Surface-bounded Exosphere Research Articles

High-energy particle observations from the Moon.

The Moon is a unique natural laboratory for the study of the deep space plasma and energetic particles environment. During more than 3/4 of its orbit around the Earth it is exposed to the solar wind. Being an unmagnetized body and lacking a substantial atmosphere, solar wind and solar energetic particles bombardthe Moon's surface, interacting with the lunar regolith and the tenuous lunar exosphere. Energetic particles arriving at the Moon's surface can be absorbed, or scattered, or can remove another particle from the lunar regolith by sputtering or desorption. A similar phenomenon occurs also with the galactic cosmic rays, which have fluxes and energy spectra representative of interplanetary space. During the remaining part of its orbit the Moon crosses the tail of the terrestrial magnetosphere. It then provides the opportunity to study in-situ the terrestrial magnetotail plasma environment as well as atmospheric escape from the Earth's ionosphere, in the form of heavy ions accelerated and streaming downtail. The lunar environment is thus a unique natural laboratory for analysing the interaction of the solar wind, the cosmic rays and the Earth's magnetosphere with the surface, the immediate subsurface, and the surface-bounded exosphere of an unmagnetized planetary body. This article is part of a discussion meeting issue 'Astronomy from the Moon: the next decades (part 2)'.

Rapid Imaging Planetary Spectrograph

The Rapid Imaging Planetary Spectrograph (RIPS) was designed as a long-slit high-resolution spectrograph for the specific application of studying atmospheres of spatially extended solar system bodies. With heritage in terrestrial airglow instruments, RIPS uses an echelle grating and order-sorting filter to obtain optical spectra at resolving powers of up to R ∼ 127,000. An ultra-narrowband image from the reflective slit jaws is captured concurrently with each spectrum on the same electron-multiplying charge-coupled device detector. The “rapid” portion of RIPS’s moniker stems from its ability to capture high frame rate data streams, which enables the established technique known as “lucky imaging” to be extended to spatially resolved spectroscopy. Resonantly scattered emission lines of alkali metals, in particular, are sufficiently bright to be measured within short integration times. RIPS has mapped the distributions of Na and K emissions in Mercury’s tenuous exosphere, which exhibits dynamic behavior coupled with the planet’s plasma and meteoroid environment. An important application is daylight observation of Mercury with solar telescopes, as the synoptic context of the exosphere’s distribution comprises valuable ground-based support for the upcoming BepiColombo orbital mission. As a conventional long-slit spectrograph, RIPS has targeted the Moon’s surface-bound exosphere, where structures in line width and brightness are observed as a function of tangent altitude. At the Galilean moons, RIPS can study the plasma interaction with Io and place new constraints on the sputtered atmosphere of Europa, which in turn provides insight into the salinity of Europa’s subsurface ocean. The instrumental design and construction are described herein, and these astronomical observations are presented to illustrate the performance of RIPS as a visiting instrument at three different telescope facilities.

Space plasma physics science opportunities for the lunar orbital platform - Gateway

The Lunar Orbital Platform - Gateway (LOP - Gateway, or simply Gateway) is a crewed platform that will be assembled and operated in the vicinity of the Moon by NASA and international partner organizations, including ESA, starting from the mid-2020s. It will offer new opportunities for fundamental and applied scientific research. The Moon is a unique location to study the deep space plasma environment. Moreover, the lunar surface and the surface-bounded exosphere are interacting with this environment, constituting a complex multi-scale interacting system. This paper examines the opportunities provided by externally mounted payloads on the Gateway in the field of space plasma physics, heliophysics and space weather, and also examines the impact of the space environment on an inhabited platform in the vicinity of the Moon. It then presents the conceptual design of a model payload, required to perform these space plasma measurements and observations. It results that the Gateway is very well-suited for space plasma physics research. It allows a series of scientific objectives with a multi-disciplinary dimension to be addressed.

Exospheric escape: A parametrical study

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

Two-dimensional description of surface-bounded exospheres with application to the migration of water molecules on the Moon.

On the Moon, water molecules and other volatiles are thought to migrate along ballistic trajectories. Here, this migration process is described in terms of a two-dimensional partial differential equation for the surface concentration, based on the probability distribution of thermal ballistic hops. A random-walk model, a corresponding diffusion coefficient, and a continuum description are provided. In other words, a surface-bounded exosphere is described purely in terms of quantities on the surface, which can provide computational and conceptual advantages. The derived continuum equation can be used to calculate the steady-state distribution of the surface concentration of volatile water molecules. An analytic steady-state solution is obtained for an equatorial ring; it reveals the width and mass of the pileup of molecules at the morning terminator.

A Sublimation-driven Exospheric Model of Ceres

After Vesta, the NASA Dawn spacecraft will visit the dwarf planet Ceres to carry out in-depth observations of its surface morphology and mineralogical composition in 2015. One of the important questions is whether Ceres has any outgassing activity that would lead to the formation of a thin atmosphere. The recent detection of water vapor emitted from localized source regions by Herschel (Küppers et al., 2014) has only underscored this point. If the localized outgassing activity observed by Herschel is totally switched off, could a sizable surface-bounded exosphere still be maintained by other source mechanisms? Our preliminary assessment is that chemical sputtering via solar wind interaction and meteoroid impact are probably not adequate because of the large injection speed of the gas at production relative to the surface escape velocity of Ceres. One potential source is a low-level outgassing effect from its subsurface due to thermal sublimation with a production rate of the order of 1024 molecules s−1 as first considered by Fanale and Salvail (1989). If the water plumes are active, the fall-back of some of the water vapor onto Ceres’ surface would provide an additional global source of water molecules on the surface with a production rate of about 1025 molecules s−1. In this work, different scenarios of building up a tenuous exosphere by ballistic transport and the eventual recycling of the water molecules to the polar cold trap are described. It turns out that a large fraction of the exospheric water could be transferred to the polar caps area as originally envisaged for the lunar polar ice storage.

Kinetic modeling of sodium in the lunar exosphere

Our knowledge about the lunar environment is based on a large volume of ground-based, remote, and in situ observations. These observations have been conducted at different times and sampled different pieces of such a complex system as the surface-bound exosphere of the Moon. Numerical modeling is the tool that can link results of these separate observations into a single picture. Being validated against previous measurements, models can be used for predictions and interpretation of future observations results.In this paper we present a kinetic model of the sodium exosphere of the Moon as well as results of its validation against a set of ground-based and remote observations. The unique characteristic of the model is that it takes the orbital motion of the Moon and the Earth into consideration and simulates both the exosphere as well as the sodium tail self-consistently. The extended computational domain covers the part of the Earth’s orbit at new Moon, which allows us to study the effect of Earth’s gravity on the lunar sodium tail.The model is fitted to a set of ground-based and remote observations by tuning sodium source rate as well as values of sticking, and accommodation coefficients. The best agreement of the model results with the observations is reached when all sodium atoms returning from the exosphere stick to the surface and the net sodium escape rate is about 5.3×1022s−1.

Energetic neutral particles detection in the environment of Jupiter’s icy moons: Ganymede’s and Europa’s neutral imaging experiment (GENIE)

The detection of Energetic Neutral Particles (ENP) above 10eV can unequivocally relate a surface-bound exosphere to surface features and can monitor instantaneously the effect of plasma precipitation onto the surface. In the framework of a mission to Jupiter's moons, 2D imaging of plasma precipitation will provide important information on the plasma circulation at the orbits of the moons. Furthermore, a joint measurement of precipitating ions will permit an estimation of the efficiency of the release process. Coupled measurements of ENP and gas composition will improve our knowledge of surface release mechanisms. Ganymede's and Europa's Neutral Imaging Experiment (GENIE) is a high-angular-resolution detector, based on the ToF (Time of Flight) technique, that can detect ENP (energy range >10eV–few keV) in the Jupiter environment thanks to an innovative design and technology. Its objective is to map the sites of origin of the ENP of the icy moons’ exospheres to investigate the interaction between the surface and the environment. Finally, coupling GENIE with an ion sensor and a mass spectrometer will be an outstanding opportunity to better understand the magnetosphere–moon coupling within the Jupiter system and compare the surface interaction with plasma in the diverse moons. In this paper, the scientific objectives and requirements of ENP detection are summarized and the description of the innovative design concept of GENIE is given, together with the signal and background noise simulation.

Source dependency of exospheric sodium on Mercury

Due to a large solar radiation effect, the sodium exosphere exhibits many interesting effects, including the formation of an extended corona and a tail-like structure. The current suite of observations allows us to study some physical properties of the sodium exosphere, such as the source rates and the interaction with the surface, both experimentally and theoretically. In order to quantify the complex variations in the sodium exosphere in more detail, we use an exospheric model with the Monte-Carlo method to examine the surface interactions of a sodium atom, including the surface thermal accommodation rate and the sticking coefficient. The source rates from different components, such as the photon stimulated desorption (PSD), the meteoroid impact vaporization (MIV), and the solar wind ion sputtering (IS), can be constrained by comparing our exospheric model calculations with the published observational data. The detected terminator to limb (TL) ratio on the disk and the tail production rate can be explained with no sticking effect and small thermal accommodation rates. We also examine the best fit of the MIV source evolution, through comparison with the disk-averaged emission. The resultant discrepancy between the observations and the model fit may reflect the surface variation in the sodium abundance. A comprehensive mapping of the surface geochemical composition of the surface by the MESSENGER and Bepi-Colombo missions should give us more information about the nature of this surface-bound exosphere.

Investigation of local time dependence of Mercury's sodium exosphere based on a numerical simulation

Mercury has a surface-bounded exosphere (SBE) similar to that of the Moon. One of the atmospheric species, sodium, was found by ground-based observations to be the most prominent component. Mercury's sodium SBE is known to be non-uniform with respect to local time (LT) in low-latitude regions: the sodium column density in the dawn-side region is larger than that in the dusk-side region, and the sodium abundance is the largest in the morning–noon region. To reveal the production processes for the exosphere near Mercury's surface, the LT dependence of the exosphere was investigated through a numerical simulation. Three data sets of sodium column densities observed for the dawn-side hemisphere, observed by Sprague et al. [1997. Distribution and abundance of sodium in Mercury's atmosphere, 1985–1988. Icarus 12, 506–527], were compared with results simulated by a 3D Monte Carlo method, and the source rates and density of sodium of the planetary surface were estimated. In the simulation, the photon-stimulated desorption (PSD) and thermal desorption (TD) processes were assumed as the release mechanisms. The sodium source rates for the three data sets, at respective heliocentric distances of about 0.33, 0.42, and 0.44 AU, were estimated as 1–4×10 8 Na/cm 2/s with weak LT dependence. In contrast, the expected sodium surface density showed clear dependence on LT and the heliocentric distance. The sodium surface density decreases from early morning to noon by a few orders, and, particularly for large heliocentric distances, the surface is in a condition of sodium excess and depletion with respect to the surface sodium density assumed by Killen et al. [2004. Source rates and ion recycling rates for Na and K in Mercury's atmosphere. Icarus 171, 1–19] in the early morning and morning–noon regions, respectively. This study implies that the decrease in sodium surface density from the early morning to noon regions might produce the characteristic LT dependence in the low-latitude dawn-side region.

Mercury’s sodium exosphere explored by the BepiColombo mission

The Mercury’s Sodium Atmosphere Spectral Imager (MSASI) on BepiColombo will address fundamental scientific questions pertaining to the Mercury’s sodium exosphere. Together, our measurements on the overall scale will provide ample new information on regolith–exosphere–magnetosphere coupling as well as new understanding of the dynamics governing the surface-bounded exosphere. We will compare the four different source mechanisms in preparation for modeling MSASI data and show the feasibility of identifying a process.

Mercury’s Atmosphere: A Surface-Bounded Exosphere

The existence of a surface-bounded exosphere about Mercury was discovered through the Mariner 10 airglow and occultation experiments. Most of what is currently known or understood about this very tenuous atmosphere, however, comes from ground-based telescopic observations. It is likely that only a subset of the exospheric constituents have been identified, but their variable abundance with location, time, and space weather events demonstrate that Mercury’s exosphere is part of a complex system involving the planet’s surface, magnetosphere, and the surrounding space environment (the solar wind and interplanetary magnetic field). This paper reviews the current hypotheses and supporting observations concerning the processes that form and support the exosphere. The outstanding questions and issues regarding Mercury’s exosphere stem from our current lack of knowledge concerning the surface composition, the magnetic field behavior within the local space environment, and the character of the local space environment.

Mercury's exosphere: A possible source for Na

Many ground-based observations of Na in Mercury's surface-bounded exosphere have been made and continued to be made in an effort to understand the sources, sinks, and distribution of Na around Mercury. These time consuming and costly efforts are made to better understand the physical processes on and around Mercury. A big step would be to discover an actual source of the Na from Mercury's crust because it is already known that meteorites and comets provide Na to the exosphere through impact. We provide ground-based CCD imagery obtained with small ground-based telescopes that show bright albedo features at locations coincident with enhanced Na emissions in Mercury's exosphere. We suggest these locations are sources for Na. We also provide a mechanism to test this hypothesis using in situ observations by instruments on the MESSENGER spacecraft during the three fly bys of Mercury that will occur in 2008 and 2009, and during the orbital mission which begins in 2011. It is necessary to prove that Na is delivered to the exosphere from one or more crustal source regions before exospheric Na can be used as a measure of the volatile content of Mercury used to infer formation and evolution from the primitive solar nebula. The same applies to other elements such as K which is known to be in Mercury's exosphere and S which is postulated to be present. We expound on the impact that the discovery of one or more source regions from Mercury's crust would have on our ability to discern between the three leading models of Mercury's formation and crustal evolution.

Observation of the sodium exosphere from mercury magnetospheric orbiter

The Mercury Sodium Atmosphere Spectral Imager (MSASI) in the BepiColombo mission will address a wealth of fundamental scientific questions pertaining to the Mercury’s exosphere. Together, our measurements on the overall scale will provide ample new information on regolith–exosphere–magnetosphere coupling as well as new understanding of the dynamics governing the ‘surface-bounded exosphere’. It arises quite clearly from ground-based observations that the regolith of Mercury releases a fraction of its content to the exosphere. Some processes are identified up to now as leading to this ejection, e.g., photon-stimulated desorption. These processes are associated with different energies of ejection and behaviors in different regions of Mercury’s surface. Therefore, different types of population are born from the surface, depending on the processes. The MSASI measurements definitely can identify the release processes, how exospheric sodium is born from the regolith. MSASI/BepiColombo is the first and unique opportunity to study the formation, circulation, and maintenance of the surface-bounded exosphere.

The Surface-Bounded Exosphere of Mercury

Almost thirty years ago, the instruments on the Mariner 10 spacecraft detected traces of hydrogen, helium, and possibly oxygen in the atmosphere of Mercury. There the matter rested until the mid-80’s, when emission lines of sodium (Potter & Morgan 1985) and potassium (Potter & Morgan 1986) were found in the spectrum of Mercury, resulting from resonance scattering of sunlight by metal atoms in the atmosphere. Searches for other metals in the Mercury atmosphere were fruitless until recently, when an emission line attributed to calcium has been observed (Bida, Killen & Morgan 2000). The densities of all these species are so low that gas phase collisions are negligible, so that the Mercury atmosphere is an exosphere bounded at its base by the surface of the planet. The fact that the exosphere is bounded by the Mercury surface means that interactions with the surface must occur. Energy exchange, surface absorption and desorption, and ion neutralization can take place, and these must be accounted for in models of the exosphere. None of these atmospheric species can survive long on Mercury, being lost mainly by photoionization followed by trapping in the solar wind and partly by solar radiation acceleration. Consequently, the species that we observe must be in a steady state, being generated as fast as they are lost to space. The metals must originate from the surface of the regolith, with the supply of fresh surface material maintained by gardening of the surface by meteoroid impact. The metals are released from the surface into the exosphere by photon-stimulated desorption (Madey et al. 1998), particle sputtering (McGrath, Johnson & Lanzerotti 1986), and meteoroid impact vaporization (Morgan, Zook & Potter 1988). Thermal evaporation of species condensed on the cold side may be important for determining their distribution over the planet (Hunten & Sprague 2002). The interaction and relative importance of all these processes has been discussed by Killen & Morgan (1993). Much can be said about these processes, but this review is focused primarily on the current status of observations of sodium, potassium, and calcium.

Exploring Mercury: the iron planet

Preface List of Figs, Tables and Plates Additional information on CD 1. The Twilight Planet 2. The Mariner 10 Mission 3. Mercury's Motions 4. Mercury's Size, Mass and Density 5. Mercury's Magnetic Field and Internal Constitution 6. Mercury's Surface-Bounded Exosphere 7. General Surface Features and Radar Characteristics 8. Surface Composition 9. The Impact Cratering Record 10. Plains: Smooth and Intercrater 11. Tectonics 12. History and Origin 13. Future Exploration of Mercury Appendix A Orbital and Physical Data for Mercury Appendix B Glossary and Terms Appendix C Names and Locations of Mercury's Surface Features Bibliography Index Appendix 1. Glossary of Terms 2. Orbital and Physical Data of Mercury 3. Names and Locations of Mercury's Surface Features CD ROM Containing 1. README explanation of CD contents and instructions for minimal image processing 2. All good 1st, 2nd , and 3rd encounter Mariner 10 Images (about 1000) 3. PC and Macintosh Software for opening and manipulating images (NIH Image public domain software) 4. Possibly Digital Maps and Mariner 10 photomosaics 5. Some MESSENGER graphics including a movie of the spacecraft trajectory.