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Energetic oxygen ions are an important component of the plasma incident onto Titan's atmosphere. Therefore, we report measurements of electron capture and ionization collisions of N2 with incident O+ over the energy range 10-100 keV. Using time of flight coincidence counting techniques we also measured the collision-induced dissociation of N2 following ionization and electron capture. The electron capture and ionization cross sections were found to have comparable magnitudes. Capture collisions are dominated by non-dissociative processes with the dissociative processes providing contributions that are only slightly smaller. In contrast, ionization is entirely dominated by the dissociative processes. The energy distributions of the N+ and N atom fragments ejected by 20, 50, and 100 keV incident O+ projectiles have also been determined. These fragments carry considerable amounts of energy and if produce in the exobase region can readily escape. The cross sections measured here have been used with Cassini energetic ion and atmospheric density data to determine the ionization by and neutralization of energetic O+ penetrating Titan's N2 rich atmosphere. Neutralization by charge exchange is found not to occur efficiently above Titan's exobase, so energetic particles with large gyroradii penetrate the atmosphere primarily as ions. When the energetic O+ flux is large, we also show it is an important source of ionization and heating at depth into Titan's atmosphere and the fragments contribute to the net atmospheric loss rate.

The entire set of observations from the Galileo Ultraviolet Spectrometer (UVS) is analyzed to look for spectral trends across the surface of Callisto, and to probe the spectral shapes in the near-UV. At low resolution, the leading hemisphere is slightly redder than the trailing hemisphere at -->λ > 280 nm; this has been interpreted by past researchers to indicate the presence of SO2 on the leading hemisphere. Here we point out that such an absorption feature can be induced when ratioing hemispherical spectra. High-resolution observations are used to detect the presence of an absorption band at high southern latitudes, interpreted to be due to some organic species that is weathered away (carbonized) at lower latitudes. The presence of CO2 in the surface and in the atmosphere of Callisto and the dark nature of the surface suggest that carbon-based species are present across the surface associated with either endogenic or delivered organics. These organics experience chemical modification by UV radiation and are mixed into the regolith by meteoritic bombardment.

Measurements of electron capture and ionization of O2 molecules in collisions with H+ and O+ ions have been made over an energy range 10-100 keV. Cross sections for dissociative and nondissociative interactions have been separately determined using coincidence techniques. Nondissociative channels leading to O product formation are shown to be dominant for both the H+ and the O+ projectiles in the capture collisions and only for the H+ projectiles in the ionization collisions. Dissociative channels are dominant for ionizing collisions involving O+ projectiles. The energy distributions of the O+ fragment products from collisions involving H+ and O+ have also been measured for the first time using time-of-flight methods, and the results are compared with those from other related studies. These measurements have been used to describe the interaction of the energetic ions trapped in Jupiter's magnetosphere with the very thin oxygen atmosphere of the icy satellite Europa. It is shown that the ionization of oxygen molecules is dominated by charge exchange plus ion impact ionization processes rather than photoionization. In addition, dissociation is predominately induced through excitation of electrons into high-lying repulsive energy states (electronically) rather than arising from momentum transfer from knock-on collisions between colliding nuclei, which are the only processes included in current models. Future modeling will need to include both these processes.

Atmospheric loss induced by an incident plasma, often called atmospheric sputtering, can significantly alter the volatile inventories of solar system bodies. Based on the present atmospheric sputtering rate, the net loss of nitrogen from Titan in the last 4 Gyr was small, consistent with Titan retaining a component of its primordial atmosphere. However, atmospheric sputtering by the magnetospheric plasma ions and by pickup ions, even at present levels, would have caused the loss of a large, residual Titan-like atmosphere from Io and Europa and a significant fraction of such an atmosphere from Ganymede. At Callisto, higher magnetospheric plasma densities would have been required for the loss of such an atmosphere. Since higher plasma densities were probable in earlier epochs, the evolution of the volatile inventories of each of the Galilean satellites has been profoundly affected by the interaction of their atmospheres with the Jovian magnetospheric plasma.

Erosion of small grains or grain mantles is important in a number of astrophysical environments. Since energy deposition events produced by energetic ions can create a "hot" region in a grain, simple thermal spike models have been applied to estimate the sputtering efficiency. Here the results of molecular dynamics (MD) simulations are used to describe sputtering from the "heat spike" formed by a heavy cosmic-ray (CR) ion. These results are compared to extrapolations of laboratory sputtering data for frozen volatiles and to earlier models for CR ion-induced desorption. The CO and H2O desorption rates for thick mantles or for whole volatile grains are given and are found to disagree with results obtained using heat spike models. Further, the rates are an order of magnitude smaller and an order of magnitude larger, respectively, than the rates typically used for desorption of CO and H2O from a thin mantle on a 0.1 μm silicate core in a molecular cloud. The model given here can now be used for calculating energetic ion-induced desorption from grains in a variety of astrophysical environments and of other compositions and sizes.

The sputtering of a small grain embedded in a plasma or hot gas in the interstellar medium (ISM) is calculated using a Monte Carlo binary collision code. This technique is used to describe the transport of the atoms set in motion in the solid by an incident ion or atom and the sputtering from the surface of a grain induced by collisional cascades. The calculations, calibrated to laboratory data, describe the enhancement in the yield due to erosion from the edges and back surfaces by the incident particle and the energetic recoils. The angular dependence of the sputtering yield typically used, Y ~ (cos θ)-1, is shown not to be valid. The grain size effect manifests itself even when the grain radius is only ~3 times larger than the mean penetration depth of the incident particle. Sputtering of a grain in a single-fluid shock is used as an example to test the effect of grain size on the erosion rate. The enhancement in the sputtering rate due to grain size exacerbates the problem of small grain destruction, placing more severe constraints on grain growth in the ISM. The results presented are also relevant to the erosion of circumstellar and circumplanetary grains by energetic ions.

The remarkable observation that Enceladus, a small icy satellite of Saturn, is actively venting has led to the suggestion that ejected water molecules are the source of the toroidal atmosphere observed at Saturn for over a decade using the Hubble Space Telescope (HST). Here we show that the venting leads directly to a new feature, a narrow Enceladus neutral torus. The larger torus, observed using HST, is populated by charge exchange, the process that limits the lifetime of the neutrals in the Enceladus torus.

Radiation-inducedbubbleformation,aprocessknowntocausethedeteriorationofreactormaterials,produces O2/O3microatmospheres in the icy surface of Ganymede. Energetic ions in Jupiter’s magnetosphere bombard this surface, producing vacancies and radicals in the ice. At the equatorial temperatures on Ganymede, the radicalsmigrateandreactefficiently,formingnewvolatilespeciesH2andO2,andthevacanciesaggregatetoform voids.WhereastheH2islostreadily,theO2accumulates,permeatingtheregolith,producingaweakatmosphere, and becoming trapped in the voids, forming gas-filled bubbles. Such bubbles (microatmospheres) form efficiently above10.3‐0.5timesthemeltingtemperatureofthematerialandcanattainahighdensityofO2,consistentwith the observation of “condensed” O2on Ganymede. Dissociation of O2in a microatmosphere by UV photons or the incident ions leads to formation of O3. Including the O( 1 D) interactions in the Chapman equations, a ratio O3/O212 310 24 is obtained, close to the estimate based on observations of Ganymede. Subject headings: planets and satellites: individual (Ganymede, Europa)—molecular processes— radiation mechanisms: nonthermal

The one-dimensional steady-state problem of thermal escape from a single-component atmosphere of mon- and diatomic gases is studied in the hydrodynamic (blow-off) regime using the direct simulation Monte Carlo method for an evaporative-type condition at the lower boundary. The simulations are performed for various depths into an atmosphere, indicated by a Knudsen number, Kn 0, equal to the ratio of the mean free path of molecules to the radial position of the source surface, ranging from 10 to 10–5, and for the range of the source Jeans parameter, λ0, equal to the ratio of gravitational and thermal energies, specific to blow-off. The results of kinetic simulations are compared with the isentropic model (IM) and the Navier-Stokes model. It is shown that the IM can be simplified if formulated in terms of the local Mach number and Jeans parameter. The simulations predict that at Kn 0 < ~ 10–3 the flow includes a near-surface non-equilibrium Knudsen layer, a zone where the flow can be well approximated by the IM, and a rarefied far field. The corresponding IM solutions, however, only approach Parker's critical solution as λ0 approaches the upper limit for blow-off. The IM alone is not capable for predicting the flow and requires boundary conditions at the top of the Knudsen layer. For small Kn 0, the scaled escape rate and energy loss rate are found to be independent of λ0. The simulation results can be scaled to any single-component atmosphere exhibiting blow-off if the external heating above the lower boundary is negligible, in particular, to sublimation-driven atmospheres of Kuiper belt objects.

Thermally-driven atmospheric escape evolves from an organized outflow (hydrodynamic escape) to escape on a molecule by molecules basis (Jeans escape) with increasing Jeans parameter, the ratio of the gravitational to thermal energy of molecules in a planet's atmosphere. This transition is described here using the direct simulation Monte Carlo method for a single component spherically symmetric atmosphere. When the heating is predominantly below the lower boundary of the simulation region, R0, and well below the exobase, this transition is shown to occur over a surprisingly narrow range of Jeans parameters evaluated at R0: {\lambda}0 ~ 2-3. The Jeans parameter {\lambda}0 ~ 2.1 roughly corresponds to the upper limit for isentropic, supersonic outflow and for {\lambda}0 >3 escape occurs on a molecule by molecule basis. For {\lambda}0 > ~6, it is shown that the escape rate does not deviate significantly from the familiar Jeans rate evaluated at the nominal exobase, contrary to what has been suggested. Scaling by the Jeans parameter and the Knudsen number, escape calculations for Pluto and an early Earth's atmosphere are evaluated, and the results presented here can be applied to thermally-induced escape from a number of solar and extrasolar planetary bodies., Comment: 16 pages,5 figures

The removal and/or redistribution of SO2 frosts on the surface of the first Galilean satellite, Io, can occur through the erosion of these frosts by the magnetosphere particle environment of the satellite. The energy, species, and temperature dependence of the erosion rates of SO2 ice films by charged particles have been studied in laboratory experiments. Rutherford backscattering and thin film techniques are used in the experiments. The ice temperature is varied between about 10 K and the sublimation temperature. The erosion rates are found to have a temperature-independent and a temperature-dependent regime and to be much greater, for 10-2000 keV ions, than those predicted by the usual sputtering process. The laboratory results are used together with measured magnetosphere particle fluxes in the vicinity of Io to estimate the erosion rates of SO2 ice films from the satellite and implications therefrom on an SO2 atmosphere on Io.