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Remote sensing observations of far-ultraviolet (FUV) emissions have been used to estimate 3D neutral hydrogen (H) density models of the terrestrial exosphere under solar minimum (2008) and solar maximum (2013 and 2015) conditions. Specifically, we used Lyman-alpha (Lyman-α, FUV at 121.6 nm) radiance data acquired by the Lyman-alpha detectors (LADs) onboard NASA's TWINS satellites, collected over several days for each of the 3 years to provide sufficient coverage of the exospheric region. The datasets included Lyman-α measurements taken only above Earth radii (Re) of 3.75, assumed to be the optically thin region of the exosphere, where the measured Lyman-α intensity along a line of sight (LOS) is linearly proportional to the atomic hydrogen column density. Based on the calibration using multiple UV-bright stars, a significant decrease in TWINS1 LAD1/2 sensitivities was found for 2013 (LAD2 ∼1/2, LAD ∼1/3) and 2015 (LAD2 ∼1/3, LAD ∼1/7) compared to 2008. The calibration uncertainty was derived to be, on average, 5%. We estimated the 3D global hydrogen density distributions from these radiance data using two different tomographic inversion methods: first, a parametric fitting method based on a spherical harmonic function of order 3 and second, a high degree-of-freedom retrieval approach to validate our retrievals of H density. Both inversion methods consistently incorporate the effects of Lyman-α absorption within the exosphere and Lyman-α re-emission from Earth's albedo. Our results reveal that H densities during solar maximum conditions are, on average, 30%-40% higher than those during solar minimum. All models showed a high concentration of atomic H on the dayside and nightside near the Sun-Earth line, which determines a nose/geotail structure consistent with theoretical effects from the solar radiation pressure. Furthermore, we identified that H-density enhancements during solar maximum with respect to solar minimum conditions occur at mid-to-high latitudes, particularly on the dusk side, while no significant enhancement seems to occur near the dayside nose. [ABSTRACT FROM AUTHOR]

Mercury's exosphere is an important target for understanding the dynamics of coupled systems in space environments, tenuous planetary atmospheres, and planetary surfaces. Magnesium (Mg) is especially crucial for establishing methods for estimating the surface chemical composition distribution through observations of the exosphere because its distribution in the exosphere and on the surface is strongly correlated. However, owing to its low radiance, the Hermean Mg exosphere has only been detected by the Mercury Atmospheric and Surface Composition Spectrometer (MASCS) onboard the Mercury Surface, Space Environment, Geochemistry, and Ranging (MESSENGER) spacecraft. Thus, we have few observation data for areas other than low latitude regions in addition to few detection cases of short‐term or sporadic fluctuations, resulting in a poor understanding of ejection and transportation mechanisms of the Mg exosphere. In this study, we analyzed the distribution of the Hermean Mg exosphere by the Probing of Hermean Exosphere by Ultraviolet Spectroscopy (PHEBUS) onboard the Mercury Planetary Orbiter of the BepiColombo mission during its second and third Mercury swing‐bys (MSBs). First, we constructed a calibration method including background subtraction and calibration using stellar observations. Mg light curves at two true anomaly angles were obtained, which were in agreement with the Chamberlain model and a three‐dimensional numerical calculation. Comparing the Mg and calcium (Ca) radiances obtained by PHEBUS during the MSBs, the exospheric Mg atoms have a lower energy than the exospheric Ca atoms. This is consistent with the lower energy necessary for producing the Mg atoms produced by molecular photodissociation than for Ca atoms. Plain Language Summary: The tenuous atmosphere of Mercury helps us to understand how space environments, thin planetary atmospheres, and planetary surfaces interact. Magnesium is particularly important because its presence in Mercury's atmosphere can tell us a lot about the planetary surface composition. However, studying the magnesium atmosphere in Mercury is challenging because of its poor visibility. In this study, we used a new ultraviolet spectrometer installed on the BepiColombo spacecraft. Analyzing solar light scattered by magnesium atoms when the spacecraft was close to Mercury, we derived distributions of Mercury's magnesium atmosphere, which are aligned well with theoretical models. Furthermore, we compared the energies of magnesium and calcium in the atmosphere, both of which were measured by the same instrument. We found that magnesium has a lower energy than calcium in Mercury's atmosphere, which is consistent with our understanding that less energy is necessary for magnesium than calcium to break apart their original molecules in photodissociation. This study also demonstrates how to improve the quality of data obtained from the BepiColombo spacecraft. Key Points: We established the calibration and background subtraction methods of the FUV channel of BepiColombo MPO/PHEBUSWe analyzed the swing‐by data and successfully obtained Mg exosphere distributions consistent with two theoretical modelsThe lower energy of the Mg exosphere than that of Ca reflects the lower energy required for photodissociation from their parent molecules [ABSTRACT FROM AUTHOR]

The most abundant neutral constituent in the exospheric region (i.e., beyond = 500 km altitude) is the atomic hydrogen (H); however, its density distributions predicted by physics-based models have been challenged by satellite-based observations of its far ultraviolet emissions. This discrepancy may impact magnetospheric ions' densities and velocities since numerous chemistry and ion-neutral coupling interactions rely sensitively on the underlying neutral hydrogen population. The Polar Wind Outflow Model a first-principled model for relevant ion species in the high-latitude ionosphere, is employed to investigate the role of neutral H on the ionospheric outflow. Specifically, variability in the outflow of ionospheric H+, He+, N+, and O+ as a response to systematic enhancement and depletion of H number densities were simulated. The altitudedependent ion density and energy partition profiles vary with neutral H density, solar activities, and ion species. These findings suggest that the exosphere plays a crucial role in controlling the production and loss of ions through ionospheric chemistry, as well as the energy contributions by altering ionneutral-electron collisions and the ambipolar electric field to the high-latitude ionospheric outflow. As a result, the escape rates of the ionospheric outflow are directly associated with exospheric distributions. This work potentially helps understand the dominant mechanisms of atmospheric escape, particularly during a hydrogen-rich early Earth's and exoplanet's atmosphere, which is known to play a significant role in understanding the evolution of Earth's atmosphere. [ABSTRACT FROM AUTHOR]

Remote sensing observations of far-ultraviolet (FUV) emissions have been used to estimate 3D neutral hydrogen (H) density models of the terrestrial exosphere under solar minimum (2008) and solar maximum (2013 and 2015) conditions. Specifically, we used Lyman-alpha (Lyman-α, FUV at 121.6 nm) radiance data acquired by the Lyman-alpha detectors (LADs) onboard NASA’s TWINS satellites, collected over several days for each of the 3 years to provide sufficient coverage of the exospheric region. The datasets included Lyman-α measurements taken only above Earth radii (Re) of 3.75, assumed to be the optically thin region of the exosphere, where the measured Lyman-α intensity along a line of sight (LOS) is linearly proportional to the atomic hydrogen column density. Based on the calibration using multiple UV-bright stars, a significant decrease in TWINS1 LAD1/2 sensitivities was found for 2013 (LAD2 ∼1/2, LAD ∼1/3) and 2015 (LAD2 ∼1/3, LAD ∼1/7) compared to 2008. The calibration uncertainty was derived to be, on average, 5%. We estimated the 3D global hydrogen density distributions from these radiance data using two different tomographic inversion methods: first, a parametric fitting method based on a spherical harmonic function of order 3 and second, a high degree-of-freedom retrieval approach to validate our retrievals of H density. Both inversion methods consistently incorporate the effects of Lyman-α absorption within the exosphere and Lyman-α re-emission from Earth’s albedo. Our results reveal that H densities during solar maximum conditions are, on average, 30%–40% higher than those during solar minimum. All models showed a high concentration of atomic H on the dayside and nightside near the Sun–Earth line, which determines a nose/geotail structure consistent with theoretical effects from the solar radiation pressure. Furthermore, we identified that H-density enhancements during solar maximum with respect to solar minimum conditions occur at mid-to-high latitudes, particularly on the dusk side, while no significant enhancement seems to occur near the dayside nose.

The most abundant neutral constituent in the exospheric region (i.e., beyond ≈ 500 km altitude) is the atomic hydrogen (H); however, its density distributions predicted by physics-based models have been challenged by satellite-based observations of its far ultraviolet emissions. This discrepancy may impact magnetospheric ions’ densities and velocities since numerous chemistry and ion-neutral coupling interactions rely sensitively on the underlying neutral hydrogen population. The Polar Wind Outflow Model a first-principled model for relevant ion species in the high-latitude ionosphere, is employed to investigate the role of neutral H on the ionospheric outflow. Specifically, variability in the outflow of ionospheric H+, He+, N+, and O+ as a response to systematic enhancement and depletion of H number densities were simulated. The altitude-dependent ion density and energy partition profiles vary with neutral H density, solar activities, and ion species. These findings suggest that the exosphere plays a crucial role in controlling the production and loss of ions through ionospheric chemistry, as well as the energy contributions by altering ion-neutral-electron collisions and the ambipolar electric field to the high-latitude ionospheric outflow. As a result, the escape rates of the ionospheric outflow are directly associated with exospheric distributions. This work potentially helps understand the dominant mechanisms of atmospheric escape, particularly during a hydrogen-rich early Earth’s and exoplanet’s atmosphere, which is known to play a significant role in understanding the evolution of Earth’s atmosphere.

One of the main features that distinguishes dusty plasma from ordinary (not containing charged dust particles) plasma is anomalous dissipation associated with the process of charging dust particles, leading to new physical phenomena, effects and mechanisms. The process of anomalous dissipation is considered in the context of describing the dynamics of dust particles in the dusty plasma of atmosphereless bodies of the Solar System. A description of the oscillations of a dust particle over the surfaces of Mercury, the Moon, and the Martian satellites Phobos and Deimos is presented, the attenuation of which is determined by the charging frequency of the dust particles, which characterizes anomalous dissipation. The possibility of using an approach that takes into account anomalous dissipation to describe plasma-dust processes in the vicinity of comets is discussed. It is shown that anomalous dissipation plays a significant role in determining the possibility of using the model of levitating dust particles in describing dusty plasma over the surfaces of atmosphereless bodies of the Solar System. The results of numerical calculations are presented, confirming the possibility of using this model for a number of atmospherelesso cosmic bodies. [ABSTRACT FROM AUTHOR]

The variability of Mars exosphere over monthly to solar‐cycle scales at 251 and 412 km altitude is quantified by analysis of 41‐Ls mean densities derived from precise orbit determination of the Mars Reconnaissance Orbiter (MRO) and Mars Odyssey (MO) satellites, respectively. The data encompass 2006–2020 (MRO) and 2002–2020 (MO). At both altitudes, most of the variance is captured by cos(Ls–ϕ), where ϕ ≈ 258°. This term represents the effects of solar heating changes due to the eccentricity of Mars orbit around the Sun, and climatological changes in heating due to lower‐atmosphere dust loading, which does not play a significant role. The remaining variability is connected with the "irregular" variability of solar flux over monthly time scales. For MO, the presence of Helium disrupts a clean correlation with these sources. Plain Language Summary: The force of continuous bombardment of atmospheric particles on a satellite slowly changes its orbit. By tracking the satellite, changes in its orbit can in turn be used to infer atmospheric particle densities. In this paper, we use tracking from NASA's Deep Space Network, an international array of giant antennas, to measure minute orbital changes of the Mars Reconnaissance Orbiter and Mars Odyssey satellite orbits (and thereby atmospheric densities) around Mars during the 2002–2020 time frame. These data are used to study how the outer regions of Mars atmosphere respond to changes in heating due to solar radation absorption by various chemical species, and by dust that is elevated into the atmosphere by strong winds near the surface. The major finding of this paper is that the variability of Mars outer atmosphere is mainly controlled by solar heating changes due to the eccentricity of Mars orbit around the Sun. Key Points: Densities from precise orbit determination of Mars Reconnaissance Orbiter and Mars Odyssey are used to quantify long‐term variability of Mars' exosphereThe annual term is the greatest contributor to total variance, followed by the irregular component of EUV/UV fluxDust‐induced inter‐annual variability is small, but the analysis method does suppress potentially important regional‐scale events [ABSTRACT FROM AUTHOR]

We present the first observations of the dayside coronal oxygen emission in far ultraviolet (FUV) measured by the Emirates Mars Ultraviolet Spectrometer (EMUS) onboard the Emirates Mars Mission (EMM). The high sensitivity of EMUS is providing an opportunity to observe the tenuous oxygen corona in FUV, which is otherwise difficult to observe. Oxygen resonance fluorescence emission at 130.4 nm provides a measurement of the upper atmospheric and exospheric oxygen. More than 500 oxygen corona profiles are constructed using the long–exposure time cross–exospheric mode (OS4) of EMUS observations. These profiles range from ∼200 km altitude up to several Mars radii (>6 RM) across all seasons and for two Mars years. Our analysis shows that OI 130.4 nm is highly correlated with solar irradiance (solar photoionizing and 130.4 nm illuminating irradiances) as well as changes in the Sun–Mars distance. The prominent short term periodicity in oxygen corona brightness is consistent with the solar rotation period (quasi–27–day). A comparison between the perihelion seasons of Mars Year (MY) 36 and MY 37 shows interannual variability with enhanced emission intensities during MY 37, due to the rise of Solar Cycle 25. These observations show a highly variable oxygen corona, which has significant implications on constraining the photochemical escape of atomic oxygen from Mars. Plain Language Summary: The highly sensitive Emirates Mars Ultraviolet Spectrometer (EMUS) onboard Emirates Mars Mission (EMM) is capable of observing ultraviolet emissions emanating from Mars. Oxygen in the Martian exosphere is hard to see because it's tenuous. In this study, the analysis of long exposure time EMUS optical observations show that the hot oxygen corona on Mars has a short term variability due to solar rotation. Hot oxygen corona also shows a long–term variability that depends on the Sun–Mars distance and the solar cycle progression. When comparing data from two Martian years, it is noticed that the oxygen corona became brighter when the Sun is more active. Key Points: Brighter O corona is observed during perihelion and dimmer during aphelion, indicating a strong relationship with the Sun‐Mars distanceThe variation in OI 130.4 nm brightness shows a linear correlation with solar EUV irradiance, with a short‐term solar rotation periodicityInterannual variability is observed from MY 36 to MY 37, showing an enhancement in O corona brightness with the rise of Solar Cycle 25 [ABSTRACT FROM AUTHOR]

The anomalous dissipation related to the effect of charging of dust particles that gives rise to new physical phenomena, effects, and mechanisms represents one of the main specific features of dusty plasma that makes it different from conventional plasma containing no charged dust particles. We analyze the process of anomalous dissipation in the context of description of the dynamics of dust particles in dusty plasma of the Mercury's exosphere. An analytical description of oscillations of a dust particle above the surface of Mercury is presented. The frequency of charging of dust particles that characterizes the anomalous dissipation determines the damping of such oscillations. It is demonstrated that the anomalous dissipation is important for substantiation of the model of levitating dust particles that is used for description of dusty plasma above Mercury. The results of numerical simulations that justify the use of the discussed model are presented. [ABSTRACT FROM AUTHOR]

An important discovery of MESSENGER is the occurrence of dayside disappearing magnetosphere (DDM) events that occur when the solar wind dynamic pressure is extremely high and the interplanetary magnetic field (IMF) is both intense and southward. In this study, we investigate the DDM events at Mercury under extreme solar wind conditions using a three‐dimensional (3‐D) global hybrid simulation model. Our results show that when the solar wind dynamic pressure is 107 nPa and the magnitude of the purely southward IMF is 50 nT, most of the dayside magnetosphere disappears within 10 s after the interaction between the solar wind and the planetary magnetic field starts. During the DDM event, the ion flux is significantly enhanced at most of the planetary dayside surface and reaches its maximum value of about 1010 cm−2 s−1 at the low‐latitude surface, which is much larger than that under normal solar wind conditions. During the DDM events, the dayside bow shock mostly disappears for about 9 s and then reappears. Moreover, the time evolution of magnetopause standoff distance under different solar wind conditions is also studied. When the solar wind dynamic pressure exceeds 25 nPa and the IMF is purely southward, a part of the dayside magnetosphere disappears. Under the same IMF, the higher the solar wind dynamic pressure, the faster the magnetopause standoff distance reaches the planetary surface. When the solar wind conditions are normal (with a dynamic pressure of 8 nPa) or the IMF is purely northward, the dayside magnetosphere does not disappear. The results provide a clear physical image of DDM events from a 3‐D perspective. Plain Language Summary: Mercury has a significant, and apparently global, magnetic field. The planetary magnetic interacts with the solar wind originating from the Sun, creating a magnetosphere. However, it has been observed that the dayside part of the planetary magnetosphere can disappear, which is referred to as dayside disappearing magnetosphere (DDM) events. These events normally occur when the solar wind dynamic pressure is extremely high and the magnetic field in the solar wind is both intense and southward. In this study, we use computer simulations to investigate the DDM events at Mercury under extreme solar wind conditions. Our results show that most of the dayside magnetosphere disappears after the interaction between the extreme solar wind and the planetary magnetic field, leading to a significant enhancement in the precipitation of ion flux on the planetary surface. Moreover, the temporal evolution of the dayside magnetosphere's size under different solar wind conditions was also studied. Under the same solar wind magnetic field, the higher the solar wind dynamic pressure, the smaller the magnetosphere. The results provide a clear physical image of DDM events from a 3‐D perspective. Key Points: Mercury's disappearing dayside magnetosphere event under extreme solar wind conditions is studied by using 3‐D global hybrid simulationsMost of the dayside magnetosphere, even the dayside bow shock, disappears shortly after the interaction between the extreme solar wind and the planetary magnetic fieldIon precipitation on the planetary surface and the time evolution of magnetopause standoff distance under different solar wind conditions are also studied [ABSTRACT FROM AUTHOR]

BepiColombo, the ESA/JAXA joint mission performed its first flyby of Mercury on 1 October 2021 and its second on 23 June 2022. PHEBUS observed the exosphere of Mercury during these flybys notably with its visible channels c404 (centered on the potassium emission line at 404 nm) and c422 (centered on the calcium emission line at 422 nm). The c422 signal shows not only an enhancement of calcium (Ca) near the dawn region but also a very extensive Ca exosphere on the morning side beyond 10,000 km. The e‐folding distance deduced from our Ca profiles (2,500–2,800 km) is in agreement with the value reported by MESSENGER at similar true anomaly angles. We use a Chamberlain model to determine the temperature and density at the exobase. Using the morning side low‐altitude data, we derived a high temperature at the exobase (>50,000 K), in agreement with the MESSENGER results. We also report a day/night asymmetry in the Ca exosphere that could indicate that the source of Ca is predominantly on the dayside or be the consequence of a shift of the main source of Ca away from the dawn region. The c404 channel detected additional species at low altitudes on the morning side during both flybys. Comparison with previous studies is inconclusive and further analysis will be necessary to identify the species. Nevertheless, we can note that the e‐folding distance deduced from our profile is relatively small (135 km) and that the Chamberlain model applied to our profiles seems to indicate a temperature at the exobase <3,000 K. Plain Language Summary: BepiColombo, the ESA/JAXA joint mission on its way to Mercury, has already flown over the planet twice in October 2021 and June 2022. PHEBUS (Probing of Hermean Exosphere By Ultraviolet Spectroscopy), the UV spectrometer onboard the spacecraft, observed Mercury's exosphere during the flybys at the closest parts of approach to the planet with its two visible channels notably. Mercury is surrounded by a tenuous collisionless atmosphere (exosphere) that contains a variety of species. We report the detection of exospheric calcium with an enhancement near the dawn region. We show that the calcium exosphere is much extended on the morning side, beyond 10,000 km. The observations also reveal a day/night asymmetry of the calcium exosphere. Furthermore, we report the detection of additional species. However, the flyby data alone do not allow us to formally identify the species. Key Points: PHEBUS observed Mercury's extended exosphere of calcium on the morning side beyond ∼10,000 kmThe observed calcium profiles may be explained by the 2D effects of altitude and local timeAdditional species were detected, potentially manganese and potassium [ABSTRACT FROM AUTHOR]

Abstract The variability of Mars exosphere over monthly to solar‐cycle scales at 251 and 412 km altitude is quantified by analysis of 41‐Ls mean densities derived from precise orbit determination of the Mars Reconnaissance Orbiter (MRO) and Mars Odyssey (MO) satellites, respectively. The data encompass 2006–2020 (MRO) and 2002–2020 (MO). At both altitudes, most of the variance is captured by cos(Ls–ϕ), where ϕ ≈ 258°. This term represents the effects of solar heating changes due to the eccentricity of Mars orbit around the Sun, and climatological changes in heating due to lower‐atmosphere dust loading, which does not play a significant role. The remaining variability is connected with the “irregular” variability of solar flux over monthly time scales. For MO, the presence of Helium disrupts a clean correlation with these sources.

Observations of Callisto's atmosphere have indicated an O2 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 O2 via radiolysis within Callisto's exposed ice patches and determine the corresponding O2 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 O2 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 O2. Our results suggest that production of O2 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 O2. We also make similar estimates for the production of H2 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. Plain Language Summary: Molecular oxygen (O2) has been inferred to exist in Callisto's atmosphere from observations taken across more than two decades by three separate instruments each using distinct measurement techniques. Exposure of Callisto's icy surface by the ions and electrons trapped in Jupiter's magnetic field is expected to produce O2, which subsequently releases into an atmosphere around Callisto. However, prior to this study, the amount of O2 produced through such pathways has not been explicitly quantified. Here, for the first time, we determine the amount of O2 produced by the irradiation of the exposed ice on Callisto's surface by Jupiter's plasma after depositing energy in the atmosphere. Our results show that this source of O2 does not produce nearly enough compared to what has been inferred from observations. Therefore, additional production mechanisms of O2 are required at Callisto. Key Points: We estimate the energy deposited by the thermal and energetic components of the Jovian magnetospheric plasma in Callisto's atmosphereWe calculate O2 column densities in Callisto's atmosphere corresponding to the energies of the plasma fluxes impacting the surfaceRadiolysis in the exposed ice on Callisto's surface is an insufficient source to produce the amount of O2 inferred from observations [ABSTRACT FROM AUTHOR]

On 1 October 2021, Bepi‐Colombo performed its first flyby of Mercury. During the maneuver, the short wavelength channel (55–155 nm) of "Probing the Hermean Exosphere by UV Spectroscopy" (PHEBUS) was activated for a total duration of 1 hr. The helium resonance line at 58.4 nm was clearly observed during the whole sequence. At large distance from the planet, the emission was due to helium atoms in the interplanetary medium (interplanetary UV glow). Just after crossing the terminator of the planet and entering the dawn side of the exosphere, PHEBUS observed a clear additional emission due to scattering of solar photons by helium atoms in the exosphere of Mercury. The first detection of the 58.4 nm line in the exosphere of Mercury was reported by Broadfoot et al. (1976, https://doi.org/10.1029/gl003i010p00577) following the Mariner 10 flybys in 1974. The PHEBUS observation of exospheric helium emissions is the first for this element since the UVS measurements. In this paper, we present the results of our analysis of the PHEBUS data at 58.4 nm. Calibration of both instruments are compared with observations of the interplanetary glow, showing that the measurements of both instruments are accurate. However, we find that the exospheric density of helium atoms deduced from the PHEBUS data is 4.5–7.5 times lower than the previous estimate from UVS on Mariner 10. Possible explanations are considered. We show that some of the helium atoms present in the exosphere of Mercury could originate from the local interstellar cloud. Plain Language Summary: On 1 October 2021, The Bepi‐Colombo mission performed its first flyby of Mercury. During this maneuver, the "Probing the Hermean Exosphere by UV Spectroscopy" (PHEBUS) ultraviolet spectrometer was activated for 1 hour, starting 30 min before the time of the closest approach to the surface of Mercury. This instrument is able to measure the emission of helium atoms either in the interplanetary medium or in the tenuous atmosphere of Mercury. The presence of helium atoms around Mercury was first discovered by the ultraviolet spectrograph (UVS) of the Mariner 10 mission in 1974. The observations of PHEBUS confirm the detection of helium atoms in the atmosphere of Mercury. However, the amount of helium atoms detected by PHEBUS is 4.5–7.5 times lower than the value reported by UVS after the Mariner 10 flybys. We discuss possible explanations for this difference. Key Points: This paper presents helium measurements obtained during the first flyby of Mercury by the Bepi‐Colombo missionCalibrations of UVS/Mariner 10 and of PHEBUS are cross‐checked with observations of the interplanetary glow at 58.4 nmThe helium content in the exosphere of Mercury derived by PHEBUS is 4.5–7.5 times lower than the previous estimate from the Mariner 10 mission [ABSTRACT FROM AUTHOR]

This study explores the impact of the exosphere hydrogen (H) density on the ionosphere-plasmasphere system using a model whose key inputs are constrained by ionosphere observations at both ends of the magnetic field line with an L-value of 1.75 in the American longitudinal sector during a period with low solar and magnetic activities. This study is the first to be validated by ground-based and satellite data in the plasmasphere and both hemispheres. The main finding is that the entire ionosphere-plasmasphere system is very sensitive to the neutral hydrogen density in the lower exosphere. It was found that an increase in the H density by a factor of 2.75 from the commonly accepted values was necessary to bring the simulated plasma density into satisfactory agreement with Arase satellite measurements in the plasmasphere and also with DMSP satellite measurements in the topside ionospheres of the northern and southern hemispheres. A factor of 2.75 increase in the H density increases the simulated plasma density in the afternoon plasmasphere up to ∼80% and in the nighttime topside ionosphere up to ∼100%. These results indicate prominently that using the commonly accepted empirical model of the H density causes unacceptable errors in the simulated plasma density of the near-Earth plasma shells. We alert the space science community of this problem.

The Peregrine Ion Trap Mass Spectrometer (PITMS) is a mass spectrometer instrument that operated during the Astrobotic Peregrine Mission-1 as part of the NASA Commercial Lunar Payload Services initiative. This paper describes the instrument and investigation design, development, and planning conducted by the PITMS team, consisting of a successful partnership between NASA Goddard Space Flight Center (GSFC), The Open University, NASA, and ESA. PITMS was designed to measure the abundance and temporal variability of volatile species in the near-surface lunar exosphere from a landed platform on the lunar surface. The PITMS instrument consisted of a European Space Agency–provided Exospheric Mass Spectrometer (including sensor, electronics, controller, and power supply boards) and a GSFC wrapper that provided structural elements, thermal control, and a deployable dust cover. PITMS was designed to operate as a passive sampler, where ambient gases would enter PITMS through an aperture, diffuse around the mass analyzer cavity, become ionized by electron impact and trapped in an RF field, and then sequentially be released to a detector to build a mass spectrum. PITMS was capable of measuring species with a mass-to-charge ratio ( m / z ) from 10 to 150 Da, with a mass resolution of approximately 0.5 amu. The PITMS science investigation was planned to be operated by GSFC with an international team of scientists. Though the mission did not achieve its lunar landing, information about the PITMS instrument and planning is provided to be able to understand and effectively use data that will be forthcoming from the investigation.

On icy bodies like the dwarf planet Ceres, impacts excavate volatile-rich material from beneath a dessicated lag layer and deposit it in the near-surface environment where higher temperatures drive sublimation. Ice has been detected in the upper meter of the ejecta blanket and interior of Occator crater, suggesting that large craters could be a significant source of exospheric water vapor. We assess the present-day exospheric contribution of a complex crater by first estimating the extent of volatile-rich deposits associated with a crater of a given size. We use a vapor diffusion model to calculate sublimation rates from the deposits, taking into account constraints on the thermophysical parameters of icy regolith from the Dawn mission. Extrapolating this model to craters formed throughout Ceres’ history, we find that the cumulative present-day sublimation rate from all complex crater deposits is ∼0.01 kg s ^−1 , a factor of a few times greater than the outgassing rate from the global ice table. The dominant source of sublimation is not the conspicuous faculae but rather the volatile-rich ejecta blankets, which cover a significantly larger area than deposits in the crater interior. Because large impacts can blanket a significant fraction of the surface with ice-rich ejecta, complex craters are crucial for understanding the background present-day exosphere and the history of sublimation on icy bodies.

Ion pickup by the solar wind is ubiquitous in space plasma. Because pickup ions are originally produced by ionization of an exospheric neutral atmosphere, their measurements contain information on the exospheric neutral abundance. Here we established a method to retrieve exospheric number densities, by analyzing the ion velocity distribution functions of pickup ions measured by the Mars Atmosphere and Volatile EvolutioN spacecraft. We successfully retrieved exospheric oxygen density distributions at altitudes ranging from 1000 to 10,000 km around Mars except for the vicinity of the bow shock. This method can be applied to other space missions to study the upper atmosphere of planets, moons, and other small bodies in our solar system, where pickup ions exist.

We supply the modelers with a database, SpuBase (doi: https://doi.org/10.5281/zenodo.10783295 ), that is based on the latest approach for obtaining solar wind ion sputter yields in agreement with experimental sputter data outlined in Jäggi et al. We include an overview of sputter results for typical Lunar and Hermean surfaces. To obtain total sputter yields for any given surface, we perform a mass balance of individual mineral sputter yields. For a set of impact angles, the angular and energy distribution data are scaled according to the sputter yield, summed up and fitted to obtain one probability distribution for each chemical element involved. Comparison of the results from different geochemical terranes on the Moon and Mercury has shown that variations in the abundance of silicates result in comparable energy and angular distribution data owing to the underlying model assumptions. The inclusion of sulfides relevant for Mercury, however, significantly affects the energy and angular distributions of sputtered particles. The application of the damage-driven sulfur diffusion rate in FeS in all sulfur-bearing minerals results in 35 times lower sulfur yields on average and a less prominent forward sputtering of sulfur at grazing incidence angles.

We provide an overview of our understanding of the dust environment at Mercury and the role that dust plays in shaping the planet's surface and exosphere. Our understanding of the role that dust impacts play in the generation of Mercury's atmosphere has evolved considerably with continued analysis of results from the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) mission. Recent models have provided evidence for the probable release of refractory species into Mercury's exosphere via impacts. However, there remain significant questions regarding the relative contributions of atoms released via impacts versus other mechanisms (e.g., photon-stimulated desorption) to the overall exospheric budget. We also discuss the state of observational and modeling efforts to constrain the dust environment at Mercury, including sources from the zodiacal cloud, cometary trails, and interstellar dust. We describe the advancements that will be made in our characterization of dust at Mercury with BepiColombo, providing observational constraints on the dust clouds themselves and the role that impacts play in exospheric generation. On Mercury's surface, there remain outstanding questions regarding the role that dust impacts play in the regolith cycling and development. We review how improved modeling efforts to understand grain lifetimes as a function of impactor flux will further our understanding of Mercury's regolith. Finally, there are few constraints on the role of dust impacts on the space weathering of Mercury's surface, particularly the expected chemical, physical, and spectral alterations to the regolith. Here we discuss the importance of laboratory experiments to simulate these processes for the interpretation of data from MESSENGER and BepiColombo.

A realistic model of physico-chemical processes during collisions between meteoroids and the Moon considering condensation of refractory elements in the form of minerals and variable adiabatic index during expansion of impact-produced clouds was developed. Quenched chemical composition of impact-produced cloud is estimated. In accordance with this model relative fraction of atoms delivered to the lunar exosphere by impacts of meteoroids is significantly higher than that previously estimated with usage of the model with constant adiabatic index and without considering condensation as a factor affecting on pressure in impact-produced clouds. • Advanced model of adiabatically expanding impact-produced hot clouds is developed. • Variable adiabatic index and condensation of minerals are considered. • Quenching parameters of the composition of impact-produced clouds are estimated. • Main species delivered to the lunar exosphere by impacts of meteoroids are determined. • Hydrogen is delivered to the exosphere by impacts mainly in the form of H and OH. [ABSTRACT FROM AUTHOR]

Neutral exospheres of large airless bodies consist of atoms or molecules on ballistic trajectories. An import example is the lunar water exosphere, thought to transport water to cold traps. In anticipation of future observational measurements, the theory of thermalized surface-bounded gravitationally-bound exospheres is further developed. The vertical density profile is calculated using thermodynamic averages of an ensemble of ballistic trajectories. When the launch velocities follow the Maxwell–Boltzmann Flux distribution, the classical density profile results. For many other probability distributions, including thermal desorption from a vertical wall, the density diverges logarithmically near the surface. Hence, an exosphere resulting from thermal desorption from a rough surface includes a ground-hugging population that appears to be colder than the surface. Another insight derived from the thermodynamic perspective is that cold traps can be interpreted in terms of the frostpoint of the water exosphere, if the long-term average of the pressure of the exosphere is considered. Ice in lunar caves is long-lasting only if the cave interior is below the cold trap temperature threshold. [ABSTRACT FROM AUTHOR]

Spacecraft flybys provide access to the chemical composition of the gaseous envelope of the planetary object. Typical relative encounter velocities range from km/s to tens of km/s in flybys. For speeds exceeding about 5 km/s, modern mass spectrometers analyzing the rapidly encountering gas suffer from intrinsic hypervelocity impact-induced fragmentation processes causing ambiguous results when analyzing complex molecules. In this case, instruments use an antechamber, inside which the incoming species collide many times with the chamber wall. These collisions cause the desired deceleration and thermalization of the gas molecules. However, these collisions also dissociate molecular bonds, thus fragmenting the molecules, and possibly forming new ones precluding scientists from inferring the actual chemical composition of the sampled gas. We developed a novel time-of-flight mass spectrometer that handles relative encounter velocities of up to 20 km/s omitting an antechamber and its related fragmentation. It analyzes the complete mass range of m/z 1 to 1000 at an instance. This innovation leads to unambiguous analysis of complex (organic) molecules. Applied to Enceladus, Europa or Io, it will provide reliable chemical composition datasets for exploration of the Solar System to determine its status, origin and evolution. [ABSTRACT FROM AUTHOR]

Current three‐dimensional, data‐based models for the terrestrial exosphere have been derived from measurements of optically thin Lyman‐alpha (Ly‐α) emissions scattered by neutral hydrogen atoms. Such models are only valid for the middle exospheric region (3–8 Earth radii geocentric distances) since the orbital paths of the space‐based platforms used to acquire Ly‐α radiance were located within the exosphere, thus precluding the proper detection of the faint outer exospheric emission. Notwithstanding, accurate specifications of density distributions beyond 8 RE are needed to support comprehensive studies of the solar‐terrestrial interactions. Two upcoming missions, the Solar wind Magnetosphere‐Ionosphere Link Explorer and the Lunar Environment Heliospheric X‐ray Imager, will image the Earth's magnetosheath in soft X‐rays, and neutral densities are crucial to extract ion distributions through inversion of the acquired images. This work develops a technique to estimate the Earth's outer exospheric density distributions using far‐ultraviolet wide‐field data acquired by the Lyman‐Alpha Imaging Camera (LAICA) onboard the Proximate Object Close Flyby with Optical Navigation mission. Our approach formulates an inverse problem based on the linearity between measurements of scattered Ly‐α flux and the local atomic hydrogen density, which is solved using the Bayesian approach known as Maximum a posteriori estimation. We use the LAICA image to derive global, 3‐D hydrogen density distributions at 6–20 RE geocentric distances. We find that the spatial structure of the outer exosphere agrees well with the predictions of radiation pressure theory. Further, we find that the hydrogen density at the 10 RE subsolar point is 26.51 atoms/cm3. Key Points: The Proximate Object Close Flyby with Optical Navigation/Lyman‐Alpha Imaging Camera (LAICA) instrument provides unprecedented data to study the vast terrestrial exosphereWe introduce a statistical inversion technique to derive 3‐D exospheric density distributions at 6–20 RE using the LAICA data setDerived exospheric spatial structure is consistent with the predictions of the radiation pressure theory [ABSTRACT FROM AUTHOR]

A map of surface molecular water was derived from long slit spectroscopy of the south polar region of the Moon using the Faint Object infraRed CAmera for the SOFIA Telescope spectrometer on the Stratospheric Observatory for Infrared Astronomy. Mean water abundances detected are about 250 μg/g over that of a mare reference surface at Mare Fecunditatis. Water abundances are locally anticorrelated with temperature. The distribution of water is consistent with derivation of water from pre‐existing hydroxyl subsequently trapped in impact glass, provided hydroxyl increases with latitude as some models and measurements suggest. The detected water cannot be in equilibrium with the exosphere because insufficient water is present in the exosphere to maintain the surface abundance. The data are consistent with a high latitude water‐bearing mineral host that may be a precursor to recently detected high latitude hematite. Plain Language Summary: If water is present in sufficient quantities on the Moon, it may be an important resource for space exploration as it can be used to make rocket fuel and sustain human presence. Water molecules on the illuminated surface of the Moon can be detected using a unique spectral signature that is obscured from telescopes on Earth's surface by water vapor in the atmosphere. The NASA/DLR Stratospheric Observatory for Infrared Astronomy is a large telescope on a 747 aircraft that operates high in the stratosphere above 99% of the water vapor, and so can detect the infrared thermal emission from water on the Moon's surface. Near the South Pole, we produced a map of water emission that shows water is present at a few hundred parts per million, and is inversely correlated with surface temperature. This is consistent both with the behavior of water free to migrate on the surface, and water bound up in glass from meteorite impacts. However, the amount of water we find probably cannot be freely exchanging with the Moon's tenuous exosphere because it would require much more water in the exosphere than has been measured. Key Points: Stratospheric Observatory for Infrared Astronomy + Faint Object infraRed CAmera for the SOFIA Telescope confirms the presence of water emission in the South Polar Region of the MoonSmall maps of water emission are presented that begin to enable tests of various hypotheses for water formation and variationObservations suggest the water cannot migrate and instead is trapped within impact glasses or in a mineral‐based host [ABSTRACT FROM AUTHOR]

The Moon and Mercury are airless bodies, thus they are directly exposed to the ambient plasma (ions and electrons), to photons mostly from the Sun from infrared range all the way to X-rays, and to meteoroid fluxes. Direct exposure to these exogenic sources has important consequences for the formation and evolution of planetary surfaces, including altering their chemical makeup and optical properties, and generating neutral gas exosphere. The formation of a thin atmosphere, more specifically a surface bound exosphere, the relevant physical processes for the particle release, particle loss, and the drivers behind these processes are discussed in this review. [ABSTRACT FROM AUTHOR]

The first maps of electron-induced X-ray emission from the dayside of Mercury's surface are presented, generated by the development of a solar X-ray flux filter. This enables the isolation of the X-ray fluorescence of calcium driven by probable electron precipitation. A catalog of such events has been generated and dayside maps of implied electron precipitation zones have been produced. We find that, similar to electron induced emission events on the nightside, these zones are strongly organized by latitude and magnetic local time. The majority of the dayside events appear in the southern hemisphere and there is a strong enhancement observed centered about local dawn (06:00 LT). There is apparent poleward continuation of emission in the north, but very few events were observed on the duskward hemisphere. These results carry implications for Mercury's magnetosphere by constraining zones of electron precipitation, for the exosphere as a potential source of exospheric species, and for surface science as an additional source of X-ray fluorescence. Plain Language Summary Mercury has a magnetic field which is similar to Earth's, and charged particles (electrons and protons) within it move in similar ways. At Earth, particles which reach the atmosphere generate the aurora borealis and aurora australis (northern and southern lights). At Mercury, there is no atmosphere so these particles reach the surface, where they produce X-rays. At night, it is relatively simple to detect these X-rays and map the regions where they are being produced, because the only source of X-rays is the particles reaching the surface. During the day, however, the signal can easily be overwhelmed because at the same time X-rays are being produced in response to illumination by the Sun. We have been able to tentatively isolate the X-ray signal coming from charged particles during the day on Mercury by applying a filter which takes into account the brightness of the Sun in X-rays at the same time. When the sun is dim in X-rays but the surface is bright, we can be confident that the surface signal comes from charged particles, and vice versa. [ABSTRACT FROM AUTHOR]

The atmosphere of the Jovian Europa being tenuous, the interaction of the energetic solar photons is a probable source of excitation for the emission of X-rays from the surface of the satellite. Solar photons in the X-region of the electromagnetic spectrum have energies greater than the binding energy of the elemental constituents of the surface, cause excitation of electrons, and on de-excitation cause emission of X-rays through X-ray fluorescence. We developed models for the computation of the solar X-ray flux during the representative phases of a solar activity cycle (1-100 Å), and using these fluxes, we computed the photon-induced X-ray fluorescent and Rayleigh scattering emission flux from the regolith of the Europa by taking into account its predominant H2O ice composition. This work observed that during the extreme case of the quiet Sun condition and an X2-class flare, the X-ray energy flux at the Europa distance of 4.96 AU vary from 1.08 x 10-7 to 5.23 x 10-4 ergs-cm-2s-1. This flux from the H2O ice composition can generate a total X-ray energy flux, inclusive of the X-ray fluorescent and Rayleigh scattering events, varying as 1.40 x 10-23 to 6.21 x 10-20 ergs-cm-2s-1. These computed numbers of the X-ray energy fluxes at the telescope of the Chandra X-ray Observatory are of lesser magnitude in comparison to its observed numbers (3.0 x 10-16). This deduction reveals the influence of a stronger excitation source than the photon-induced mechanisms of the generation of X-rays from the surface of the Europa. [ABSTRACT FROM AUTHOR]

In a seminal analysis, Fuselier et al. (2020) combined subsolar in situ magnetosheath plasma observations with remote energetic neutral atom (ENA) observations to derive an exospheric neutral density of 11 cm−3 in the vicinity of the subsolar magnetopause. This commentary extends the analysis of Fuselier et al. (2020) to (a) employ predicted and observed magnetosheath plasma parameters at times other than those immediately following an MMS magnetopause crossing, (b) take into account the component of the plasma bulk flow transverse to the ENA spacecraft's line‐of‐sight through the flank magnetosheath, and (c) consider the cooling that results from the antisunward expansion of the magnetosheath plasma. With these additional considerations, subsolar exospheric neutral densities rise by a factor of 4.1–4.5 to 45–50 cm−3, consistent with results from previous studies. Plain Language Summary: Because the number of energetic neutral atoms generated when protons exchange electrons with hydrogen atoms depends on the number of protons available for charge exchange and the neutral density, neutral densities in the Earth's outer atmosphere can (with the use of models) be deduced from simultaneous remote observations of energetic neutral atoms and in situ plasma observations. When proton parameters typical of the time interval studied and the motion of the protons are taken into account, neutral densities in the vicinity of the subsolar point on the Earth's magnetopause are found to be on the order of 50 cm−3. Key Points: The strength of energetic neutral atom and soft X‐ray emissions from the magnetosheath depends upon exospheric neutral densitiesAn augmentation of a previous MMS/IBEX study indicates that neutral densities may significantly exceed those originally derivedExospheric neutral densities suffice to ensure the success of future soft X‐ray imaging missions [ABSTRACT FROM AUTHOR]

The community has leveraged satellite accelerometer data sets in previous years to estimate neutral mass density and exospheric temperatures. We utilize derived temperature data and optimize a nonlinear machine‐learned (ML) regression model to improve upon the performance of the linear EXospheric TEMPeratures on a PoLyhedrAl gRid (EXTEMPLAR) model. The newly developed EXTEMPLAR‐ML model allows for exospheric temperature predictions at any location with one model and provides performance improvements over its predecessor. We achieve reductions in mean absolute error of 2 K on an independent test set while providing similar error standard deviation values. Comparing the performance of both EXTEMPLAR models and the Naval Research Laboratory Mass Spectrometer and Incoherent Scatter radar Extended model (NRLMSISE‐00) across different solar and geomagnetic activity levels shows that EXTEMPLAR‐ML has the lowest mean absolute error across 80% of conditions tested. A study for spatial errors demonstrated that at all grid locations, EXTEMPLAR‐ML has the lowest mean absolute error for over 60% of the polyhedral grid cells on the test set. Like EXTEMPLAR, our model's outputs can be utilized by NRLMSISE‐00 (exclusively) to more closely match satellite accelerometer‐derived densities. We conducted 10 case studies where we compare the accelerometer‐derived temperature and density estimates from four satellites to NRLMSISE‐00, EXTEMPLAR, and EXTEMPALR‐ML during major storm periods. These comparisons show that EXTEMPLAR‐ML generally has the best performance of the three models during storms. We use principal component analysis on EXTEMPLAR‐ML outputs to verify the physical response of the model to its drivers. Plain Language Summary: Density in the upper atmosphere is highly variable and difficult to model. Empirical density models often rely on temperature profile predictions to determine species and mass densities. One of three key parameters in determining the temperature profiles is the asymptotic value at the top of the thermosphere called the exospheric temperature. By using temperatures derived from satellite acceleration measurements, we develop a machine‐learned global temperature model called EXospheric TEMPeratures on a PoLyhedrAl gRid Machine Learned (EXTEMPLAR‐ML). We achieve a 2 K reduction in mean absolute error on the independent test set relative to the model's predecessor. Additional analyses showed that EXTEMPLAR‐ML was more accurate than linear EXTEMPLAR across a majority of conditions and grid locations. We also look at temperatures and densities along satellite orbits during 10 major geomagnetic storms from the 21st century. In this study, we see major improvements over a significant empirical model called NRLMSISE‐00 and the linear predecessor to EXTEMPLAR‐ML. We leveraged a mathematical decomposition tool on the model outputs to assess its internal formulation. This shows that EXTEMPLAR‐ML is most heavily driven by solar activity and the seasons. Key Points: We develop a nonlinear global model for exospheric temperature prediction called EXospheric TEMPeratures on a PoLyhedrAl gRid Machine Learned (EXTEMPLAR‐ML)We leverage principal component analysis to improve our understanding of the EXTEMPLAR‐ML temperature formulationEXTEMPLAR‐ML shows increased accuracy relative to satellite observations across a majority of conditions, locations, and during geomagnetic storms [ABSTRACT FROM AUTHOR]

The EXospheric TEMperatures on a PoLyhedrAl gRid (EXTEMPLAR) method predicts the neutral densities in the thermosphere. The performance of this model has been evaluated through a comparison with the Air Force High Accuracy Satellite Drag Model (HASDM). The Space Environment Technologies (SET) HASDM database that was used for this test spans the 20 years 2000 through 2019, containing densities at 3 hr time intervals at 25 km altitude steps, and a spatial resolution of 10° latitude by 15° longitude. The upgraded EXTEMPLAR that was tested uses the newer Naval Research Laboratory MSIS 2.0 model to convert global exospheric temperature values to neutral density as a function of altitude. The revision also incorporated time delays that varied as a function of location, between the total Poynting flux in the polar regions and the exospheric temperature response. The density values from both models were integrated on spherical shells at altitudes ranging from 200 to 800 km. These sums were compared as a function of time. The results show an excellent agreement at temporal scales ranging from hours to years. The EXTEMPLAR model performs best at altitudes of 400 km and above, where geomagnetic storms produce the largest relative changes in neutral density. In addition to providing an effective method to compare models that have very different spatial resolutions, the use of density totals at various altitudes presents a useful illustration of how the thermosphere behaves at different altitudes, on time scales ranging from hours to complete solar cycles. Plain Language Summary: A recently developed computer model predicts the mass density of atoms and molecules in upper atmosphere, in the region known as the thermosphere. Changes in this "neutral density" following geomagnetic storms can perturb the orbits of the many satellites in this region, leading to imprecise knowledge of their paths and risk of collisions. This model uses measurements of the solar wind and the embedded magnetic field to predict the level of heating in the upper atmosphere, and the resulting expansion of the atmosphere to higher altitudes. In order to test the capabilities of the new model, its calculations were compared with density values derived by an Air Force data assimilation system based on radar tracking of multiple objects in Earth orbit over a 20‐year period. The results of this comparison show an excellent agreement, particularly at the higher altitudes where geomagnetic storms have the greatest influence. Key Points: Thermosphere neutral densities from the EXospheric TEMperatures on a PoLyhedrAl gRid (EXTEMPLAR) model are compared with the SET HASDM density database for a 20 year time periodThe use of mean densities on spherical shells at several altitudes is an effective way to compare the modelsThe EXTEMPLAR model performs well at altitudes of 400 km and above where geomagnetic storms produce the largest changes in neutral density [ABSTRACT FROM AUTHOR]

Detailed description of the original pointing software used for spectral observations of the lunar exosphere is given. The software allows to point the spectrograph of 2-m telescope of Terskol Observatory inside and outside the lunar disk with accuracy of about 2″. Observations of Na lines in the exosphere of the Moon on January 8 and October 7, 2017, do not reveal quick variability of Na lines at time scales of about 20 minutes. On January 8, 2017, the intensity of Na lines above the south pole of the Moon was stronger than that above the north pole of the Moon.

The fate of Mercury’s exospheric volatiles and, in a lesser way, of the refractory particles absorbed in the first few centimeters of the surface both depend highly on the temperature profile with depth and its diurnal variation. In this paper, we review several mechanisms by which the surface temperature might control the surface/exosphere interface. The day/night cycle of the surface temperature and its orbital variation, the temperature in the permanent shadow regions, and the subsurface temperature profiles are key thermal properties that control the fate of the exospheric volatiles through the volatile ejection mechanisms, the thermal accommodation, and the subsurface diffusion. Such properties depend on the solar illumination from large to small scales but also on the regolith structure. The regolith is also space-weathered by the thermal forcing and by the thermal-mechanical processing. Its composition is changed by the thermal conditions. We conclude by discussing key characteristics that need to be investigated theoretically and/or in the laboratory: the dependency of the surface spectra with respect to temperature, the typical diffusion timescale of the volatile species, and the thermal dependency of their ejection mechanisms.

Mars has undergone a substantial water loss, transforming from the early warm and wet state to the current cold and arid state. Observations and modeling efforts suggest that hydrogen escape is a metric of water loss on Mars. As a consequence of the vertical transport of water vapor by deep convection, hydrogen escape is significantly enhanced during Martian global dust storms. Motivated by the established scenario that the horizontal wind field could substantially enhance thermal escape, here we evaluate, for the first time, how the escape of H and H _2 on Mars during a typical global dust storm is modified by the enhanced horizontal wind field during the period. By combining kinetic model calculations and the Mars Climate Database outputs, we reach the conclusion that a nonnegligible enhancement of the H and H _2 escape flux could be driven by horizontal winds near the exobase, reaching 15% for H and 60% for H _2 at dawn near the equator during the dust storm. Although the enhancement of the global hydrogen escape rate by the horizontal wind is insignificant, it plays a crucial role in the redistribution of H and H _2 escape flux. The results presented here make useful contributions to a thorough understanding of enhanced hydrogen escape during the global dust storms.

Rocky planets and moons experiencing solar wind sputtering are continuously supplying their enveloping exosphere with ejected neutral atoms. To understand the quantity and properties of the ejecta, well-established binary collision approximation Monte Carlo codes like TRIM with default settings are used predominantly. Improved models such as SDTrimSP have come forward, and together with new experimental data, the underlying assumptions have been challenged. We introduce a hybrid model, combining the previous surface binding approach with a new bulk binding model akin to Hofsäss & Stegmaier. In addition, we expand the model implementation by distinguishing between free and bound components sourced from mineral compounds such as oxides or sulfides. The use of oxides and sulfides also enables the correct setting of the mass densities of minerals, which was previously limited to the manual setting of individual atomic densities of elements. All of the energies and densities used are thereby based on tabulated data, so that only minimal user input and no fitting of parameters are required. We found unprecedented agreement between the newly implemented hybrid model and previously published sputter yields for incidence angles up to 45° from surface normal. Good agreement is found for the angular distribution of mass sputtered from enstatite MgSiO _3 compared to the latest experimental data. Energy distributions recreate trends of experimental data of oxidized metals. Similar trends are to be expected from future mineral experimental data. The model thus serves its purpose of widespread applicability and ease of use for modelers of rocky body exospheres.

Solar wind (SW) ion irradiation on airless bodies can play an important role in altering their surface properties and surrounding exosphere. Much of the ion sputtering data needed for exosphere studies come from binary collision approximation sputtering models such as TRansport of Ions in Matter and its more recent extension, SDTrimSP. These models predict the yield and energy distribution of sputtered atoms, along with the depth of deposition and damage of the substrate, all as a function of the incoming ion type, impact energy, and impact angle. Within SDTrimSP there are several user-specific inputs that have been applied differently in previous SW ion sputtering simulations. These parameters can influence the simulated behavior of both the target and sputtered atoms. Here, we have conducted a sensitivity study into the SDTrimSP parameters in order to determine a best practice for simulating SW ion impacts onto planetary surfaces. We demonstrate that ion sputtering behavior is highly sensitive to several important input parameters including the ion impact angle and energy distribution and the ejected atom surface binding energy. Furthermore, different parameters can still result in similarities in the total sputtering yield, potentially masking large differences in other sputtering-induced behaviors such as the elemental yield, surface concentration, and damage production. Therefore, it is important to consider more than just the overall sputtering behavior when quantifying the importance of different parameters. This study serves to establish a more consistent methodology for simulations of SW-induced ion sputtering on bodies such as Mercury and the Moon, allowing for more accurate comparisons between studies.

We identify magnetic waves in the initial insertion orbits of the Mars Atmosphere and Volatile EvolutioN (MAVEN) mission spacecraft far upstream of the Martian bow shock. Overall, the majority of the observed magnetic waves are elliptically polarized and more perpendicular than parallel relative to the mean magnetic field. We find and analyze numerous waves near 1 Hz. The handedness of these 1 Hz waves tends toward left‐handed as the angle between the solar wind velocity and the magnetic field decrease. Similar waves are not seen in the Juno cruise data when it passed Mars' orbit (while Mars was more than 1 AU away). The presence of these waves out to distances of 14 Mars radii as observed by MAVEN and their absence in the Juno cruise data suggests the 1 Hz waves are amplified and/or created by the Martian environment. We also find a second, separate class of intermittent waves with frequencies near the proton cyclotron gyrofrequency. These waves are primarily right‐handed polarized in the spacecraft frame. The spatial distribution of the waves suggests no clear association with the well‐known plume of escaping ions that flows in the direction of the solar wind electric field. There are no obvious correlations between the wave amplitudes and the direction of the local interplanetary magnetic field or on their proximity to Mars. These characteristics lead us to conclude that the majority of these near‐proton cyclotron gyrofrequency waves are likely whistler‐mode waves that are not directly associated with Mars. Key Points: Waves near 1 Hz far upstream of the Martian bow shock are enhanced and/or created by the Martian environmentWaves near the proton cyclotron frequency far upstream of the Martian bow shock are intermittent and likely of solar wind originMars' plasma influence extends to at least 14 Mars radii [ABSTRACT FROM AUTHOR]

Data collected by the Fast Imaging Plasma Spectrometer (FIPS) aboard the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft showed singly charged Na+‐group ions at energies of between 1 and 13 keV in Mercury's northern planetary cusp. Most of these ions are likely formed by either photoionization or charge exchange of exospheric Na atoms, with initial energies of approximately 1 eV or less. FIPS observations did not establish which acceleration mechanism most reasonably accounts for this energy gain. Using the Adaptive Mesh Particle Simulator (AMPS) model, we undertake kinetic simulations of 1 eV Na+ test particles through the electric and magnetic fields output from the Block Adaptive Tree Solar wind Roe‐type Upwind Scheme (BATSRUS) global magnetohydrodynamic (MHD) model of Mercury's magnetosphere, in search of plausible explanations for the source of this energization. We find that Na+ with initial energy of 1 eV are readily picked up by the Dungey cycle return flow in the dayside magnetosphere. In some cases, this flow provides the energy for the ions to escape into the magnetosheath, and in other cases it energizes the ions to hundreds of eV before they pass immediately into the cusp. Those that escape can be rapidly picked up into the magnetosheath flow, where they are accelerated by pickup again up to tens of keV. These one‐ and two‐stage pickup processes on Mercury's dayside can account for the energies of many of the Na+ ions observed in Mercury's northern magnetospheric cusp by MESSENGER. Plain Language Summary: Data collected in orbit at Mercury showed sodium ions in the northern planetary cusp at high energies. The processes that likely create these ions are only responsible for 0.01%–0.1% of that high energy, and no mechanism previously known to operate at Mercury can account for the difference. We model the Mercury system, and the paths of ions through that system, in search of such a mechanism. We find two mechanisms, both involving energization into proton flows, that explain the data observations. Key Points: Sodium ions (Na+) with initial energy of 1 eV are energized by Dungey cycle return flow in the dayside dipolar regionNa+ are not trapped within the dayside dipolar region of MercuryOne‐ and two‐stage pickup processes can energize Na+ up to tens of keV entirely on the dayside [ABSTRACT FROM AUTHOR]

We utilize measurements from instruments on the Mars Atmosphere and Volatile EvolutioN (MAVEN) mission to investigate singly ionized helium formed by charge exchange between solar wind alpha particles and neutral hydrogen in the region upstream from Mars. We show that the observed helium ion signal varies with solar wind speed and spatial location in a manner consistent with expectations for a charge exchange source. We find that the ratio of singly to doubly ionized helium varies with Martian season, with a peak in the southern summer season. The inferred neutral hydrogen column density and the seasonal variation thereof agree with the results of previous studies based on other measurement techniques. The MAVEN helium ion measurements provide a new method of probing the hydrogen corona, with nearly continuous coverage of the Martian seasonal cycle across the entire mission, enabling study of the interannual variability of the Martian exosphere. Plain Language Summary: Thanks to Mars' small size and weak gravity, the upper reaches of its atmosphere extend far above the surface. The lightest element, hydrogen, forms a corona around Mars many times larger than the planet. The solar wind that flows from the Sun, composed mainly of protons and alpha particles (doubly ionized helium), interacts directly with this hydrogen corona. Reactions between solar wind alpha particles and neutral hydrogen in the corona can form singly ionized helium. We utilize measurements from instruments on the Mars Atmosphere and Volatile EvolutioN (MAVEN) mission to investigate the singly ionized helium signature in the region upstream from Mars. We show that the measured signal varies with solar wind speed and distance from Mars as expected, and utilize it to probe the structure and seasonal variability of the Martian hydrogen corona. These measurements will ultimately allow us to better understand the Martian seasonal cycle and its role in the escape of hydrogen from the Martian atmosphere. Key Points: Singly ionized helium is produced upstream from Mars by charge exchange between solar wind alpha particles and Martian neutral hydrogenThe observed ratio of singly to doubly ionized helium varies as expected with solar wind speed, spatial location, and Martian seasonHelium ion measurements can be utilized to estimate the column density of hydrogen in the Martian corona and its seasonal variability [ABSTRACT FROM AUTHOR]

Plumes spewing water high above Europa's surface have been inferred from several observation campaigns in the past decade. Whereas the occasional existence of plumes on Europa has thus been confirmed, the origin of the plumes remains uncertain. Most notably, it is still unclear whether the inferred plumes are of oceanic origin, possibly containing information about the ocean's habitability, or if they are of surficial nature, offering information on the highly processed surface instead. In this study, we use a Monte‐Carlo model to analyze three different plume models, two of which are surficial (near‐surface liquid inclusion and diapir), and one of which originates in the sub‐surface ocean (oceanic plume). We analyze all relevant Lyman‐α and OI 1304 Å emission mechanisms to determine the three models' emission profiles. These profiles are then compared to the Hubble Space Telescope/Space Telescope Imaging Spectograph measurements presented by Roth, Retherford, et al. (2014, https://doi.org/10.1073/pnas.1416671111) and Roth, Saur, et al. (2014, https://doi.org/10.1126/science.1247051) with the goal of determining which ab initio model fits these measurements best. Our analyses show that all three models investigated produce similar Lyman‐α and OI 130.4 nm emission profiles, with differences being perceivable only on scales well below the HST resolution and sensitivity. Since none of the models contradict the Roth, Retherford, et al. (2014, https://doi.org/10.1073/pnas.1416671111) and Roth, Saur, et al. (2014, https://doi.org/10.1126/science.1247051) observations, none of them can be ruled out as being the acting force behind the observed plumes. The currently available optical measurements are thus not sufficient to determine the physical nature of Europa's plumes, identification of which can only be achieved through higher resolution images or in situ measurements. Plain Language Summary: In this study, we model three different Europa plume types using a Monte‐Carlo model. Two of the analyzed plume types have their origin in the near‐surface layer (near‐surface liquid inclusion and diapir), whereas one model originates in the sub‐surface ocean (oceanic plume). To compare our model results to Hubble Space Telescope observations, we compute for each plume type the Lyman‐alpha and OI 130.4 nm emission profile. Our analysis shows that all three plume types fit the plumes observed by the Hubble Space Telescope equally well. It is thus impossible to distinguish from the Hubble Space Telescope observations what the origin of the observed plumes is. To do so, images with higher resolution, or in situ measurements are required. Key Points: Roth observations agree well with plume modeling (200 km height, 1.5e16 cm−2 column densities, two distinct plumes ∼500 km apart)No difference in Ly‐α and OI 130.4 nm emission profiles of three different plume → source cannot be determined from available observationsHigher resolution images and mass spectrometer measurements would allow dis‐tinction; prepare for data becoming available [ABSTRACT FROM AUTHOR]

The Naval Research Laboratory (NRL) Sami2 is Another Model of the Ionosphere (SAMI2) and Sami3 is Also a Model of the Ionosphere (SAMI3) ionosphere/plasmasphere codes have shown that thermosphere composition and winds significantly affect H+ outflows from the topside ionosphere. In particular, O density inhibits upward diffusion of O+ from the ionosphere F layer, especially during solar maximum conditions. In addition, winds affect the quiet-time latitudinal extent of the F layer, affecting densities at mid-to-high latitudes that are the source of plasmasphere refilling outflows. Evidence for these effects is reviewed and prospects for forecasting these outflows are explored. Open questions for future research are highlighted.

Volatiles and refractories represent the two end-members in the volatility range of species in any surface-bounded exosphere. Volatiles include elements that do not interact strongly with the surface, such as neon (detected on the Moon) and helium (detected both on the Moon and at Mercury), but also argon, a noble gas (detected on the Moon) that surprisingly adsorbs at the cold lunar nighttime surface. Refractories include species such as calcium, magnesium, iron, and aluminum, all of which have very strong bonds with the lunar surface and thus need energetic processes to be ejected into the exosphere. Here we focus on the properties of species that have been detected in the exospheres of inner Solar System bodies, specifically the Moon and Mercury, and how they provide important information to understand source and loss processes of these exospheres, as well as their dependence on variations in external drivers. [ABSTRACT FROM AUTHOR]

BepiColombo Mio (previously called MMO: Mercury Magnetospheric Orbiter) was successfully launched by Ariane 5 from Kourou, French Guiana on October 20, 2018. The Mercury Plasma/Particle Experiment (MPPE) is a comprehensive instrument package onboard Mio spacecraft used for plasma, high-energy particle and energetic neutral atom measurements. It consists of seven sensors including two Mercury Electron Analyzers (MEA1 and MEA2), Mercury Ion Analyzer (MIA), Mass Spectrum Analyzer (MSA), High Energy Particle instrument for electron (HEP-ele), High Energy Particle instrument for ion (HEP-ion), and Energetic Neutrals Analyzer (ENA). Significant efforts were made pre-flight to calibrate all of the MPPE sensors at the appropriate facilities on the ground. High voltage commissioning of MPPE analyzers was successfully performed between June and August 2019 and in February 2020 following the completion of the low voltage commissioning in November 2018. Although all of the MPPE analyzers are now ready to begin observation, the full service performance has been delayed until Mio's arrival at Mercury. Most of the fields of view (FOVs) of the MPPE analyzers are blocked by the thermal shield surrounding the Mio spacecraft during the cruising phase. Together with other instruments on Mio including Magnetic Field Investigation (MGF) and Plasma Wave Investigation (PWI) that measure plasma field parameters, MPPE will contribute to the comprehensive understanding of the plasma environment around Mercury when BepiColombo/Mio begins observation after arriving at the planet Mercury in December 2025. [ABSTRACT FROM AUTHOR]

The lunar environment while in the Earth's magnetosphere represents a unique plasma regime, in which we can directly detect the Moon's effects on the ambient plasma environment. To a much greater degree than in the solar wind, the presence of the Moon and its exosphere perturb the ambient plasma environment. We identified and analyzed a variety of different effects of this interaction. We mapped four different interaction signatures and analyzed the solar and solar wind parameters during their occurrence. Confirming previous work, we found that the lunar events investigated in this paper tend to occur above the lunar dayside surface. In addition, we found that magnetospheric activity indices are higher during the categorized lunar events. This suggests that when the terrestrial magnetosphere is more active, as a result of solar wind activity, the terrestrial magnetospheric plasma and electromagnetic fields interact with lunar plasma to create the observed perturbations. [ABSTRACT FROM AUTHOR]

We explore the velocity distribution of impact generated water ice ejecta from planetary surface. We show that the ejecta velocity distribution should be modeled by a mixed Maxwellian distribution, rather than the commonly used single Maxwellian distribution, in order to better capture the outcome of the different emission processes and the dynamics of the clustered water molecules in the ejecta. We find that, comparing to the mixed Maxwellian distribution, the single Maxwellian distribution may lead to a significant underestimation of the impact-ejected water molecules that stay gravitationally bound for exosphere models. While the relative difference is within 10% for celestial bodies with a large escape velocity such as Mercury, it can be about 1000% for celestial bodies with a small escape velocity such as Enceladus. • We present a new framework for analyzing the impact generated exosphere of icy bodies. • The new framework takes into account of clustering in the ejecta. • A mixed Maxwellian distribution funciton is proposed to describe the ejecta dynamics. • Model prediction of surface bounded exosphere is strongly affected by clustering in the ejecta. [ABSTRACT FROM AUTHOR]

The present issue of Earth and Planetary Physics is dedicated to the near-space neutral and plasma environments of Mars. The issue includes nine papers that present new results on the properties of the Martian exosphere, ionosphere, and magnetosphere, from both observational and modeling points of view. Due to the similarity between the two objects, the issue also includes two additional papers on the near-Venus plasma environment.