Ion Neutral Mass Spectrometer Research Articles

Triton's photochemical model: Update

Titan, Triton, and Pluto are the Solar System objects with the nitrogen-ethane atmospheres. The progress in Titan's photochemistry, stimulated by the very successful Cassini-Huygens mission, facilitated photochemical interpretation of Pluto's atmosphere that was observed by the New Horizons mission and supporting ground-based measurements. Here is an update of Triton's atmospheric and ionospheric photochemistry that combines the previous photochemical model (Krasnopolsky and Cruikshank 1995, Photochemistry of Triton's atmosphere and ionosphere, J. Geophys. Res. 100, 21,271-21,286) with findings in Titan's and Pluto's photochemistry and progress in chemical kinetics.Our modeling of Triton's photochemistry is restricted to the conditions of the Voyager 2 flyby in 1989, when data on vertical profiles of temperature, haze, and densities of N2, CH4, N, and electrons were obtained using radio- and solar UVS occultations and imaging.Significant differences between the composition and photochemistry of Triton's atmosphere and those of Titan and Pluto are mostly caused by the low CH4 tropospheric abundance of 100 ppm, smaller than those on Titan and Pluto by two orders of magnitude. This methane is strongly depleted photochemically to a ppb level in the upper atmosphere of Triton, while it is enriched by diffusive separation on Titan and Pluto. Therefore atomic species and their chemistry are much more abundant on Triton than on Titan and Pluto. Atomic ions C+ and N+ with slow recombination rates dominate in Triton's ionosphere with electron densities, significantly exceeding those on Titan and Pluto.The proposed model agrees with all observational constraints and presumes thermal escape of H2 + H with a rate of 45 g cm−2 Byr−1, loss of nitrogen of 40 g cm−2 Byr−1 on thermal escape and precipitation of nitriles, and sublimation and loss of methane of 270 g cm−2 Byr−1.C2H6 and HCN are minor species in the photochemical precipitations on Triton, and their detection in Triton's ice may refer to either the primordial composition or ice photochemistry.Seven versions of the model are calculated to study its sensitivity to some basic input parameters. The model can be used for planning future studies of Triton's atmosphere. An analog of the Ion-Neutral Mass Spectrometer onboard Triton's orbiter looks promising.

Empirical Photochemical Modeling of Saturn’s Ionization Balance Including Grain Charging

Abstract We present a semianalytical photochemical model of Saturn’s near-equatorial ionosphere and adapt it to two regions (∼2200 and ∼1700 km above the 1 bar level) probed during the inbound portion of Cassini’s orbit 292 (2017 September 9). The model uses as input the measured concentrations of molecular hydrogen, hydrogen ion species, and free electrons, as well as the measured electron temperature. The output includes upper limits, or constraints, on the mixing ratios of two families of molecules, on ion concentrations, and on the attachment rates of electrons and ions onto dust grains. The model suggests mixing ratios of the two molecular families that, particularly near ∼1700 km, differ notably from what independent measurements by the Ion Neutral Mass Spectrometer suggest. Possibly connected to this, the model suggests an electron-depleted plasma with a level of electron depletion of around 50%. This is in qualitative agreement with interpretations of Radio Plasma Wave Science/Langmuir Probe measurements, but an additional conundrum arises in the fact that a coherent photochemical equilibrium scenario then relies on a dust component with typical grain radii smaller than 3 Å.

Molecular hydrogen in the upper atmospheres of Saturn and Titan

Molecular hydrogen is the third most abundant species in Titan's atmosphere with a tropospheric/lower stratospheric mole fraction of 0.001 derived from Voyager and Cassini measurements and the major species in Saturn's atmosphere. The motivation for this paper is to explore the hypothesis that the corrected H2 density measurements reported by Cassini Ion Neutral Mass Spectrometer (INMS) team were too high by a factor of 2.2, the factor that the INMS team reported in Teolis et al. (2015) as applicable to all species with no exceptions. For Titan, this would lead to the resolution of a possible missing H2 source and eliminate the need for a large downward gas phase H2 flux. For Saturn this would bring the INMS in-situ and UVIS stellar occultation H2 density measurements into agreement in the low latitude thermosphere and attribute the additional mass density derived from Cassini Attitude and Articulation Control Subsystem (AACS) exceeding the reduced INMS H2 mass density to infalling ring particles.

Wavelike perturbations in Titan’s ionosphere and their compositional variation: A preliminary survey of Cassini Ion Neutral Mass Spectrometer measurements

Wavelike perturbations in the ionosphere of Titan, the largest satellite of Saturn, are explored based on the Cassini Ion Neutral Mass Spectrometer (INMS) measurements. Strong wavelike perturbations are identified for more than twenty ion species, from simple ones such as N⁺ and CH₄⁺ to complex ones such as C₂H₃CNH⁺ and C₄H₇⁺. Simultaneous wavelike perturbations in background N₂, indicative of atmospheric gravity waves, are also observed, motivating us to speculate that the INMS-derived ion perturbations are wave-driven. The amplitudes of the ion perturbations are found to be larger than that of the N₂ perturbations. Clear compositional variation is revealed by the data: heavier ion species exhibit greater amplitudes. Such observations might be understood based on considerations either of force balance or chemical loss in Titan’s ionosphere.

Energy deposition in Saturn’s equatorial upper atmosphere

We construct Saturn equatorial neutral temperature and density profiles of H, H2, He, and CH4, between 10−12 and 1 bar using measurements from Cassini’s Ion Neutral Mass Spectrometer (INMS) taken during the spacecraft’s final plunge into Saturn’s atmosphere on 15 September 2017, combined with previous deeper atmospheric measurements from the Cassini Composite InfraRed Spectrometer (CIRS) and from the UltraViolet Imaging Spectrograph (UVIS). These neutral profiles are fed into an energy deposition model employing soft X-ray and Extreme UltraViolet (EUV) solar fluxes at a range of spectral resolutions (Δλ=4×10−3nm to 1 nm) assembled from TIMED/SEE, from SOHO/SUMER, and from the Whole Heliosphere Interval (WHI) quiet Sun campaign. Our energy deposition model calculates ion production rate profiles through photo-ionisation and electron-impact ionisation processes, as well as rates of photo-dissociation of CH4. The ion reaction rate profiles we determine are important to obtain accurate ion density profiles, meanwhile methane photo-dissociation is key to initiate complex organic chemical processes. We assess the importance of spectral resolution in the energy deposition model by using a high-resolution H2 photo-absorption cross section, which has the effect of producing additional ionisation peaks near 800 km altitude. We find that these peaks are still formed when using low-resolution (Δλ=1nm) or mid-resolution (Δλ=0.1nm) solar spectra, as long as high-resolution cross sections are included in the model.

Precipitating Solar Wind Hydrogen as Observed by the MAVEN Spacecraft: Distribution as a Function of Column Density, Altitude, and Solar Zenith Angle

AbstractThe Mars Atmosphere and Volatile EvolutioN (MAVEN) mission observes precipitating solar wind hydrogen ENAs in the upper Martian ionosphere with various instruments onboard. These precipitating solar wind hydrogen atoms result from the direct interaction between the solar wind and Mars's extensive hydrogen corona, creating a beam of particles traveling at solar wind speed that can penetrate deep into the ionosphere of Mars. Using particle data collected by both the Solar Wind Ion Analyzer (SWIA) and Neutral Gas and Ion Mass Spectrometer (NGIMS) onboard MAVEN, we seek to characterize the global behavior of these particles. By using a combination of energy spectra and neutral gas density data, we are able to determine the behavior of this particle population as a function of altitude, column density, and solar zenith angle. Through our analysis, we see that these precipitating solar wind hydrogen ENAs undergo multiple charge exchange interactions and ultimately scatter significantly at an altitude of 160 km or column density of 5 × 1016 cm−2. This observed column density at which equilibrium is reached is at a value ∼17 times greater than the theoretical value. We also demonstrate that this particle population's energy flux follows a cosine dependence of solar zenith angle, as is predicted by models.

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

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) on board 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.

Formation of Complex Organic Molecules (COMs) from Polycyclic Aromatic Hydrocarbons (PAHs): Implications for ISM IR Emission Plateaus and Solar System Organics

The presence of complex organic molecules (COMs) in a variety of solar system objects has prompted various theories regarding their origins. This paper reports on a series of proton, electron, and UV radiation experiments, conducted singly and in combination, on polycyclic aromatic hydrocarbon (PAH) thin films to determine if PAH-related materials could be related to COMs. Fourier transform infrared (FTIR) and mass spectra reveal that the PAHs are fragmented and altered, producing new infrared features and a mass spectral pattern similar to those from small aliphatic and larger saturated hydrocarbon chain and ring systems. Mass spectra after proton and electron irradiation exhibit significant differences. The proton irradiation products appear to contain more oxygen-related species, possibly the result of higher H2O concentrations in the vacuum chamber, whereas electron irradiation generates a more abundant, larger mass organic species mass spectral pattern. Combined irradiation produces a superposition of the proton and electron results, with some subtle differences. Mass spectral patterns resulting from electron and combined irradiation compare favorably to Cassini ion neutral mass spectrometer (INMS) measurements of particles falling from Saturn’s inner rings into its upper atmosphere. FTIR results are compared to PAH emission from interstellar sources and absorption components in IR spectra observed toward low-mass young stellar objects (YSOs). Notably, some newly reported broad IR features appear very similar in structure to the plateaus underlying the astronomical 3.2−3.6, 6−9, and 10–15 μm emission plateaus and several of the absorption components found in spectra observed toward YSOs. The studies also indicate that the presence of other species, such as H2O, may significantly impact the radiation products. These results point to the fact that a top-down synthesis of solar system COMs is possible via irradiative processing of PAHs.

The petitSat mission – Science goals and instrumentation

The mid- and low-latitude ionosphere is home to a variety of plasma density irregularities, including depletions (bubbles), enhancements (blobs), and small-scale scintillation. Previous studies of plasma density enhancements observed using ROCSAT data have posited that these structures are the direct result of the formation of bubbles near the geomagnetic equator. However, more recent observations from the C/NOFS satellite suggest that multiple mechanisms are responsible for forming plasma enhancements, with wave action in the ionosphere and thermosphere as a significant driver of the enhanced densities. Indeed, statistical analysis of enhancements observed from satellites resembles the statistics of Medium-Scale Traveling Ionosphere Disturbances (MSTIDs) with respect to seasonal variability and solar activity.petitSat is a CubeSat mission designed to examine the link between MSTIDs and plasma enhancements. The mission will provide in situ measurements of the plasma density, 3D ion drift, as well as ion and neutral composition. The instrument suite includes a combined retarding potential analyzer and cross-track drift meter and an ion-neutral mass spectrometer. This instrument suite will provide comprehensive information about the fluctuations in plasma, as well as changes in the neutral profile. petitSat will launch into a 51 deg inclination orbit at 400 km (consistent with an International Space Station deployment), allowing for numerous conjunctions with the Boston University All-Sky Imager network over the mission lifetime.

Cassini INMS constraints on the composition and latitudinal fractionation of Saturn ring rain material

During Cassini's Grand Finale, the spacecraft flew between Saturn's upper atmosphere and its innermost ring, the D ring. Throughout these final orbits, Cassini encountered material flowing from the D ring into Saturn's atmosphere. Here, we present a compositional analysis of this material using data collected by Cassini's Ion Neutral Mass Spectrometer. We confirm the identification of CH4, CO2, CO, N2, H2O, NH3, and organics in the ring material, and provide upper limits for organic compounds of interest. The H/C, O/C, N/C, and S/C ratios of ring material are constrained using three different kinds of model spectra: automated fits, hand fits of inorganics + hydrocarbons, and hand fits of inorganics + organics with priority given to O-, N-, and S-bearing organics. Additionally, we compare data from the final plunge to earlier orbits, and find that ring material can be divided into gas and dust constituents, with CO2 at the volatility boundary between the two phases. At increasing distance above the equatorial plane, the gas/dust molar ratio increases.

A photochemical model of Pluto's atmosphere and ionosphere

The model for Titan by Krasnopolsky (2014, Icarus 236, 83-91) has been adjusted to Pluto's conditions during the New Horizons flyby. The model includes 419 reactions of 83 neutrals and 33 positive ions plus 10 reactions of CO+ and HCO+ that were missing for Titan because of the low CO abundance. The model has 289 altitude steps up to the exobase at 1600 km with thermal escape for neutrals and diffusion velocities for ions as the upper boundary conditions. The model involves condensation on the haze and the surface with sticking coefficient γ = 0.002 for hydrocarbons, H2O, and CO2, γ = 0.01 for nitriles except HC3N with γ = 0.1. The photochemistry is driven by the solar EUV and UV radiation, the interplanetary Lyman-alpha emission, and the galactic cosmic rays. The adopted eddy diffusion K = 3 × 104 cm2 s−1 facilitates transport of C2H4 and C2H6, their condensation on the surface, and does not require the revision of the laboratory data on saturated vapor densities of C2H4 and C2H6 by orders of magnitude to fit the New Horizons observations. The CH4 homopause is at 110 km for this K, and the CH4 vertical profile is mostly controlled by molecular diffusion and agrees with the NH observations. Productions and losses of major hydrocarbons, nitriles, oxygen species, and hydrogen are briefly discussed. The daytime ionosphere is predicted with a maximum electron density of 800 cm−3 at 750 km. The most abundant ions are HCNH+ and C9H11+ above and below 600 km, respectively. Chemical effects of the ion reactions on some neutral species are significant. The predicted ion densities are measurable by an analog of the Cassini ion-neutral mass spectrometer. Evolution of the atmosphere includes thermal escape of H2 + H, CH4 + CH3, and N2 + N (92, 203, and 1 g cm−2 Byr−1, respectively), condensation of hydrocarbons and nitriles (306 and 101 g cm−2 Byr−1, respectively), and polymerization (30 g cm−2 Byr−1). (Annually mean values are halves of those.) The composition and photochemistry of Pluto during the New Horizons flyby are very different from those of Triton during the Voyager 2 flyby and controlled by the methane mole fraction near the surface. Seasonal variations of the atmospheric methane should cover both states of the atmospheres on both Pluto and Triton.

On the H2 abundance and ortho-to-para ratio in Titan's troposphere

We have analyzed spectra recorded between 50 and 650 cm−1 by the Composite Infrared Spectrometer (CIRS) aboard the Cassini spacecraft at low and high emission angles to determine simultaneously the H2 mole fraction and ortho-to-para ratio in Titan's troposphere. We used constraints from limb spectra between 50 and 900 cm−1 and from in situ measurements by the Huygens probe to characterize the temperature, haze and gaseous absorber profiles. We confirm that the N2-CH4 collision-induced absorption (CIA) coefficients used up to now need to be increased by about 52% at temperatures of 70–85 K. We find that the N2-N2 CIA coefficients are also too low in the N2 band far wing, beyond 110 cm−1, in agreement with recent quantum mechanical calculations. We derived a H2 mole fraction equal to (0.88 ± 0.13) × 10−3, which pertains to the ~1–34 km altitude range probed by the S0(0) and S0(1) lines. This result agrees with a previous determination based only on the H2-N2 dimer transition in the S0(0) line, and with the in situ measurement by the Gas Chromatograph Mass Spectrometer (GCMS) aboard Huygens. It is 3–4 times smaller than the value measured in situ by the Ion Neutral Mass Spectrometer (INMS) of Cassini at 1000–1100 km. The H2 para fraction is close to equilibrium in the 20-km region. CIRS spectra can be fitted assuming ortho-to-para (o-p) H2 thermodynamical equilibrium at all levels or a constant para fraction in the range 0.49–0.53. We have investigated different mechanisms that may operate in Titan's atmosphere to equilibrate the H2 o-p ratio and we have developed a one-dimensional model that solves the continuity equation in presence of such conversion mechanisms. We conclude that exchange with H atoms in the gas phase or magnetic interaction of H2 in a physisorbed state on the surface of aerosols are too slow compared with atmospheric mixing to play a significant role. On the other hand, magnetic interaction of H2 with CH4, and to a lesser extent N2, can operate on a timescale similar to the vertical mixing time in the troposphere. This process is thus likely responsible for the o-p equilibration of H2 in the mid-troposphere implied by CIRS measurements. The model can reproduce the inferred o-p ratio in the 20-km region, assuming low atmospheric mixing in the troposphere down to 15–20 km and conversion rates with CH4 or N2 slightly larger than obtained from an extrapolation of natural ortho-para conversion rate measured in gaseous hydrogen.

Titan's ionospheric chemistry, fullerenes, oxygen, galactic cosmic rays and the formation of exobiological molecules on and within its surfaces and lakes

We discuss the formation of aerosols within Titan's thermosphere-ionosphere and the different chemical pathways. Negative ion measurements by the Cassini Plasma Spectrometer (CAPS) Electron Spectrometer (ELS) give evidence for formation of unsaturated anion carbon chains, while positive ion measurements of the Cassini Ion Neutral Mass Spectrometer (INMS) indicate formation of more aromatic cation hydrocarbons. There is presently no direct observational evidence for large neutral molecule growth in Titan's thermosphere-ionosphere. The hydrocarbon cations are expected to form Polycyclic Aromatic Hydrocarbons (PAH), those with the addition of nitrogen being called PAHNs. We theorize anion carbon chains can eventually become long enough to fold into fullerene C60,70 carbon shells, of various charge states. Based on laboratory data the fullerenes can trap incoming O+ magnetospheric ions that have relatively high energy collisions with the fullerenes and, once trapped, protect the oxygen atom from Titan's reducing thermosphere-ionosphere. The fullerenes can form into larger onion fullerenes and condense into larger embryo aerosols (i.e., m/q > 10,000 amu/q anions as observed by CAPS/ELS) eventually falling onto Titan's surface and precipitating to the bottom of its hydrocarbon lakes. Molecule production composed of H, C, N is known to occur in Titan's atmosphere with energy input from the magnetosphere, solar UV, and deep-penetrating irradiation from galactic cosmic rays (GCR). Space radiation effects by GCR irradiation of Titan's surface and lakes can lead to the manufacture of exobiological molecules with oxygen as the new ingredient. We have developed a model of galactic cosmic ray irradiation of Titan's atmosphere, surface, subsurface and bottoms of Titan lakes. GCR would provide further energy for processing of the aerosols into more complex organic forms such as tholins and precursor molecules for amino acids. A second process called hydrolysis then converts the precursor molecules into amino acids. Hydrolysis is provided via meteor impacts with size >10 km and cryovolcanism both which can produce liquid water on Titan's surface for episodic periods > several 100 to 1000 years. Our model shows that GCR secondary particles can penetrate ~ 100 m below the ice surface (including the bottom of Titan's less dense hydrocarbon lakes ~ 150 m depths) and produce chemically significant dosages over very long timescales ~ 450 Myrs. The GCR model is combined with laboratory data from experiments in which dry methyl ices were irradiated to doses producing prebiotic amino acids such as glycine. The model calculations show glycine can form to ~ 2.5 ppb levels near the surface after ~ 450 Myrs of GCR proton irradiation and potentially to 5 ppb if heavy-ion GCRs up through Fe are included. If such molecules were detected, this would not only confirm this model but indicate that life forms different from ours may not be required.

Species-dependent ion escape on Titan

Cassini observations over the past ten years have revealed that Titan possesses a chemically complex ionosphere. In this study, we investigate the relative contributions of different ion species to the total ion escape on Titan, by dividing all ion species probed by the Cassini Ion Neutral Mass Spectrometer (INMS) into six groups according to their mass-to-charge ratios (M/Z). For the three lightest ion groups, with characteristic M/Z of 22, 41, and 52 daltons , the observed scale heights tend to be lower than the scale heights predicted by assuming diffusive equilibrium; for the three heavier groups, observed and predicted scale heights are in general agreement, implying that most ion escape from Titan is by relatively light species, with M/Z < 60 daltons. A diffusion model is constructed to describe the density distribution of each ion group in regions where the effect of ionospheric chemistry could be neglected. The data model comparison predicts an optimal total ion escape rate of 3.1×1024 s–1, of which more than 99% is contributed by relatively light ions with M/Z < 32 daltons.

The Structure of Titan’s N2 and CH4 Coronae

Abstract In this study, we analyze the structures of Titan’s N2 and CH4 coronae using a large data set acquired by the Ion Neutral Mass Spectrometer (INMS) instrument on board Cassini. The N2 and CH4 densities measured from the exobase up to 2000 km imply a mean exobase temperature of 146 K and 143 K, respectively, which is lower than the mean upper atmospheric temperature by 4 and 7 K. This indicates that on average, Titan possesses a subthermal rather than suprathermal corona. A careful examination reveals that the variability in corona structure is not very likely to be solar driven. Within the framework of the collisionless kinetic model, we investigate how the CH4 energy distribution near the exobase could be constrained if strong CH4 escape occurs on Titan. Several functional forms for the CH4 energy distribution are attempted, assuming two representative CH4 escape rates of s−1 and s−1. We find that the double Maxwellian and power-law distributions can reproduce the shape of the CH4 corona structure as well as the imposed CH4 escape rate. In both cases, the escape rate is contributed by a suprathermal CH4 population on the high-energy tail, with a number fraction below 5% and a characteristic energy of 0.1–0.6 eV per suprathermal CH4 molecule. The coexistence of the subthermal CH4 corona revealed by the INMS data and substantial CH4 escape suggested by some previous works could be reconciled by a significant departure in the exobase CH4 energy distribution from ideal Maxwellian that enhances escape and causes a noticeable redistribution of the corona structure.

Enceladus Plume Structure and Time Variability: Comparison of Cassini Observations.

During three low-altitude (99, 66, 66 km) flybys through the Enceladus plume in 2010 and 2011, Cassini's ion neutral mass spectrometer (INMS) made its first high spatial resolution measurements of the plume's gas density and distribution, detecting in situ the individual gas jets within the broad plume. Since those flybys, more detailed Imaging Science Subsystem (ISS) imaging observations of the plume's icy component have been reported, which constrain the locations and orientations of the numerous gas/grain jets. In the present study, we used these ISS imaging results, together with ultraviolet imaging spectrograph stellar and solar occultation measurements and modeling of the three-dimensional structure of the vapor cloud, to constrain the magnitudes, velocities, and time variability of the plume gas sources from the INMS data. Our results confirm a mixture of both low and high Mach gas emission from Enceladus' surface tiger stripes, with gas accelerated as fast as Mach 10 before escaping the surface. The vapor source fluxes and jet intensities/densities vary dramatically and stochastically, up to a factor 10, both spatially along the tiger stripes and over time between flyby observations. This complex spatial variability and dynamics may result from time-variable tidal stress fields interacting with subsurface fissure geometry and tortuosity beyond detectability, including changing gas pathways to the surface, and fluid flow and boiling in response evolving lithostatic stress conditions. The total plume gas source has 30% uncertainty depending on the contributions assumed for adiabatic and nonadiabatic gas expansion/acceleration to the high Mach emission. The overall vapor plume source rate exhibits stochastic time variability up to a factor ∼5 between observations, reflecting that found in the individual gas sources/jets. Key Words: Cassini at Saturn—Geysers—Enceladus—Gas dynamics—Icy satellites. Astrobiology 17, 926–940.

Dayside ionosphere of Titan: Impact on calculated plasma densities due to variations in the model parameters

A one dimensional photochemical model for the dayside ionosphere of Titan has been developed for calculating the density profiles of ions and electrons under steady state photochemical equilibrium condition. We concentrated on the T40 flyby of Cassini orbiter and used the in-situ measurements from instruments onboard Cassini as input to the model. An energy deposition model is employed for calculating the attenuated photon flux and photoelectron flux at different altitudes in Titan’s ionosphere. We used the Analytical Yield Spectrum approach for calculating the photoelectron fluxes. Volume production rates of major primary ions, like, N2+, N+, CH4+, CH3+, etc due to photon and photoelectron impact are calculated and used as input to the model. The modeled profiles are compared with the Cassini Ion Neutral Mass Spectrometer (INMS) and Langmuir Probe (LP) measurements. The calculated electron density is higher than the observation by a factor of 2 to 3 around the peak. We studied the impact of different model parameters, viz. photoelectron flux, ion production rates, electron temperature, dissociative recombination rate coefficients, neutral densities of minor species, and solar flux on the calculated electron density to understand the possible reasons for this discrepancy. Recent studies have shown that there is an overestimation in the modeled photoelectron flux and N2+ ion production rates which may contribute towards this disagreement. But decreasing the photoelectron flux (by a factor of 3) and N2+ ion production rate (by a factor of 2) decreases the electron density only by 10 to 20%. Reduction in the measured electron temperature by a factor of 5 provides a good agreement between the modeled and observed electron density. The change in HCN and NH3 densities affects the calculated densities of the major ions (HCNH+, C2H5+, and CH5+); however the overall impact on electron density is not appreciable ( < 20%). Even though increasing the dissociative recombination rate coefficients of the ions C2H5+ and CH5+ by a factor of 10 reduces the difference between modeled and observed densities of the major ions, the modeled electron density is still higher than the observation by ∼ 60% at the peak. We suggest that there might be some unidentified chemical reactions that may account for the additional loss of plasma in Titan’s ionosphere.

Ion and aerosol precursor densities in Titan's ionosphere: A multi-instrument case study.

The importance of the heavy ions and dust grains for the chemistry and aerosol formation in Titan's ionosphere has been well established in the recent years of the Cassini mission. In this study we combine independent in situ plasma (Radio Plasma and Wave Science Langmuir Probe (RPWS/LP)) and particle (Cassini Plasma Science Electron Spectrometer, Cassini Plasma Science Ion Beam Spectrometer, and Ion and Neutral Mass Spectrometer) measurements of Titan's ionosphere for selected flybys (T16, T29, T40, and T56) to produce altitude profiles of mean ion masses including heavy ions and develop a Titan-specific method for detailed analysis of the RPWS/LP measurements (applicable to all flybys) to further constrain ion charge densities and produce the first empirical estimate of the average charge of negative ions and/or dust grains. Our results reveal the presence of an ion-ion (dusty) plasma below ~1100 km altitude, with charge densities exceeding the primary ionization peak densities by a factor ≥2 in the terminator and nightside ionosphere (n e /n i ≤ 0.1). We suggest that ion-ion (dusty) plasma may also be present in the dayside ionosphere below 900 km (n e /n i < 0.5 at 1000 km altitude). The average charge of the dust grains (≥1000 amu) is estimated to be between -2.5 and -1.5 elementary charges, increasing toward lower altitudes.

THE VARIABILITY OF HCN IN TITAN’S UPPER ATMOSPHERE AS IMPLIED BY THE CASSINI ION-NEUTRAL MASS SPECTROMETER MEASUREMENTS

ABSTRACT HCN is an important constituent in Titan’s upper atmosphere, serving as the main coolant in the local energy budget. In this study, we derive the HCN abundance at the altitude range of 960–1400 km, combining the Ion-Neutral Mass Spectrometer data acquired during a large number of Cassini flybys with Titan. Typically, the HCN abundance declines modestly with increasing altitude and flattens to a near constant level above 1200 km. The data reveal a tendency for dayside depletion of HCN, which is clearly visible below 1000 km but weakens with increasing altitude. Despite the absence of convincing anti-correlation between HCN volume mixing ratio and neutral temperature, we argue that the variability in HCN abundance makes an important contribution to the large temperature variability observed in Titan’s upper atmosphere.

Dione and Rhea seasonal exospheres revealed by Cassini CAPS and INMS

A Dione O2 and CO2 exosphere of similar composition and density to Rhea's is confirmed by Cassini spacecraft Ion Neutral Mass Spectrometer (INMS) flyby data. INMS results from three Dione and two Rhea flybys show exospheric spatial and temporal variability indicative of seasonal exospheres, modulated by winter polar gas adsorption and desorption at the equinoxes. Cassini Plasma Spectrometer (CAPS) pickup ion fluxes also show exospheric structure and evolution at Rhea consistent with INMS, after taking into consideration the anticipated charge exchange, electron impact, and photo-ionization rates. Data-model comparisons show the exospheric evolution to be consistent with polar frost diffusion into the surface regolith, which limits surface exposure and loss of the winter frost cap by sputtering. Implied O2 source rates of ∼45(7)×1021s−1 at Dione(Rhea) are ∼50(300) times less than expected from known O2 radiolysis yields from ion-irradiated pure water ice measured in the laboratory, ruling out secondary sputtering as a major exospheric contributor, and implying a nanometer scale surface refractory lag layer consisting of concentrated carbonaceous impurities. We estimate ∼30:1(2:1) relative O2:CO2 source rates at Dione(Rhea), consistent with a stoichiometric bulk composition below the lag layer of 0.01(0.13) C atoms per H2O molecule, deriving from endogenic constituents, implanted micrometeoritic organics, and (in particular at Dione) exogenous H2O delivery by E-ring grains. Impact deposition, gardening and vaporization may thereby control the global O2 source rates by fresh H2O ice exposure to surface radiolysis and trapped oxidant ejection.