 and a 9 u/q gap between C20 and the C21 major peaks. However as there are uncertainties of ±3 u/q due to the energy resolution of IBS at these masses, it is plausible that the spacing is closer to 12 or 14 u/q. Alternatively, there is a minor peak in the C20 group at 253 u/q, which would correspond to 12 and 13 u/q gaps between the C20 peak and its neighboring groups.

Above 270 u/q no clear peak and trough structure can be seen in Figure 6. Several significant peaks do exist, one such peak is seen at 280 u/q that would be associated with an ion containing 22 heavy atoms. At the highest masses studied here peaks are seen at 294 ± 3, 299 ± 3 and 304 ± 3 u/q. These peaks however do not have a distinct gap between them due to the uncertainties in measurement. Given the range of masses, it is likely there are multiple peaks and these are likely associated with ions containing 23 or 24 heavy atoms.

3.2.1. Peak Spacing

Further analysis of the mass spacing between peaks was performed to aid the understanding of growth pathways. This was done by finding the mass difference between each possible pair of peaks and dividing each mass difference by the number of assumed heavy atoms, for example, the mass difference for the 178 and 217 u/q peaks is 39 u/q and the difference in assumed heavy atoms is 3, generating a peak spacing of 13 u/q. This is repeated for all possible peak pairs and then the peak spacing is averaged. The results from this can be seen as a box and whisker plot in Figure 7, with the mean plotted in red. The lower and upper bounds of the whiskers are due to the combined uncertainty from both peak measurements. For example, the 203 and 217 u/q peaks both have an uncertainty of ±2 u/q, meaning the peak spacing for this pair could be between 10 and 18 u/q. The value shown for the lower bound is the average of this minimum spacing across all peak pairs, a similar calculation gives the value for the upper bound. Some peaks have large interquartile ranges such as 257, 262, and 299 u/q, these are caused by neighboring peaks with small mass differences. These peaks could also be indicative of a molecule containing a heavier atom than carbon, such as nitrogen or oxygen, which would then affect the spacing between the peaks.

Zoom In Zoom Out Reset image size

The mean spacing across all peaks is 12.4 u/q and the lower to upper quartile range typically covers the 12–13 u/q. These masses would be consistent with a C or CH addition to the molecular ions.

Here we discuss the composition of the positive ions and a possible growth mechanism. The C14 and C15 group peaks have masses consistent with cationic three ringed PACs. Examining the C14 group across all flybys, the major peak lies between 176 and 181 u/q in the IBS data. Ali et al. (2015) suggested hydrocarbon cations of C₁₄H₉⁺, C₁₄H₁₁⁺ and C₁₄H₁₃⁺ as well as nitrogen-bearing C₁₃H₁₀N⁺ and C₁₃H₁₂N⁺ ions. These molecules lie within the 176 and 181 u/q range given the associated errors, meaning we cannot distinguish between the PAHs and nitrogen-containing PACs. Similarly, the cations in the C16, C17, and C18 groups can be associated with four ringed PAC cations and the C19, C20, and C21 with five ringed PAC cations.

The C15 group frequently occurs in all flybys generating a strong peak at 190 ± 2 u/q. Ali et al. (2015) describe the mechanism of substituting CH₂⁺ in place of a hydrogen atom in a neutral PAC to generate cations, this would increase the mass of the ion by 13 u/q, consistent with our peak spacing finding of 12 to 13 u/q. Subsequent reactions with neutral ethylene creates larger cations alongside ejection of one or two H₂ molecules. A similar mechanism is also proposed by Westlake et al. (2011), who investigated both ion–molecule reactions with neutral acetylene and ethylene.

The same CH₂⁺ substitution mechanism can be applied to generate all observed ion group peaks. From Figure 8 for example, possible growth chains of CH₂⁺ substitutions could be C₁₄H₉ $\to$ C₁₅H₁₀⁺ followed by C₁₅H₁₀ $\to$ C₁₆H₁₁⁺. These chains could also exist at the upper end of the studied mass range as well, C₂₀H₁₄ $\to$ C₂₁H₁₃⁺ alongside a H₂ ejection followed by C₂₁H₁₃ $\to$ C₂₂H₁₄⁺. These chains could similarly occur for nitrogen- or oxygen- bearing PACs. Although this growth pathway is consistent with the observed peak spacing it does not rule out other reactions occurring with PAC cations.

Oxygen ions entering the ionosphere and the presence of H₂O, CO₂ and CO (Cui et al. 2009; Hörst et al. 2012) imply that oxygen-bearing polycyclic aromatic molecules are plausible, although are likely to have a small contribution compared with PAHs or PANHs. These oxygen ions charge transfer with neutrals generated thermal O atoms (Vuitton et al. 2019) and interstellar medium experiments have found reaction pathways between atomic oxygen and the C₁₀H₈⁺ naphthalene cation (Lavvas et al. 1999a). Le Page et al. (1999b) studied reactions between various pyrene cations and atomic oxygen and found that C₁₆H₉O⁺ and C₁₆H₁₀O⁺ could be created. The reaction with CO could be significant and has been shown to incorporate oxygen into tholins in atmospheric simulation experiments (Fleury et al. 2014). Other reactions involving OH, H₂O and other oxygen-bearing molecules cannot be ruled out but are not explored here. The formation pathways and reaction rates for oxygen-bearing polycyclic aromatic molecules are poorly understood but their presence cannot be ruled out by this study.

Examining the different characteristics for the five studied flybys: the closest approach altitudes were between 955 and 967 km, the closest approach longitudes were within a 5 degree range and the closest approach local times were all close to 22:00. However, the SZA values range from 103 to 149, which is primarily due to the flyby latitude increasing from −19° during T55, to −55° during T59. There is also variation in the percentage occurrence of some observed peaks between different flybys. This could be a signature of varying production, loss, and transport rates of the ions associated with the peaks. Furthermore, it could be linked to neutral and anion chemistry that occurs, which varies between the day and nightside of Titan (Vuitton et al. 2019). Examining Figure 9, six of the thirteen significant peaks display clear variations. Some peaks occur more frequently with more solar radiation while one peak occurs less frequently. Furthermore, some peaks can be seen to occur less frequently in the transition between illuminated and unilluminated conditions.

Zoom In Zoom Out Reset image size

From Figure 9, we can see that the 229 ± 2 and 304 ± 3 u/q peaks occur more frequently in the T59 flyby. Examining the percentage occurrence of the 229 u/q peak in Figure 6, there is a 35% increase for T59 compared with T55. The trend for the 304 ± 3 u/q peak is not as clear, with the significant increase in occurrence occurring only during the T59 flyby.

In contrast, panel B) in Figure 9 shows the only example of a significant decrease in percent occurrence, associated with the 178 ± 2 u/q peak in the C14 ion mass group. Between the dark T55 flyby and the illuminated T59 flyby the peak occurrence drops by around 20%. The C14 group is the least frequently seen group in the 100–200 u/q range in both this study and Crary et al. (2009), which could be related to this observed variation.

The third panel in Figure 9 shows three peaks which have an occurrence minimum during the five studied flybys. The 190 ± 2 and 266 ± 3 u/q peaks have a minimum percentage occurrence during the T57 flyby while 203 ± 2 u/q minimum is during T58.

Due to Titan's extended atmosphere the effects of solar EUV radiation on ions are seen past the terminator, up to SZA values of 135° at 1000 km (Ågren et al. 2009; Cui et al. 2009; Coates et al. 2009, 2011; Wellbrock et al. 2009, 2013; A. Wellbrock 2021, in preparation), although at high SZA values the solar radiation is attenuated by passing through more of Titan's atmosphere Cui et al. (2009) studied the diurnal variations of positive ions, finding that light ions were strongly depleted on the nightside while the heavy ions were moderately depleted and the most significant diurnal variations were seen at low altitudes. These depletions are linked to the loss of ion production from solar EUV radiation.

Similar to the light cations, light anions (<50 u/q) have been shown to have higher densities on the dayside (Wellbrock et al. 2012; Mihailescu et al. 2020; A. Wellbrock 2021, in preparation). On the other hand, Wellbrock et al. (2009), A. Wellbrock (2021, in preparation) and Coates et al. (2009, 2011) show that heavy anion densities under illuminated conditions past the terminator are lower than unilluminated conditions in the deeper nightside ionosphere, as observed during flybys such as T57. The higher densities of heavy anions in unilluminated conditions could provide a competing pathway to heavy cation production. This could be through higher anion production rates or through lower anion loss rates, an example of a loss process would be photodetachment. Photodetachment efficiency increases with ion size (Lavvas et al. 2013; Wellbrock et al. 2019), which would remove heavy anions from the dayside allowing for higher heavy cation densities. Ionization energy is the energy needed to remove an electron from a molecule while electron affinity is the amount of energy released when an electron is added to a molecule. There is a general trend of increasing electron affinity and decreasing ionization energy with increasing PAH mass (Christodoulides et al. 1984; Dabestani & Ivanov 1999), meaning larger PAHs can more readily form positive or negative ions under favorable conditions.

Wellbrock et al. (2013) identified several mass groups of negative ions, with the 190–625 u/q mass group displaying the highest density of all groups at an average peak altitude of 1000 km. The flybys in this study are at a similar altitude and the mass range studied covers this negative ion group.

Due to the complex cation-neutral-anion chemistry and lack of information on reaction rates at these high masses, we cannot distinguish between different factors such as anion chemistry, ion transport, and changing ion-neutral pathways. Future investigations that examine flybys in the noon/midnight regions would help identify photochemical effects. Furthermore, future chemical models could aid identification of these heavy molecular ions through examining recombination rates and ion-neutral reactions to see which ions are consistent with observed variations.

López-Puertas et al. (2013) studied an IR emission from Titan and applied a fitting algorithm to identify neutral PACs in the upper atmosphere. 7 of their 19 most abundant PACs lie in the 170–310 u/q mass range. These seven molecules are C₁₃H₉N (179 u/q), C₁₂H₈N₂ (180 u/q), C₁₄H₁₆ (184 u/q), C₁₆H₁₀N₂ (230 u/q), C₂₀H₁₀ (250 u/q), C₂₀H₁₄ (254 u/q) and C₂₂H₁₆ (280 u/q). These neutral molecules are in the even numbered mass groups: C14, C18, C20, and C22.

Three of these molecules are in the C14 group, C₁₃H₉N, C₁₂H₈N₂ and C₁₄H₁₆. The first two of these neutrals could be related to the major C14 peak in the IBS data at 178 u/q. López-Puertas et al. (2013) reported C₁₄H₁₆ to be present at a low concentration and in this study we find a minor peak (percentage occurrence below 10%) at 184 ± 2 u/q that occurs in four of the studied flybys. This peak could result from the ionization of the reported C₁₄H₁₆ neutral.

C₁₆H₁₀N₂ is in the C18 group and could be related to the ion mass peak at 229 u/q. The two molecules in the C20 group, C₂₀H₁₀ and C₂₀H₁₄, were some of the least abundant molecules reported by López-Puertas et al. (2013). The major peak in the C20 group is at 257 ± 3 u/q with a minor peak at 253 ± 3 u/q. This puts both molecules within the error of the ion peak. However, there is no clear peak structure at 250 u/q with the region between 248 and 251 u/q being a trough between the C20 and C21 groups. This could indicate that we see a higher level of hydrogenation in the cationic phase.

Although we do not find clear structure for the C22 group, there is a peak at 280 ± 3 u/q therefore a similar mass to the neutral C₂₂H₁₆ at 280 u/q. This peak strongly occurs during the T58 and T59 flybys, peaking at 279 u/q and 280 u/q respectively.

In summary, from the comparison between the ionic and neutral phases, the masses of previously observed neutral PACs in Titan's atmosphere correlate with cations in the ionosphere. The only exception found was the neutral PAH C₂₀H₁₀. The comparisons between the observed abundant neutral molecules and the frequently occurring ion peaks indicate that there is a need for combined study of the neutral and cationic phases to fully characterize Titan's ionosphere. In addition, further study of what neutral PAHs exist in Titan's ionosphere could aid with interpretation in future ion compositional studies.