Ion And Neutral Mass Spectrometer Research Articles

Utilizing Helium Ion Chemistry to Derive Mixing Ratios of Heavier Neutral Species in Saturn's Equatorial Ionosphere

AbstractA surprisingly strong influx of organic‐rich material into Saturn's upper atmosphere from its rings was observed during the proximal obits of the Grand Finale of the Cassini mission. Measurements by the Ion and Neutral Mass Spectrometer (INMS) gave insights into the composition of the material, but it remains to be resolved what fraction of the inferred heavy volatiles should be attributed as originating from the fragmentation of dust particles in the instrument versus natural ablation of grains in the atmosphere. In the present study, we utilize measured light ion and neutral densities to further constrain the abundances of heavy volatiles in Saturn's ionosphere through a steady‐state model focusing on helium ion chemistry. We first show that the principal loss mechanism of He+ in Saturn's equatorial ionosphere is through reactions with species other than H2. Based on the assumption of photochemical equilibrium at altitudes below 2,500 km, we then proceed by estimating the mixing ratio of heavier volatiles down to the closest approaches for Cassini's proximal orbits 288 and 292. Our derived mixing ratios for the inbound part of both orbits fall below those reported from direct measurements by the INMS, with values of ∼2 × 10−4 at closest approaches and order‐of‐magnitude variations in either direction over the orbits. This aligns with previous suggestions that a large fraction of the neutrals measured by the INMS stems from the fragmentation of infalling dust particles that do not significantly ablate in the considered part of Saturn's atmosphere and are thus unavailable for reactions.

Identifying Shadowing Signatures of C Ring Ringlets and Plateaus in Cassini Data from Saturn’s Ionosphere

For orbits 288 and 292 of Cassini’s Grand Finale, clear dips (sharp and narrow decreases) are visible in the densities measured by the Ion and Neutral Mass Spectrometer (INMS). In 2017, the southern hemisphere of Saturn was shadowed by its rings and the substructures within. Tracing a path of the solar photons through the ring plane to Cassini’s position, we can identify regions in the ionosphere that were shadowed by the individual ringlets and plateaus (with increased optical depths) of Saturn’s C ring. The calculated shadowed altitudes along Cassini’s trajectory line up well with the dips in the data when adjusting the latter based on a detected evolving shift in the INMS timestamps since 2013, illustrating the potential for verification of instrument timings. We can further estimate the mean optical depths of the ringlets/plateaus by comparing the dips to inbound densities. Our results agree well with values derived from stellar occultation measurements. No clear dips are visible for orbits 283 and 287, whose periapsides were at higher altitudes. This can be attributed to the much longer chemical lifetime of at these higher altitudes, which in turn can be further used to estimate a lower limit for the flow speed along Cassini’s trajectory. The resulting estimate of ∼0.3 km s−1 at an altitude of ∼3400 km is in line with prior suggestions. Finally, the ringlet and plateau shadows are not associated with obvious dips in the electron density, which is expected due to their comparatively long chemical (recombination) lifetime.

CubeSat measurements of thermospheric plasma: spacecraft charging effects on a plasma analyzer

Spacecraft charging affects the accuracy of in-situ plasma measurements in space. We investigate the impact of spacecraft charging on upper thermospheric plasma measurements captured by a 2U CubeSat called Phoenix. Using the Spacecraft Plasma Interactions Software (SPIS), we simulate dayside surface potentials of − 0.6 V, and nightside potentials of − 0.2 V. We also observe this charging mechanism in the distribution function captured by the Ion and Neutral Mass Spectrometer (INMS) on-board Phoenix. Whilst negative charging in the dense ionosphere is known, the diurnal variation in density and temperature has resulted in dayside potentials that are smaller than at night. We apply charging corrections in accordance with Liouville’s theorem and employ a least-squares fitting routine to extract the plasma density, bulk speed, and temperature. Our routine returns densities that are within an order of magnitude of the benchmarks above, but they carry errors of at least 20%. All bulk speeds are greater than the expected range of 60–120 m/s and this could be due to insufficient charging corrections. Our parameterised ion temperatures are lower than our empirical benchmark but are in-line with other in-situ measurements. Temperatures are always improved when spacecraft charging corrections are applied. We mostly attribute the shortcomings of the findings to the ram-only capture mode of the INMS. Future work will improve the fitting routine and continue to cross-check with other in-flight data.

Detection of Cyclopropenylidene on Titan with ALMA

Abstract We report the first detection on Titan of the small cyclic molecule cyclopropenylidene (c-C3H2) from high-sensitivity spectroscopic observations made with the Atacama Large Millimeter/submillimeter Array. Multiple lines of cyclopropenylidene were detected in two separate data sets: ∼251 GHz in 2016 (Band 6) and ∼352 GHz in 2017 (Band 7). Modeling of these emissions indicates abundances of 0.50 ± 0.14 ppb (2016) and 0.28 ± 0.08 (2017) for a 350 km step model, which may either signify a decrease in abundance, or a mean value of 0.33 ± 0.07 ppb. Inferred column abundances are (3–5) × 1012 cm−2 in 2016 and (1–2) × 1012 cm−2 in 2017, similar to photochemical model predictions. Previously the C3H ion has been measured in Titan’s ionosphere by Cassini’s Ion and Neutral Mass Spectrometer (INMS), but the neutral (unprotonated) species has not been detected until now, and aromatic versus aliphatic structure could not be determined by the INMS. Our work therefore represents the first unambiguous detection of cyclopropenylidene, the second known cyclic molecule in Titan’s atmosphere along with benzene (C6H6) and the first time this molecule has been detected in a planetary atmosphere. We also searched for the N-heterocycle molecules pyridine and pyrimidine finding nondetections in both cases, and determining 2σ upper limits of 1.15 ppb (c-C5H5N) and 0.85 ppb (c-C4H4N2) for uniform abundances above 300 km. These new results on cyclic molecules provide fresh constraints on photochemical pathways in Titan’s atmosphere, and will require new modeling and experimental work to fully understand the implications for complex molecule formation.

Compositional Measurements of Saturn's Upper Atmosphere and Rings from Cassini INMS

AbstractThe Cassini spacecraft's last orbits directly sampled Saturn's thermosphere and revealed a much more chemically complex environment than previously believed. Observations from the Ion and Neutral Mass Spectrometer (INMS) aboard Cassini provided compositional measurements of this region and found an influx of material raining into Saturn's upper atmosphere from the rings. We present here an in‐depth analysis of the CH4, H2O, and NH3 signal from INMS and provide further evidence of external material entering Saturn's atmosphere from the rings. We use a new mass spectral deconvolution algorithm to determine the amount of each species observed in the spectrum and use these values to determine the influx and mass deposition rate for these species.

Low-mass nitrogen-, oxygen-bearing, and aromatic compounds in Enceladean ice grains

ABSTRACT Saturn’s moon Enceladus is erupting a plume of gas and ice grains from its south pole. Linked directly to the moon’s subsurface global ocean, plume material travels through cracks in the icy crust and is ejected into space. The subsurface ocean is believed to be in contact with the rocky core, with ongoing hydrothermal activity present. The Cassini spacecraft’s Ion and Neutral Mass Spectrometer (INMS) detected volatile, gas phase, organic species in the plume and the Cosmic Dust Analyser (CDA) discovered high-mass, complex organic material in a small fraction of ice grains. Here, we present a broader compositional analysis of CDA mass spectra from organic-bearing ice grains. Through analogue experiments, we find spectral characteristics attributable to low-mass organic compounds in the Enceladean ice grains: nitrogen-bearing, oxygen-bearing, and aromatic. By comparison with INMS results, we identify low-mass amines [particularly (di)methylamine and/or ethylamine] and carbonyls (with acetic acid and/or acetaldehyde most suitable) as the best candidates for the N- and O-bearing compounds, respectively. Inferred organic concentrations in individual ice particles vary but may reach tens of mmol levels. The low-mass nitrogen- and oxygen-bearing compounds are dissolved in the ocean, evaporating efficiently at its surface and entering the ice grains via vapour adsorption. The potentially partially water soluble, low-mass aromatic compounds may alternatively enter ice grains via aerosolization. These amines, carbonyls, and aromatic compounds could be ideal precursors for mineral-catalysed Friedel–Crafts hydrothermal synthesis of biologically relevant organic compounds in the warm depths of Enceladus’ ocean.

The Ion Composition of Saturn's Equatorial Ionosphere as Observed by Cassini

AbstractThe Cassini Orbiter made the first in situ measurements of the upper atmosphere and ionosphere of Saturn in 2017. The Ion and Neutral Mass Spectrometer (INMS) found molecular hydrogen and helium as well as minor species including water, methane, ammonia, and organics. INMS ion mode measurements of light ion species (H+, H2+, H3+, and He+) and Radio and Plasma Wave Science instrument measurements of electron densities are presented. A photochemical analysis of the INMS and Radio and Plasma Wave Science data indicates that the major ion species near the ionospheric peak must be heavy and molecular with a short chemical lifetime. A quantitative explanation of measured H+ and H3+ densities requires that they chemically react with one or more heavy neutral molecular species that have mixing ratios of about 100 ppm.

Climatology of CH4, HCN and C2H2 in Titan's upper atmosphere from Cassini/VIMS observations

The Visual and Infrared Mapping Spectrometer (VIMS) measurements of non-Local Thermodinamic Equilibrium (non-LTE) emissions of CH4, HCN and C2H2 in the near-infrared represent a dataset with unique coverage to study Titan's upper atmosphere in the altitude range from 500 to 1000 km. This region is the key to a better understanding of the middle atmosphere circulation. The assessment of the latitudinal and seasonal variations in the distributions of HCN and C2H2 could give important hints about it and complement the Composite InfraRed Spectrometer (CIRS) observations, that target the atmosphere below 500 km. The measurement of CH4 and HCN above 900 km can also help in understanding the Ion and Neutral Mass Spectrometer (INMS) observations near and above 1000 km. We have analysed the VIMS daytime spectra in the 2.9–3.4 μm region measured from 2004 to 2012 that span from the mid-northern winter through mid-northern spring Titan seasons. The measurements exhibit very strong non-LTE emissions of CH4, HCN and C2H2. Non-LTE vibrational temperatures have been calculated for the emitting levels of the three molecules, and have been used in the inversion of their abundances in the 500–1100 km region. The retrieved abundances have been averaged in latitudinal bins and in two seasons: north hemisphere winter (2004–2009) and north hemisphere early spring (2010−2012). The C2H2 average profile, retrieved for the first time from VIMS measurements, shows good agreement with UltraViolet Imaging Spectrograph (UVIS) measurements below 850 km, while higher values are found above that altitude. The HCN volume mixing ratio (VMR) shows an enhancement at the north pole during winter at all altitudes (from ∼700 km up to near 1000 km) and at the north and south poles during spring below 900 km. Also, the average C2H2 abundance is larger at both poles during spring below 800 km, while no significant variation is seen during winter. For CH4 we found enhancements above 800 km at the north pole during winter and at both poles during spring. Our results suggest that the middle atmosphere circulation on Titan extends well above the 500 km level, up to 750–800 km or even higher if we assume that the CH4 concentration above 800 km is mainly controlled by the circulation. The behavior of CH4 at the winter pole above 800 km is in line with the strong zonal winds suggested to explain INMS observations, although it can also be explained by the slower photo-dissociation occurring during polar winter. Despite the poor statistic at the polar regions, our results represent a unique dataset that could help to improve our understanding of Titan's atmosphere above 500 km.

Spatial variations in the dust-to-gas ratio of Enceladus’ plume

On day 138 of 2010, the plume of dust and gas emerging from Enceladus’ South Polar Terrain passed between the Sun and the Cassini spacecraft. This solar occultation enabled Cassini’s Ultraviolet Imaging Spectrograph (UVIS) and the Visual and Infrared Mapping Spectrometer (VIMS) to obtain simultaneous measurements of the plume’s gas and dust components along the same lines of sight. The UVIS measurements of the plume’s gas content are described in Hansen et al. (2011, GRL 38:11202) , while this paper describes the VIMS data and the information they provide about the plume’s particle content. Together, the VIMS and UVIS measurements reveal that the plume material above Baghdad and Damascus sulci has a dust-to-gas mass ratio that is roughly an order of magnitude higher than the material above Alexandria and Cairo sulci. Similar trends in the plume’s dust-to-gas ratio are also found in data obtained when Cassini flew through the plume in 2009, during which time the Ion and Neutral Mass Spectrometer (INMS), Radio and Plasma Wave Science instrument (RPWS) and Cosmic Dust Analyzer (CDA) instruments made in-situ measurements of the plume’s gas and dust densities (Dong et al. 2015 JGR 120:915-937). These and other previously-published systematic differences in the material erupting from different fissures likely reflect variations in subsurface conditions across Encealdus’ South Polar Terrain.

Laboratory investigations of Titan haze formation: In situ measurement of gas and particle composition

Prior to the arrival of the Cassini-Huygens spacecraft, aerosol production in Titan’s atmosphere was believed to begin in the stratosphere where chemical processes are predominantly initiated by far ultraviolet (FUV) radiation. However, measurements taken by the Cassini Ultraviolet Imaging Spectrograph (UVIS) and Cassini Plasma Spectrometer (CAPS) indicate that haze formation initiates in the thermosphere where there is a greater flux of extreme ultraviolet (EUV) photons and energetic particles available to initiate chemical reactions, including the destruction of N2. The discovery of previously unpredicted nitrogen species in measurements of Titan’s atmosphere by the Cassini Ion and Neutral Mass Spectrometer (INMS) indicates that nitrogen participates in the chemistry to a much greater extent than was appreciated before Cassini. The degree of nitrogen incorporation in the haze particles is important for understanding the diversity of molecules that may be present in Titan’s atmosphere and on its surface. We have conducted a series of Titan atmosphere simulation experiments using either spark discharge (Tesla coil) or FUV photons (deuterium lamp) to initiate chemistry in CH4/N2 gas mixtures ranging from 0.01% CH4/99.99% N2 to 10% CH4/90% N2. We obtained in situ real-time measurements using a high-resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS) to measure the particle composition as a function of particle size and a proton-transfer ion-trap mass spectrometer (PIT-MS) to measure the composition of gas phase products. These two techniques allow us to investigate the effect of energy source and initial CH4 concentration on the degree of nitrogen incorporation in both the gas and solid phase products. The results presented here confirm that FUV photons produce not only solid phase nitrogen bearing products but also gas phase nitrogen species. We find that in both the gas and solid phase, nitrogen is found in nitriles rather than amines and that both the gas phase and solid phase products are composed primarily of molecules with a low degree of aromaticity. The UV experiments reproduce the absolute abundances measured in Titan’s stratosphere for a number of gas phase species including C4H2, C6H6, HCN, CH3CN, HC3N, and C2H5CN.

Solar cycle variations in ion composition in the dayside ionosphere of Titan

AbstractOne Titanian year spans over two complete solar cycles, and the solar irradiance has a significant effect on ionospheric densities. Solar cycle 24 has been one of the quietest cycles on record. In this paper we show data from the Cassini ion and neutral mass spectrometer (INMS) and the radio and plasma wave science Langmuir probe spanning the time period from early 2005, at the declining phase of solar cycle 23, to late 2015 at the declining phase of solar cycle 24. Densities of different ion species measured by the INMS show a consistent enhancement for high solar activity, particularly near the ionospheric peak. The density enhancement is best seen in primary ion species such as CH3+ rather than heavier ion species such as HCNH+. Unlike at Earth, where the ionosphere and atmosphere thermally expand at high solar activity, at Titan the altitude of the ionospheric peak decreases, indicating that the underlying neutral atmosphere was less extensive. Among the major ion species, CH5+ shows the largest decrease in peak altitude, whereas heavy ions such as C3H5+ show very little decrease. We also calculate the ion production rates using a theoretical model and a simple empirical model using INMS data and show that these effectively predict the increased ion production rates at high solar activity.

Ion energy distributions and densities in the plume of Enceladus

Enceladus has a dynamic plume that is emitting gas, including water vapor, and dust. The gas is ionized by solar EUV radiation, charge exchange, and electron impact and extends throughout the inner magnetosphere of Saturn. The charge exchange collisions alter the plasma composition. Ice grains (dust) escape from the vicinity of Enceladus and form the E ring, including a portion that is negatively charged by the local plasma. The inner magnetosphere within 10RS (Saturn radii) contains a complex mixture of plasma, neutral gas, and dust that links back to Enceladus. In this paper we investigate the energy distributions, ion species and densities of water group ions in the plume of Enceladus using test particle and Monte Carlo methods that include collisional processes such as charge exchange and ion-neutral chemical reactions. Ion observations from the Cassini Ion and Neutral Mass Spectrometer (INMS) for E07 are presented for the first time. We use the modeling results to interpret observations made by the Cassini Plasma Spectrometer (CAPS) and the INMS. The low energy ions, as observed by CAPS, appear to be affected by a vertical electric field (EZ=−10µV/m) in the plume. The EZ field may be associated with the charged dust and/or the pressure gradient of plasma. The model results, along with the results of earlier models, show that H3O+ ions created by chemistry are predominant in the plume, which agrees with INMS and CAPS data, but the INMS count rate in the plume for the model is several times greater than the data, which we do not fully understand. This composition and the total ion count found in the plume agree with INMS and CAPS data. On the other hand, the Cassini Langmuir Probe measured a maximum plume ion density more than 30,000cm−3, which is far larger than the maximum ion density from our model, 900cm−3. The model results also demonstrate that most of the ions in the plume are from the external magnetospheric flow and are not generated by local ionization. The origin of the ions in the plume was investigated using two different velocity models. Most ions were created by the interaction with background magnetospheric plasma and by photoionization. INMS and CAPS also detected water cluster ions. We will interpret these observations as a result of ion collisions with neutral water clusters, (H2O)n, originating in the tiger stripe vents as suggested by Tokar et al. (2009). We also estimated the process of generating cluster ions based on the INMS observations. We suggest that the most likely source is reaction of H3O+ with neutral water clusters or dimers such as (H2O)2 formed in the plume vents.

Modeling insights into the locations of density enhancements from the Enceladus water vapor jets

Monte Carlo modeling of the vapor erupting from Enceladus' south polar region is presented to demonstrate the influence of physical characteristics of the emitted vapor on the distribution of particles at altitude. The modeled sources include localized jets and eruptions distributed along the surface features labeled “tiger stripes.” The modeling reveals that density enhancements are displaced from the source location caused not only by the angle of emission but also by superposition of material from nearby sources. The altitude of the superposition is mass dependent and contributes to differences observed in the mass 28 and mass 44 channels of the Cassini Ion and Neutral Mass Spectrometer (INMS) during Enceladus encounters. INMS data are well modeled using only the tiger stripe sources; however, certain regions require additional sources to reproduce some features of the data. In particular, an excess source is required for 44 unified atomic mass unit (u) on the Saturn-facing hemisphere of Baghdad Sulcus. It is apparent in three parallel Cassini flybys. A relative decrease in the source rate is observed for mass 28 u for E14. The more diffuse nature of the 28 u INMS observations compared to the more collimated structure of the 44 u INMS observations is consistent with increased thermal spreading for low-mass constituents of the plume.

Influence of local ionization on ionospheric densities in Titan's upper atmosphere

AbstractTitan has the most chemically complex ionosphere of the solar system. The main sources of ions on the dayside are ionization by EUV solar radiation and on the nightside include ionization by precipitated electrons from Saturn's magnetosphere and transport of ions from the dayside, but many questions remain open. How well do models predict local ionization rates? How strongly do the ionization processes drive the ionospheric densities locally? To address these questions, we have carried out an analysis of ion densities from the Ion and Neutral Mass Spectrometer (INMS) from 16 close flybys of Titan's upper atmosphere. Using a simple chemical model applied to the INMS data set, we have calculated the ion production rates and local ionization frequencies associated with primary ions and . We find that on the dayside the solar energy deposition model overestimates the INMS‐derived production rates by a factor of 2. On the nightside, however, the model driven by suprathermal electron intensities from the Cassini Plasma Spectrometer Electron Spectrometer sometimes agrees and other times underestimates the INMS‐derived production rates by a factor of up to 2–3. We find that below 1200 km, all ion number densities correlate with the local ionization frequency, although the correlation is significantly stronger for short‐lived ions than long‐lived ions. Furthermore, we find that, for a given N2 local ionization frequency, has higher densities on the dayside than on the nightside. We explain that this is due to CH4 being more efficiently ionized by solar photons than by magnetospheric electrons for a given amount of N2 ionization.

Cassini INMS measurements of Enceladus plume density

During six encounters between 2008 and 2013, the Cassini Ion and Neutral Mass Spectrometer (INMS) made in situ measurements deep within the Enceladus plumes. Throughout each encounter, those measurements contained density variations that reflected the nature of the source, particularly of the high-velocity jets. Since the dominant constituent of the vapor, H2O, interacted with the walls of the INMS inlet, we track changes in the external vapor density by using more-volatile species that responded promptly to those changes. However, the most-abundant volatiles, at 28u and 44u, behaved differently from each other in the plume. At least a portion of their differences may be attributed to mass-dependent thermal velocity that affects Mach number in the high-velocity jets. Variations between volatiles place an emphasis on modeling as a means to construct overall plume density from the volatile densities and to investigate the velocity, gas temperature, and location of the jets. Ice grains, entering the INMS aperture add complexity and uncertainty to the physical interpretation of the data because the grains modified the INMS measurements. A comparison of data from the last three encounters, E14, E17, and E18, are consistent with the VIMS observation of variability in jet production and a slower, more diffuse gas flux from the four sulci or tiger stripes. We provide and describe the INMS data, its processing, and its uncertainty.

Ion composition in Titan's exosphere via the Cassini Plasma Spectrometer I: T40 encounter

AbstractWe investigate the complex interaction between Saturn's magnetosphere and Titan's upper ionosphere using ion data acquired by the Cassini Plasma Spectrometer (CAPS) during the T40 encounter. Bounds on ion‐group abundances at altitudes between ~2733 and ~12,541 km are determined by fitting mass spectra with model functions derived from instrument calibration data. The spectra are dominated by H+, H2+, H3+, and two hydrocarbon groups with mass ranges 12–19 and 24–32 amu, respectively. Notably, this constitutes the first reported observation of H3+ in Titan's exosphere. These measurements are discussed in the context of data from the CAPS electron spectrometer (ELS) and the Ion and Neutral Mass Spectrometer (INMS), which fortuitously sampled the ionospheric outflow during the T40 encounter at altitudes between ~2225 and ~3034 km. The CAPS data reveal a composition that is constitutively similar to that sampled by INMS, with hydrocarbon ions first observed as far as ~11,000 km from Titan and increasing in density by more than an order of magnitude along Cassini's inbound trajectory. In addition, we juxtapose the CAPS ion data with numerical results from three different interaction models and show that it is consistent with the location of the field‐draping boundary described by Ulusen et al. (2012) and the Saturnward ion tail predicted by Sillanpää et al. (2006).

The Vaporization Threshold: Hypervelocity Impacts of Ice Grains into a Titanium Cassini Spacecraft Instrument Chamber

The Cassini spacecraft, currently in orbit around Saturn, has an Ion and Neutral Mass Spectrometer (INMS) on board as part of the scientific payload. There have been some unexpected readings of the instrument in flybys of the moon Enceladus. In relative encounter velocity, these flybys range from 7 to 18km/s, and it has been suggested that ice grain impacts in the instrument could have a velocity-dependent response that influences the materials that the instrument records. To explore the physics of the impacts, computations were performed with CTH. Small ice grains (1 micron across) were impacted into a titanium alloy at a range of speeds of interest. Results are that the formation of a titanium vapor plume begins at impact velocities of 16km/s. The transition to the formation of vapor of target material is fairly sharp. We explore the transition to determine the influence of ice grain geometry on the vapor formation transition. Efforts have been made to quantify the titanium vapor and titanium solid and liquid ejecta at various impact speeds, as all of these may influence chemistry in the instrument's antechamber and thus affect what ions or molecules are seen by the INMS.

The role of ion‐molecule reactions in the growth of heavy ions in Titan's ionosphere

AbstractThe Ion and Neutral Mass Spectrometer (INMS) and Cassini Plasma Spectrometer (CAPS) have observed Titan's ionospheric composition and structure over several targeted flybys. In this work we study the altitude profiles of the heavy ion population observed by the Cassini Plasma Spectrometer‐Ion Beam Spectrometer (CAPS‐IBS) during the nightside T57 flyby. We produce altitude profiles of heavy ions from the C6–C13 group (Ci indicates the number, i, of heavy atoms in the molecule) using a CAPS‐IBS/INMS cross calibration. These altitude profiles reveal structure that indicates a region of initial formation and growth at altitudes below 1200 km followed by a stagnation and dropoff at the lowest altitudes (1050 km). We suggest that an ion‐molecule reaction pathway could be responsible for the production of the heavy ions, namely reactions that utilize abundant building blocks such as C2H2 and C2H4, which have been shown to be energetically favorable and that have already been identified as ion growth patterns for the lighter ions detected by the INMS. We contrast this growth scenario with alternative growth scenarios determining the implications for the densities of the source heavy neutrals in each scenario. We show that the high‐mass ion density profiles are consistent with ion‐molecule reactions as the primary mechanism for large ion growth. We derive a production rate for benzene from electron recombination of C6H7+ of 2.4 × 10−16 g cm−2 s−1 and a total production rate for large molecules of 7.1 × 10−16 g cm−2 s−1.

The observed composition of ions outflowing from Titan

We report on Cassini Ion and Neutral Mass Spectrometer (INMS) observations above Titan's exobase at altitudes of 2225 km to 3034 km. We observe significant densities of CH5+, HCNH+ and C2H5+that require ion‐molecule reactions to be produced in the quantities observed. The measured composition and ion velocity (about 0.8–1.5 km/s) suggest that the observed ions must have been created deep inside Titan's ionosphere (below the exobase) and then transported to the detection altitude. Plasma motion from below Titan's exobase to large distances can be driven by a combination of thermal pressure and magnetic forces. The observed outward flows may link the main ionosphere with the more distant wake and provide a source of hydrocarbon ions in the Saturnian system.

Modeling of electron fluxes in the Enceladus plume

Observations by instruments onboard the Cassini spacecraft of Saturn's icy satellite Enceladus have shown that a plume containing water vapor and ice grains is present in the southern hemisphere. Energy distributions of electrons in this plume were measured by the electron sensor part of the Cassini plasma spectrometer (CAPS – ELS). A significant suprathermal electron population was detected. The nature of the electron population is important for understanding the composition and chemistry of the plume plasma because the electron‐ion recombination rate depends on the energy distribution and because ionizing collisions by energetic electrons creates new plasma. We present the results of a two‐stream electron transport model for plume electrons that includes neutral densities that agree with Cassini Ion and Neutral Mass Spectrometer (INMS) data. Electron production within the plume due to photoionization by solar radiation and by electron impact ionization was included, as were energy losses due to electron‐neutral collisions. Model cases were considered that both included and did not include electron inputs from outside the plume. Comparisons are made of model fluxes with measured fluxes by CAPS – ELS on October 9, 2008. The model‐data comparisons indicate that photoelectrons (10 eV–70 eV energies) locally produced within the plume can explain the data. The possible role of electron‐grain collisions was also explored and it was determined that nanograin densities in excess of 106 cm−3 would be needed to affect the electron distribution.