Cassini Observations Research Articles

Modeling the Emission of Energetic Neutral Atoms in Titan's Dynamic Magnetospheric Environment

AbstractTo provide context for Cassini observations of energetic neutral atoms (ENAs) generated by charge exchange between Titan's atmosphere and energetic magnetospheric ions, we compute synthetic ENA images for a variety of uniform and draped electromagnetic field configurations, flyby scenarios, and viewing geometries. Starting with the idealized case of the magnetospheric field near Titan pointing southward, we calculate the observable ENA emission morphology for a large set of detector locations, each mimicking the properties of Cassini's Ion and Neutral Camera. Regardless of whether the electromagnetic fields near Titan are treated as uniform or draped, ENA production occurs within a thin shell of radial thickness 85 km, located directly above the moon's exobase. The observable ENA morphology is largely controlled by the angle between the detector's boresight vector and Saturn's magnetospheric field. Both the radius and the sense of energetic ion gyration carve distinct gaps in the ENA emission pattern. Field line draping around Titan leaves a discernible imprint in the ENA emissions only for a narrow range of viewing geometries. We also apply the model to study the ENA images taken during the T20 and T24 flybys. During both encounters, the ambient magnetospheric field displayed frequent transitions between Saturn's northern and southern magnetodisk lobes. We show that the notion of a steady‐state interaction between Titan and an averaged magnetospheric background field is not suitable to explain the observed ENA morphology. Instead, ambient field variations measured more than 1 hr before ENA image acquisition still leave an imprint in the emission pattern.

The D/H ratio in Titan’s acetylene from high spectral resolution IRTF/TEXES observations

We report observations of deuterated acetylene (C2HD) at 19.3μm (519 cm−1) with the Texas Echelon Cross Echelle Spectrograph on the NASA Infrared Telescope Facility in July 2017. Six individual lines from the Q-branch of the ν4 band were clearly detected with a S/N ratio up to 10. Spectral intervals around 8.0μm (745 cm−1) and 13.4μm (1247 cm−1) containing acetylene (C2H2) and methane (CH4) lines respectively, were observed during the same run to constrain the disk-averaged C2H2 abundance profile and temperature profile. Cassini observations with the Composite Infrared Spectrometer (CIRS) were used to improve the flux calibration and help to constrain the atmospheric model. The measured D/H ratio in acetylene, derived from the C2HD/C2H2 abundance ratio, is (1.22−0.21+0.27)× 10−4, consistent with that in methane obtained in previous studies. Possible sources of fractionation at different steps of the acetylene photochemistry are investigated.

Measurements of Titan’s Stratospheric Winds during the 2009 Equinox with the eSMA

Saturn’s moon Titan possesses stratospheric zonal winds that places it among a sparse class of planetary bodies known to have superrotation in their atmospheres. Few measurements have been made of these speeds in the upper stratosphere, leaving their seasonal variations still not well understood. We examined observations made with the extended Submillimeter Array in 2009 March (L s = 355°) and 2010 February (L s = 5°), shortly before and after Titan's northern spring equinox. Cassini observations and atmospheric models find equinoctial periods to be especially dynamic. Zonal wind calculations, derived from the Doppler frequency shift of CH3CN near 349.4 GHz, yielded speeds of 128 ± 27 m s−1 in 2009 and 209 ± 48 m s−1 in 2010. We estimated the measured emission to originate from vertical altitudes of 336−88+112 km, equivalent to pressures of 3.8−3.4+19.2 Pa, commensurate with Titan’s upper stratosphere/lower mesosphere. This suggests a possible increase in zonal speeds during this period. The results are then compared to those from previous Cassini-inferred and direct-interferometric observations of winds, as well as general circulation model simulations, to form a more complete picture of the seasonal cycle of stratospheric zonal winds.

Effect of Intermittent Structures on Electron Heating in Saturn's Magnetosphere: Cassini Observations

AbstractIn Saturn's magnetosphere, the plasma temperature increases with radial distance, requiring a heating mechanism to counteract the adiabatic cooling effect of expanding plasma. To explore potential heating source, we perform a statistical study about intermittent structures and intermittent heating in Saturn's magnetosphere based on the observations from the Cassini spacecraft. Partial Variance of Increments (PVI) technique is used to measure the intermittency of magnetic field and identify the intermittent structures. It is found that the electron temperature has a rising trend as the increase of magnetic field intermittency, implying the occurrence of electron heating in the intermittent structures. Additionally, the turbulence heating rate also exhibits an increasing trend as the increase of probability density of intermittent structures. Our results evidence the effect of intermittent heating in the Saturn's magnetosphere, and suggest that intermittent structures with high magnetic field intermittency play an important role in turbulence heating.

Giant Planet Lightning in Nonideal Gases

Lightning has been directly observed or inferred on all giant planets, generally accepted to be occurring in their water clouds. However, much as Earth has both cloud–cloud and cloud–ground lightning, this does not mean all flashes occur in a narrow altitude range: on Jupiter, the Galileo spacecraft detected lightning flashes apparently below the cloud base, explicable as lightning due to precipitation, and the Juno SRU detected small flashes far above it, at pressures of only 1–2 bars. We use a computationally light 1D entraining plume model, incorporating particle growth and noninductive charging, which predicts this wide range of Jovian lightning provides freezing point depressions caused by ammonia, and modify it to use a van der Waals equation of state instead of an ideal gas, as well as integrating the evaporation of rain; this allows modeling of planets colder than Jupiter, where clouds and lightning occur at greater pressures. For Saturn, the uppermost lightning is predicted at 3–4 bars; unlike on Jupiter, ammonia is not required to match the Cassini observations. For Uranus and Neptune, depending on their convective structures, very high rates of lightning are possible in the deep water clouds; while deeper than on Jupiter or Saturn, lightning is predicted likely to peak above the water cloud base, at pressures around 100 bars. Voyager 2's radio observations of Uranian and Neptunian sferics may thus be either due to attenuation of deep water lightning of this type, or due to lightning in the shallow ammonia clouds; future observations are required to resolve this dichotomy.

Constraints on the initial mass, age and lifetime of Saturn’s rings from viscous evolutions that include pollution and transport due to micrometeoroid bombardment

The Cassini spacecraft provided key measurements during its more than twelve year mission that constrain the absolute age of Saturn’s rings. These include the extrinsic micrometeoroid flux at Saturn, the volume fraction of non-icy pollutants in the rings, and a measurement of the ring mass. These observations taken together limit the ring exposure age to be ≲ a few 100 Myr if the flux was persistent over that time (Kempf et al., 2022). In addition, Cassini observations during the Grand Finale further indicate the rings are losing mass (Hsu et al., 2018; Waite et al., 2018) suggesting the rings are ephemeral as well. In a companion paper (Durisen and Estrada, 2022), we show that the effects of micrometeoroid bombardment and ballistic transport of their impact ejecta can account for these loss rates for reasonable parameter choices. In this paper, we conduct numerical simulations of an evolving ring in a systematic way in order to determine initial conditions that are consistent with these observations.We begin by revisiting the ancient massive ring scenario of Salmon et al. (2010). Here, we model not just the viscous evolution, but we subject the ring to pollution by micrometeoroid bombardment over the age of the Solar System. We find that regardless of initial mass, the ring always ends up with more pollutant than is currently observed, because the ring spends the majority of its lifetime at relatively low mass where it is most susceptible to darkening. We then show that models with initial disk masses of ∼1–3 Mimas masses reach volume fractions of pollutant consistent with the observed volume fractions of non-icy material in the A and B rings within a time scale of ∼ a few 100 Myr.Finally, we use the analysis of Durisen and Estrada (2022) to add the dynamical effects of meteoroid bombardment into the evolution equations, namely, mass loading and ballistic transport. The treatment of mass loading is exact, while ballistic transport is handled in an approximate way. Simulations show that: (1) mass loading and ballistic transport applied to an initially high optical depth annulus inevitably produce a lower density C ring analog interior to the annulus; and (2) high density rings subject to persistent micrometeoroid bombardment do not have an asymptotic mass but instead have an asymptotic lifetime much shorter than the age of the Solar System. This is because micrometeoroid bombardment and ballistic transport drive the dynamical evolution of the ring once viscosity weakens, indicating that the exposure age of the rings and their dynamical age are connected.

Numerical simulations of heat exchange and vapor flow in ice fractures on Enceladus

Cassini spacecraft observed plume of water vapor and ice particles above the south polar terrain (SPT) of Saturn's moon Enceladus in 2005. The area of the SPT is marked by four prominent linear depressions in the ice dubbed “tigers stripes” (TS) that appear to be the source of the plume. The area around TS is also the source of anomalously high heat flux. The current consensus based on analysis of Cassini observations and modeling suggests that TS are fractures in the ice that penetrate to warm sub-ice ocean. Escaping vapor laden with ice particles forms the visible plume and latent heat of condensing vapor serves as the source of the surface thermal anomaly. In the present work previously developed numerical model of vapor flow and heat exchange inside TS fractures is refined and used to explain Cassini observations. Model improvements include consideration of a possible snow layer on the surface and inclusion of surface ice sublimation into energy balance model. Results of the simulations with improved model suggest that Cassini observations of vapor mass flow and endogenic heat flux can be explained if the width of the ice fractures is in the range 0.05–0.1 m, similar to previously published results. Wider fractures are possible if only a small portion of TS total length produces vapor plume, but the whole ~500 km length of TS fractures still contributes to the surface thermal anomaly. Alternatively, wider fractures are also possible in fractures with high effective friction due to wall roughness or channel tortuosity. Simulated heat output of fractures with different widths and depths in the range of 1.5–3 km is ~2.5–3.0 GW and is only weakly dependent on fracture width. A 2-m-thick surface snow layer covering the fracture can significantly lower outgoing heat flux to 0.9–1.9 GW and reduce surface temperatures by 20–40 K. The total heat flow from different fractures (including conduction from the water-filled portion) is remarkably similar - it is ~3.3 GW and 2.3 GW for fractures without and with a surface snow layer, respectively. The simulated heat flow is lower than the observed heat flow, suggesting that each TS depression may host 2 to 6 narrow ice fractures. The long-standing puzzle of the large difference between the observed heat flow from near TS (~4.2 GW) and from the whole SPT (~15.8 GW) is explained by conduction of ~11 GW of heat from the ocean through the ~5 km-thick ice shell underlying the SPT. Predicted high surface temperatures near fracture exit result in high rates of ice sublimation, which may produce a secondary source of water vapor feeding the observed plume. Fracture outlet likely evolves into a cone-like cross-section widening towards the surface on timescales from 1 to 100 years.

Excitation of Saturnian ECH Waves Within Remote Plasma Injections: Cassini Observations

AbstractBased on Cassini observations, we report representative electrostatic electron cyclotron harmonic (ECH) wave events observed in Saturn's magnetosphere within remote plasma injections. Unlike local injections, remote injections are “older” injection events that have evolved to form a dispersed signature in particle energy spectrum. We show that Saturnian ECH waves within remote injections present a strong fundamental band and much weaker high harmonic bands. By calculating the linear wave growth rates based on the measured electron velocity distributions, we indicate that Saturnian ECH waves can be excited by the loss cone distribution of remotely injected, hot electrons of ∼100 eV to several keV. We find that ECH waves tend to intensify with increased fluxes of injected hot electrons but weakening with increased evolution time of the flux tubes, which is consistent with the results of wave growth rates and improves the current understanding of the generation of ECH waves at Saturn.

On the potential variation of charged dust in Saturn's magnetosphere: The dust-plasma interaction

We presented a theory describing interaction between cold plasma and dust grain based on Cassini observations of Enceladus in Saturn's E ring. We assumed a finite-sized dust cloud with inside plasma to simulate the environment. The dust cloud that results from this scenario has a different potential than the surrounding plasma. Cassini's observations revealed the cold plasma characteristics of the Enceladus plume, which include a high plasma density for both electrons and ions at sufficient low temperature. Therefore, the electron fluid is incorporated in the cloud to form a quantum electron gas governed through Fermi-Dirac statistics. According to observations, more electrons bound to the dust grain results in a collective dust plasma interaction with surrounding cold plasma. Dust-plasma interaction predominantly occurred at negative sub-micrometer sized dust grain. • Using Cassini observations of Enceladus in Saturn's E ring as a groundwork, a theory is presented in this article. • The Enceladus plume has cold plasma characteristics. We considered ions as Boltzmannian distributed particles and Fermi-Dirac distribution is applicable for electrons. • We derived relations for the potential variation in the Enceladus plume region and the E-rings. We found that more electrons bound to the dust grain results in a collective dust plasma interaction. • Analytical analysis and the numerical results form the expressions for the given environment is presented.

Influence of observed seasonally varying composition on Titan’s stratospheric circulation

Titan’s atmosphere exhibits variations in composition as it progresses through its seasons. Past observations have shown a substantial enrichment in short-lived molecules in the winter stratosphere above 100 km. Seasonal variations in Titan’s stratospheric dynamics also lead to a transient detached haze layer (DHL) above 400 km. The seasonal variations in aerosol opacity and molecular abundance lead to varying radiative heating rates in both the shortwave and longwave spectral regions. In this paper, we report on the effects of a new dataset for aerosol opacity and trace gas abundance derived from Cassini observations on simulations of Titan’s stratospheric dynamics with the Titan Atmospheric Model (TAM). We find that including seasonally varying radiative species (SVRS) decreases the autumn and winter polar stratopause temperature by up to 10 K poleward of 60°. We also find a similar increased seasonality in the zonal winds with the early autumn polar jet strengthening by 60 m s−1, while the mid winter jet is unaffected. While including the observationally derived SVRS dataset increases the strength of the seasonal variations of the polar jet, it does not substantially affect the timing of the onset or dissipation of the jet or other seasonal phenomena.

Statistics of Water-group Band Ion Cyclotron Waves in Saturn's Inner Magnetosphere Based on 13 yr of Cassini Measurements

Based on Cassini observations from 2004 to 2016, we perform a comprehensive analysis of the statistical distribution of the occurrence rate, averaged amplitude, wave normal angle (WNA), ellipticity, and power spectral intensity of ion cyclotron waves in Saturn's inner magnetosphere. Our results show that ion cyclotron waves mainly occur between the orbits of Enceladus and Dione near the equatorial region (∣λ∣ < 20°), with higher occurrence rates in the northern hemisphere than the southern hemisphere. The averaged wave amplitudes vary between 0.1 and 2 nT with a strong day–night asymmetry and a pronounced minimum at the equator. Saturnian ion cyclotron waves are predominantly left-handed polarized with small WNAs near the equator and become linearly polarized with larger WNAs at higher latitudes. The major wave power occurs frequently at frequencies of 0.5–1.2 , where is the equatorial gyrofrequency of H2O+ ions, with the strongest intensity (>∼10 nT2 Hz−1) at L ∼ 6.5 statistically present in the midnight sector.

Energy Exchanges in Saturn's Polar Regions From Cassini Observations: Eddy-Zonal Flow Interactions.

Saturn's polar regions (polewards of ∼63° planetocentric latitude) are strongly dynamically active with zonal jets, polar cyclones and the intriguing north polar hexagon (NPH) wave. Here we analyze measurements of horizontal winds, previously obtained from Cassini images by Antuñano et al. (2015), https://doi.org/10.1002/2014je004709, to determine the spatial and spectral exchanges of kinetic energy (KE) between zonal mean zonal jets and nonaxisymmetric eddies in Saturn's polar regions. Eddies of most resolved scales generally feed KE into the eastward and westward zonal mean jets at rates between 4.3 × 10−5 and 1.4 × 10−4 W kg−1. In particular, the north polar jet (at 76°N) was being energized at a rate of ∼10−4 W kg−1, dominated by the contribution due to the zonal wavenumber m = 6 NPH wave itself. This implies that the hexagon was not being driven at this time through a barotropic instability of the north polar jet, but may suggest a significant role for baroclinic instabilities, convection or other internal energy sources for this feature. The south polar zonal mean jet KE was also being sustained by eddies in that latitude band across a wide range of m. In contrast, results indicate that the north polar vortex may have been weakly barotropically unstable at this time with eddies of low m gaining KE at the expense of the axisymmetric cyclone. However, the southern axisymmetric polar cyclone was gaining KE from non‐axisymmetric components at this time, including m = 2 and its harmonics, as the elliptical distortion of the vortex may have been decaying.

Reflection and Refraction of the L-O Mode 5kHz Saturn Narrowband Emission by the Magnetosheath.

The reflection‐by‐sheath mechanism of 5 kHz narrowband emissions (NB) at Saturn is confirmed by Cassini observations during several crossings of the magnetopause, which show that the 5 kHz NB can be prevented from escaping Saturn's magnetosphere. The L‐O mode 5 kHz NB remained visible in areas of low plasma density but disappeared in regions of high plasma density. In three cases, NB disappeared immediately after the crossings of Saturn's magnetopause. A possible reflected NB event observed near the magnetosheath is discussed. This mechanism can help explain the 5 kHz NB observed at low latitudes outside the Enceladus plasma torus and their upper frequency limit variations. This mechanism significantly improves the current understanding of the 5 kHz NB.

Polar and mid-latitude vortices and zonal flows on Jupiter and Saturn

Zonal flow on Jupiter and Saturn consists of equatorial super-rotation and alternating East–West jet streams at higher latitudes. Interacting with these zonal flows, numerous vortices occur with various sizes and lifetimes. The Juno mission and Cassini’s grand finale have shown that the zonal jets of Jupiter and Saturn extend deeply into their molecular envelopes. On Jupiter, the vast majority of low and mid-latitude jovian vortices are anticyclonic, whereas cyclones appear at polar latitudes. Cassini mission observations revealed a similar pattern on Saturn; its North and South polar vortices are cyclonic, whereas anticyclones occur at mid-latitudes. We use the recently developed code Rayleigh to run high-resolution simulations of rotating convection in 3D spherical shells. Four models are presented that result in dynamical flows that are comparable to those on the giant planets. We confirm previous results, finding that deep convective turbulence can explain the structure of jets. However, the latitude and the strength and depth of stable stratification can modify jet morphologies and affect the formation and dynamics of vortices. Lower latitudes favour shallow anticyclonic vortices that form due to upward and divergent flow near the outer boundary. These anticyclones are typically shielded by cyclonic filaments associated with downwelling return flow. In contrast, a single polar cyclone, or clusters of cyclones form near the poles. All of our simulations have this global pattern; a strong preference for shallow anticyclones in the first anticyclonic shear zone away from the equatorial jet (corresponding to the region of the Great Red Spot on Jupiter and Storm Alley on Saturn), cyclonic and anticyclonic vortices at higher mid-latitudes, and a deeply seated cyclone or cyclone clusters at each pole. Our results show that Juno and Cassini observations of cloud-level flow can be explained in terms of deep convective dynamics in the molecular envelopes of Jupiter and Saturn.

Magnetic Flux Circulation in the Saturnian Magnetosphere as Constrained by Cassini Observations in the Inner Magnetosphere

AbstractIn steady state, magnetic flux conservation must be maintained in Saturn’s magnetosphere. The Enceladus plumes add mass to magnetic flux tubes in the inner magnetosphere, and centrifugal force pulls the mass‐loaded flux tubes outward. Those flux tubes are carried outward to the magnetotail where they deposit their mass and return to the mass loading region. It may take days for the magnetic flux to be carried outward to the tail, but the return of the nearly empty flux tubes can last only several hours, with speeds of inward motion around 200 km/s. Using time sequences of Cassini particle count rate, the difference in curvature drift and gradient drift is accounted for to determine the return speed, age, and starting dipole L‐shell of return flux tubes. Determination of this flux‐return process improves our understanding of the magnetic flux circulation at Saturn and provides insight into how other giant planets remove the mass added by their moons.

Global Spatial Distribution of Dipolarization Fronts in the Saturn's Magnetosphere: Cassini Observations

AbstractDipolarization front (DF), characterized by a sharp increase of the south‐north component of the magnetic field, is suggested to play an important role in transferring plasmas, magnetic fluxes, and energy in the planetary magnetosphere. Using the measurements from the Cassini spacecraft between January 1, 2005 and June 15, 2011, we successfully selected 96 DF events, and obtained the global spatial distribution of DFs in the Saturn's magnetosphere. For the first time, we found that DFs are distributed not only in the nightside magnetotail but also in the dayside magnetosphere. The dayside DF events are mainly located from X = 10 RS to X = 30 RS (RS is the Saturn's radius) while the nightside events have a wide range, up to X∼−50 RS. Moreover, the DFs are observed to be asymmetric in the south‐northern hemisphere: ∼70% of events in the northern hemisphere and ∼30% of events in the southern hemisphere, which is likely to be due to the asymmetric orbit coverage of the Cassini in south‐northern hemisphere. The occurrence of dayside DFs provides a strong evidence that the magnetic reconnection could also occur in the dayside of Saturn's magnetosphere. Thus, our results concerning the location and distribution of DFs are helpful for the study of the site of magnetic reconnection and energy transport/dissipation in Saturn's magnetosphere.

The 3D Structure of Saturn Magnetospheric Neutral Tori Produced by the Enceladus Plumes

AbstractThis research provides updated and validated spatial distributions of Enceladus‐generated H2O, OH, and O neutral tori. These tori are produced by the primary neutral source, Enceladus, and consist of populations of coorbiting neutral particles that provide the bulk of heavy molecules to Saturn's (neutral particle dominant) magnetosphere. We conducted self‐consistent neutral particle and plasma modeling that is constrained and validated by all relevant Cassini and Hubble Space Telescope observations. Our results include a three dimensional (3D) spatial distribution of all three neutral tori. Significant findings include (1) short‐term plume variability combines to produce an average H2O source rate of ∼280 kg/s; (2) Enceladus plume variability does not impact the neutral tori distribution because it occurs on time scales less than the orbital periodicity (&lt;17 h); (3) H2O source torus particles quickly dissociate in &lt;65 days while only ∼22% are directly ionized; (4) the resulting OH torus dissociates in &lt;40 days and only ∼11% are directly ionized; (5) ∼59% of the significantly dispersed O torus is directly ionized; and (6) the near plume environment is very important and relatively small changes (&lt;0.5 eV) in the core electron population within the plume region dramatically change the H3O+ abundance.

Selection and Characteristics of the Dragonfly Landing Site near Selk Crater, Titan

Abstract The factors contributing to the initial selection of a dune site near the Selk impact structure on Titan as the first landing site for the Dragonfly mission are described. These include arrival geometry and aerodynamic/aerothermodynamic considerations, illumination, and Earth visibility, as well as the likely presence of exposed deposits of water-rich material, potentially including materials where molten ice has interacted with organics. Cassini observations of Selk are summarized and interpreted: near-infrared reflectance and microwave emission data indicate water-rich materials in and around the crater. Radar topography data shows the rim of Selk to have slopes on multi-km scales reaching only ∼2° degrees, an order of magnitude shallower than early photoclinometric estimates.

HCN Production in Titan’s Atmosphere: Coupling Quantum Chemistry and Disequilibrium Atmospheric Modeling

Abstract Hydrogen cyanide (HCN) is a critical reactive source of nitrogen for building key biomolecules relevant for the origin of life. Still, many HCN reactions remain uncharacterized by experiments and theory, and the complete picture of HCN production in planetary atmospheres is not fully understood. To improve this situation, we develop a novel technique making use of computational quantum chemistry, experimental data, and atmospheric numerical simulations. First, we use quantum chemistry simulations to explore the entire field of possible reactions for a list of primary species in N2-, CH4-, and H2-dominated atmospheres. In this process, we discover 33 new reactions with no previously known rate coefficients. From here, we develop a consistent reduced atmospheric hybrid chemical network (CRAHCN) containing experimental values when available and our calculated rate coefficients otherwise. Next, we couple CRAHCN to a 1D chemical kinetic model (ChemKM) to compute the HCN abundance as a function of atmospheric depth on Titan. Our simulated atmospheric HCN profile agrees very well with the Cassini observations. CRAHCN contains 104 reactions; however, nearly all of the simulated atmospheric HCN profile can be obtained using a scaled-down network of only 19 dominant reactions. From here, we form a complete picture of HCN chemistry in Titan’s atmosphere, from the dissociation of the main atmospheric species, down to the direct production of HCN along four major channels. One of these channels was first discovered and characterized in Pearce et al. and this work.

Nondetection of Radio Emissions From Titan Lightning by Cassini RPWS

AbstractThe Saturn‐orbiting Cassini spacecraft completed 126 close Titan flybys from 2004 until 2017. During almost all of them the Cassini Radio and Plasma Wave Science (RPWS) instrument was turned on to search for radio emissions attributed to Titan lightning. Here we report about their nondetection after close inspection of all Titan flybys throughout the Cassini mission. We also infer new and strong constraints on the permissible flash energy and flash rate of potential Titan lightning. The nondetection of lightning flashes by Cassini observations implies that any lightning on Titan must be either very weak, very rare, or does not exist at all, and the latter could be due to cloud electric fields being too low to initiate a discharge. This finding holds important implications for the prebiotic chemistry of Titan and also implies that lightning will not be a significant hazard to the upcoming Dragonfly mission.