Direct Simulation Monte Carlo (DSMC) calculations of acoustic gravity wave propagation into the exobase region of a Mars-like atmosphere reveal that radial geometry can reduce wave-driven heating compared to a Cartesian model. We examine two acoustic wave (AW) modes with periods of 11 min (AW1) and 5.5 min (AW2) propagating from 100 to 320 km altitude using a radial molecular kinetics model. The wave-driven heating was reduced by 40-56 % with cycle-averaged temperature gradient (dT/dr) decreasing from 9.4 K per scale height H0 to 5.6 K/H0 for AW1 and from 4.4 K/H0 to 1.9 K/H0 for AW2 when accounting for planetary curvature. While the growth in wave density amplitude was attenuated for the 1D radial geometry as well, the heating differences are more pronounced, with both effects driven by geometric spreading accumulating as waves propagate into increasingly rarefied regions. These findings suggest that accounting for curvature effects is crucial when conducting DSMC estimates of acoustic wave contributions to thermospheric heating and atmospheric escape, as Cartesian-based derived counterparts may be overestimated by factors of 1.7-2.3 for these frequencies.

We describe the physical processes that affect the formation, trapping, and outgassing of O2 at Europa and Ganymede. Following Voyager measurements of their ambient magnetospheric plasmas, laboratory data indicated that the observed ions, which were mostly ejected from volcanic Io, would in turn impact and sputter their surfaces. This would decompose the ice and produce thin oxygen atmospheres. More than a decade later, Europa's O2 atmosphere was inferred from observations of the O aurora, and "condensed" O2 bands were observed in Ganymede's icy surface at 5773 and 6275 Å. More than another decade later, their atmospheres were shown to have a dusk/dawn enhancement, confirmed by recent Juno data. Although the incident plasma produces these observables, processes that occur within the topmost surface are still not well understood. Here, we note that the incident plasma particles produce a nonequilibrium defect density locally in the surface ice grains. Defect diffusion within these grains leads to the formation of voids and molecular products, some of which are volatile. Although some volatiles are released into the satellite atmospheres, others are trapped at defect sites or trapped in voids, which create gas bubbles whose lifetimes (in steady state) are limited by the plasma-induced destruction rate. Here, we discuss how trapping competes with the annealing of the radiation damage. We describe the differences observed at Europa and Ganymede and roughly determine the observed trend with latitude of O2 bands observed on Ganymede's trailing hemisphere. This understanding is used to discuss the relative importance of "condensed" O2 and O2 adsorbed on regolith grains as atmospheric sources, accounting for dusk/dawn enhancements and temporal variability reported in "condensed" O2 band depths. Since plasma-induced damage and thermal annealing timescales drive oxidant variability on icy moons (likely also Callisto, Dione, and Rhea), they can help determine volatile downwelling, a potentially metabolic source for their oceans, and upwelling of other trapped oxidants (e.g., CO2), suggestive of ongoing geologic activity.

We examine the transition from diffusion-limited to drag-off escape using a direct simulation Monte Carlo (DSMC) model developed to simulate planetary atmospheres, referred to as Harrah. A 1D spherically symmetric DSMC model is used to simulate two-component atmospheres to focus on the impact of rarefaction on thermal diffusion. The results for the escape rates and thermal structure are characterized using a mass-averaged Jeans parameter, λ _avg,0 , and Knudsen number, Kn _a,0 , defined at a reference radial distance r _0 . As a test of the model, we simulated the escape of N _2 and H _2 self-consistently for conditions of Pluto’s upper atmosphere observed during the New Horizons mission. As expected, the DSMC result for H _2 escape is consistent with the diffusion-limited approximation, and H _2 escape has little effect on the background N _2 atmosphere. Then we examined the transition from diffusion-limited to drag-off escape for two-component atmospheres characterized by mass-averaged Knudsen numbers and Jeans parameters of Kn _a,0 ≈ 0.01–0.1 and λ _avg,0 ≈ 0.5–13, respectively. We found that escape transitioned from hydrodynamic escape to an enhanced Jeans-like escape at λ _avg,0 ≈ 3.0–3.6. For values of λ _avg,0 < 3.0–3.6, escape was hydrodynamic and in the blow-off regime. For values of λ _avg,0 >≈ 3.6, the DSMC results indicated that using the diffusion-limited and drag-off approximations can lead to inaccurate estimates of the escape rate.

Fractures and vents in the ice crust of Europa, exposing the sub-surface ocean or liquid-water inclusions to the vacuum, might be responsible for the generation of water-vapor plumes. During its passage through the ice, the plume vapor is expected to partially condense on the cold ice walls. Together with other effects (water spillage, compression forces, etc.) this mechanism likely contributes to sealing the vent. In this work, we develop a simple lumped-parameter model that can quantify how quickly a hypothetical vent of prescribed width would be sealed via water-vapor deposition. As an example, we apply our model to the vent size and density conditions inferred from the 2012 Hubble Space Telescope plume detection, predicting a sealing time of about 30 min. This suggests that the actual ice fracture might have been larger than originally proposed and/or the plume density at the vent might have been lower. While many other effects could have been present and responsible for sealing the vent, our estimates indicate that vapor deposition might have played a major role in eventually shutting off the observed plume. A map of sealing times vs. plume density, mass flow rate and aperture areas is given. Plume quantities from the literature are analyzed and compared to our results. For a given plume density/mass flow rate, small apertures would be sealed quickly by vapor deposition and are thus incompatible with observations.

Following spacecraft encounters with comets 67P/C-G and 1P/Halley, it was surprising that O2, expected to be a very minor species in their comas, was observed to outgas at a few percent abundance during their ice sublimation phases. This challenged the direct connection suggested between comets and material in the interstellar medium (ISM), which exhibits a very low O2/H2O gas-phase abundance, leading to a number of papers suggesting novel sources for O2. Since these eccentrically orbiting comets have lost significant amounts of their evaporating surfaces over their lifetimes, the O2 observed must have been stably trapped down to significant depths in these primordial icy bodies. O2 was seen in the coma by Rosetta, along with other volatiles, long after water ice sublimation began to subside. Here we note that the extensive observations of the icy satellites of Jupiter (Europa, Ganymede, and Callisto) exhibit radiolytic and outgassing processes that provide certain direct parallels to interpretations of recent comet observations. Given that O2 is regularly observed in the atmospheres of icy Jovian satellites, as well as stably trapped as ‘bubbles’ (Johnson and Jesser, 1997) in their water ice surfaces, their spectral observations can help constrain the environment in which Jupiter-family and Oort cloud comets formed given that the observed O2/H2O abundances at both types of comets and icy moons are nearly identical. Based on the approximate charged particle radiation required to produce the observed steady-state concentrations of O2, we suggest that comets likely formed in a far more energetic environment than the ISM. While grains can be irradiated for longer timescales in the neutral ISM, small grains are expected to evaporate before significant O2 formation and trapping occurs. Independent of celestial dynamics then, an unknown radiation source, may provide insight to the first population of oxidized water ice grains in the early solar system.

The parameter space for the very uncertain composition of sublimated H2O and its photochemical products H and H2 in Callisto's atmosphere is examined using the Direct Simulaton Monte Carlo (DSMC) method.We focus on two significantly different versions of H2O production in which:(1) the ice and dark, non-ice/ice-poor material are intimately mixed and H2O sublimates at Callisto's warm day-side temperatures (e.g., as in most atmospheric modeling efforts at Callisto to date [1-4]); and(2) the ice and dark, non-ice/ice-poor material are segregated (e.g., consistent with interpretations of images of Callisto's surface taken by Voyager [5, 6] and Galileo [7]) and H2O sublimates at "ice" temperatures [8].Our 2D molecular kinetic models track the motion H2O, whose sublimation yield varies several orders of magnitude depending on the description of Callisto's surface, its photochemical products H and H2, and a relatively dense O2 component. Whereas H is assumed to react in the regolith on return to the surface, H2 is assumed to thermalize and re-enter the atmosphere.We compare the simulated LOS column densities of H to the detected H corona at Callisto [9], which was suggested to be produced primarily by photodissociation of sublimated H2O. Our goal is to use the corona observations to help constrain the source rate for H2O from Callisto’s complex surface.References[1] Liang et al., 2005. Atmosphere of Callisto. Journal of Geophysical Research: Planets.[2] Vorburger et al., 2015. Monte-Carlo simulation of Callisto’s exosphere. Icarus.[3] Hartkorn et al., 2017. Structure and density of Callisto’s atmosphere from a fluid-kinetic model of its ionosphere: Comparison with Hubble Space Telescope and Galileo observations. Icarus.[4] Carberry Mogan et al., 2021 (under review). A tenuous, collisional atmosphere on Callisto. Icarus.[5] Spencer and Maloney, 1984. Mobility of water ice on Callisto: Evidence and implications. Geophysical Research Letters.[6] Spencer, 1987. Thermal segregation of water ice on the Galilean satellites. Icarus.[7] Moore et al., 1999. Mass movement and landform degradation on the icy Galilean satellites: Results of the Galileo nominal mission. Icarus.[8] Grundy et al., 1999. Near-infrared spectra of icy outer solar system surfaces: Remote determination of H2O ice temperatures. Icarus.[9] Roth et al., 2017. Detection of a hydrogen corona at Callisto. Journal of Geophysical Research: Planets.

Neutral sodium (Na i) is an alkali metal with a favorable absorption cross section such that tenuous gases are easily illuminated at select transiting exoplanet systems. We examine both the time-averaged and time-series alkali spectral flux individually, over 4 nights at a hot Saturn system on a similar to 2.8 day orbit about a Sun-like star WASP-49 A. Very Large Telescope/ESPRESSO observations are analyzed, providing new constraints. We recover the previously confirmed residual sodium flux uniquely when averaged, whereas night-to-night Na i varies by more than an order of magnitude. On HARPS/3.6 m Epoch II, we report a Doppler redshift at v Gamma,NaD = + 9.7 +/- 1.6 km s-1 with respect to the planet's rest frame. Upon examining the lightcurves, we confirm night-to-night variability, on the order of similar to 1%-4% in NaD, rarely coinciding with exoplanet transit, not readily explained by stellar activity, starspots, tellurics, or the interstellar medium. Coincident with the similar to+10 km s-1 Doppler redshift, we detect a transient sodium absorption event dF NaD/F star = 3.6% +/- 1% at a relative difference of Delta F NaD(t) similar to 4.4% +/- 1%, lasting Delta t NaD greater than or similar to 40 minutes. Since exoplanetary alkali signatures are blueshifted due to the natural vector of radiation pressure, estimated here at roughly similar to-5.7 km s-1, the radial velocity is rather at +15.4 km s-1, far larger than any known exoplanet system. Given that the redshift magnitude v Gamma is in between the Roche limit and dynamically stable satellite orbits, the transient sodium may be a putative indication of a natural satellite orbiting WASP-49 A b.

Observations of the Jovian upper atmosphere at high latitudes in the UV, IR and mm/sub-mm all indicate that the chemical distributions and thermal structure are broadly influenced by auroral particle precipitations. Mid-IR and UV observations have shown that several light hydrocarbons (up to 6 carbon atoms) have altered abundances near Jupiter's main auroral ovals. Ion-neutral reactions influence the hydrocarbon chemistry, with light hydrocarbons produced in the upper stratosphere, and heavier hydrocarbons as well as aerosols produced in the lower stratosphere. One consequence of the magnetosphere-ionosphere coupling is the existence of ionospheric jets that propagate into the neutral middle stratosphere, likely acting as a dynamical barrier to the aurora-produced species. As the ionospheric jets and the background atmosphere do not co-rotate at the same rate, this creates a complex system where chemistry and dynamics are intertwined. The ion-neutral reactions produce species with a spatial distribution following the SIII longitude system in the upper stratosphere. As these species sediment down to the lower stratosphere, and because of the progressive dynamical decoupling between the ionospheric flows and the background atmosphere, the spatial distribution of the auroral-related species progressively follows a zonal distribution with increasing pressures that ultimately produces a system of polar and subpolar hazes that extends down to the bottom of the stratosphere. This paper reviews the most recent work addressing different aspects of this environment.

Abstract Results for 2D molecular kinetics models of single- and multi-component Callisto-like atmospheres are presented. The evolution of these neutral atmospheres is driven by the diurnal changes in surface temperatures, intermolecular collisions, and thermal escape. Galileo and Hubble Space Telescope (HST) observations of Callisto, the outermost Galilean moon of Jupiter, have shown it possesses an atmosphere. However, the origin, composition, and evolution of this atmosphere are still not well understood. It ranges from a surface-bound exosphere to a collisional atmosphere with an exobase located above the surface. These spatial and temporal variations are driven by the diurnal changes in surface temperatures as well as by incident plasma and UV photons. Galileo first observed a tenuous CO2 atmosphere on Callisto via airglow emissions [1]. Several processes, such as radiolysis, were suggested to be the source of the atmosphere, and its extent was suggested to be global, driven by the diurnal variations of surface temperatures and the volatility and mobility of CO2. Galileo radio occultations indicated the presence of a substantial ionosphere via measured electron density profiles near the terminator region [2]. Analogous to the O2 atmosphere on Europa, a much thicker O2-dominated atmosphere was suggested to exist at Callisto. UV auroral emissions were not detected in subsequent HST-Space Telescope Imaging Spectrograph (STIS) observations, but upper limits of C and O were estimated [3]. Based on these observations, a range of chemical and photochemical models were applied to Callisto-like atmospheres [4]. Photoionizaton of the observed CO2 was shown to be insufficient to generate the observed electron density profiles and, thus, a more dense, predominantly O2 atmosphere was suggested to exist at Callisto. In addition, chemical reactions in a porous regolith were shown to recycle the H2O, whose photochemical products suppressed the accumulation of O2, thereby preventing the O column density from exceeding the estimated upper limit. Using the HST-Cosmic Origins Spectrograph (COS), O emissions from Callisto's atmosphere were detected, likely generated by photo-electrons in an O2-dominated atmosphere [5]. The density of this inferred O2 atmosphere was about an order of magnitude lower than was previously inferred. This difference in densities is likely due to Callisto's orbit relative to Jupiter and the Sun: the HST-COS observations occurred when Callisto's sunlit hemisphere was opposite its ram-side hemisphere, whereas the radio occultations occurred when the two hemispheres aligned. The HST-STIS observations were recently revisited and faint emissions from an atomic hydrogen corona, likely a photochemical product of H2O vapor, were detected [6]. Contrary to the aforementioned O2 asymmetries, the derived H corona was larger on the sunlit leading hemisphere than on the sunlit trailing hemisphere. This was suggested to be a result of the leading hemisphere's lower albedo and, thus, higher surface temperatures, which would enhance the H2O sublimation rate. A Monte-Carlo model was used to simulate surface-bound Callisto-like ballistic exospheres, which varied in surface temperature and composition as well as atmospheric sources and sinks [7]. A subsequent study improved this model by differentiating between the cold and hot parts of the Jovian magnetosphere and considering the influence of ionospheric shielding [8]. Our recent study demonstrated the influence of collisions and thermal escape in single- and multi-component 1D Callisto-like neutral atmospheres [9]. Therein we assumed that the constituents of the atmospheres (O2, CO2, H2) were radiolytic products which thermally desorb from the surface according to the local temperature and on returning to the surface they permeate the porous regolith and become trapped in the radiation-damaged ice. Using the direct simulation Monte Carlo (DSMC) method [10] we calculated translational and internal energy exchanges via intermolecular collisions between test particles. Our results demonstrated that collisions can suppress or enhance H2 thermal escape relative to Jeans (ballistic) escape and collisions between the escaping H2 and O2 and CO2 affected the heavier species' structure, producing non-isothermal profiles. We also compared these results to ballistic models and demonstrated where the latter breaks down. Model Here we expand our previous models to 2D to include the diurnal variation of surface temperatures as well as include H2O sublimation, which is extremely sensitive to Callisto's surface temperatures, varying ~15 orders of magnitude from noon (T0 = 155 K) to midnight (T0 = 80 K). The additional dimension in these models varies along Callisto's subsolar latitude (SSL), where the surface temperature varies from noon (SSL = 0) to midnight (SSL = 180). We track test particles from their original SSL to the SSL they return to the surface at or, in the case of H2, they escape from. Thus, local as well as global fluxes and subsequent return and escape rates can be calculated as a means to better understand the distribution of Callisto's neutral atmosphere. Moreover, thermal winds induced by collisions of particles from the warmer regions of the atmosphere with particles from the colder regions are observed, thereby influencing the local distribution. References [1] Carlson, R.: A tenuous carbon dioxide atmosphere on Jupiter's moon Callisto, Science, 1999. [2] Kliore, A., et al.: Ionosphere of Callisto from Galileo radio occultation observations, Journal of Geophysical Research: Space Physics, 2002. [3] Strobel, D., et al.: Hubble Space Telescope space telescope imaging spectrograph search for an atmosphere on Callisto: A Jovian unipolar inductor, The Astrophysical Journal Letters, 2002. [4] Liang, M.-C., et al.: Atmosphere of Callisto, Journal of Geophysical Research: Planets, 2005. [5] Cunningham, N., et al.: Detection of Callisto's oxygen atmosphere with the Hubble Space Telescope, Icarus, 2015. [6] Roth, L., et al.: Detection of a hydrogen corona at Callisto, Journal of Geophysical Research: Planets, 2017. [7] Vorburger, A., et al.: Monte-Carlo simulation of Callisto's exosphere, Icarus, 2017. [8] Vorburger, A., et al.: 3D-modeling of Callisto's surface sputtered exosphere environment, Journal of Geophysical Research: Space Physics, 2019. [9] Carberry Mogan, S., et al.: The influence of collisions and thermal escape in Callisto's atmosphere, Icarus, 2020. [10] Bird, G.: Molecular gas dynamics and the direct simulation of gas flows, Clarendon Press, 1994.

Juno flew over the northern mid-latitudes of Ganymede during orbit 34 of the Juno mission, reaching an altitude of 1,053 km (16:56:07.972 UTC) at a sub spacecraft latitude/longitude of 33.66N, 57.5W degrees on 7 June 2021. Between 16:43 and 17:02 UT, Juno pierced Ganymede's magnetosphere at a velocity relative to Ganymede of 18.57 km s-1. Juno's instrumentation provided a unique opportunity to sample the local environment of Ganymede and its magnetosphere. We present measurements of the composition of the polar ionospheric outflow and the energetic electrons that penetrate Ganymede's atmosphere and produce its aurora. When these new observations are combined with modeling, conclusions can be drawn that affect our understanding of the atmosphere of Ganymede. The measured JADE precipitating plasma electrons provide an energy flux beyond that needed to create the observed oxygen emissions measured by UVS, but the electron energy spectrum is optically thin to the sparse atmosphere and does not provide the observed oxygen ultraviolet emission unless the O2 column density is increased by over an order of magnitude compared to previous atmospheric models. More than 99% of the electron energy flux passes through the atmosphere into the ice, thereby increasing the H2 and O2 content of the atmosphere. The increased H2 and O2 production is largely responsible for increasing the oxygen column density to a level that produces within known uncertainties the OI135.6 and OI130.4 nm emissions when bombarded by the electron energy flux observed by JADE. This suggests that past modeling efforts have underestimated the density of the atmosphere by over an order of magnitude. Juno flew by the Jovian satellite Ganymede on orbit 34 of the mission coming within 1,053 km of the surface at high northern latitudes. Juno provides a unique data set from the particle, field, and ultraviolet imaging experiments that can be utilized to study the interaction between the magnetosphere of Ganymede and the magnetosphere of Jupiter. Compositionally diverse ion outflow was observed in the northern polar cap that can be linked to the exchange of plasma and energetic particles between Jupiter and Ganymede. Furthermore, an electron spectrum produced by magnetic reconnection processes was observed that can be linked to the aurora at Ganymede. These observations provide new information about Ganymede's atmosphere. They strongly suggest an increase in the column density of O2 in Ganymede's atmosphere of an order of magnitude relative to all previous models. The latter has implications for all water ice satellites in the outer solar system. Multiple data sets from Juno's flyby of Ganymede indicate energetic particle impact of the surface and atmosphere resulting in ion outflow Reconnection processes on the Jupiter-facing flank of the magnetospheric interaction provide constraints on the auroral excitation Initial modeling strongly suggests an order of magnitude increase in atmospheric O2 column density relative to previous models

The origin and evolution of Saturn's rings is critical to understanding the Saturnian system as a whole. Here, we discuss the physical and chemical composition of the rings, as a foundation for evolutionary models described in subsequent chapters. We review the physical characteristics of the main rings, and summarize current constraints on their chemical composition. Radial trends are observed in temperature and to a limited extent in particle size distribution, with the C ring exhibiting higher temperatures and a larger population of small particles. The C ring also shows evidence for the greatest abundance of silicate material, perhaps indicative of formation from a rocky body. The C ring and Cassini Division have lower optical depths than the A and B rings, which contributes to the higher abundance of the exogenous neutral absorber in these regions. Overall, the main ring composition is strongly dominated by water ice, with minor silicate, UV absorber, and neutral absorber components. Sampling of the innermost D ring during Cassini's Grand Finale provides a new set of in situ constraints on the ring composition, and we explore ongoing work to understand the linkages between the main rings and the D ring. The D ring material is organic- and silicate-rich and water-poor relative to the main rings, with a large population of small grains. This composition may be explained in part by volatile losses in the D ring, and current constraints suggest some degree of fractionation rather than sampling of the bulk D ring material.

The JUpiter ICy moons Explorer (JUICE) of the European Space Agency will investigate Jupiter and its icy moons Europa, Ganymede, and Callisto, with the aim to better understand the origin and evolution of our Solar System and the emergence of habitable worlds around gas giants. The Particle Environment Package (PEP) on board JUICE is designed to measure neutrals, ions, electrons, and energetic particles over an energy range from eV to MeV.In the vicinity of Callisto, PEP will characterize the Jovian plasma environment and the outer parts of Callisto’s atmosphere and ionosphere. Roughly twenty Callisto flybys with closest approaches between 200 km and 5000 km altitude are planned over the course of the JUICE mission. This study aims at optimizing the scientific insight gained from the foreseen flybys by combining the input from the PEP science team and operation planning with recent model efforts for Callisto’s atmosphere, the plasma environment and the production of Energetic Neutral Atoms. The results of this study will inform both science operation planning of PEP and JUICE and they will guide future model development for Callisto’s atmosphere, ionosphere, and their interaction with the plasma environment.

The vertical propagation of wave energy into a planet’s exosphere, a process that affects atmospheric evolution, is calculated here using 1D molecular kinetic simulations. Effects sensitive to molecular interactions are examined by comparing simulation results to solutions of linear fluid models for steady wave activity using parameters associated with Mars’ upper atmosphere. In addition to correctly describing the wave behavior in the exobase region, these simulations directly yield nonlinear effects such as atmospheric heating. They also readily include the transient behavior due to the onset and decay of waves propagating into the rarefied region of a planet’s atmosphere. This is a first step in understanding the effects of variable wave activity in the region where the atmosphere evolves from collisional to collisionless.

Observations of Callisto's atmosphere have indicated an O 2 component should exist, but the evolution from its initial source to its inferred steady‐state abundance is not well understood. Herein we constrain the production of O 2 via radiolysis within Callisto's exposed ice patches and determine the corresponding O 2 column density. To do so, for the first time we simulate the thermal and energetic components of the Jovian magnetospheric plasma irradiating Callisto's atmosphere and estimate energy deposited therein by the impinging charged particles along their trajectories to the surface. We then calculate O 2 source fluxes corresponding to the energy of the impacting plasma fluxes, which is coupled with estimated atmospheric lifetimes to determine the steady‐state abundance of O 2 . Our results suggest that production of O 2 via radiolysis within the exposed ice on Callisto's surface does not produce a sufficiently dense atmosphere relative to the column densities inferred from observations by about 2–3 orders of magnitude. To resolve this discrepancy between estimated and observed abundances, we provide the first estimates for other potential sources of atmospheric O 2 . We also make similar estimates for the production of H 2 in Callisto's atmosphere relative to constraints provided in the literature, and the conclusion is the same: a sufficiently dense atmosphere is not produced. Thus, we have shown that a better understanding of the production and fate of radiolytic products in Callisto's regolith is required in order to place firmer constraints on the generation mechanisms of its atmosphere in preparation for future observations.

We explore the parameter space for the contribution to Callisto's H corona observed by the Hubble Space Telescope from sublimated H2O and radiolytically produced H2 using the Direct Simulation Monte Carlo (DSMC) method. The spatial morphology of this corona produced via photo- and magnetospheric electron impact-induced dissociation is described by tracking the motion of and simulating collisions between the hot H atoms and thermal molecules including a near-surface O2 component. Our results presented indicate that sublimated H2O produced from the surface ice, whether assumed to be intimately mixed with or distinctly segregated from the dark non-ice or ice-poor regolith, cannot explain the observed structure of the H corona. On the other hand, a global H2 component can reproduce the observation, and is also capable of producing the enhanced electron densities observed at high altitudes by Galileo's plasma-wave instrument, providing the first evidence of H2 in Callisto's atmosphere. Finally, we discuss the implications of these results, in particular how they compare to Europa and Ganymede.

&lt;p&gt;&lt;strong&gt;Introduction: &lt;/strong&gt;Space weathering by ion irradiation is ubiquitous on the surfaces of airless bodies in the Solar System. Sputtering occurs when solar wind (SW) or magnetosphere ions (MI) impact the suraces of bodies in space. Asteroids and moons are too small to maintain a significant atmosphere, and therefore they are directly exposed to ionizing radiation from the solar wind and magnetospheric plasmas. Incident ions can transfer sufficient energy to surface species to cause them to desorb and potentially escape to space. A small fraction of the sputtered species can escape as ions, called sputtered secondary ions (SSI). Mass, charge, and energy analysis of the sputtered ions using secondary ion mass spectrometry is highly diagnostic of the irradiated surface composition. The upcoming JAXA MMX mission will carry a Mass Spectral Analyzer (MSA) instrument will be capable of making measurements of SSI around its target bodies Phobos and Deimos (P&amp;D). However, there is currently limited estimates of SSI yields from relevant surface compositions under relevant irradiation conditions, and the expected SSI fluxes around P&amp;D are not well constrained.&lt;/p&gt; &lt;p&gt;&lt;strong&gt;Background:&lt;/strong&gt; Although P&amp;D are exposed to both the SW and MI and SSI are expected to be present throughout their orbits. However, several challenges arise when attempting to derive a precise surface composition from a measured SIMS spectra, or when estimating the expected count rates and elemental ratios that will be observed by MSA for a given composition: (i) the relative abundances measured by SIMS are not directly correlated with the actual surface composition, and (ii) the relative and absolute SSI yields (# of SSI ejected per incident ion) likely depend on the surface chemistry and exposure history, and on the incident ion type and energy.&lt;/p&gt; &lt;p&gt;&lt;strong&gt;Results: &lt;/strong&gt;A combined computational and experimental approach has been used in order to better constrain the solar wind sputtering rates of small, rocky bodies. First, a series of SIMS measurements in the laboratory were carried out to determine the relative ion sputtering ratios from several lunar samples of known composition. Then, using Monte Carlo simulations of sputtering due to both solar wind and magnetosphere ions and the measured SSI energy distributions to determine the total sputtering yields, the total abundance and relative composition of sputtered ions can be determined for an arbitrary small body. This work will &lt;strong&gt;(1) estimate the the SSI yields from analog Mars and Carbonaceous Chondrite analog materials and correlate the expected yields with the surface composition, and (2) provide estimates the SSI fluxes and densities during their orbits around Mars.&lt;/strong&gt; Further, this work will demonstrate how measurement of the elemental ratios of SSI can be used to estimate the potential origins scenarios for small bodies.&lt;/p&gt;

Electron impact ionization is critical in producing the ionospheres on many planetary bodies and, as discussed here, is critical for interpreting spacecraft and telescopic observations of the tenuous atmospheres of the icy Galilean satellites of Jupiter (Europa, Ganymede, and Callisto), which form an interesting planetary system. Fortunately, laboratory measurements, extrapolated by theoretical models, were developed and published over a number of years by K. Becker and colleagues (see Deutsch et al. in Adv At Mol Opt Phys 57:87–155, 2009) to provide accurate electron impact ionization cross sections for atoms and molecules, which are crucial to correctly interpret these measurements. Because of their relevance for the Jovian icy satellites, we provide useful fits to the complex, semiempirical Deutsch–Märk formula for energy-dependent electron impact ionization cross sections of gas-phase water products (i.e., H _2 O, H _2 , O _2 , H, O). These are then used with measurements of the thermal plasma in the Jovian magnetosphere to produce ionization rates for comparison with solar photo-ionization rates at the icy Galilean satellites.

Juno flew within 1053 km of the surface of Ganymede on June 7, 2021. A unique data set of the interaction of its magnetosphere with the magnetosphere of Jupiter was obtained during the flyby. Auroral imaging was carried out by the UVS experiment simultaneous with the in-situ sampling of the polar cap ionosphere by the Waves, MAG, JEDI, and JADE experiments onboard Juno. Significant outflow of Ganymede’s polar cap ionosphere was observed as well as an in-situ sampling of reconnection processes near the magnetospheric boundary on the flank of the trailing side of the magnetospheric interaction region. Assuming that the electrons measured in the reconnection/interaction region are representative of the electrons producing the aurora, we use the UVS auroral vertical profiles obtained from the flyby and modeling to dramatically improve our understanding of the Ganymede atmosphere. The results of the relevant flyby measurements and the modeling of the atmosphere and aurora will be presented in this talk.