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EUV radiation is primarily responsible for driving hydrodynamic mass loss and thus determining whether an exoplanet can retain its atmosphere and water. The Lyman- α flux is primarily responsible for photo-dissociating water and methane, and therefore plays a major role in determining the chemistry in the upper atmospheres of exoplanets. Since interstellar hydrogen absorbs much of the EUV and Lyman- α radiation, reconstruction and theoretical techniques are needed to determine the intrinsic flux levels received by exoplanets. We describe the techniques and their limitations for estimating the extreme ultraviolet (EUV) spectral energy distribution (10–91.2 nm) and hydrogen Lyman- α flux (121.6 nm) emitted by host stars and incident on exoplanet atmospheres. We evaluate how each reconstruction technique can match the observed solar EUV spectral energy distribution and Lyman- α flux. Each technique has its limitations, but the techniques that can reconstruct the observed solar emission and are based on stellar activity observables that are not affected by interstellar absorption should best explain the intrinsic EUV and Lyman- α stellar emission. [ABSTRACT FROM AUTHOR]

Water ice, abundant in the outer solar system, is volatile in the inner solar system. On the largest airless bodies of the inner solar system (Mercury, the Moon, Ceres), water can be an exospheric species but also occurs in its condensed form. Mercury hosts water ice deposits in permanently shadowed regions near its poles that act as cold traps. Water ice is also present on the Moon, where these polar deposits are of great interest in the context of future lunar exploration. The lunar surface releases either OH or H2O during meteoroid showers, and both of these species are generated by reaction of implanted solar wind protons with metal oxides in the regolith. A consequence of the ongoing interaction between the solar wind and the surface is a surficial hydroxyl population that has been observed on the Moon. Dwarf planet Ceres has enough gravity to have a gravitationally-bound water exosphere, and also has permanently shadowed regions near its poles, with bright ice deposits found in the most long-lived of its cold traps. Tantalizing evidence for cold trapped water ice and exospheres of molecular water has emerged, but even basic questions remain open. The relative and absolute magnitudes of sources of water on Mercury and the Moon remain largely unknown. Exospheres can transport water to cold traps, but the efficiency of this process remains uncertain. Here, the status of observations, theory, and laboratory measurements is reviewed. [ABSTRACT FROM AUTHOR]

Observations of the plasma environment outside of the Earth's magnetosphere by the Magnetospheric Multiscale Mission Hot Plasma Composition Analyzers (MMS‐HPCAs) allow density measurements of geocoronal hydrogen pickup ions (PUIs). A study of this environment from December 4, 2019 to December 6, 2019, part of a single orbital period of the MMS spacecraft, revealed an inverse square dependence on PUI density. This dependence corresponds well with an inverse‐cube dependence on the neutral hydrogen density within the geocorona. Plain Language Summary: Measuring pickup ions produced from the Earth's geocorona provide an in‐situ analysis of their geocentric density dependence. These measurements also provide a model‐dependent evaluation of the neutral geocorona's expanse and geocentric dependence, which is part of the Earth's extended atmosphere, also known as the exosphere. Key Points: Pickup ions (PUIs) produced from the hydrogen exosphere are observable with near‐Earth plasma instrumentsPickup ions are distinguishable from solar wind ions through their respective velocity distributionsGeocoronal PUI density decreases with geocentric radius and corresponds to an inverse radius cube neutral dependence [ABSTRACT FROM AUTHOR]

This manuscript represents a review on progress made over the past decade concerning our understanding of meteoroid bombardment on airless solar system bodies as one of the sources of the formation of their exospheres. Specifically, observations at Mercury by MESSENGER and at the Moon by LADEE, together with progress made in dynamical models of the meteoroid environment in the inner solar system, offer new tools to explore in detail the physical phenomena involved in this complex relationship. This progress is timely given the expected results during the next decade that will be provided by new missions such as DESTINY+, BepiColombo, the Artemis program or the Lunar Gateway. [ABSTRACT FROM AUTHOR]

Abstract The evolution of Europa's water-product exosphere over its 85-h day, based on current models, has not been shown to exhibit any diurnal asymmetries. Here we simulate Europa's exosphere using a 3-D Monte Carlo routine including, for the first time, the role of Europa's rotation on the evolution of exospheric molecules tracked throughout the orbit. In this work we focus on understanding the behavior of a single atmospheric constituent, O 2 , sputtered by a trailing hemisphere source with a temperature-dependence under isotropic plasma conditions. Under rotation, the O 2 is also subject to the centrifugal and Coriolis forces in addition to the standard gravitational forces by Jupiter and Europa in our model. We find that the O 2 component, while global, is not homogeneous in Europa local time. Rather, the O 2 consistently accumulates along the direction of Europa's rotation at the dusk hemisphere. When rotation is explicitly excluded in our simulations, no diurnal asymmetries exist, and any accumulation is due to the prescribed geometry of the sputtering source. We find that the assumed thermal-dependence on the O 2 source is critical for a diurnal asymmetry: the diurnal surface temperature profile is imprinted on to the near-surface O 2 atmosphere, due to small hop times for the non-adsorbing O 2 , which then effectively rotates with Europa as subsequently described in Oza et al., 2018; Johnson et al., 2019. Simulation tests demonstrate that the diurnal asymmetry is not driven by the thermal inertia of the ice, found to have only a weak dependence (< 7 %). Altogether, the various test cases presented in this work conclude that the dusk-over-dawn asymmetry is driven by Europa's day-night O 2 cycle synchronized with Europa's orbital period based on our model assumptions on O 2 production and loss. This conclusion is in agreement with the recent understanding that a non-adsorbing, rotating O 2 source peaking at noon will naturally accumulate from dawn-to-dusk, should the O 2 lifetime be sufficiently long compared to the orbital period. Lastly we compare hemispherically-averaged dusk-over-dawn ratios to the recently observed oxygen emission data by the Hubble Space Telescope. We find that while the simulations are globally consistent with brighter oxygen emission at dusk than at dawn, the orbital evolution of the asymmetries in our simulations can be improved by ameliorating the O 2 source & loss rates, and possibly adsorption onto the regolith. Highlights • A 3-D rotation-dependent model of Europa's water-product atmosphere is presented. • Europa's near-surface O 2 undergoes a diurnal cycle peaking at dusk, consistent with HST oxygen aurorae observations. • An atmospheric O 2 bulge persists at dusk due to the O 2 lifetime being roughly synchronized to the orbital time. • O 2 preferentially adsorbing on to regolith grains due to possible leading/trailing surface differences is evidenced. [ABSTRACT FROM AUTHOR]

Highlights • We show the immediate fractional escape for species of mass <50 AMU at primary bodies <10,000 km in radius. • We compare escape fractions of volatiles escaping via two different sputter functions. • We present specific studies for Ceres, the Moon and Mercury. Abstract The study of exospheres can help us understand the long-term loss of volatiles from planetary bodies due to interactions of planets, satellites, and small bodies with the interplanetary medium, solar radiation, and internal forces including diffusion and outgassing. Recent evidence for water and OH on the Moon has spurred interest in processes involving chemistry and sequestration of volatile species at the poles and in voids. In recent years, NASA has sent spacecraft to asteroids including Vesta and Ceres, and ESA sent Rosetta to comet 67P/Churyumov–Gerasimenko and the asteroids Lutetia and Steins. Japan's Hayabusa spacecraft returned a sample from asteroid Itakowa, and OSIRIS-REX will return a sample from a primitive asteroid, Bennu, to Earth. In a surface-bounded exosphere, the gases are derived from the surface and thus reflect the composition of the body's regolith, although not in a one-to-one ratio. Observation of an escaping exosphere, termed a corona, is challenging. We have therefore embarked on a parametrical study of exospheres as a function of mass of the exospheric species, mass of the primary body and source velocity distribution, specifically thermal (Maxwell-Boltzmann) and sputtering. The goal is to provide a quick look to determine under what conditions and for what mass of the primary body the species of interest are expected to be bound or escaping and to quickly estimate the observability of exospheric species. This work does not provide a comprehensive model but rather serves as a starting point for further study. These parameters will be useful for mission planning as well as for students beginning a study of planetary exospheres. [ABSTRACT FROM AUTHOR]

The study of multiscale pickup ion phase-mixing in the lunar plasma wake with a hybrid model is the main subject of our investigation in this paper. Photoionization and charge exchange of protons with the lunar exosphere are the ionization processes included in our model. The computational model includes the self-consistent dynamics of the light ( H + or H 2 + and He + ), and heavy ( Na + ) pickup ions. The electrons are considered as a fluid. The lunar interior is considered as a weakly conducting body. In this paper we considered for the first time the cumulative effect of heavy neutrals in the lunar exosphere (e.g., Al, Ar), an effect which was simulated with one species of Na + but with a tenfold increase in total production rates. We find that various species produce various types of plasma tail in the lunar plasma wake. Specifically, Na + and He + pickup ions form a cycloid-like tail, whereas the H + or H 2 + pickup ions form a tail with a high density core and saw-like periodic structures in the flank region. The length of these structures varies from 1.5 R M to 3.3 R M depending on the value of gyroradius for H + or H 2 + pickup ions. The light pickup ions produce more symmetrical jump in the density and magnetic field at the Mach cone which is mainly controlled by the conductivity of the interior, an effect previously unappreciated. Although other pickup ion species had little effect on the nature of the interaction of the Moon with the solar wind, the global structure of the lunar tail in these simulations appeared quite different when the H 2 + production rate was high. [ABSTRACT FROM AUTHOR]

A latitudinal and radial study of the lunar sodium exosphere has been performed utilizing observations made from two different methods: (1) observations made at targeted altitudes using a Fabry-Perot Spectrometer (FPS) and (2) observations made from a coronagraph. The FPS observations made from the National Solar Observatory McMath-Pierce Solar Telescope, Kitt Peak, Arizona and the coronagraph observations were made at the Winer Observatory, Sonoita, Arizona. A small subset of the high resolution FPS observations were made concurrently with coronagraph measurements. Measured linewidths and linewidth-derived temperatures from FPS observations were compared to temperatures derived from the coronagraphic intensity altitude profiles, with FPS linewidth-derived temperatures shown to be consistently lower. We suggest that the coronagraph method samples a velocity distribution perpendicular to the FPS's LOS, while the FPS samples a velocity distribution tangential to the lunar limb (i.e., along the FPS LOS). We also suggest that the coronagraph measurements may be more sensitive to the escaping population of atoms as the population close to the surface is not observed. The concurrent FPS measurements sit below the occulting disk of the coronagraph and measure the atoms closer to the surface. Furthermore, both the FPS linewidth-derived temperatures and the coronagraph scale heights show an increase towards high latitudes, an effect which is attributed to particle transport and/or contributions from a source like meteoroid impact vaporization. FPS linewidths decrease as a function of altitude, a result confirmed through a simulation of velocity distributions from nonthermal source mechanisms. And, finally, Linewidths are largest when looking over the dawn/dusk terminator. These results will enable improved characterization of the sources for the lunar sodium exosphere. • Lunar exospheric measurements from two different instruments and methods are compared. • Concurrent linewidth-derived temperatures and coronagraph scale heights increase towards high latitudes. • Observed linewidths decrease as a function of altitude. • 1-D velocity distributions simulated near local noon predict linewidths that decrease monotonically with altitude. • Near sunrise, an increase in linewidth is occasionally observed at ∼1500 km, perhaps indicating a transition to a more energetic source process. [ABSTRACT FROM AUTHOR]

We report here on the first observational evidence of Al and Fe in the exosphere of Mercury, based on measurements of resolved emission lines of these metals with Keck-1/HIRES. Al emission was observed on two separate runs, in 2008 and 2013, with tangent column densities of 3.1 ± 1.0 and 4.0 ± 1.5 × 10 7 Al atoms cm −2 at altitudes of 1185 and 1870 km (1.5 and 1.75 R M ). The Al radiative intensity was seen to increase where the slit crossed the planetary penumbral shadow, and then decrease monotonically with altitude. Fe emission has been observed once, in 2009, indicating an extended source. We also present observed 3-σ Ca + upper limits near Mercury's equatorial anti-solar limb, from which an abundance limit of 4.0 × 10 6 cm −2 at 1650 km altitude is derived for the Ca ion. A simple model for zenith column abundances of the neutral species yields 1.9−5.2 × 10 7 Al cm −2 , and 8.2 × 10 8 Fe cm −2 . The observations appear to be consistent with production of these species by impact vaporization, with a large fraction of the Al ejecta in molecular form, and that for Fe in mixed atomic and molecular forms. The scale height of the Al gas is consistent with a kinetic temperature of 6100–8000 K. The apparent high temperature and low density of the Al gas would suggest that it may be produced by dissociation of molecules. [ABSTRACT FROM AUTHOR]

In this paper, we review the scientific aspects of planetary space weather at different regions of our Solar System, performing a comparative planetology analysis that includes a direct reference to the circum-terrestrial case. Through an interdisciplinary analysis of existing results based both on observational data and theoretical models, we review the nature of the interactions between the environment of a Solar System body other than the Earth and the impinging plasma/radiation, and we offer some considerations related to the planning of future space observations. We highlight the importance of such comparative studies for data interpretations in the context of future space missions (e.g. ESA JUICE; ESA/JAXA BEPI COLOMBO). Moreover, we discuss how the study of planetary space weather can provide feedback for better understanding the traditional circum-terrestrial space weather. Finally, a strategy for future global investigations related to this thematic is proposed.

Abstract: In this report we discuss the self-consistent dynamics of pickup ions in the solar wind flow around the lunar-like object. In our model the solar wind and pickup ions are considered as a particles, whereas the electrons are described as a fluid. Inhomogeneous photoionization, electron-impact ionization and charge exchange are included in our model. The Moon will be chosen as a basic object for our modeling. The current modeling shows that mass loading by pickup ions , and may be very important in the global dynamics of the solar wind around the Moon. In our hybrid modeling we use exponential profiles for the exospheric components. The Moon is considered as a weakly conducting body. Special attention will be paid to comparing the modeling pickup ion velocity distribution with ARTEMIS observations. Our modeling shows an asymmetry of the Mach cone due to mass loading, the upstream flow density distribution and the magnetic field. The pickup ions form an asymmetrical plasma tails that may disturb the lunar plasma wake. [Copyright &y& Elsevier]

Abstract: Europa's exosphere is a mixture of different species among which sputtered H2O and H2 dominate in the highest altitudes and O2, formed mainly by radiolysis of ice and subsequent release of the produced molecules, prevails at lower altitudes. Europa's O2 exosphere has been demonstrated through both observation and simulation-based techniques to be spatially non-uniform. In the present study we investigate Europa's exospheric O2 characteristics under the external conditions that are likely in the Jupiter's magnetospheric environment, applying the Europa Global model of Exospheric Outgoing Neutrals (EGEON, Plainaki et al., 2012) for different configurations between the positions of Europa, Jupiter and the Sun. We demonstrate for the first time that the spatial distribution of Europa's exosphere is explicitly time-variable due to the time-varying relative orientations of solar illumination and the incident plasma direction. We show that the O2 release efficiency depends both on solar illumination and plasma impact direction. The modeled densities at different orbital phases of Europa are compared, a posteriori, with the analysis results from two observations in order to validate the model. Using the outputs of EGEON we also calculate the supply of neutral oxygen atoms of exospheric origin to Europa's neutral cloud. [Copyright &y& Elsevier]

Abstract: In this report we discuss the self-consistent dynamics of pickup ions in the solar wind flow around the lunar-like object. In our model the solar wind and pickup ions are considered as a particles, whereas the electrons are described as a fluid. Inhomogeneous photoionization, electron-impact ionization and charge exchange are included in our model. The Moon will be chosen as a basic object for our modeling. The current modeling shows that mass loading by pickup ions and may be very important in the global dynamics of the solar wind around the Moon. In our hybrid modeling we use exponential profiles for the exospheric components. The Moon is considered as a weakly conducting body. Special attention will be paid to comparing the modeling pickup ion velocity distribution with ARTEMIS observations. Our modeling shows an asymmetry of the Mach cone due to mass loading, the upstream flow density distribution and the magnetic field. The pickup ions form an asymmetrical plasma tails that may disturb the lunar plasma wake. [Copyright &y& Elsevier]

Abstract: The ion-sputtering (IS) process is active in many planetary environments in the solar system where plasma precipitates directly on the surface (for instance, Mercury, Moon and Europa). In particular, solar wind sputtering is one of the most important agents for the surface erosion of a near-Earth object (NEO), acting together with other surface release processes, such as photon stimulated desorption (PSD), thermal desorption (TD) and micrometeoroid impact vaporization (MIV). The energy distribution of the IS-released neutrals peaks at a few eVs and extends up to hundreds of eVs. Since all other release processes produce particles of lower energies, the presence of neutral atoms in the energy range above 10eV and below a few keVs (sputtered high-energy atoms (SHEA)) identifies the IS process. SHEA easily escape from the NEO, due to NEO''s extremely weak gravity. Detection and analysis of SHEA will give important information on surface-loss processes as well as on surface elemental composition. The investigation of the active release processes, as a function of the external conditions and the NEO surface properties, is crucial for obtaining a clear view of the body''s present loss rate as well as for getting clues on its evolution, which depends significantly on space weather. In this work, an attempt to analyze processes that take place on the surface of these small airless bodies, as a result of their exposure to the space environment, has been realized. For this reason, a new space weathering model (space weathering on NEO-SPAWN) is presented. Moreover, an instrument concept of a neutral-particle analyzer specifically designed for the measurement of neutral density and the detection of SHEA from a NEO is proposed. [Copyright &y& Elsevier]

Observations and simulations show that Mars' atmosphere has large seasonal variations. Total atmospheric density can have an order of magnitude latitudinal variation at exobase heights. By numerical simulations we show that these latitude variations in exobase parameters induce asymmetries in the hydrogen exosphere that propagate to large distances from the planet. We show that these asymmetries in the exosphere produce asymmetries in the fluxes of energetic neutral atoms (ENAs) and soft X-rays produced by charge exchange between the solar wind and exospheric hydrogen. This could be an explanation for asymmetries that have been observed in ENA and X-ray fluxes at Mars. [ABSTRACT FROM AUTHOR]

In view of the low H2O abundance in the present Venusian and Martian atmospheres several observations by spacecraft and studies suggest that both planets should have lost most of their water over the early active period of the young Sun. During the first Gyr after the Sun arrived at the Zero- Age-Main-Sequence high X-ray and EUV fluxes between 10 and 100 times that of the present Sun were responsible for much higher temperatures in the thermosphere-exosphere environments on both planets. By applying a diffusive-gravitational equilibrium and thermal balance model for investigating radiation impact on the early thermospheres by photodissociation and ionization processes, due to exothermic chemical reactions and cooling by CO2 IR emission in the 15μm band we found expanded thermospheres with exobase levels between about 200 km (present) and 2000 km (4.5 Gyr ago). The higher temperatures in the upper atmospheres of both planets could reach “blow-off” conditions for H atoms even at high CO2 mixing ratios of 96%. Lower CO2/N2 mixing ratio or higher contents of H2O vapor in the early atmospheres could have had a dramatic impact from the loss of atmosphere and water on both planets. The duration of this phase of high thermal loss rates essentially depended on the mixing ratios of CO2, N2, and H2O in the early atmospheres and could have lasted between about 150 and several hundred Myr. [ABSTRACT FROM AUTHOR]