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

Richard P. Haythornthwaite Mullard Space Science Laboratory, Department of
Space and Climate Physics, University College London, Dorking, UK  The Centre
for Planetary Sciences at UCL/Birkbeck, London, UK  Andrew J. Coates Mullard
Space Science Laboratory, Department of Space and Climate Physics, University
College London, Dorking, UK  The Centre for Planetary Sciences at
UCL/Birkbeck, London, UK  Geraint H. Jones Mullard Space Science Laboratory,
Department of Space and Climate Physics, University College London, Dorking,
UK  The Centre for Planetary Sciences at UCL/Birkbeck, London, UK  Anne
Wellbrock Mullard Space Science Laboratory, Department of Space and Climate
Physics, University College London, Dorking, UK  The Centre for Planetary
Sciences at UCL/Birkbeck, London, UK  J. Hunter Waite Space Science and
Engineering Division, Southwest Research Institute, San Antonio, Texas, 78228,
USA  Véronique Vuitton Institut de Planétologie et d’Astrophysique de
Grenoble, Univ. Grenoble Alpes, CNRS, Grenoble 38000, France  Panayotis Lavvas
Université de Reims Champagne Ardenne, CNRS, GSMA UMR 7331, 51097 Reims,
France

(Received 18 September 2020; Revised 11 December 2020; Accepted 15 December
2020)

###### 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.

Ionosphere — Planetary science — Saturnian satellites

††journal: PSJ††software: astropy (Astropy Collaboration et al., 2013, 2018),
matplotlib (Caswell et al., 2020), NumPy (van der Walt et al., 2011), SciPy
(Virtanen et al., 2020)

##  1 Introduction

Titan is the largest moon of Saturn and the only known moon in the solar
system with a dense atmosphere. Since its discovery by Christiaan Huygens it
has been observed using both telescopes and spacecraft including the Pioneer
11, Voyager 1 & 2 spacecraft and most recently Cassini-Huygens. Data from the
Voyager flybys revealed Titan to possess a thick atmosphere (150 kPa) rich in
nitrogen and hydrocarbon compounds (Tyler et al., 1981; Lindal et al., 1983).
During Cassini’s tour of the Saturnian system between 2004 and 2017 the
Cassini spacecraft performed over 100 flybys of Titan, taking in-situ
measurements of its ionosphere and upper atmosphere. These in-situ
measurements helped demonstrate the connection between ion-neutral reactions
generating large charged molecules in the upper atmosphere and the formation
of aerosols (Waite et al., 2005, 2007; Coates et al., 2007).

Titan’s atmosphere is nitrogen-rich (95-98%) and contains minor species of
methane (2-5%) and hydrogen (0.1%). 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
oxygen-bearing molecules are thought to originate from O+ ions and
micrometeorites entering Titan’s atmosphere (Hartle et al., 2006; Hörst et
al., 2012).

Noticeable features of Titan’s atmosphere are the haze layers that give rise
to the moon’s orange appearance. The layers are optically thick in most
visible wavelengths making remote observations of Titan’s surface difficult
(Porco et al., 2005). The organic haze is made up of aerosols and from early
experiments using N2/CH4 mixtures, a combination of polycyclic aromatic
hydrocarbons (PAHs) and polycyclic aromatic nitrogen heterocycles (PANHs) was
proposed as the composition of the aerosols (Sagan et al., 1993). 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
also suggested the presence of PAHs/PANHS up to several hundred amu in the
atmosphere (López-Puertas et al., 2013).

Previous ion composition studies by the Ion and Neutral Mass Spectrometer
(INMS) and Cassini Plasma Spectrometer (CAPS) instruments revealed “families”
of ions around particular mass values and a regular spacing of 12 to 14 u/q
between mass group peaks characteristic of spectra of complex organic
compounds and related to a carbon or nitrogen backbone that dominates the ion
chemistry (Waite et al., 2005; Cravens et al., 2006; Vuitton et al., 2007;
Crary et al., 2009). Early Titan flybys by Cassini revealed positive ion
masses up to 350 u/q, obtained using measurements from the CAPS Ion Beam
Spectrometer (Waite et al., 2007). Later flybys demonstrated the existence of
even larger positive ions up to 1100 u/q (Coates et al., 2010). In addition to
positive ions, negative ions have been detected in Titan’s ionosphere (Coates
et al., 2007). These negative ions extend to higher masses than the positive
ions with masses up to 13,800 u/q (Coates et al., 2009; Wellbrock et al.,
2013, 2019). The negative ions are thought to be composed of carbon chains,
with the higher masses having a suspected PAH/PANH composition (Coates et al.,
2007; Desai et al., 2017).

The studies by Crary et al. (2009); Mandt et al. (2012) and Westlake et al.
(2014) used IBS data to study various aspects of ion composition at Titan.
These compositional studies covered positive ions (1-200 u/q) in Titan’s
ionosphere and showed that the likely origin of the ions above 100 u/q were
aromatic compounds created through ion-molecule reactions. Crary et al. (2009)
discuss possible ion composition for mass groups up to C13, where C13
indicates the mass group where there are 13 “heavy” (carbon, nitrogen or
oxygen) atoms present. The heaviest mass group they reported was the C15 group
although the possible ion composition of this material was not discussed.

There have been several experimental efforts to replicate the organic haze
present at Titan along with measurements of the ion-molecule gas-phase
chemistry (Dubois et al., 2020). Some of these experiments have measured
products up to 400 amu (Berry et al., 2019). Chemical models based on CAPS
measurements have proposed molecules up to the C17 group using a mechanistic
approach to explain the carbocation/carboanion chemistry (Ali et al., 2015).
The schemes studied included hydrocarbons and nitriles and various reaction
paths for the growth of these molecules.

Ions with masses between 170 and 310 u/q are examined in this study with ion
mass groups identified up to 275 u/q. Prominent groups identified contain 15,
16, 18 and 21 heavy atoms. Several ion groups also show variation in their
occurrence between flybys.

##  2 Methodology

Figure 1: An example of an IBS energy spectra during the T57 flyby. The error
bars shown represent the uncertainty due to Poisson counting error. Red x’s
indicate the peaks as identified by the peak finding algorithm. Due to the
logarithmic energy scale, at low energies the ion beams can be seen over a
number of bins, while at the high masses the beams can only be seen in a
single bin.

###  2.1 Cassini Plasma Spectrometer

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, $150^{\circ}\times 1.4^{\circ}$)
which are tilted at $30^{\circ}$ to each other. The energy resolution of IBS
is 0.014 $\Delta E/E$ 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.

###  2.2 Data Analysis and Ion Winds

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.

Figure 2: An energy/charge spectrogram from the T59 Titan flyby with intensity
shown in differential energy flux. Peaks identified by the algorithm are over
plotted with dots. The mass groups identified can clearly be seen by the
horizontal lines of dots. Small variations in the energy of the identified
peaks are due to effects such as ion velocity, spacecraft potential and slight
shifts in spacecraft orientation. The energy shift on the inbound leg of the
flyby (before 15:30) is due to the acceleration of ions away from Titan.

During the Titan flybys, Cassini had a high velocity (6 km/s) relative to the
low ion thermal (150K) and wind velocities (<230 m/s) 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 $\alpha$, occurs at energy
$E_{\alpha}$ as approximated by equation (1) (Crary et al., 2009; Mandt et
al., 2012; Westlake et al., 2014),

$E_{\alpha}=\frac{1}{2}m_{\alpha}(v_{sc}+v_{wind})^{2}+e\Phi_{sc}+8kT.$ (1)

Equation (1) takes into account various effects that alter the peak energy
flux such as ion temperature (8kT), spacecraft potential (e$\Phi_{sc}$) 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),

$\frac{M}{\Delta
M}\approx\frac{v_{sc}+v_{wind}}{\sqrt{\frac{2kT}{m_{\alpha}}}}.$ (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 $\pm$ 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 +0.2V. Also, a
fixed temperature of 150K was applied during the analysis. This value is
representative of the temperatures seen in Titan’s ionosphere which are
between 100 and 200K (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),

$\Phi^{{}^{\prime}}\approx\Phi+\frac{mv_{sc}}{e}v_{wind},$ (3)

and by differentiating we find

$v_{wind}=\frac{e}{v_{sc}}\frac{d\Phi^{{}^{\prime}}}{dm},$ (4)

where $\Phi$ is the spacecraft potential, $\Phi^{{}^{\prime}}$ 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).

###  2.3 High Mass Methodology and Uncertainties

Figure 3: These three panels are used to illustrate the difference between
observing low mass and high mass ion beams with the IBS sensor. For all three
panels, the red lines represent the IBS response, with each gaussian curve
representing a different IBS energy bin. The black line represents an observed
ion beam, idealized with a gaussian and the width being determined by equation
2. The blue lines are each of the IBS energy bins (red) multiplied by the ion
beam (black) and are representative of the spectra generated by IBS.

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 $\Delta E/E$ energy resolution at Full Width Half Maximum
(FWHM) of the IBS sensor. IBS was designed with a $\Delta E/E$ of 0.014 (Young
et al., 2004) and calibration tests measured a $\Delta E/E$ of 0.02 (Vilppola
et al., 2001). Here we use a $\Delta E/E$ of 0.017, which was the operational
$\Delta E/E$ value (Young et al., 2004) and was used in Crary et al. (2009). A
$\Delta E/E$ of 0.017 results in the effective $M/\Delta M$ 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
to 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.

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 centred
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 $\pm$ 2 or $\pm$ 3 u/q compared to $\pm$ 0.5 u/q from the binning.

###  2.4 Studied Flybys

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.

=-1in

Flyby |  UTC times |  Titan shadow exit UTC time |  Number of sweeps |  Min altitude (km) |  Max altitude (km) |  Lat(∘) |  Long(∘) |  SZA Range (∘) |  Titan Local Time
---|---|---|---|---|---|---|---|---|---
T55 |  21:25:13 - 21:28:08 |  21:28:06 |  88 |  966 |  1001 |  -19.1 |  176.1 |  134-149 |  22:00:27
T56 |  19:58:37 - 20:01:21 |  20:00:19 |  83 |  967 |  1000 |  -28.0 |  175.4 |  127-143 |  22:00:57
T57 |  18:30:57 - 18:34:09 |  18:31:37 |  97 |  955 |  1000 |  -34.9 |  173.1 |  119-137 |  22:07:31
T58 |  17:02:39 - 17:05:25 |  17:01:21 |  84 |  966 |  999 |  -44.7 |  172.4 |  112-128 |  22:07:42
T59 |  15:32:26 - 15:35:39 |  15:29:03 |  96 |  957 |  1000 |  -54.7 |  171.6 |  103-120 |  22:08:20

Table 1: Table displaying the data used in this study. The time where Cassini
emerged from Titan’s geometric shadow is shown in the under ”Titan shadow exit
UTC time”. Min and Max altitudes demonstrate that these samples were taken at
some of the lowest regions of Titan’s ionosphere traversed by Cassini.
Latitude(Lat) and Longitude(Long) are displayed at the closest approach during
the flyby. Solar Zenith Angle(SZA) Range is the range of SZA values observed
in the sample range. Titan Local Time is shown in the last column.

##  3 Results

Figure 4: a) An altitude-mass plot for every peak identified by the peak
finding algorithm at the altitudes examined. The error bars represent the
instrument uncertainty for the peak detection. Each color represents a
different flyby, T55 - blue, T56 - orange, T57 - green, T58 - red, T59 -
purple. Panel b) is a section of panel a) between 200 and 300 u/q. Clear
grouping of ions can be seen, notably around 203, 217, 229, 241 and 266 u/q.

Through the use of the previously described algorithm across the 448 CAPS IBS
energy sweeps, the occurrence of mass groups and the group peaks can be
studied. Shown in Figure 4a is every peak identified across the five flybys,
plotted against altitude. The previously found trend of heavier ions occurring
at lower altitudes (Crary et al., 2009) can be seen. Figure 4b displays the
peaks between 200 and 300 u/q and below 1000km. With this plot several groups
can be seen with distinct ion clustering around 203, 217, 228, 241 and 266
u/q. The T57 and T59 flybys have closest approaches of 955 and 957 km, around
10 km lower than the other three flybys that can be seen in the figure.

The low mass ions from 0-100 u/q have been well studied by both the CAPS and
INMS instruments (Mandt et al., 2012; Westlake et al., 2014). Groups of
intermediate mass (100-200 u/q) positive ions were found along with major
peaks with a 12-14 u/q spacing between peaks. This study focuses on the high
masses between 170 and 310 u/q outside the mass range of the INMS instrument.
The structure is less clearly defined than the lower mass ranges and therefore
only mass groups and significant peaks are focused on in this study.

###  3.1 Previously Reported Ion Groups (100 - 200 u/q)

Figure 5: The black line represents the summed percent occurrence of the peaks
over the flybys with 1 u/q binning from the present study. The blue line
represents the percent occurrence from Crary et al. (2009). Good agreement can
be seen, with the present study showing more defined peaks for the ion groups
around 180 and 190 u/q.

For the comparison to 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 to 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.

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 January 2008 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.

Figure 6: The top panel displays the total number of times a peak occurs in
each 1 u/q bin. Vertical lines indicate the boundaries between the different
ion groups, with the boundaries being defined by local minima in the total
number of occurrences. Each group is labeled with C#, where # indicates the
number of heavy (carbon/nitrogen/oxygen) ions present. The bottom panel
represents the percentage occurrence by flyby, with the same group definitions
as the top panel. The percentage occurrence is by flyby, for example the large
peak for the T59 flyby in the C18 group occurs in 44% of the 96 sweeps
examined. The vertical error bars in the top panel shown are three times the
standard error relating to Poisson counting statistics, meaning there is a 99%
probability that the number of occurrences lies within this range. The
horizontal error bars are a combination of the uncertainty resulting from the
energy resolution and the uncertainty from a 1 u/q binning.  Figure 7: A box
and whisker plot demonstrating the mass spacing between the significant
observed peaks under the assumption of a number of “heavy atom”
(carbon/nitrogen/oxygen) in a molecular ion. This is calculated by finding the
mass difference between pairs of peaks and dividing through by the difference
in “heavy atoms”. The mean spacing is plotted in red and is largely in the 12
and 13 u/q range. The box shown represents the interquartile range of the peak
spacing along with the median. The low mean spacing of 11 u/q for the 299 u/q
peak is due to the close neighboring masses at 294 and 304 u/q, a similar
effect can be seen for the 257 and 262 u/q peaks.

###  3.2 High Mass Ions (170 - 310 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 to 14 u/q agrees with the trend
observed at lower masses.

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 to 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 $\pm$ 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 $\pm$ 3, 299 $\pm$ 3 and 304 $\pm$ 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 $\pm$ 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.

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

##  4 Discussion

Many structures have been hypothesized for the large molecular ions in Titan’s
ionosphere. These have included polyacetylene, polyynes, cyanopolyynes (Wilson
& Atreya, 2003; Lavvas et al., 2008a, b; Vuitton et al., 2019). (HCN)n
polymers have been proposed (Wilson & Atreya, 2003), as well as HC3N/C2H2
copolymers (Lebonnois et al., 2002; Lavvas et al., 2008b). PAHs and PANHs have
been proposed as well (López-Puertas et al., 2013; Zhao et al., 2018),
resulting from the detection of benzene in Titan’s upper atmosphere (Waite et
al., 2007). Amines and imine polymers have been proposed to contribute to
Titan’s haze due to either the prerequisite monomers being found in Titan’s
atmosphere or the molecules being detected in tholin experiments (Vuitton et
al., 2006; Cable et al., 2014; Skouteris et al., 2015). Fullerenes have also
been proposed to exist (Sittler et al., 2020).

For assessing the composition of the ion peaks, we compare the masses
associated with the ion peaks, with the masses consistent with different
structures. This includes those structures mentioned above as well as other
possible structures such as long aliphatic hydrocarbons with different levels
of saturation. These aliphatic hydrocarbon chains include alkanes, alkenes,
dienes, trienes and alkynes. A table summarizing the comparison of peaks to
possible structures is shown in Figure 8. Combinations of aliphatic
hydrocarbons and various polymers can explain the observed ion peaks but the
chemical structures that are consistent with all peaks are Polycyclic Aromatic
Compounds (PACs). Furthermore, chemical structures such as (HCN)n polymers for
example, are consistent with peaks for n = 7, 8, 9 and 11, but are not present
for n = 10, which is unlikely to occur. This does not rule out the presence of
aliphatic compounds but rather that the most abundant ions observed in this
study are consistent with PACs. PACs represent a combination of PAH and/or
polycyclic aromatic heterocycles containing nitrogen. Oxygen-bearing
polycyclic aromatic molecules are also possible. There are also likely
methylene substituted derivatives of these compounds, as proposed in Ali et
al. (2015).

PACs possibly have a role in the formation of aerosols (Lavvas et al., 2011).
Early experiments found that Titan tholin analogs contain PAHs (Sagan et al.,
1993) and also that PAHs may adsorb onto the tholins. Later studies have also
found PANHs and PAHs in Titan tholin analogs (Trainer et al., 2013; Yoon et
al., 2014; Mahjoub et al., 2016; Sciamma-O’Brien et al., 2017). Gautier et al.
(2017) discuss how aerosol growth is affected depending on the number of
aromatic rings in the reactant composition, finding that benzene and pyridine
caused production of co-polymeric structures while double-ringed aromatic
produced quasi-pure polymeric structures. Other studies have also investigated
the impact of ions interacting with charged aerosols, demonstrating that they
contribute to the growth of these particles (Lavvas et al., 2013). These
findings indicate that the composition of Titan’s aerosols is likely affected
by the composition of the positive PAC ions studied here.

Westlake et al. (2014) examined three hypotheses for the production of high
mass ions in Titan’s ionosphere. They concluded that the most viable method
for creating the large molecules were through ion-molecule reactions with
neutral acetylene, ethylene and hydrogen cyanide. Ali et al. (2015) further
investigated the cation, anion and neutral chemistry that could be present in
Titan’s ionosphere through ion-molecule reactions. Low-temperature formation
pathways of PAHs through radical-neutral reactions have been shown to exist
for molecules with up to four six-membered rings (Zhao et al., 2018).

Comparing the findings presented here with the ion-molecule reactions proposed
in Westlake et al. (2014) and Ali et al. (2015) we find the ion group peak
masses agree with the expected masses resulting from ion-molecule reactions.
Furthermore, comparing with proposed neutral PAHs/PANHs from López-Puertas et
al. (2013), we find the reported neutral molecules have comparable masses,
suggesting ionization and electron recombination are a factor. These two
comparisons suggest a coupling between the cationic and neutral phases at the
studied mass range.

=10mm

Figure 8: A table showing what structures are consistent with significant
peaks. The structures are broken into three groups and highlighted in
different colors for clarity: ringed structures containing aromatic and cyclic
molecules, nitrogen-bearing aliphatic molecules and aliphatic hydrocarbon
molecules. Any molecule that lies within the mass uncertainty associated with
a peak is considered plausible. Polycyclic Aromatic Compounds are consistent
with all observed ion peaks, followed by a graphite- or graphene- like
structure, consistent with nine of the thirteen observed peaks. The list of
polycyclic aromatic molecules considered are from the NASA Ames PAH IR
database (except for the 294 u/q peak) and should be viewed as not-exhaustive.

###  4.1 Ion Mass Group Composition

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
$\pm$ 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 $\rightarrow$ C15H10+ followed by C15H10
$\rightarrow$ C16H11+. These chains could also exist at the upper end of the
studied mass range as well, C20H14 $\rightarrow$ C21H13+ alongside a H2
ejection followed by C21H13 $\rightarrow$ 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 to 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 (Le Page et al., 1999b). Le Page et al. (1999)
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.

###  4.2 Variations between Flybys

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.

From Figure 9, we can see that the 229 $\pm$ 2 and 304 $\pm$ 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 to T55. The
trend for the 304 $\pm$ 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 $\pm$ 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 $\pm$ 2 and 266 $\pm$ 3 u/q peaks have
a minimum percentage occurrence during the T57 flyby while 203 $\pm$ 2 u/q
minimum is during T58.

Figure 9: The change in percent occurrence of the observed peaks, as compared
to T55. The uncertainties are Poisson counting statistics, with the dark grey
and light grey regions representing one and two times the standard deviation
respectively. The peaks are categorized into four groups, panel A) shows peaks
which occurred more frequently under illuminated conditions. Conversely, panel
B) shows a peak which occurred more frequently under unilluminated conditions.
Panel C) shows the peaks which had a minimum during the flybys and panel D)
are the remaining seven observed peaks that show no clear variation, with all
points lying within one standard deviation.

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 1000km (Ågren et
al., 2009; Cui et al., 2009; Coates et al., 2009, 2011; Wellbrock et al.,
2009, 2013; Wellbrock & et al., 2021, in prep), 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; Wellbrock & et al.,
2021, in prep; Mihailescu et al., 2020). On the other hand, Wellbrock et al.
(2009); Wellbrock & et al. (2021, in prep) 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 1000km. 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.

###  4.3 Comparison with Neutral PACs in Titan’s Atmosphere

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 $\pm$ 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 $\pm$ 3 u/q with a minor peak at 253 $\pm$ 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 $\pm$ 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.

##  5 Conclusions

The composition of positive ions between 170 and 310 u/q has been examined
during five CAPS actuator-fixed flybys that took place during 2009. The
composition was investigated by examining how frequently a peak occurs at a
given mass. Up to 275 u/q there is a clear ion group structure of peaks and
troughs, where the peaks are ion masses with a high relative abundance.
Conversely, the troughs are associated with ion of low relative abundance.
Between 275 and 310 u/q, several prominent peaks are found but with no clearly
associated group structure. The identified ion peaks are consistent with
various aliphatic compounds, however, polycylic aromatic compounds, such as
PAHs/PANHs, are found to be consistent with all identified peaks. The spacing
between peaks was found to be between 12 and 13 u/q, with an average spacing
of 12.4 u/q consistent with the addition of C or CH.

Variation in the occurrence of prominent peaks indicates changes in the
photochemistry, most likely due to differing solar radiation conditions, at
the studied 170-310 u/q mass range. Comparisons with neutral PAHs/PANHs
findings in the atmosphere reveal that the newly found ion peaks are found at
similar masses to the abundant molecules in the neutral phase. This implies
coupling between the ion and neutral phases at these masses.

These are the largest distinct positive ion groups reported so far in Titan’s
ionosphere. The discovery of these groups will aid future atmospheric chemical
models of Titan through the identification of what heavy positive ions are
prominent, aiding the understanding of which reaction pathways are plausible
to create the previously found heavy negative ions and neutrals. The future
Dragonfly mission to Titan will study the surface composition and prebiotic
chemistry. Given that the complex organic compounds created in Titan’s
atmosphere form the particles that comprise the haze found at lower altitudes
and that these particles are expected to fall onto the surface, understanding
the origin of these organic compounds gives helpful insights into the
interpretation of future results.

R. P. Haythornthwaite is supported by Science and Technology Facilities
Council (STFC) studentship 2062537. A. J. Coates, G. H. Jones and A. Wellbrock
acknowledge support from the STFC consolidated grant to UCL‐MSSL ST/S000240/1.
We acknowledge funding from ESA via the UK Space Agency for the Cassini CAPS
Operations team. Work in the USA was supported by Southwest Research Institute
internal funding. Cassini CAPS data are available from the https://pds-
ppi.igpp.ucla.edu/mission/Cassini-Huygens (catalog NASA PDS service). We thank
the reviewers for their helpful comments that have improved the paper.

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