Solar Wind Bombardment Research Articles

Modeling the production of submicroscopic iron in the lunar highlands

Micrometeoroid impacts, solar wind bombardment, and dielectric breakdown driven by solar energetic particles all potentially alter the optical properties of the lunar regolith by creating submicroscopic metallic iron (smFe0), which includes both nanophase (<33nm) and microphase (>33nm) iron. We create a simple model that describes the time-dependent accumulation of optically active smFe0 in the lunar highlands. Our model synthesizes recent analyses of how space weathering processes form smFe0-bearing agglutinates and rims on soil grains and how impact gardening controls the exposure time of these grains. It successfully reproduces the results of a prior analysis of the formation of smFe0 in the highlands, particularly in regard to nanophase iron, showing that there is consistency among diverse analyses of Apollo samples and of orbital observations. We find that the results of our model are not consistent with the solar wind directly forming smFe0 (although the solar wind may play a role in optical maturation via hydrogen implantation). Our model results are consistent with smFe0 in the lunar highlands being created mainly by micrometeoroid impacts, with a possible contribution from dielectric breakdown weathering.

Temperature programmed desorption comparison of lunar regolith to lunar regolith simulants LMS-1 and LHS-1

Water and molecular hydrogen evolution from Apollo sample 14163 and lunar regolith simulants LMS-1, a mare simulant, and LHS-1, a highlands simulant, were examined using Temperature Programmed Desorption (TPD) in ultra-high vacuum. LMS-1, LHS-1, and Apollo 14163 released water upon heating, whereas only the Apollo sample directly released measurable quantities of molecular hydrogen. The resulting H2O and H2 TPD curves were fit using a model which considers desorption at the vacuum grain interface, transport in the void space between grain-grain boundaries, molecule formation via recombination reactions and sub-surface diffusion. The model yielded a most probable H2O formation and desorption effective activation energy of ∼150 kJmol−1 for all samples. The probability distribution widths of the effective activation energies were ∼100–400, ∼100–350, and ∼100–300 kJmol−1 for LMS-1, LHS-1, and Apollo 14163, respectively. In addition to having the narrowest energy distribution width, the Apollo sample released the least amount to water (103 ppm) relative to LMS-1 (176 ppm) and LHS-1 (195 ppm). Since essentially no molecular hydrogen was observed from the simulants, the results indicate that LMS-1 and LHS-1 display water surface formation, binding, and transport interactions similar to actual regolith but not the desorption chemistry associated with the implanted hydrogen from the solar wind. Overall, these terrestrial surrogates are useful for understanding the surface and interface interactions of lunar regolith grains, which are largely dominated by the terminal hydroxyl sites under both solar wind bombardment and terrestrial preparation conditions.

Evidence that Earth’s magnetotail affects dielectric breakdown weathering on the Moon

The standard view of space weathering on the Moon is that the solar wind and micrometeoroid impacts alter the optical properties of lunar soil. A third process—dielectric breakdown driven by solar energetic particles (SEPs)—has also been suggested to contribute to space weathering. It has been difficult to determine the relative roles of these processes. The Earth’s magnetotail, however, provides a way to distinguish between them, because it affects only charged particles. Earth’s magnetotail blocks the solar wind, and here we show that it also likely reduces the flux of SEPs traveling across the tail and impacting the tail-facing hemisphere of the Moon when it is entering or leaving. Consequently, we make two predictions that distinguish how the tail affects dielectric breakdown weathering patterns from how it affects solar wind weathering patterns. First, the magnetotail should create two minima in the total amount of breakdown weathering that has occurred: one near ≳60∘ and a deeper one near ≲−60∘ longitude. Second, the tail should create east–west asymmetries in the breakdown weathering of crater walls, with the greatest asymmetries occurring at ±75∘ longitude. Although the first prediction has proven difficult to test, we find that the second prediction is supported by observations. Therefore, we conclude that investigations of space weathering must consider, not only micrometeoroid and solar wind bombardment, but also dielectric breakdown.

Near-UV and near-IR reflectance studies of lunar swirls: Implications for nanosize iron content and the nature of anomalous space weathering

We performed an analysis of spacecraft multispectral images for lunar swirls in order to gain an improved understanding of optical space weathering on the Moon and its causes. LROC WAC data provide information on the slope of the spectrum in the near-UV (NUV), as measured by the 321-nm/415-nm or 321-nm/360-nm reflectance ratios. Kaguya MI data were used to assess the near-infrared (NIR) continuum slope (1548-nm/749-nm reflectance ratio). Context for interpreting the spectral variations found in the remotely observed regions of the lunar surface is provided by laboratory reflectance spectra of lunar rocks and soils, as well as spectra for transparent silica gel analogs (Noble et al., 2007) containing different sizes and abundances of nanometer-sized iron (nsFe) particles. We gain additional insights into the spectral effects of sample maturity by considering the ferromagnetic resonance parameter (Is) values for mare and highland soils, as well as the number density of nsFe particles in the silica gels.We examined a set of three mare swirls (Reiner Gamma, Ingenii, and Mare Marginis) and three highland swirls (Airy, Descartes, and Gerasimovich). The NIR continuum slopes of both mare and highland swirls are shallower than those of the nearby normal mature regolith. Bright swirl surfaces have higher NIR slopes than normal fresh material of the same albedo. The NUV ratios within mare swirls are lower than in the mature background, but for highland swirls, the NUV ratios are approximately the same as the mature background. We do not see definitive evidence for "over-maturation" (excessive darkening and reddening beyond that found in the normal background surfaces) in dark lanes at the swirls we examined, although saturation of weathering effects at a high-iron location like Reiner Gamma could prevent over-maturation from appearing – even if enhanced solar-wind bombardment related to deflection by local magnetic fields is taking place.Evaluation of the NIR character of swirls and comparison with lab spectra of lunar soils and nsFe-bearing silica gel analogs leads to the conclusion that swirl materials contain abundances of nsFe that are lower than that of normal non-swirl background surfaces; the nsFe content of swirls corresponds to immature (though not pristine) or submature soils. However, the size distribution of nsFe in swirls is anomalous compared with normal lunar surfaces, with a deficiency in the smaller size range (< ~15 nm), as inferred from the NUV character of swirls. Because the flux of solar-wind ions reaching the surface in swirls is attenuated by shielding by crustal magnetic fields, we conclude that solar-wind exposure is the primary agent for production of small nsFe in normal lunar space weathering. Micrometeoroid bombardment, which is unimpeded by the presence of magnetic fields, is mainly responsible for production of larger nsFe in space weathering.

Semiannual Oscillation of the Lunar Exosphere: Implications for Water and Polar Ice

A time‐dependent simulation of the argon‐40 exosphere of the Moon shows that the semiannual oscillation of argon detected by the neutral mass spectrometer on the Lunar Atmosphere and Dust Environment Explorer spacecraft is consistent with adsorptive respiration in seasonal cold traps near the lunar poles. The magnitude of the oscillation requires that high‐energy adsorption sites on soil grain surfaces at polar latitudes be as free of water contamination as soils at low latitudes. This requirement is met by the combination of two generally ignored water removal mechanisms: solar wind bombardment of exposed adsorption sites and the serpentinization reaction of water with olivine. The significance of these processes is supported by the lack of evidence of water in Lunar Atmosphere and Dust Environment Explorer data, which, in turn, establishes an upper bound for exospheric transport of water to polar traps at less than 1014 molecules/Ga.

Coordinated Nanoscale Compositional and Oxidation State Measurements of Lunar Space‐Weathered Material

AbstractSpace weathering on airless bodies includes a number of processes, such as micrometeorite impacts and solar wind bombardment, which leads to a variety of microscale to nanoscale alteration features, including vapor deposited layers on grain and rock surfaces and creation of nanophase opaque inclusions. The nanophase inclusions cause reddening and darkening of the visible to near‐infrared spectra of space weathered material, features associated with increasing space exposure of many airless body regoliths. On the Moon, most nanophase inclusions are metallic iron (npFe0), but recent work using aberration‐corrected scanning transmission electron microscopy and electron energy loss spectroscopy has provided evidence of oxidized nanoparticles in space weathered lunar soil grains. We examined three different lunar soils in order to confirm the finding of oxidized nanophase inclusions and to provide detailed elemental and mineralogical information about the surrounding material. Our data show that substrate and rim composition are key factors in determining whether highly localized oxidation occurs; for example, nanophase inclusions in rims on low Fe substrates are more prone to oxidation. Detailed understanding of the phases and features present in these samples is necessary for the correct interpretation of remotely sensed data as well as extrapolation from processes on the lunar surface to those on other airless bodies.

Production of Negative Hydrogen Ions Within the MMS Fast Plasma Investigation Due to Solar Wind Bombardment

AbstractThe particle data delivered by the Fast Plasma Investigation instrument aboard National Aeronautics and Space Administration's Magnetospheric Multiscale (MMS) mission allow for exceptionally high‐resolution examination of the electron and ion phase space in the near‐Earth plasma environment. It is necessary to identify populations which originate from instrumental effects. Using Fast Plasma Investigation's Dual Electron Spectrometers, we isolate a high‐energy (approximately kiloelectron volt) beam, present while the spacecraft are in the solar wind, which exhibits an azimuthal drift with period associated with the spacecraft spin. We show that this population is consistent with negative hydrogen ions H− generated by a double charge exchange interaction between the incident solar wind H+ ions and the metallic surfaces within the instrument. This interaction is likely to occur at the deflector plates close to the instrument aperture. The H− density is shown to be approximately 0.2–0.4% of the solar wind ion density, and the energy of the negative ion population is shown to be 70% of the incident solar wind energy. These negative ions may introduce errors in electron velocity moments on the order of 0.2–0.4% of the solar wind velocity and significantly higher errors in the electron temperature.

Biogenic oxygen from Earth transported to the Moon by a wind of magnetospheric ions

For five days of each lunar orbit, the Moon is shielded from solar wind bombardment by the Earth’s magnetosphere, which is filled with terrestrial ions. Although the possibility of the presence of terrestrial nitrogen and noble gases in lunar soil has been discussed based on their isotopic composition1, complicated oxygen isotope fractionation in lunar metal2,3 (particularly the provenance of a 16O-poor component) re­mains an enigma4,5. Here, we report observations from the Japanese spacecraft Kaguya of significant numbers of 1–10 keV O+ ions, seen only when the Moon was in the Earth’s plasma sheet. Considering the penetration depth into metal of O+ ions with such energy, and the 16O-poor mass-independent fractionation of the Earth’s upper atmosphere6, we conclude that biogenic terrestrial oxygen has been transported to the Moon by the Earth wind (at least 2.6 × 104 ions cm−2 s−1) and implanted into the surface of the lunar regolith, at around tens of nanometres in depth3,4. We suggest the possibility that the Earth’s atmosphere of billions of years ago may be preserved on the present-day lunar surface. The authors present evidence that the lunar soil contains oxygen ions from Earth that escape our atmosphere and reach the Moon through the Earth’s magnetospheric wind. The lunar surface possibly contains clues about the history of Earth’s atmosphere.

Grain‐scale supercharging and breakdown on airless regoliths

AbstractInteractions of the solar wind and emitted photoelectrons with airless bodies have been studied extensively. However, the details of how charged particles interact with the regolith at the scale of a single grain have remained largely uncharacterized. Recent efforts have focused upon determining total surface charge under photoemission and solar wind bombardment and the associated electric field and potential. In this work, theory and simulations are used to show that grain‐grain charge differences can exceed classical sheath predictions by several orders of magnitude, sometimes reaching dielectric breakdown levels. Temperature‐dependent electrical conductivity works against supercharging by allowing current to leak through individual grains; the balance between internal conduction and surface charging controls the maximum possible grain‐to‐grain electric field. Understanding the finer details of regolith grain charging, conductive equilibrium, and dielectric breakdown will improve future numerical studies of space weathering and dust levitation on airless bodies.

Simulation of the Neutral Particle Converter of the ARIES-L Device

This paper presents the concept of Lunar regolith surface layer composition analysis using solar wind bombardment and registration of the sputtered and knocked out atoms converted into ions with solid-state converters and subsequent registration of ions by high-aperture electrostatic analyzer. Monte Carlo binary collision approximation code is used to calculate angular and energy distributions of scattered and sputtered atoms. Literature data are used for evaluation of charged fractions of scattered and sputtered components and SIMION code is used for calculation of charged particles trajectories in electrostatic deflector. The total sensitivity of the proposed scheme is evaluated.

The effect on the lunar exosphere of a coronal mass ejection passage

Solar wind bombardment onto exposed surfaces in the solar system produces an energetic component to the exospheres about those bodies. The solar wind energy and composition are highly dependent on the origin of the plasma. Therefore, using the measured composition of the slow wind, fast wind, solar energetic particle (SEP) population, and coronal mass ejection (CME), we have estimated the total sputter yield for each type of solar wind. We show that the heavy ions, especially the He++ and O+7, can greatly enhance the total sputter yield during times when the heavy ion population is enhanced. Folding in the flux with the yield of individual ions, we compute the source rate for several species during different types of solar wind. Finally, we use a Monte Carlo model developed to simulate the time‐dependent evolution of the lunar exosphere to study the sputtering component of the exosphere under the influence of a CME passage. We simulate the background exosphere of Na, K, Ca, and Mg. Simulations indicate that sputtering increases the mass of those constituents in the exosphere more than ten times the background values. The escalation of atmospheric density occurs within an hour of onset. The decrease in atmospheric density after the CME passage is also rapid, although takes longer than the increase. Sputtered neutral particles have a high probability of escaping the Moon, by both leaving the Hill Sphere and photoionization. Density and spatial distribution of the exosphere can be tested with the LADEE mission.

Hydrogen migration to the lunar poles by solar wind bombardment of the moon

We investigate the deposition of hydrogen at the lunar poles from the incident solar wind proton flux. We follow an incident solar wind proton as it interacts with the lunar surface, is emitted into the atmosphere, and migrate through a series of ballistic hops across the surface of the Moon. We trace the path of the particle until it is removed from the system by photo-processes such as ionization or dissociation, by thermal escape, by surface chemistry, or by reaching a cold trap. Iterating with various compounds formed through surface chemistry, we follow all hydrogen atoms input into the system, regardless of their molecular composition. Accumulating statistics on the outcomes for various input particles, we determine that .6 of the hydrogen forms H 2 through chemical sputtering. Most of the remaining H thermally desorbs as atomic in a fraction of .27. Although the amount converted to OH is small (10%), most hydrogen reaches the pole through OH migration, .008 OH/H +. Finally, we calculate the amount of time required for the solar wind to supply the amount of hydrogen detected at the poles by the Lunar Prospector Neutron Spectrometer to be 7 Myr. If the poles only retain water due to the low freezing points of the other molecules, the time required is 100 Myr.

Loss of Water on the Young Venus: The Effect of a Strong Primitive Solar Wind

An enhanced primitive solar wind, such as may have prevailed during the first few 100 million years of the solar system history, is shown to have had the potential to stimulate strong thermal atmospheric escape from the young Venus. Due to heating by solar wind bombardment of an extended dense planetary corona, typically 10 times more extensive than the solid planet, an escape flux of pure atomic hydrogen as large as 3 × 10 14cm −2sec −1is found to be possible, provided the solar wind was ≈10 3–10 4more intense than now. Even if escape was diffusion-limited, an enhanced primitive solar UV flux (a factor of ≈5 above present level), absorbed by ≈0.3 mbar of thermospheric water vapor, was able to supply the flow at the required rate. For these high escape rates, oxygen was massively dragged off along with hydrogen, and water molecules could be lost at a rate of ≈6 × 10 13molecules cm −2sec −1. Because, at this rate, a terrestrial-type ocean was completely lost in ≈10 million years, short compared to typical accretion and outgassing times, water was lost “as soon” as it was outgassed. This mechanism could explain the present lack of oxygen in the Venus atmosphere. Because it is expected to affect all sunlike stars in the early phase of planet formation, abiotic oxygen atmospheres could be rare in the universe.

The temporal development of microcrater and accretionary grain populations on lunar rocks subjected to meteoroid and solar wind bombardment—I. Theory

The development, with time, of microcrater and accretionary particle distributions is investigated for lunar rocks subjected to meteoroid and solar wind bombardment. Equations are derived which enable the evaluation of crater and accreta lifetimes and also allow crater and accreta production, transition and equilibrium populations to be calculated.

The temporal development of microcrater and accretionary grain populations on lunar rocks subjected to meteoroid and solar wind bombardment—II. Observations on the lunar surface and Apollo samples

The development, with time, of microcrater and accretionary particle distributions is investigated for lunar rocks subjected to meteoroid and solar wind bombardment. Experimental observations of the impact crater size distributions and accretionary particle populations on specially selected areas of Apollo Lunar Samples are used to derive incident fluxes for the theory of topological development described in Paper I. ( ibid.). Observations show that a delineation and quantitative characterisation of erosion by impact, solar wind sputter and accretionary build-up leads to features typical of lunar surface rocks. The dominance of specific erosion mechanisms is shown to be size dependent. Monte Carlo simulations of these processes are developed to mimic the surface development of populations under arbitrary exposure conditions. Surface dust and splash (accreta) build-up significantly affects observed parameters; it may be used also as a sample surface exposure age indicator. Sputter by the solar wind is shown to modify both accreta and microcrater populations up to dimensions of one micron.

Solar-wind sputtering of the martian atmosphere

IN the sputtering process an incident particle beam loses part of its energy to recoil motion of target atoms, some of which may escape through a nearby surface. The sputtering yield, S, is defined as the number of atoms ejected per incident particle. In the Solar System, sputtering will occur whenever the solar wind, consisting mainly of 1 keV AMU−1 hydrogen and helium ions, strikes a material body. Many years ago, Wehner et al.1 suggested that solar wind-induced sputtering of the lunar surface should be an important cause of erosion; recently, analyses2 of returned lunar material have been interpreted quantitatively3 in terms of such solar-wind sputtering. Mars provides another example of the interaction of the solar wind with a planetary body. However, in contrast to the lunar surface, the martian surface is largely protected from direct solar wind bombardment by its atmosphere. The primarily CO2 atmosphere is thin by terrestrial standards but still opaque to the solar wind. We discuss here whether solar-wind sputtering of the martian atmosphere is a mechanism leading to significant mass loss.

Solar wind and micrometeorite effects in the lunar regolith

Using available data from the literature, we formulate an outline of the major physical and chemical effects expected during solar wind bombardment of the lunar regolith. In agreement with the results of Auger and e.s.c.a. analyses of the composition of lunar grain surfaces, this outline predicts that solar wind sputtering will tend to clean exposed grain surfaces by ejecting material at velocities exceeding lunar escape velocity. We also discuss results showing that Fe is partially reduced in the outer few 10 nm of grain surfaces and that this reduced Fe forms 10 nm diameter range metal spheres throughout the glass during agglutinate formation by micrometeorite impacts. These metal spheres give the agglutinates their distinctive optical and magnetic properties and are partially responsible for the decreasing albedo of the lunar surface with exposure age.

Investigation of a possible solar-wind darkening of the lunar surface by photoelectron spectroscopy

Detailed sequential study by photoelectron spectroscopy of the change in valence states of iron in Fe2O3, Fe3O4, FeO, and Fe foil samples, showing that chemical reduction does take place on the sample surface when argon ion bombardment occurs. These spectra give evidence that, after prolonged ion bombardment, the surface of all four samples consisted predominantly of iron in the metallic state. Argon ion sputtering of 15 silicate and oxide samples under similar experimental conditions produced surface darkening only in samples containing iron. Monitoring of the photoelectron peak of carbon during the experiment indicates that ion-bombardment darkening is not due to the buildup of hydrocarbon surface contaminants in the oil-free high-vacuum system used by the authors. On the basis of these studies, it is concluded that solar-wind bombardment should be included as one of the possible mechanisms for producing darkening of the lunar surface.

On the development of the crater population on the moon with time under meteoroid and solar wind bombardment

In this paper a theory is evaluated to describe the development of the lunar crater population with time under the bombardment by meteoroids and solar wind. Starting from a general mass distribution law a differential equation has been established and solved separately for meteoroid impact and solar wind bombardment. The theory permits the calculation of absolute formation ages of the lunar surface as well as the particle flux, supposing the crater distributions on the moon have been measured. As an important result it includes a D −2 equilibrium crater distribution law ( D =crater diameter), actually measured in Mare Tranquillitatis and Oceanus Procellarum. Additionally, the exponential decrease of particle flux with time is confirmed.

Hydrogen and helium ion implantation into olivine and enstatite: Retention coefficients, saturation concentrations, and temperature-release profiles

Low-energy ions, as from the solar wind, penetrate the surface of exposed samples. The retention coefficients, saturation concentrations, and gas-release curves for the predominant solar-wind species, hydrogen and helium, have been determined for olivine and enstatite. Forsterite, fayalite, and enstatite samples that had been irradiated with 1.8-kev helium ions and forsterite that had been irradiated with 2-kev hydrogen ions were degassed in controlled heating experiments. The concentration of the evolved gas was measured with a thermal conductivity detector gas chromatograph. In forsterite the hydrogen retention coefficient for an incident dose of 5.4 × 1016 ions/cm2 was found to be approximately 0.9. For 6.1 ×1016 helium ions/cm2 the observed retention coefficients are approximately 0.2 for enstatite, 0.2 for single-crystal forsterite, and 0.4 for dunite (fine-grained forsterite). Decreased retention with increased dose (saturation effect) is observed; saturation of 2-kev hydrogen ions in single-crystal forsterite occurred at 5 × 1017 cm−2; saturation of 1.8-kev helium ions in single-crystal forsterite, fayalite, and enstatite occurred at about 6 × 1016 cm−2. Saturation of 1.8-kev helium ions in dunite (fine-grained forsterite) was not detected, but it was concluded to occur above 1 × 1017 cm−2. The temperature for the onset of release of the trapped gas was found to correspond approximately to the irradiation temperature. Increased sample temperature at a given irradiation dose resulted in decreased retention. For any comparable irradiation, the helium release occurs at a lower temperature than the hydrogen release. The saturation concentration of 1.8-kev helium in enstatite or olivine, about 6 × 1016 cm−2for a uniform irradiation of 30-μ diameter grains, would give 1.3 cc STP He/cm2. Saturation would be reached in a few hundred years of solar-wind bombardment at 1 AU.