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Habitability at the surface of a planet depends on having an atmosphere long enough for life to develop. The loss of atmosphere to space is an important component in assessing planetary surface habitability. Current models of atmospheric escape from exoplanets are not well constrained by observations. Atmospheric escape observations from the terrestrial planets are available in public data archives. We recast oxygen escape rates from Earth derived from an instrument on Dynamics Explorer‐1 as function of solar wind and compare them to similar data from Mars. Analysis demonstrates that oxygen escape rates from Mars are not as sensitive to variations in solar power components as those from Earth. Available data from Venus can confirm or refute the assertion that oxygen escape from magnetized planets is more sensitive than that from unmagnetized planets. Plain Language Summary: Habitability of a planet depends on having an atmosphere long enough for life to develop. NASA and ESA data archives contain information about atmospheric escape from the terrestrial planets. For these planets oxygen ions dominate atmospheric escape. The data archives are just beginning to be analyzed and presented in a form that allows comparison with, and validation of, models of the interaction of stellar winds with exoplanets. We derive oxygen escape rates from Earth as a function of solar power components from a recasting of Dynamics Explorer‐1 data and compare them to similar data from Mars. Our analysis demonstrates that oxygen escape rates from Mars are not as sensitive to variations in the solar power components as those from Earth. These data and similar data from Venus will prove to be important constrains on models of stelar wind/atmosphere interactions and atmospheric escape from exoplanets. Key Points: We recast oxygen escape rates from Earth derived from an instrument on Dynamics Explorer‐1 as a function of solar energy inputsWe compare escape rates for a magnetized planet (Earth) and an unmagnetized planet (Mars) as a function of solar energy inputsOxygen escape rates from Mars are not as sensitive to variations in the solar power components as those from Earth [ABSTRACT FROM AUTHOR]

Io, the closest of Jupiter's four Galilean moons, suffers from intense ion bombardment from Jupiter's magnetosphere. The constant atmospheric erosion by energetic ion precipitation, referred to atmospheric sputtering, serves as an important mechanism of Io's atmospheric escape. This study is devoted to a state‐of‐the‐art study of atmospheric sputtering at Io, with the aid of constantly accumulated understandings of Io's space environment and atmospheric photochemistry, as well as the updated laboratory measurements. A Monte Carlo model is constructed to track the energy degradation of incident S++ and O+ and atmospheric recoils from which the sputtering yields of different atmospheric species are determined. Our calculations suggest a total escape rate of 3 × 1029 atom s−1 on Io, and SO2 is the dominant sputtered species. Further investigations reveal that S++ is the most efficient species for atmospheric sputtering on Io, and sputtering yields increase substantially with increasing incident ion mass, energy, and incidence angle. The model sensitivity to different influence factors is also discussed, including scattering angle distribution, atmospheric column density, proton precipitation, inelastic process, and surface sputtering, of which the former two dominate. Plain Language Summary: As the closest Galilean moon of Jupiter, Io is strongly influenced by high energy ions from Jupiter's magnetosphere. The collisions between these high energy ions and background atmospheric particles can produce considerable high energy atmospheric recoils. A large portion of these particles are able to escape to space when their kinetic energies exceed local escape energy. Such a phenomenon is named atmospheric sputtering. In this study, we simulate this process on Io with the aid of parameters from laboratory measurements and a Monte Carlo model. Our calculations show that sputtering is the most important process driving atmospheric loss at Io. We also analyze the effects of various incident plasma populations and model setups on the atmospheric sputtering at Io. Key Points: Atmospheric sputtering is the most important mechanism driving atmosphere escape at IoOur calculations suggest a total escape rate of 3 × 1029 atom s−1 on IoSputtering yields vary substantially with incident plasma and atmospheric conditions, yet are insensitive to proton precipitation and inelastic process [ABSTRACT FROM AUTHOR]

Heavy cold ions at Mars are gravitationally bound to the planet unless some process provides energy to them. Observations show that cold (<20 eV) and dense (∼>1 cm−3) O+/O2+ ions with bulk velocities equal to energies ∼1 keV can reach deep into the nightside Martian magnetosheath. These ions are co‐located with a change of the sign of the sunward component of the magnetic field. This magnetic field topology implies the persistence of a localized planetary ions escape channel associated with draped magnetic field lines that are convecting tailward. The observed ion populations propagate approximately in the same direction as surrounding magnetosheath flow and are likely to be almost unheated ionospheric ions from low altitudes. The paper discusses planetary ion energization via Hall electric field originated from ions and electron separation associated with magnetic field curvature. Plain Language Summary: In‐situ observations above the nightside of Mars show the presence of localized dense planetary ion fluxes at altitudes exceeding 2,000 km and escaping from the planet at high energies, comparable to that of the solar wind. These fluxes are accompanied by the reversal of sunward component of magnetic field. Unlike most atmospheric escape channels, the reported phenomenon is characterized by an increase in heavy to light ions density ratio with the distance from the planet at the observed altitudes up to nearly 5,000 km, as well as an increase in overall plasma number density inside this escape channel relative to the ambient sheath environment. This behavior is consistent with acceleration process initiated by a bent magnetic flux tube. Key Points: Ions of different species gain similar energies in the Martian magnetosheath by Hall electric fields associated with magnetic curvatureA high concentration of ionospheric ions correlates with a near void of shocked solar wind protons and a magnetic field reversalNumber density at the reversal increases with distance from Mars in comparison to the surrounding sheath at least till two Martian radii [ABSTRACT FROM AUTHOR]

• M5 is a five-craft Mars mission with a solar wind monitor and four smaller spacecraft. • Focus on largely unexplored magnetotail region and atmospheric escape processes. • Four craft formation allows current and plasma wave variations to be studied in 3D. • Detailed assessment of scientific need and mission and spacecraft design presented. • Developed during FFG/ESA Alpbach Summer School 2022 on Comparative Plasma Physics. Mars, lacking an intrinsic dynamo, is an ideal laboratory to comparatively study induced magnetospheres, which can be found in other terrestrial bodies as well as comets. Additionally, Mars is of particular interest to further exploration due to its loss of habitability by atmospheric escape and possible future human exploration. In this context, we propose the Mars Magnetospheric Multipoint Measurement Mission (M5), a multi-spacecraft mission to study the dynamics and energy transport of the Martian induced magnetosphere comprehensively. Particular focus is dedicated to the largely unexplored magnetotail region, where signatures of magnetic reconnection have been found. Furthermore, a reliable knowledge of the upstream solar wind conditions is needed to study the dynamics of the Martian magnetosphere, especially the different dayside boundary regions but also for energy transport phenomena like the current system and plasma waves. This will aid the study of atmospheric escape processes of planets with induced magnetospheres. In order to resolve the three-dimensional structures varying both in time and space, multi-point measurements are required. Thus, M5 is a five spacecraft mission, with one solar wind monitor orbiting Mars in a circular orbit at 5 Martian radii, and four smaller spacecraft in a tetrahedral configuration orbiting Mars in an elliptical orbit, spanning the far magnetotail up to 6 Mars radii with a periapsis just outside the Martian magnetosphere of 1.8 Mars radii. We not only present a detailed assessment of the scientific need for such a mission but also show the resulting mission and spacecraft design taking into account all aspects of the mission requirements and constraints such as mass, power, and link budgets. Additionally, different aspects of the mission programmatics like a possible mission timeline, cost estimates, or public outreach are shown. The common requirements for acceptance for an ESA mission are considered. The mission outlined in this paper was developed during the Alpbach Summer School 2022 on the topic of "Comparative Plasma Physics in the Universe". [ABSTRACT FROM AUTHOR]

Abstract Habitability at the surface of a planet depends on having an atmosphere long enough for life to develop. The loss of atmosphere to space is an important component in assessing planetary surface habitability. Current models of atmospheric escape from exoplanets are not well constrained by observations. Atmospheric escape observations from the terrestrial planets are available in public data archives. We recast oxygen escape rates from Earth derived from an instrument on Dynamics Explorer‐1 as function of solar wind and compare them to similar data from Mars. Analysis demonstrates that oxygen escape rates from Mars are not as sensitive to variations in solar power components as those from Earth. Available data from Venus can confirm or refute the assertion that oxygen escape from magnetized planets is more sensitive than that from unmagnetized planets.

We develop a new model of diffusively modulated hydrodynamic escape to predict oxygen isotopic fractionations caused by the loss of water from a steam atmosphere of Venus. The chief technical advance over previous work is including CO 2 as a major species. We find that ordinary (δ 18 O) and mass-independent (Δ 17 O) fractionations depend mostly on the extent of lithospheric buffering and the ferocity of EUV heating when escape took place, and relatively little on the size of the lost ocean(s). It is likely that Δ 17 O evolved significantly from its birth state, not only in the atmosphere but also in the silicates of the crust and upper mantle. If both δ 18 O and Δ 17 O of Venus are identical to Earth and Moon, we may conclude that Venus and Earth accreted from a common pool. But differences in δ 18 O and Δ 17 O can be attributed to escape rather than to genetics. If the differences are large enough, they can be used to constrain when escape took place and the extent of volatile exchange with the lithosphere. Neon and argon systematics are most consistent with minimal escape, especially if an Ar-rich source, possibly derived from comets, is added. However, we also find a novel class of solutions in which Ne and Ar of Venus, Earth, and Mars are evolved from a common source material subject to different vigors of hydrodynamic escape, least extreme for Earth and most extreme for Mars. These alternative models require that Venus was always rather dry (<10% of an Earth ocean) and its water lost very early (before <100 Myrs). The two styles of escape – minimal or extreme – should be readily distinguished by an unambiguous measurement of the Ar/Kr ratio. Finally, we find that predicted D/H enrichments are of order 100 for almost all model parameters. This result, a direct consequence of diffusion-limited escape of H and D, provides support for the overall scenario. • Oxygen isotopes can be strongly fractionated by hydrodynamic escape. • There can be no expectation that Venus retains an interpretable record of its birth composition in its oxygen isotopes. • Diffusion-limited hydrogen escape correctly predicts D/H fractionation on Venus. • We develop a novel explanation of Ne/Ar systematics that can encompass Venus, Earth, Mars, and chondrites. [ABSTRACT FROM AUTHOR]

Abstract Heavy cold ions at Mars are gravitationally bound to the planet unless some process provides energy to them. Observations show that cold (1 cm−3) O+/O2+ ions with bulk velocities equal to energies ∼1 keV can reach deep into the nightside Martian magnetosheath. These ions are co‐located with a change of the sign of the sunward component of the magnetic field. This magnetic field topology implies the persistence of a localized planetary ions escape channel associated with draped magnetic field lines that are convecting tailward. The observed ion populations propagate approximately in the same direction as surrounding magnetosheath flow and are likely to be almost unheated ionospheric ions from low altitudes. The paper discusses planetary ion energization via Hall electric field originated from ions and electron separation associated with magnetic field curvature.

Here we review the last advances in our understanding of exoplanetary upper atmospheres, with a focus on the evaporation of exoplanets orbiting close to their stars. The atmospheric escape takes a significant part on the phenomena that sculpt the population of planets with short orbital distances.We also observe evaporation of minor bodies in young planetary systems when they approach to their star. These “exocomets” have been studied since the mid 80’s, yielding a large amount of observational data. In particular, in the case of exocomets orbiting the young star $\beta $ Pictoris, it has been shown that there are two different families of comets, tracing two different dynamical histories. Most recently, photometric observations with the NASA TESS space observatory allowed the detection of the dust tails produced by the evaporation of the exocomets’ nuclei. Using numerical simulation these observations allowed the derivation of the comets nuclei size distribution, which is found to be strikingly similar to the one observed in the Solar system and to the one expected for a collisionally relaxed population of minor bodies.

In this work, the kinetic model of aeronomy of the upper atmosphere of an exoplanet is extended by including the effect of stellar wind plasma on the extended hydrogen corona of a hot sub-Neptune. For this purpose, the precipitation of high-energy protons and hydrogen atoms into planetary atmospheres was studied using previously developed kinetic Monte Carlo models. The kinetic model was adapted to the upper atmospheres of hot sub-Neptunes, which made it possible to calculate the energy deposition rate of stellar wind plasma in the planetary corona and to refine estimates of the rate of non-thermal loss of the atmosphere caused by the stellar wind of the parent star. Calculations carried out for the hot sub-Neptune π Men c showed that the energy of the flux of energetic neutral hydrogen atoms (ENA H) penetrating into the atmosphere, formed during the charge exchange of stellar wind protons with thermal atoms of the hydrogen corona, mainly goes to heating the hydrogen corona of the hot exoplanet. [ABSTRACT FROM AUTHOR]

Incoherent scatter radars (ISRs) represent the only instrument (both ground and space based) capable of making high temporal and spatial resolution measurements of multiple atmospheric parameters—such as densities, temperatures, particle velocities, mass flux—over an altitude range covering the entire mesosphere/lower thermosphere/ionosphere (MLTI) system on a quasi-continuous basis. The EISCAT Svalbard incoherent scatter radar (ESR), located just outside Longyearbyen (78.15 ∘ N) on Svalbard, is the only currently operating facility capable of making such measurements inside the polar cusp—an area of significant energy input into the atmosphere and characterized by heating instabilities and turbulence. The ESR was built in the mid-1990s and has provided valuable data for the international experimental and modelling communities. New radar technologies are now available, in the form of phased array systems, which offer new data products and operational flexibility. This paper outlines the achievements and current research focus of the ESR and provides scientific arguments, compiled from inputs across the international scientific community, for a new phased array ISR facility on Svalbard. In addition to the fundamental scientific arguments, the paper discusses additional benefits of continued ISR observations on Svalbard, building on the key findings of the ESR. Svalbard has a large network of complementary instrumentation both focused on the MLTI system (e.g. the Kjell Henriksen auroral Observatory, the Svalbard SuperDARN radar and the Svalrak sounding rocket launch facility) with synergies to other research fields, such as meteorology and oceanography. As a further holistic system science view of the Earth becomes more important, a new ISR on Svalbard will be important also in this respect with its ability to provide datasets with a wide range of scientific applications. Increased activity in space has highlighted problematic issues such as space debris. A changing Arctic has also seen increased human activity via the opening up of new shipping routes, which are reliant on GNSS technology that is effected by severe turbulence in the MLTI system. As such, societal applications of a future ISR are also presented. The accessibility and logistical support for such a facility is also briefly discussed. [ABSTRACT FROM AUTHOR]

TOI‐700 d is the first Earth‐sized planet in the habitable zone (HZ) discovered by the Transiting Exoplanet Survey Satellite. Here, we assess whether a Venus‐like exoplanet at the TOI‐700 d location could retain an atmosphere for a time comparable to the age of the host star based on multispecies magnetohydrodynamics simulations. We investigate the effects of X‐ray and EUV (XUV) radiation from the host star, the interplanetary magnetic field (IMF) orientation, and the planetary intrinsic magnetic field. In unmagnetized cases, major ion loss is caused by O+ escape through a ring‐shaped region by the mass loading process after the ionization of the extended oxygen corona. As the IMF Parker spiral angle increases, the escape flux in the magnetotail shows stronger enhancement around the meridional current sheet, and the escape rate of molecular ions (O2+ ${\mathrm{O}}_{\mathrm{2}}^{+}$ and CO2+ $\mathrm{C}{\mathrm{O}}_{\mathrm{2}}^{+}$) increases by an order of magnitude due to acceleration in the ionosphere by magnetic tension forces. In magnetized cases, the intrinsic magnetic field suppresses ion pickup loss from the neutral oxygen corona by deflecting the stellar wind and preventing ion pickup while promoting cusp‐origin escape from the lower ionosphere. These results suggest that the unmagnetized exoplanet would have difficulty retaining its atmosphere over a few billion years under extreme conditions where XUV is 30 times stronger than at the current Earth. However, the dipole intrinsic magnetic field of 1,000 nT at the equatorial surface reduces the escape rate and would help the exoplanet to retain its atmosphere even under strong XUV conditions. Plain Language Summary: The recent discovery of Earth‐sized planets in the habitable zones (HZ) of M dwarfs has garnered attention over whether liquid water and life exist on these planets. We investigate the habitability of planets around M dwarfs in terms of atmospheric retention. Planetary atmospheres are necessary not only to retain liquid water on the surface but also to protect life from high‐energy particles and radiation. In this paper, we assess the feasibility of atmospheric retention on a Venus‐like exoplanet at the location of TOI‐700 d, which is the first Earth‐sized planet in the HZ discovered by the Transiting Exoplanet Survey Satellite. Our simulations show that the unmagnetized exoplanet cannot retain an atmosphere under extreme conditions where the planet is exposed to the intense light from the host star that energizes the upper layers of the planetary atmosphere, but the presence of an intrinsic magnetic field might make this possible. The escape rate of atmospheric ions, especially molecular ions, is suppressed by the interplanetary magnetic field nearly parallel to the stellar wind velocity, which may be typical as the planet and the host star get closer. Key Points: Stellar X‐ray and EUV (XUV) must be within 30 times that of Earth to retain a Venus‐like atmosphere for an unmagnetized exoplanetIn unmagnetized cases, the Parker spiral angle affects molecular ion escape due to the tension force of draping magnetic field linesA strong intrinsic magnetic field suppresses ion escape, allowing the atmosphere to be retained even when XUV is 50 times that of Earth [ABSTRACT FROM AUTHOR]

Hydrogen loss to space is a key control on the evolution of the Martian atmosphere and the desiccation of the red planet. Thermal escape is thought to be the dominant loss process, but both forward modeling studies and remote sensing observations have indicated the presence of a second, higher‐temperature "nonthermal" or "hot" hydrogen component, some fraction of which also escapes. Exothermic reactions and charge/momentum exchange processes produce hydrogen atoms with energy above the escape energy, but H loss via many of these mechanisms has never been studied, and the relative importance of thermal and nonthermal escape at Mars remains uncertain. Here we estimate hydrogen escape fluxes via 47 mechanisms, using newly developed escape probability profiles. We find that HCO+ dissociative recombination is the most important of the mechanisms, accounting for 30%–50% of the nonthermal escape. The reaction CO2+ ${\text{CO}}_{2}^{+}$ + H2 is also important, producing roughly as much escaping H as momentum exchange between hot O and H. Total nonthermal escape from the mechanisms considered amounts to 39% (27%) of thermal escape, for low (high) solar activity. Our escape probability profiles are applicable to any thermospheric hot H production mechanism and can be used to explore seasonal and longer‐term variations, allowing for a deeper understanding of desiccation drivers over various timescales. We highlight the most important mechanisms and suggest that some may be important at Venus, where nonthermal escape dominates and much of the literature centers on charge exchange reactions, which do not result in significant escape in this study. Plain Language Summary: The climate of Mars has become drier over time, partially due to hydrogen and oxygen atoms escaping from the planet's upper atmosphere. This can happen when the atoms have sufficient energy to overcome Mars' gravitational pull. Some fraction of escaping hydrogen—known as "hot" hydrogen—gains this energy through chemical processes, but the extent to which this contributes to the total hydrogen escape rate remains uncertain. Here we estimate the rate of hydrogen escape from 47 different chemical mechanisms, by considering the altitude above the surface at which they are produced and the energy that they might be given when they are formed. We find the mechanisms producing the most escaping hydrogen and estimate how quickly hot hydrogen is being lost to space. The most important mechanism, of those we consider, is HCO+ + e− → CO + H, which contributes up to half of the escaping hot H. We speculate that this is also important at Venus. Key Points: We quantify nonthermal hydrogen escape from 47 sources, many for the first time, using Monte Carlo model‐derived escape probability profilesTotal nonthermal escape from these sources is 27%–39% of the thermal escape flux of 8.7 × 107–1.1 × 108 cm−2 s−1HCO+ dissociative recombination is the biggest contributor to photochemical H escape at Mars, and likely at Venus and some exoplanets [ABSTRACT FROM AUTHOR]

Although deuterium (D) on Mars has received substantial attention, the deuterated ionosphere remains relatively unstudied. This means that we also know very little about non‐thermal D escape from Mars, since it is primarily driven by excess energy imparted to atoms produced in ion‐neutral reactions. Most D escape from Mars is expected to be non‐thermal, highlighting a gap in our understanding of water loss from Mars. In this work, we set out to fill this knowledge gap. To accomplish our goals, we use an upgraded 1D photochemical model that fully couples ions and neutrals and does not assume photochemical equilibrium. To our knowledge, such a model has not been applied to Mars previously. We model the atmosphere during solar minimum, mean, and maximum, and find that the deuterated ionosphere behaves similarly to the H‐bearing ionosphere, but that non‐thermal escape on the order of 8,000–9,000 cm−2 s−1 dominates atomic D loss under all solar conditions. The total fractionation factor, f, is f = 0.04–0.07, and integrated water loss is 147–158 m global equivalent layer. This is still less than geomorphological estimates. Deuterated ions at Mars are likely difficult to measure with current techniques due to low densities and mass degeneracies with more abundant H ions. Future missions wishing to measure the deuterated ionosphere in situ will need to develop innovative techniques to do so. Plain Language Summary: Our knowledge of ions in the Martian atmosphere that contain deuterium (D), an isotope of hydrogen (H), is limited, lacking measurements and dedicated computer models. This is a problem because the expectation is that most D that escapes to space does so by gaining extra energy from ion reactions. H and D mostly exist in water on Mars, so identifying how much H and D have escaped this way is important to understanding how Mars lost all the water that it once had, transforming it from a planet that could have supported life into the hostile environment it is today. Here, we present the first one dimensional model of the Mars atmosphere that includes D‐bearing ions. We report the amounts and types of H and D escape and confirm that most D gets its escape energy from ion chemistry. We also identify the specific chemical reactions that are most important, and show how many D‐bearing ions we expect to find in the atmosphere that might be detectable by future missions. Finally, we calculate that a layer of water 147–158 m deep has been lost from Mars. This is still less than the amount calculated by geological studies. Key Points: We present the first photochemical modeling study of the deuterated ionosphere of MarsNon‐thermal escape dominates D loss under all solar conditions, and the processes producing hot D are similar to those producing hot HThe combined D/H fractionation factor is f = 0.04–0.07, indicating 147–158 m global equivalent layer (GEL) of water loss, still less than geological estimates [ABSTRACT FROM AUTHOR]

The exobase is a key concept for understanding atmospheric escape and evolution on Mars. With the aid of neutral density data measured via the Neutral Gas and Ion Mass Spectrometer onboard the Mars Atmosphere and Volatile Evolution spacecraft, we calculated the exobase temperatures of four relatively abundant species on Mars: CO2, O, N2, and CO. Our calculations revealed that the exobase temperatures are highly variable, with median temperatures of 174, 152, 195, and 193 K for CO2, O, N2, and CO, respectively. Moreover, the calculated exobase temperatures for the four species increased systematically with an increase in the exobase altitude or a decrease in the exobase density. Further investigations indicated strong diurnal and solar cycle variations in the exobase temperature: (1) the exobase temperature on the day side was higher than that on the night side, revealing a maximum temperature near the local time of 14 h and a minimum temperature near the local time of 2 h; and (2) the exobase temperatures of the four species increased with the solar extreme ultraviolet (EUV) flux, a feature that was restricted to the day side and absent on the night side, where non-solar energy inputs are likely important. These variations originate from the varying degrees of upper atmospheric heating on Mars in response to the different solar EUV energy inputs.

The rotation of the interplanetary magnetic field (IMF) provides details of the escape mechanism that varies with the interaction between the intrinsic magnetic field and the IMF. A multispecies magnetohydrodynamic simulation is conducted on a Mars‐like planet under the conditions of a weak intrinsic magnetic field and the IMF rotating by 180° over 12 hr, which is comparable to the coronal mass ejection (CME) time scale. The total ion escape rate increases from ∼5 × 1023 s−1 to ∼3 × 1025 s−1 during the IMF rotation from due north (parallel to the assumed intrinsic magnetic field) to due south (antiparallel to the assumed intrinsic magnetic field). The escape rate increases significantly with the IMF rotating from due north to clock angles of 45°–75°, followed by a gentle increase until the IMF rotates to due south. The trigger for the large increase is multiple reconnections in the magnetospheric flank region, and the subsequent increase is due to the expansion of the reconnection area into the equatorial region. The IMF mass loading in the ionosphere is also responsible for the gentle increase with IMF clock angles above 90° in addition to the flank reconnection. The IMF rotation is a general feature for a CME. Exoplanets orbiting in the vicinity of M‐dwarfs might be frequently affected by IMF rotation, as they could be exposed to severe stellar winds such as in CME environments, and the current study could be helpful for future planned observations of the exoplanetary environment. Key Points: During interplanetary magnetic field rotation, the ion escape rate does not change much in the unmagnetized case, but increases by 50 times in the magnetized caseA rapid increase in the escape rate occurs at a 45–75° clock angle, triggered by the multiple reconnections on the magnetospheric flanksThe gentle increase in the escape rate above 90° clock angle is due to the contribution of both reconnection and mass loading [ABSTRACT FROM AUTHOR]

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Abstract Japan Aerospace Exploration Agency (JAXA) plans a Phobos sample return mission (MMX: Martian Moons eXploration). In this study, we review the related works on the past climate of Mars, its evolution, and the present climate and weather to describe the scientific goals and strategies of the MMX mission regarding the evolution of the Martian surface environment. The MMX spacecraft will retrieve and return a sample of Phobos regolith back to Earth in 2029. Mars ejecta are expected to be accumulated on the surface of Phobos without being much shocked. Samples from Phobos probably contain all types of Martian rock from sedimentary to igneous covering all geological eras if ejecta from Mars could be accumulated on the Phobos surface. Therefore, the history of the surface environment of Mars can be restored by analyzing the returned samples. Remote sensing of the Martian atmosphere and monitoring ions escaping to space while the spacecraft is orbiting Mars in the equatorial orbit are also planned. The camera with multi-wavelength filters and the infrared spectrometer onboard the spacecraft can monitor rapid transport processes of water vapor, dust, ice clouds, and other species, which could not be traced by the previous satellites on the sun-synchronous polar orbit. Such time-resolved pictures of the atmospheric phenomena should be an important clue to understand both the processes of water exchange between the surface/underground reservoirs and the atmosphere and the drivers of efficient material transport to the upper atmosphere. The mass spectrometer with unprecedented mass resolution can observe ions escaping to space and monitor the atmospheric escape which has made the past Mars to evolve towards the cold and dry surface environment we know today. Together with the above two instruments, it can potentially reveal what kinds of atmospheric events can transport tracers (e.g., H2O) upward and enhance the atmospheric escape. Graphical Abstract

NASA's Endurance sounding rocket (yard No. 47.001) will launch from Ny Ålesund, Svalbard in May 2022 on a solid fueled Oriole III-A launch vehicle. Its ∼ 19 minute flight will carry it to an altitude of ∼ 780 km above Earth's sunlit polar cap. Its objective is to make the first measurement of the weak "ambipolar" electric field generated by Earth's ionosphere. This field is thought to play a critical role in the upwelling and escape of ionospheric ions, and thus potentially in the evolution of Earth's atmosphere. The results will enable us to determine the importance to ion escape of this previously unmeasured fundamental property of our planet, which will aid in a better understanding of what makes Earth habitable. Endurance will carry six science instruments (with 16 sensors) that will measure the total electrical potential drop below the spacecraft, and the physical parameters required to understand the physics of what generates the ambipolar field. The mission will be supported by simultaneous observations of solar and geomagnetic activity. [ABSTRACT FROM AUTHOR]

Measuring electric fields in the space environments of the Earth and other planets helps us to understand the electric field structures and plasma waves in the space. An addition usage of data from electric field measurements is to derive the cold ions in the magnetosphere which are invisible to ordinary energy spectrometers. The cold ions of ionospheric origin dominate the ion population in the magnetosphere. They are accelerated by the ambipolar electric field in the ionosphere to escape velocities. The loss of cold ions plays an important role in evolution of the atmosphere and habitability. In this review, we summarize two types of electric field instruments being widely used in space exploration, and the method to derive parameters of cold ions in the magnetosphere from the measurements made by the two types of electric field instruments. Based on existing technologies, we propose a new instrument to measure the ambipolar electric field in the polar region with a magnitude of about 10−6 Vm−1. This new instrument may be onboard a future satellite operating in the polar region, and give us clues on evolution of atmosphere and habitability.

We investigated the formation mechanism of a molecular ion plume and its contribution to ion escape based on Mars Atmosphere and Volatile EvolutioN (MAVEN) observations from November 2014 to October 2019 and numerical models. Here, we report a CO2+‐rich plume event and a statistical study of the molecular ion plume. MAVEN observed a CO2+‐rich plume event, in which the maximum CO2+ escape flux is approximately 4.2 × 106 cm−2s−1, on 28 August 2015 under strong solar wind dynamic pressure conditions. A numerical simulation using strong solar wind dynamic pressure conditions from the event suggested that the molecular ion plume is formed by deep penetration of the solar wind‐induced electric field, which is caused by strong solar wind dynamic pressure. A statistical study showed that CO2+ plume events tend to be observed under high solar wind dynamic pressure and strong electric field conditions. This tendency is consistent with the formation mechanism of the molecular ion plume suggested by the event study. The O2+ plume does not show the same tendency. This is because O2+ ions are abundant in the high‐altitude ionosphere, and O2+ plumes can be formed even under weak solar wind conditions. The subsolar crustal magnetic fields tend to prevent the formation of the molecular ion plume by shielding the ionosphere from the solar wind. The escape rate ratio O+:O2+:CO2+ $\left({\mathrm{O}}^{+}:{\mathrm{O}}_{2}^{+}:{\text{CO}}_{2}^{+}\right)$ is approximately 45:53:3 during the whole statistical survey period, suggesting that a molecular ion plume from the ionosphere is a non negligible ion escape channel from Mars. Plain Language Summary: It has been considered that Mars had a thick atmosphere to sustain liquid water on the surface approximately 4 billion years ago, while the Martian atmosphere is thin at the present time. Ion escape to space is one of the candidates that contributes to atmospheric loss. Both atomic oxygen ions (O+) and molecular ions, such as O2+ and CO2+, can contribute to ion escape from Mars. However, the escape mechanism and contribution of molecular ions are far from understood. Based on observations by the Mars Atmosphere and Volatile EvolutioN mission, we investigate energetic ion escape channels from Mars, which are accelerated by electric fields. The results show that the strong pressure of the solar wind facilitates energetic molecular ion escape, while crustal magnetization on the Martian surface prevents it. The escape rate ratio O+:O2+:CO2+ $\left({\mathrm{O}}^{+}:{\mathrm{O}}_{2}^{+}:{\text{CO}}_{2}^{+}\right)$ is approximately 45:53:3, suggesting the importance of energetic molecular ion escape from Mars. Key Points: When observed, polar plume flux of O2+ ${\mathrm{O}}_{2}^{+}$ (CO2+ ${\text{CO}}_{2}^{+}$) is 4 (2) times higher than O+, but they are 4 (23) times less frequently observed than O+The CO2+ ${\text{CO}}_{2}^{+}$ plume becomes detectable only under high solar wind dynamic pressure conditions, while the O2+ ${\mathrm{O}}_{2}^{+}$ plume is detected more frequentlySubsolar crustal magnetic fields tend to prevent the formation of molecular ion plumes, especially under low dynamic pressure conditions [ABSTRACT FROM AUTHOR]

One of the major discoveries of NASA's 1979–1991 Pioneer Venus Orbiter is that the nightside ionosphere becomes filamentary at high altitude, forming comet‐like tail rays. Pioneer Venus Orbiter could not establish how much farther into the wake of Venus tail rays extend, nor understand how they form. Here we present plasma and fields data from the fourth flyby of Venus by NASA's Parker Solar Probe consistent with an intercept with an ionospheric tail ray. The observations unambiguously demonstrate that Venusian Ionotail Rays can extend to at least 7,800 km in altitude. Using the new Parker observations we are able to identify a tail ray encounter within the Venus Express dataset. We thus present a unified picture of the structure of the magnetotail of Venus based on combined Venus Express, Pioneer Venus Orbiter, and Parker observations, and recent hybrid modeling. Plain Language Summary: Like a comet, the planet Venus has a tail made of plasma. This tail was first discovered and explored by NASA's 1979–1991 Pioneer Venus Orbiter (PVO) mission. The next mission capable of measuring plasmas at Venus was the 2006–2014 ESA Venus Express (VEX) mission. However, by the time of VEX, the technology of space plasma analyzers had advanced so much that even if PVO and VEX flew through the same phenomenon, each would present data very differently. On February 20, 2021 NASA's Parker Solar Probe made its fourth close flyby of Venus, spending 10 min in the wake on the nightside. For two of these minutes, Parker encountered a plume of cold plasma escaping from Venus with all the properties of a Venusian tail ray. These measurements confirm the predictions of recent simulations which predict that tail rays can extend away from the nightside of Venus to an altitude equivalent to the radius of the planet. Furthermore, some of the modern instrumentation aboard Parker are similar to that flown aboard Venus Express enabling us to identify tail rays in that dataset as well, allowing us to directly compare and combine VEX and PVO datasets. Key Points: During its fourth flyby of Venus, NASA's Parker Solar Probe flew through a structure consistent with the tail rays discovered by Pioneer VenusTail rays are a distinct phenomenon that sometimes forms along the current sheet in the Venusian wake, extending to over 7,800 km altitudeThe tail rays discovered by Pioneer Venus Orbiter (PVO) and "Bursty Flows" later reported by Venus Express are likely one and the same phenomenon [ABSTRACT FROM AUTHOR]

Japan Aerospace Exploration Agency (JAXA) plans a Phobos sample return mission (MMX: Martian Moons eXploration). In this study, we review the related works on the past climate of Mars, its evolution, and the present climate and weather to describe the scientific goals and strategies of the MMX mission regarding the evolution of the Martian surface environment. The MMX spacecraft will retrieve and return a sample of Phobos regolith back to Earth in 2029. Mars ejecta are expected to be accumulated on the surface of Phobos without being much shocked. Samples from Phobos probably contain all types of Martian rock from sedimentary to igneous covering all geological eras if ejecta from Mars could be accumulated on the Phobos surface. Therefore, the history of the surface environment of Mars can be restored by analyzing the returned samples. Remote sensing of the Martian atmosphere and monitoring ions escaping to space while the spacecraft is orbiting Mars in the equatorial orbit are also planned. The camera with multi-wavelength filters and the infrared spectrometer onboard the spacecraft can monitor rapid transport processes of water vapor, dust, ice clouds, and other species, which could not be traced by the previous satellites on the sun-synchronous polar orbit. Such time-resolved pictures of the atmospheric phenomena should be an important clue to understand both the processes of water exchange between the surface/underground reservoirs and the atmosphere and the drivers of efficient material transport to the upper atmosphere. The mass spectrometer with unprecedented mass resolution can observe ions escaping to space and monitor the atmospheric escape which has made the past Mars to evolve towards the cold and dry surface environment we know today. Together with the above two instruments, it can potentially reveal what kinds of atmospheric events can transport tracers (e.g., H2O) upward and enhance the atmospheric escape. [ABSTRACT FROM AUTHOR]

Martian crustal magnetic fields influence the solar wind interaction with Mars in a way that is not fully understood. In some locations, crustal magnetic fields act as "mini‐magnetospheres," shielding the planet's atmosphere, while in other locations they act as channels for enhanced energy input and particle escape. The net effect of this system is not intuitively clear, but previous modeling studies have suggested that crustal fields likely decrease global ion escape from Mars. In this study, we use data from the Mars Atmosphere and Volatile EvolutioN spacecraft to analyze how crustal magnetic fields influence both global and local ion escape at Mars. We find that crustal fields only increase ion escape if ions are not bound tightly to the magnetic field. Specifically, ion escape is increased only if closed magnetic fields trap 35% or less of energized oxygen ions. In any other case, crustal fields decrease both global and local ion escape by as much as 40% and 80%, respectively. This suggests that the presence of crustal magnetic fields has had a moderate impact on atmospheric ion loss throughout Martian history, potentially influencing the planet's atmospheric evolution and habitability. Plain Language Summary: The loss of the Martian atmosphere over time has transformed Mars from a potentially warm and wet planet to the cold, dry world we observe today. This atmospheric loss is often suggested to be the result of Mars losing its global magnetic field 3 billion years ago. However, the loss of a global dynamo did not leave the Martian system devoid of planetary magnetic fields. Rather, the crust of Mars still contains scattered pockets of magnetic field that extend outward into the planet's atmosphere. In some areas, these magnetic fields shield the planetary atmosphere much in the same way as the Earth's magnetic field, while in other areas the magnetic fields channel energy down into the planet's atmosphere, potentially driving enhanced atmospheric loss. In this study, we use spacecraft data from Mars Atmosphere and Volatile EvolutioN to analyze the extent to which Martian crustal magnetic fields affect atmospheric escape at Mars. We show that the shielding provided by crustal magnetic fields reduces present‐day ion escape by as much as 40%, and suggest that over time this may have been an important factor in the total amount of atmosphere lost from the planet. Key Points: We present a new method for estimating the influence of Martian crustal magnetic fields on ion escapeMartian crustal magnetic fields affect global ion escape by at most 40%, decreasing escape under typical conditionsMartian crustal magnetic fields affect local ion escape by at most 80%, decreasing escape under typical conditions [ABSTRACT FROM AUTHOR]

Cassini observations over the past ten years have revealed that Titan possesses a chemically complex ionosphere. In this study, we investigate the relative contributions of different ion species to the total ion escape on Titan, by dividing all ion species probed by the Cassini Ion Neutral Mass Spectrometer (INMS) into six groups according to their mass-to-charge ratios (M/Z). For the three lightest ion groups, with characteristic M/Z of 22, 41, and 52 daltons , the observed scale heights tend to be lower than the scale heights predicted by assuming diffusive equilibrium; for the three heavier groups, observed and predicted scale heights are in general agreement, implying that most ion escape from Titan is by relatively light species, with M/Z < 60 daltons. A diffusion model is constructed to describe the density distribution of each ion group in regions where the effect of ionospheric chemistry could be neglected. The data model comparison predicts an optimal total ion escape rate of 3.1×1024 s–1, of which more than 99% is contributed by relatively light ions with M/Z < 32 daltons.

Lower atmospheric global dust storms affect the small‐ and large‐scale weather and variability of the whole Martian atmosphere. Analysis of the CO2 density data from the Neutral Gas and Ion Mass Spectrometer instrument on board NASA's Mars Atmosphere Volatile EvolutioN (MAVEN) spacecraft show a remarkable increase of gravity wave (GW)‐induced density fluctuations in the thermosphere during the 2018 major dust storm with distinct latitude and local time variability. The mean thermospheric GW activity increases by a factor of two during the storm event. The magnitude of relative density perturbations is around 20% on average and 40% locally. One and a half months later, the GW activity gradually decreases. Enhanced temperature disturbances in the Martian thermosphere can facilitate atmospheric escape. For the first time, we estimate observationally that, for a 20% and 40% GW‐induced disturbances, the net increase of Jeans escape flux of hydrogen is a factor of 1.3 and 2, respectively. Plain Language Summary: Atmospheric gravity waves play an important dynamical and thermodynamical role in coupling the different atmospheric layers, especially on Earth and Mars. Using data from NASA's Mars Atmosphere Volatile EvolutioN (MAVEN) mission, we study the effects of a planet‐encircling major dust storm on thermospheric gravity wave activity and observationally estimate for the first time a potential influence of gravity waves on atmospheric escape on Mars. Gravity wave activity measured in terms of relative density fluctuations increases by a factor of two during the peak phase of the storm. We show that larger‐amplitude gravity waves facilitate atmospheric escape of hydrogen from Mars' upper atmosphere. For 40% gravity wave‐induced relative disturbances of temperature, the net escape rate doubles. Key Points: Thermospheric gravity wave activity doubles during the dust stormGravity wave induced density fluctuations in the thermosphere are up to 40% during the peak storm phaseGravity waves significantly increase hydrogen escape flux by modulating temperature fluctuations [ABSTRACT FROM AUTHOR]

The accumulation of detailed ion flux measurements from long-lived spacecraft orbiting the solar system's terrestrial planets have enabled recent studies to estimate the rate of solar wind driven atmospheric ion escape from Venus, Earth, and Mars, as well as the influence of solar wind and solar extreme ultraviolet (EUV) ionizing radiation on the atmospheric ion escape rates. Here, we introduce the basic forces and processes of ion escape, review the recent studies of ion escape rates, and provide a general framework for understanding ion escape as a process that can be limited by potential bottlenecks, such as ion supply, solar wind energy transfer, and transport efficiency, effectively determining the state of the ion escape process at each planet. We find that ion escape from Venus and Earth is energy-limited, though exhibit different dependencies on solar wind and EUV, revealing the influence of Earth's intrinsic magnetic field. In contrast, ion escape from Mars is in a supply-limited state, mainly due to its low gravity, and has likely contributed relatively little to the total loss of the early Martian atmosphere, in comparison to neutral escape processes. Contrary to the current paradigm, the comparisons between the solar system planets show that an intrinsic magnetic dipole field is not required to prevent stellar wind-driven escape of planetary atmospheres, and the presence of one may instead increase the rate of ion escape. Anticipating the atmospheres of the exoplanets that will begin to be characterized over the coming decade, and the need to explain their evolution, we argue that a modern, nuanced, and evidence-based view of the magnetic field's role in atmospheric escape is required. [ABSTRACT FROM AUTHOR]

It is not yet entirely clear whether Mars began as a warm and wet planet that evolved towards the present-day cold and dry body or if it always was cold and dry with just some sporadic episodes of liquid water on its surface. An important clue into this question can be gained by studying the earliest evolution of the Martian atmosphere and whether it was dense and stable to maintain a warm and wet climate or tenuous and susceptible to strong atmospheric escape. In this review we therefore discuss relevant aspects for the evolution and stability of a potential early Martian atmosphere. This contains the EUV flux evolution of the young Sun, the formation timescale and volatile inventory of the planet including volcanic degassing, impact delivery and removal, the loss of the catastrophically outgassed steam atmosphere, atmosphere-surface interactions, as well as thermal and non-thermal escape processes affecting a potential secondary atmosphere at early Mars. While early non-thermal atmospheric escape at Mars before 4 billion years ago is poorly understood, in particular in view of its ancient intrinsic magnetic field, research on thermal escape processes and the stability of a CO2-dominated atmosphere around Mars against high EUV fluxes indicate that volatile delivery and volcanic degassing cannot counterbalance the strong atmospheric escape. Therefore, a catastrophically outgassed steam atmosphere of several bars of CO2 and H2O, or CO and H2 for reduced conditions, through solidification of the Martian magma ocean could have been lost within just a few million years. Thereafter, Mars likely could not build up a dense secondary atmosphere during its first ∼ 400 million years but might only have possessed an atmosphere sporadically during events of strong volcanic degassing, potentially also including SO2. This indicates that before ∼ 4.1 billion years ago Mars indeed might have been cold and dry with at maximum short and sporadic warmer periods. A denser CO2- or CO-dominated atmosphere, however, might have built up afterwards but must have been lost later-on due to non-thermal escape processes and sequestration into the ground. [ABSTRACT FROM AUTHOR]

The discovery of thousands of highly irradiated, low-mass, exoplanets has led to the idea that atmospheric escape is an important process that can drive their evolution. Of particular interest is the inference from recent exoplanet detections that there is a large population of low mass planets possessing significant, hydrogen dominated atmospheres, even at masses as low as ∼ 2 M ⊕ . The size of these hydrogen dominated atmospheres indicates the envelopes must have been accreted from the natal protoplanetary disc. This inference is in contradiction with the Solar System terrestrial planets, that did not reach their final masses before disc dispersal, and only accreted thin hydrogen dominated atmospheres. In this review, we discuss the evidence for hydrogen dominated atmospheres on terrestrial mass (≲ 2 M ⊕ ) planets. We then discuss the possible origins and evolution of these atmospheres with a focus on the role played by hydrodynamic atmospheric escape driven by the stellar high-energy emission (X-ray and EUV; XUV). [ABSTRACT FROM AUTHOR]

The escape of heavy ions from the Earth atmosphere is a consequence of energization and transport mechanisms, including photoionization, electron precipitation, ion‐electron‐neutral chemistry, and collisions. Numerous studies considered the outflow of O+ ions only, but ignored the observational record of outflowing N+. In spite of 12% mass difference, N+ and O+ ions have different ionization potentials, ionospheric chemistry, and scale heights. We expanded the Polar Wind Outflow Model (PWOM) to include N+ and key molecular ions in the polar wind. We refer to this model expansion as the Seven Ion Polar Wind Outflow Model (7iPWOM), which involves expanded schemes for suprathermal electron production and ion‐electron‐neutral chemistry and collisions. Numerical experiments, designed to probe the influence of season, as well as that of solar conditions, suggest that N+ is a significant ion species in the polar ionosphere and its presence largely improves the polar wind solution, as compared to observations. Plain Language Summary: Nitrogen is the most abundant element in the Earth's atmosphere. Around 78% N2 and 21% O2 form the air we breathe and expand into high‐altitude atmosphere, the thermosphere, and eventually the ionosphere. The neutral molecules in the ionosphere are ionized by solar radiation, and some of them break up into atoms, and others become charged particles. The ionospheric ions with sufficient energy can flow out into space, and the abundances of these outflowing ionospheric ions highly impact the near‐Earth plasma properties. Studies focused on outflowing O+ ions have been conducted for several years. However, the contribution of N+ ions to the outflow solution is still largely unknown due to the instrumental limitations. This letter addresses the transport and energization of ionospheric N+ ions based on theoretical predictions, and examines the role of N+ ions to the collision and chemistry in the topside ionosphere. This study shows that N+ ions are important components in the high‐altitude polar ionosphere and their abundances can be affected by the sunlight and seasonal variations. Key Points: We developed a seven‐ion polar wind model (7iPWOM), which solves for the outflowing N+ ionsN+ ions are the second most abundant ionospheric species, up to 1,200 kmThe presence of N+ ions improves the overall polar wind solution, when compared with observations [ABSTRACT FROM AUTHOR]

Uranus and Neptune are the least-explored planets in our Solar System. This paper summarizes mysteries about these incredibly intriguing planets and their environments spurred by our limited observations from Voyager 2 and Earth-based systems. Several of these observations are either inconsistent with our current understanding built from exploring other planetary systems, or indicate such unique characteristics of these Ice Giants that they leave us with more questions than answers. This paper specifically focuses on the value of all aspects of magnetospheric measurements, from the radiation belt structure to plasma dynamics to coupling to the solar wind, through a future mission to either of these planets. Such measurements have large interdisciplinary value, as demonstrated by the large number of mysteries discussed in this paper that cover other non-magnetospheric disciplines, including planetary interiors, atmospheres, rings, and moons. [ABSTRACT FROM AUTHOR]

The development of habitable conditions on Earth is tightly connected to the evolution of its atmosphere, which is strongly influenced by atmospheric escape. We investigate the evolution of the polar ion outflow from the open field line bundle, which is the dominant escape mechanism for the modern Earth. We perform Direct Simulation Monte Carlo (DSMC) simulations and estimate the upper limits on escape rates from the Earth's open field line bundle starting from 3 gigayears ago (Ga) to present assuming the present‐day composition of the atmosphere. We perform two additional simulations with lower mixing ratios of oxygen of 1% and 15% to account for the conditions shortly after the Great Oxydation Event (GOE). We estimate the maximum loss rates due to polar outflow 3 gigayears ago of 3.3×1027 s−1 and 2.4×1027 s−1 for oxygen and nitrogen, respectively. The total integrated mass loss equals to 39% and 10% of the modern atmosphere's mass, for oxygen and nitrogen, respectively. According to our results, the main factors that governed the polar outflow in the considered time period are the evolution of the XUV radiation of the Sun and the atmosphere's composition. The evolution of the Earth's magnetic field plays a less important role. We conclude that although the atmosphere with the present‐day composition can survive the escape due to polar outflow, a higher level of CO2 between 3.0 and 2.0 Ga was likely necessary to reduce the escape. Key Points: Polar outflow escape of the nitrogen ions 3 gigayears ago increases by 2 orders of magnitude compared to its present valuePolar outflow of oxygen ions from the Earth's open field line bundle varies greatly depending on the oxygen mixing ratioPolar outflow is governed primarily by the evolution of the solar short‐wavelength radiation and the atmosphere's composition [ABSTRACT FROM AUTHOR]

Here we discuss the current state of knowledge on how atmospheric escape processes can fractionate noble gas isotopes and moderately volatile rock-forming elements that populate primordial atmospheres, magma ocean related environments, and catastrophically outgassed steam atmospheres. Variations of isotopes and volatile elements in different planetary reservoirs keep information about atmospheric escape, composition and even the source of accreting material. We summarize our knowledge on atmospheric isotope ratios and discuss the latest evidence that proto-Venus and Earth captured small H2-dominated primordial atmospheres that were lost by hydrodynamic escape during and after the disk dispersed. All relevant thermal and non-thermal atmospheric escape processes that can fractionate various isotopes and volatile elements are discussed. Erosion of early atmospheres, crust and mantle by large planetary impactors are also addressed. Further, we discuss how moderately volatile elements such as the radioactive heat producing element 40K and other rock-forming elements such as Na can also be outgassed and lost from magma oceans that originate on large planetary embryos and accreting planets. Outgassed elements escape from planetary embryos with masses that are ≤ M Moon directly, or due to hydrodynamic drag of escaping H atoms originating from primordial- or steam atmospheres at more massive embryos. We discuss how these processes affect the final elemental composition and ratios such as K/U, Fe/Mg of early planets and their building blocks. Finally, we review modeling efforts that constrain the early evolution of Venus, Earth and Mars by reproducing their measured present day atmospheric 36Ar/38Ar, 20Ne/22Ne, noble gas isotope ratios and the role of isotopes on the loss of water and its connection to the redox state on early Mars. [ABSTRACT FROM AUTHOR]

A review of the results of planetary atmospheres studies performed by Russian scientists in 2015–2018 prepared in the Commission on planetary atmospheres of the National Geophysical Committee for the National Report on Meteorology and Atmospheric Sciences to the XXVII General Assembly of the International Union of Geodesy and Geophysics (Montreal, June 8–18, 2019) is presented. [ABSTRACT FROM AUTHOR]

Measurements of the limiting escape rate of atomic hydrogen (H) atoms at Earth and the relative significance of thermal evaporation and nonthermal escape mechanisms, such as charge exchange and polar wind, have long been lacking. Our recent development of sophisticated radiative transport analysis techniques now enables the reliable interpretation of remotely sensed measurements of optically thick H emission, such as those acquired along the Earth's limb by the Global Ultraviolet Imager (GUVI) onboard the NASA Thermosphere Ionosphere Mesosphere Energetics and Dynamics (TIMED) spacecraft, in terms of physical parameters such as exobase density and, crucially, vertical diffusive flux. In this work, we present results from a systematic investigation of H Lya emission measured by TIMED/GUVI along the Earth's dayside limb from 2002–2007, which we use to derive the vertical H flux and associated density distribution from 250 km out to 1 Earth radius. Our analysis reveals that the vertical flux of thermospheric H is nearly constant over a large range of solar activity and typically exceeds the calculated thermal evaporative flux, suggesting that terrestrial H escape is indeed limited by its vertical diffusion. The excess supply of H atoms to the exobase associated with large observed vertical fluxes requires that nonthermal escape mechanisms be operative for steady‐state continuity balance. We find that such nonthermal processes are a particularly significant component of total H escape during low solar activity, when thermal evaporation is weakest. Key Points: Radiative transfer inversion of Lyα emission measured by TIMED/GUVI is used for estimation of vertical H flux and density from 250 km to 1 RETotal terrestrial H escape is observed to be diffusion‐limited at a nearly constant rate of ∼1.2×108 atoms cm−2s−1 over the solar cycle.Jeans' thermal escape comprises up to ∼84% of the total flux during solar maximum, whereas non‐thermal processes dominate in solar minimum. [ABSTRACT FROM AUTHOR]

The habitability of Earth significantly depends on the evolution of its atmospheric properties, which are strongly influenced by escape processes. Polar outflow, which encompasses the atmospheric escape of ions through the open field lines of the Earth's magnetosphere, is the dominant escape process for modern Earth and likely played a significant role in Earth's early history as well. In this article, we investigate the evolution of Earth's polar outflow during the Archean eon 3-4 gigayears (Ga) ago for an N 2 -CO 2 atmosphere. For this, we use a kinetic direct simulation Monte Carlo (DSMC) particle code to simulate the interactions between the solar wind and the Earth's upper atmosphere. We explore the effects of different CO 2 mixing ratios (10%, 25%, 50%, 75%, 99%) on the polar outflow escape rates of oxygen, nitrogen and carbon. Using the ion production rates as an estimate for polar outflow, our results indicate that during the time 3.6-4.0 Ga ago, a CO 2 mixing ratio larger than 25% may have been necessary to avoid excessive escape via polar outflow. Close to 4 Ga ago, even 50% CO 2 may have been necessary due to the stronger solar winds. Our results align with other studies also suggesting a similarly high CO 2 mixing ratio in the early Archean. • Polar outflow is a non-negligible atmospheric escape process for magnetized planets • Early Archean Earth may have had 25%-50% CO 2 to prevent excessive polar outflow • Studies on thermal and non-thermal escape suggest similar levels of necessary CO 2 [ABSTRACT FROM AUTHOR]

The ion composition boundary layer that separates solar wind from ionospheric plasma at Mars typically contains a smooth compositional transition. However, ~10% of boundary crossings display large nonmonotonic variations in density, indicating an instability. We find that most of these instabilities occur on the flanks or downstream of the planet, and many fall into two distinct classes, which occur in different locations, under different preferred conditions. Plume events, which occur where the solar wind electric field points outward, likely represent the motion of the escaping ion plume across the spacecraft. Snowplow events, which occur where the electric field points inward, likely represent the escape of discrete parcels of plasma, which could account for ~10% of the total ion loss from Mars. Snowplow events occur under conditions that favor the development of ionospheric irregularities, which could form a seed for the snowplow mechanism that detaches and accelerates ionospheric plasma downstream. Plain Language Summary: The solar wind, a hot ionized gas—or plasma—that flows from the Sun, interacts directly with plasma derived from the Martian atmosphere, across a boundary layer known as the ion composition boundary. Usually, this boundary layer has a stable structure and contains smoothly varying densities of planetary and solar wind ions. However, at some times, large nonmonotonic variations in density across this boundary layer suggest a rippled and/or unsteady boundary, indicating the presence of some instability. We investigate these unstable cases, and find that they fall into two distinct classes. Plume events likely represent a flapping motion of a discrete plume of escaping ions. Snowplow events likely represent the escape of discrete clouds of plasma from the boundary layer. These two types of events occur in different locations, under different conditions. These trends help reveal the coupling between the Martian upper atmosphere, the ionosphere, the magnetosphere, and the solar wind. Key Points: Instabilities occur near the boundary layer between ions of solar wind origin and ions of ionospheric origin at MarsWe identify two distinct classes of boundary layer instabilities, which have different characteristics, and occur in different locationsSnowplow events, a potentially significant ion loss mechanism, occur under conditions that favor an irregular ionospheric structure [ABSTRACT FROM AUTHOR]

It is important to include the effects of cold ions when we consider heavy ion outflows from Mars. We here report on statistical properties of heavy ion outflows (including cold ion outflows) observed by Mars Atmosphere and Volatile EvolutioN in the optical wake region. Using data from July 2015 to December 2017, we statistically investigate the effects of solar wind convection electric fields and crustal magnetic fields on the heavy ion outflows. Results show that the average density ratio of O+:O2+:CO2+ is ~29:68:04. In the southern hemisphere where the strong crustal magnetic fields are located, the heavy ion outflow flux becomes smaller and the relative contribution of molecular ions to heavy ion outflows is larger than the northern hemisphere. The solar wind convection electric field strongly affects the heavy ion outflows. Heavy ion density is larger in the −E (electric field) hemisphere than in the +E hemisphere, while the dependence of velocity is opposite. Acceleration by the solar wind convection electric field in the +E hemisphere is expected to cause these dependencies. The heavy ion flux is larger in the −E (total O: 8.6 × 106 cm–2/s) than in the +E hemisphere (2.9 × 106 cm–2/s) due to the large density in the −E hemisphere. Velocity ratios of O+ to O2+ suggest that heavy ion outflows with large velocities tend to have the same energy to each other, while the O+ to O2+ ions are more likely to have the same velocity in outflows with small velocities. Plain Language Summary: While Martian climate is cold and dry at present, it is thought that Martian climate was relatively warm and wet about 4 billion years ago. In order to cause this climate change, a vast majority of CO2 existing in the ancient Mars must have been removed from Martian atmosphere. Ion escape from the Martian upper atmosphere to space is one of the candidate mechanisms to remove CO2. However, the detailed characteristics of ion escape are far from understood. Based on observations in the Martian‐induced magnetotail by the Mars Atmosphere and Volatile EvolutioN mission, we here report on the statistical trend of heavy ion outflows. The effects of electric fields induced by the motion of solar winds and crustal magnetic fields on heavy ion outflows are investigated. Key Points: Heavy ion outflows in the wake region are denser and slower in −E (electric field) hemisphere than in +E hemisphereHeavy ion flux is smaller in the southern hemisphere than the northern, while the relative contribution of molecular ions is largerFast outflows tend to have the same energy for O+ and O2+ ions, while they have more chance to have the same velocity in slow outflows [ABSTRACT FROM AUTHOR]

We present a mathematical model for the propagation of the shock waves that occur during planetary collisions. Such collisions are thought to occur during the formation of terrestrial planets, and they have the potential to erode the planet’s atmosphere. We show that, under certain assumptions, this evolution of the shock wave can be determined using the methodologies of Type II self similar solutions. In such solutions, the evolution of the shock wave is determined by boundary conditions at the shock front and a singular point in the shocked region. We show how the evolution can be determined for different equations of state, allowing these results to be readily used to calculate the atmospheric mass loss from planetary cores made of different materials. We demonstrate that, as a planetary shock converges to the self similar solution, it loses information about the collision that created it, including the impact angle for oblique collisions.

The magnetization of a planet significantly changes the nature of atmospheric escape, and the question of whether a planetary field shields atmosphere from erosion by the solar wind or not remains open. Ion escape processes from Mars are investigated under two hypothetical conditions, namely, no intrinsic magnetic field and weak intrinsic dipole field cases under present solar wind conditions based on multispecies magnetohydrodynamics simulations. The existence of a weak dipole field results in an enhancement of the tailward flux of planetary ions through the four escape channels. Two channels are associated with the open fields at the poles, and the others are caused by magnetic reconnection between the planetary and solar wind magnetic fields at the flank magnetopause. The ion escape rate with the weak dipole is greater than in the no‐dipole case. The enhancement is significant for O2+ and CO2+, suggesting ion escape from the low‐altitude ionosphere. Plain Language Summary: Present Mars has a thin atmosphere and little water on the surface. Space missions have provided some evidences for the existence of liquid water and a thick atmosphere on ancient Mars. These evidences suggest Mars has experienced significant atmospheric loss up to the present. Ion outflow is one of the important atmospheric loss mechanisms. In the present day, Mars does not have a global magnetic field such as that of Earth, and thus, ions escape by the direct interaction with the solar wind (influenced to some extent by Mars' crustal fields). In contrast, it is expected that ancient Mars had a global magnetic field. The global magnetic field forms the magnetosphere around the planet, perhaps changing the ion escape mechanisms. In this study, a numerical simulation of the Mars‐solar wind interaction is used to compare ion escape for the case of no planetary field with a case where a weak global magnetic field is assumed. The weak global magnetic field yields four escape routes associated with open magnetic fields and magnetic reconnection. The escape rate of heavy ions increases by ~25% compared to the nondipole case, particularly facilitating the escape of O2+ and CO2+ present in the low‐altitude ionosphere. Key Points: The ion escape rate increases when a weak intrinsic dipole magnetic field is introducedThe weak intrinsic field facilitates four escape routes, through cusps and magnetic reconnections between the IMF and planetary fieldsThe enhancement of the escape rate is more pronounced for O2+ and CO2+ than O+, suggesting the ion escape from the low‐altitude ionosphere [ABSTRACT FROM AUTHOR]

The hydrogen isotopic (D/H) ratio reflects the global cycling and evolution of water on Earth as it fractionates through planetary processes. We model the water cycle taking seafloor hydrothermal alteration, chemical alteration of continental crust, slab subduction, hydrogen escape from the early Earth, and degassing at mid-ocean ridges, hot spots, and arcs into account. The differences in D/H ratios between present-day oceans, oceanic and continental crust, and mantle are thought to reflect isotopic fractionation through seafloor alteration, chemical alteration, and slab dehydration. However, if the speed of plate tectonics has been nearly constant throughout Earth's history, the degassing and regassing rates are too small to reach the present-day D/H ratios. We show that (a) hydrogen escape from reduced early atmosphere, (b) secular net regassing, or (c) faster plate tectonics on early Earth is needed to reproduce the present-day D/H ratios of the water reservoirs. The low D/H ratio of Archean seawater at 3.8 Ga has previously been interpreted as a signature of (a) hydrogen escape, but we find it can also be explained either by (b) secular net degassing or by (c) faster plate tectonics on early Earth. The rates of hydrogen escape from early Earth and secular regassing on present-day Earth are constrained to be lower than 2.1 × 10 11 kg/yr and 3.9 × 10 11 kg/yr. Consequently, the volume of water in the present-day mantle could result entirely from the regassing through Earth's history. In that case, the volume of initial oceans could be 2 to 3 times larger than that of current Earth. We suggest that, in addition to the D/H ratio of Archean seawater, identifying the D/H ratios of both seawater and mantle throughout Earth's history would allow to distinguish these evolutionary scenarios. [ABSTRACT FROM AUTHOR]

Abstract: Cold ion outflow is one of the candidate processes to cause significant atmospheric escape from Mars. We here report on the cold dense heavy ion outflow event observed in Martian‐induced magnetotail by Mars Atmosphere and Volatile EvolutioN (MAVEN) on 4 December 2014. In the outflow event, the cold dense heavy ion outflow was observed only in south dusk downward convection electric field (E) lobe. During the event, the strong crustal magnetic field was located on dayside of Mars. It suggests that combination of minimagnetosphere and downward E facilitates the cold dense heavy ion outflow. In order to estimate the CO2+ density, we adopted a fitting method of log‐normal distribution to the O2+ time of flight distributions to eliminate O2+ contamination in the CO2+ time of flight data. Contribution of O2+, O+, and CO2+ to the total heavy ions (~134 cm−3) is about 71%, 26%, and 3%, respectively. Average heavy ion flux was ~5.4 × 107 cm−2s−1, assuming the escape velocity at 2,000‐km altitude. [ABSTRACT FROM AUTHOR]

Abstract: Mars likely lost a significant part of its atmosphere to space during its history. The sputtering of the atmosphere, by precipitating planetary heavy pickup ions accelerated by the solar wind, is one of the processes that could have significantly contributed to this atmospheric escape, in particular since the cessation of its global magnetic field, 4.0–4.1 Gyr ago. We present a 2 year baseline analysis of Mars Atmosphere and Volatile EvolutioN (MAVEN) observations of the precipitating flux. We use this measurement to model the expected escape rate and exospheric structure induced by this precipitation. We conclude that sputtering signatures in the dayside exosphere will be difficult to identify by MAVEN, and the induced atmospheric escape of O atoms remains orders of magnitude smaller than the expected rate induced by dissociative recombination of O2+ in Mars's ionosphere. On the contrary, deep in the nightside, Mars's sputtering might be the main source of the nonthermal part of the exospheric density profiles of species with mass larger or equal to Ar. [ABSTRACT FROM AUTHOR]

The presence of an atmosphere over sufficiently long timescales is widely perceived as one of the most prominent criteria associated with planetary surface habitability. We address the crucial question of whether the seven Earth-sized planets transiting the recently discovered ultracool dwarf star TRAPPIST-1 are capable of retaining their atmospheres. To this effect, we carry out numerical simulations to characterize the stellar wind of TRAPPIST-1 and the atmospheric ion escape rates for all of the seven planets. We also estimate the escape rates analytically and demonstrate that they are in good agreement with the numerical results. We conclude that the outer planets of the TRAPPIST-1 system are capable of retaining their atmospheres over billion-year timescales. The consequences arising from our results are also explored in the context of abiogenesis, biodiversity, and searches for future exoplanets. In light of the many unknowns and assumptions involved, we recommend that these conclusions must be interpreted with due caution. [ABSTRACT FROM AUTHOR]