Seasonal variation is significant in Titan's atmosphere owing to the large change of solar insolation resulting from Titan's 26.7° axial tilt relative to the plane of Saturn's orbit. Here we present an investigation of hydrocarbon and nitrile species in Titan's upper atmosphere at 400–1200 km, which includes the mesosphere and the lower thermosphere, over more than one-fourth of Titan's year (2006–2014, L S = 318°–60°), using 18 stellar occultation observations obtained by Cassini/Ultraviolet Imaging Spectrograph. Vertical profiles of eight chemical species (CH 4 , C 2 H 2 , C 2 H 4 , C 2 H 6 , C 4 H 2 , C 6 H 6 , HCN, HC 3 N) and haze particles are retrieved from these observations using an instrument forward model, which considers the technical issue of pointing motion. The Markov Chain Monte Carlo algorithm is used to obtain the posterior probability distributions of parameters in the retrieval, which inherently tests the extent to which species profiles can be constrained. The results show that no change of the species profiles is noticeable before the equinox, while the decrease of atmospheric temperature and significant upwelling in the summer hemisphere are found five terrestrial years afterward. Altitude of the detached haze layer decreases toward the vernal equinox and then disappears, and no reappearance is identified within the time range of our data, which is consistent with observations from Cassini/Imaging Science Subsystem. This study provides observational constraints on the seasonal change of Titan's upper atmosphere and suggests further investigations of the atmospheric chemistry and dynamics therein.

Abstract Cassini/Ultraviolet Imaging Spectrograph (UVIS) Far‐UV observations of stellar occultations at Titan are well suited for probing its atmospheric composition and structure. However, due to instrument pointing motion, only 5 out of tens of observations have been analyzed. We present an innovative retrieval method that corrects for the effect of pointing motion by forward modeling the Cassini/UVIS instrument response function with the pointing motion value obtained from the SPICE C‐kernel along the spectral dimension. To illustrate the methodology, an occultation observation made during flyby T52 is analyzed, when the Cassini spacecraft had insufficient attitude control. A high‐resolution stellar model and an instrument response simulator that includes the position of the point source on the detector are used for the analysis of the pointing motion. The Markov chain Monte Carlo method is used to retrieve the line‐of‐sight abundance profiles of eleven species (CH4, C2H2, C2H4, C2H6, HCN, C4H2, C6N2, C6H6, haze, HC3N, and C2N2) in the spectral vector fitting process. We obtain tight constraints on all of the species aside from C2H6, C2N2, and C6N2, for which we only retrieved upper limits. This is the first time that the T52 occultation was used to derive abundances of major hydrocarbon and nitrile species in Titan's upper and middle atmosphere, as pointing motion prohibited prior analysis. With this new method, nearly all of the occultations obtained over the entire Cassini mission could yield reliable profiles of atmospheric composition, allowing exploration of Titan's upper atmosphere over seasons, latitudes, and longitudes.

The state-specific predissociation rates of the c (3)Pi(u)(v,N,J) state by b (3)Sigma(+)(u) and the D (1)Pi(+)(u)(v,J) state by the B' (1)Sigma(+)(u) continuum of various isotopologues of molecular hydrogen have been calculated from accurate ab initio potential energy curves and electronic coupling matrix elements. Lifetimes and predissociation rates of the c (3)Pi(u)(v,N,J) and D (1)Pi(+)(u)(v,J) levels and accurate energies of the c (3)Pi(-)(u)(v,N) and D (1)Pi(-)(u)(v,J) levels of the isotopologues have been obtained. Significant isotope dependence of state specific predissociation rate has been found even after adjustment for Franck-Condon factors and reduced mass. The use of average electronic matrix elements of H-2 for other isotopologues underestimates the c (3)Pi(+)(u) - b (3)Sigma(+)(u) predissociation rates of the HD, HT, D-2, DT and T-2 molecules by similar to 12%, similar to 16%, similar to 30%, similar to 40%, similar to 52%, respectively, and the D (1)Pi(+)(u) - B' (1)Sigma(+)(u) rates of the HD, HT, D-2, DT and T2 molecules by similar to 10%, similar to 15%, similar to 26%, similar to 35% and similar to 45%, respectively. When compared at similar rotation, vibration and kinetic energies, the underestimation is nearly independent of the kinetic energy. The absolute value of the average electronic coupling matrix element increases with the reduced mass while that of the vibrational overlap integral decreases with the reduced mass. This accidental substantial cancellation in the D (1)Pi(+)(u) - B' (1)Sigma(+)(u) system is responsible for the experimental observation that the relative D (1)Pi(+)(u) predissociation rate of two isotopologues approximately equals the squared inverse reduced mass ratio. The origin and implications of the isotope dependence of the averaged electronic coupling matrix elements are discussed.

The state-specific predissociation rates of the c 3 Π u (v,N,J) state by b 3 Σ u + and the D 1 Π u + (v,J) state by the B ′ 1 Σ u + continuum of various isotopologues of molecular hydrogen have been calculated from accurate ab initio potential energy curves and electronic coupling matrix elements. Lifetimes and predissociation rates of the c 3 Π u (v,N,J) and D 1 Π u + (v,J) levels and accurate energies of the c 3 Π u − (v,N) and D 1 Π u − (v,J) levels of the isotopologues have been obtained. Significant isotope dependence of state specific predissociation rate has been found even after adjustment for Franck-Condon factors and reduced mass. The use of average electronic matrix elements of H2 for other isotopologues underestimates the c 3 Π u + − b 3 Σ u + predissociation rates of the HD, HT, D2, DT and T2 molecules by ∼12%, ∼16%, ∼30%, ∼40%, ∼52%, respectively, and the D 1 Π u + − B ′ 1 Σ u + rates of the HD, HT, D2, DT and T2 molecules by ∼10%, ∼15%, ∼26%, ∼35% and ∼45%, respectively. When compared at similar rotation, vibration and kinetic energies, the underestimation is nearly independent of the kinetic energy. The absolute value of the average electronic coupling matrix element increases with the reduced mass while that of the vibrational overlap integral decreases with the reduced mass. This accidental substantial cancellation in the D 1 Π u + − B ′ 1 Σ u + system is responsible for the experimental observation that the relative D 1 Π u + predissociation rate of two isotopologues approximately equals the squared inverse reduced mass ratio. The origin and implications of the isotope dependence of the averaged electronic coupling matrix elements are discussed.

The state-specific predissociation rates of the (v,N,J) state by and the (v,J) state by the continuum of various isotopologues of molecular hydrogen have been calculated from accurate ab initio potential energy curves and electronic coupling matrix elements. Lifetimes and predissociation rates of the (v,N,J) and (v,J) levels and accurate energies of the (v,N) and (v,J) levels of the isotopologues have been obtained. Significant isotope dependence of state specific predissociation rate has been found even after adjustment for Franck-Condon factors and reduced mass. The use of average electronic matrix elements of H2 for other isotopologues underestimates the − predissociation rates of the HD, HT, D2, DT and T2 molecules by ∼12%, ∼16%, ∼30%, ∼40%, ∼52%, respectively, and the − rates of the HD, HT, D2, DT and T2 molecules by ∼10%, ∼15%, ∼26%, ∼35% and ∼45%, respectively. When compared at similar rotation, vibration and kinetic energies, the underestimation is nearly independent of the kinetic energy. The absolute value of the average electronic coupling matrix element increases with the reduced mass while that of the vibrational overlap integral decreases with the reduced mass. This accidental substantial cancellation in the − system is responsible for the experimental observation that the relative predissociation rate of two isotopologues approximately equals the squared inverse reduced mass ratio. The origin and implications of the isotope dependence of the averaged electronic coupling matrix elements are discussed.

Transition probabilities of H-2, HD, and D-2 c (3)Pi(-)(u) -a (3)Sigma(+)(g) electric dipole, c (3)Pi(-)(u) -b (3)Sigma(+)(u) discrete-continuum magnetic dipole, and electric quadrupole transitions have been calculated using accurate energies and ro-vibrational wave functions obtained from precise ab initio potential energy curves. The predissociation rates of the c (3)Pi(-)(u) (v, N) levels by direct and indirect spin-spin and spin-orbit coupling between c (3)Pi(u) -b (3)Sigma(+)(u) fine structure levels, have been also determined. The present investigation achieved good agreement with measured lifetimes of the c (3)Pi(-)(u) fine structure levels without adjustment. A comparison of the calculated and observed lifetimes of metastable H-2, HD, and D-2 suggests that the c (3)Pi(-)(u) -b (3)Sigma(+)(u) magnetic dipole and electric quadrupole transition moments underestimate the spontaneous emission rate of the metastable levels by similar to 370 s(-1). The measured and calculated lifetimes of H-2, HD, and D-2 fine structure levels are in very good agreement after the adjustment of 370 s(-1) to the spontaneous decay rate of the c (3)Pi(-)(u) -b (3)Sigma(+)(u) transition. The calculated energies, transition probabilities, and predissociation rates obtained in the present work, along with the c (3)Pi(u) state excitation function, are sufficient to determine the c (3)Pi(u) state emission cross section, the kinetic energy distribution of H(1s) atoms, and the energy deposition rate of the (1)Sigma(+)(g) -c (3)Pi(u) excitation. In a previous investigation by Berg and Ottinger (1994 J. Chem. Phys. 100 8746), the authors were forced to insert a large scale factor into the predissociation rate in order to reconcile with measured lifetimes. Errors introduced in the approximations made in the previous investigations are discussed in the text. The H-2 c (3)Pi(u) state has the second largest triplet state excitation cross section. Predissociation and spontaneous emission of the c (3)Pi(u) state plays an important role in the energy deposition of H-2-dominated atmospheres.

The c (3)Pi(u) state of the hydrogen molecule has the second largest triplet-state excitation cross-section, and plays an important role in the heating of the upper thermospheres of outer planets by electron excitation. Precise energies of the H-2, D-2, and HD c (3)Pi(-)(u)(v, N) levels are calculated from highly accurate ab initio potential energy curves that include relativistic, radiative, and empirical non-adiabatic corrections. The emission yields are determined from predissociation rates and refined radiative transition probabilities. The excitation function and excitation cross-section of the c (3)Pi(u) state are extracted from previous theoretical calculations and experimental measurements. The emission cross-section is determined from the calculated emission yield and the extracted excitation cross-section. The kinetic energy (E-k) distributions of H atoms produced via the predissociation of the c (3)Pi(u) state, the c (3)Pi(-)(u)-b (3)Sigma(+)(u) dissociative emission by the magnetic dipole and electric quadrupole, and the c (3)Pi(u) - a (3)Sigma(+)(g) - b (3)Sigma(+)(u) cascade dissociative emission by the electric dipole are obtained. The predissociation of the c (3)Pi(+)(u) and c (3)Pi(-)(u) states both produce H(1s) atoms with an average E-k of similar to 4.1 eV/atom, while the c (3)Pi(-)(u)-b (3)Sigma(+)(u) dissociative emissions by the magnetic dipole and electric quadrupole give an average E-k of similar to 1.0 and similar to 0.8 eV/atom, respectively. The c (3)Pi(u) - a (3)Pi(+)(u) - b (3)Sigma(+)(u) cascade and dissociative emission gives an average E-k of similar to 1.3 eV/atom. On average, each H-2 excited to the c (3)Pi(u) state in an H-2-dominated atmosphere deposits similar to 7.1 eV into the atmosphere while each H-2 directly excited to the a (3)Sigma(+)(g) and d (3)Pi(u) states contribute similar to 2.3 and similar to 3.3 eV, respectively, to the atmosphere. The spectral distribution of the calculated continuum emission arising from the X (1)Sigma(+)(g) - c (3)Pi(u) excitation is significantly different from that of direct a (3)Sigma(+)(g) or d (3)Pi(u) excitations.

An occultation of ε Orionis by Enceladus' plume was observed with Enceladus at an orbital longitude near apoapsis in order to investigate whether water vapor flow is modulated diurnally, similar to ice particles. The occultation showed that the bulk water vapor emanating from Enceladus changes little with orbital position. The amount of gas in at least one supersonic jet increased significantly, implying that the increase in the number of particles lofted at apoapsis could be due to more gas coming from the supersonic jets and not the overall gas flux from the tiger stripe fissures that cross Enceladus' south polar region.

ABSTRACT Electron-impact excitation of H2 triplet states plays an important role in the heating of outer planet upper thermospheres. The state is the third ungerade triplet state, and the – emission is the largest cascade channel for the state. Accurate energies of the (v, J) levels are calculated from an ab initio potential energy curve. Radiative lifetimes of the (v, J) levels are obtained by an accurate evaluation of the – transition probabilities. The emission yields are determined from experimental lifetimes and calculated radiative lifetimes and are further verified by comparing experimental and synthetic – spectra at 20 eV impact energy. Spectral analysis revealed that multipolar components beyond the dipolar term are required to model the – excitation, and significant cascade excitation occurs at the (v = 0,1) levels. Kinetic energy (E k ) distributions of H atoms produced via predissociation of the state and the − − cascade dissociative emission are obtained. Predissociation of the state produces H atoms with an average E k of 2.3 ± 0.4 eV/atom, while the E k distribution of the − − channel is similar to that of the – − channel and produces H(1s) atoms with an average E k of 1.15 ± 0.05 eV/atom. On average, each H2 excited to the state in an H2-dominated atmosphere deposits 3.3 ± 0.4 eV into the atmosphere, while each H2 directly excited to the state gives 2.2–2.3 eV to the atmosphere. The spectral distribution of the calculated – continuum emission due to the – excitation is significantly different from that of direct excitation.

Deep extreme ultraviolet spectrograph exposures of the plasma sheet at the orbit of Europa, obtained in 2001 using the Cassini Ultraviolet Imaging Spectrograph experiment, have been analyzed to determine the state of the gas. The results are in basic agreement with earlier results, in particular with Voyager encounter measurements of electron density and temperature. Mass loading rates and lack of detectable neutrals in the plasma sheet, however, are in conflict with earlier determinations of atmospheric composition and density at Europa. A substantial fraction of the plasma species at the Europa orbit are long-lived sulfur ions originating at Io, with ∼25% derived from Europa. During the outward radial diffusion process to the Europa orbit, heat deposition forces a significant rise in plasma electron temperature and latitudinal size accompanied with conversion to higher order ions, a clear indication that mass loading from Europa is very low. Analysis of far ultraviolet spectra from exposures on Europa leads to the conclusion that earlier reported atmospheric measurements have been misinterpreted. The results in the present work are also in conflict with a report that energetic neutral particles imaged by the Cassini ion and neutral camera experiment originate at the Europa orbit. An interpretation of persistent energetic proton pitch angle distributions near the Europa orbit as an effect of a significant population of neutral gas is also in conflict with the results of the present work. The general conclusion drawn here is that Europa is geophysically far less active than inferred in previous research, with mass loading of the plasma sheet ⩽4.5 × 1025 atoms s−1 two orders of magnitude below earlier published calculations. Temporal variability in the region joining the Io and Europa orbits, based on the accumulated evidence, is forced by the response of the system to geophysical activity at Io. No evidence for the direct injection of H2O into the Europa atmosphere or from Europa into the magnetosphere system, as has been observed at Enceladus in the Saturn system, is obtained in the present investigation.

We present retrievals of Titan haze optical properties derived from Cassini UVIS stellar occultation observations of the upper atmosphere above 300 km. These measurements focus on the wavelength region in the far ultraviolet (FUV) between 1850 - 1900 A, where absorption by other hydrocarbon species is minimal. While this work does not uniquely estimate haze particle absorptivity and number density separately, we provide robust estimates of the combined eects of these two physical parameters over multiple Titan ybys. The results provide valuable constraints for models of Titan haze formation throughout this region of the upper atmosphere.