Micrometeoroid Impact Research Articles

The Phobos neutral and ionized torus

AbstractCharged particle sputtering, micrometeoroid impact vaporization, and photon‐stimulated desorption are fundamental processes operating at airless surfaces throughout the solar system. At larger bodies, such as Earth's Moon and several of the outer planet moons, these processes generate tenuous surface‐bound exospheres that have been observed by a variety of methods. Phobos and Deimos, in contrast, are too gravitationally weak to keep ejected neutrals bound and, thus, are suspected to generate neutral tori in orbit around Mars. While these tori have not yet been detected, the distribution and density of both the neutral and ionized components are of fundamental interest. We combine a neutral Monte Carlo model and a hybrid plasma model to investigate both the neutral and ionized components of the Phobos torus. We show that the spatial distribution of the neutral torus is highly dependent on each individual species (due to ionization rates that span nearly 4 orders of magnitude) and on the location of Phobos with respect to Mars. Additionally, we present the flux distribution of torus pickup ions throughout the Martian system and estimate typical pickup ion fluxes. We find that the predicted pickup ion fluxes are too low to perturb the ambient plasma, consistent with previous null detections by spacecraft around Mars.

Adhering grains and surface features on two Itokawa particles

We investigated the surface texture and chemical compositions of two ~40-μm particles returned from the surface regolith of asteroid Itokawa (RB-DQ04-0062 and RB-DQ04-0091) by the Japan Aerospace Exploration Agency’s Hayabusa mission. We identified splash melts, surface blistering, and many small adhering particles. Seven focused ion beam sections were extracted from both Itokawa particles, targeting one splash melt and ten adhering particles to investigate their composition and provenance and the role of micrometeoroid impacts on Itokawa’s surface. Based on the particle’s structure, mineralogy, and interface between the adhering particle and host grain, we identified lithic fragments and particles deposited by impact. These have morphologies and compositions consistent with impact-generated deposits: two have morphologies and compositions that are consistent with impact-generated silica glass, and one was a Ni-free, metallic Fe, and S-rich assemblage that was likely generated by vapor recondensation during a micrometeoroid impact. This study shows that, even though its regolith is young, micrometeoroid impacts have altered the regolith of asteroid Itokawa.

Reactive Oxygen Species (ROS) generation by lunar simulants

The current interest in human exploration of the Moon and past experiences of Apollo astronauts has rekindled interest into the possible harmful effects of lunar dust on human health. In comparison to the Apollo-era explorations, human explorers may be weeks on the Moon, which will raise the risk of inhalation exposure. The mineralogical composition of lunar dust is well documented, but its effects on human health are not fully understood. With the aim of understanding the reactivity of dusts that may be encountered on geologically different lunar terrains, we have studied Reactive Oxygen Species (ROS) generation by a suite of lunar simulants of different mineralogical–chemical composition dispersed in water and Simulated Lung Fluid (SLF). To further explore the reactivity of simulants under lunar environmental conditions, we compared the reactivity of simulants both in air and inert atmosphere. As the impact of micrometeorites with consequent shock-induced stresses is a major environmental factor on the Moon, we also studied the effect of mechanical stress on samples. Mechanical stress was induced by hand crushing the samples both in air and inert atmosphere. The reactivity of samples after crushing was analyzed for a period of up to nine days. Hydrogen peroxide (H2O2) in water and SLF was analyzed by an in situ electrochemical probe and hydroxyl radical (•OH) by Electron Spin Resonance (ESR) spectroscopy and Adenine probe. Out of all simulants, CSM-CL-S was found to be the most reactive simulant followed by OB-1 and then JSC-1A simulant. The overall reactivity of samples in the inert atmosphere was higher than in air. Fresh crushed samples showed a higher level of reactivity than uncrushed samples. Simulant samples treated to create agglutination, including the formation of zero-valent iron, showed less reactivity than untreated simulants. ROS generation in SLF is initially slower than in deionized water (DI), but the ROS formation is sustained for as long as 7.5h. By contrast ROS is formed rapidly within 30min when simulants are dispersed in DI, but then the concentration either stabilizes or decreases over time. The results indicate that mechanical stress and the absence of molecular oxygen and water, which are important environmental characteristics of the lunar environment, can lead to enhanced production of ROS in general. However, compositional difference among simulants is the most important factor in governing the production of ROS. Simulants with glass content in excess of 40wt% appear to produce as much as of order of magnitude more ROS than simulants with lower glass content.

New experimental capability to investigate the hypervelocity micrometeoroid bombardment of cryogenic surfaces.

Ice is prevalent throughout the solar system and beyond. Though the evolution of many of these icy surfaces is highly dependent on associated micrometeoroid impact phenomena, experimental investigation of these impacts has been extremely limited, especially at the impactor speeds encountered in space. The dust accelerator facility at the Institute for Modeling Plasmas, Atmospheres, and Cosmic Dust (IMPACT) of NASA's Solar System Exploration Research Virtual Institute has developed a novel cryogenic system that will facilitate future study of hypervelocity impacts into ice and icy regolith. The target consists of a copper block, cooled by liquid nitrogen, upon which layers of vapor-deposited ice, pre-frozen ice, or icy regolith can be built in a controlled and quantifiable environment. This ice can be grown from a variety of materials, including H2O, CH3OH, NH3, and slurries containing nanophase iron. Ice temperatures can be varied between 96 K and 150 K and ice thickness greater than 150 nm can be accurately measured. Importantly, the composition of ion plumes created during micrometeoroid impacts onto these icy layers can be measured even in trace amounts by in situ time-of-flight mass spectroscopy. In this paper, we present the fundamental design components of the cryogenic target chamber at IMPACT and proof-of-concept results from target development and from first impacts into thick layers of water ice.

A large-scale anomaly in Enceladus’ microwave emission

The Cassini spacecraft flew by Enceladus on 6 November 2011, configured to acquire synthetic aperture RADAR imaging of most of the surface with the RADAR instrument. The pass also recorded microwave thermal emission from most of the surface. We report on global patterns of thermal emission at 2.17cm based on this data set in the context of additional unresolved data both from the ground and from Cassini.The observed thermal emission is consistent with dielectric constants of pure water or methane ice, but cannot discriminate between the two. The emissivity is similar to those of other icy satellites (≈0.7), consistent with volume scattering. The most intriguing result, however, is an anomaly in the thermal emission of Enceladus’ leading hemisphere. Evidence presented here suggests the anomaly is buried at depths on the order of a few meters. This anomaly is located in similar geographic location to anomalies previously detected with the CIRS and ISS instruments on Mimas, Tethys, and Dione (Howett, C.J.A. et al. [2011]. Icarus 216, 221–226; Howett, C.J.A. et al. [2012]. Icarus 221, 1084–1088; Howett, C.J.A. et al. [2014]. Icarus 241, 239–247; Schenk, P. et al. [2011]. Icarus 211, 740–757), but also corresponds with a geological feature on Enceladus’ leading terrain (Crow-Willard, E., Pappalardo, R.T. [2011]. Global geological mapping of Enceladus. In: EPSC-DPS Joint Meeting 2011. p. 635). Simple models show that the Crow-Willard and Pappalardo (Crow-Willard, E., Pappalardo, R.T. [2011]. Global geological mapping of Enceladus. In: EPSC-DPS Joint Meeting 2011. p. 635) model is a better fit to the data. Our best-supported hypothesis is that the leading hemisphere smooth terrain is young enough (<75–200Myr old) that the micrometeorite impact gardening depth is shallower than the electromagnetic skin depth of the observations (≈3–5m), a picture consistent with ground and space radar measurements, which show no variation at 2cm, but an increase in albedo in the anomaly region at 13cm.

Frontiers in Modeling and Design of Bio-Inspired Armors

Impact behavior of materials and structures is of crucial interest. Indeed, every solid may experience collisions in its “mechanical” life. The topic involves a wide range of engineering applications, even in our everyday life: sports protectors [e.g., helmets, Milne et al. (2014)], sensitive portable electronics (Tempelman et al., 2012), bulletproof body armors (Cuniff, 1999), protection systems for buildings and machineries in the civil or defense sectors (NIST, 2005), improvement of crashworthiness in automotive (Schweizerhof et al., 1992), protection of spacecraft and satellite structures from high-velocity micrometeorite or orbital debris impact (NASA, 2015) are some of the most representative. Parallel to the primary requirement of an effective protection, straightforwardly achievable with a massive armor, research efforts are aimed at weight saving due to essential and binding needs, such us better ergonomics and flexibility (body armors), transportability (vehicles), and in general a more judicious use of materials. Thus, the real goal is the high specific toughness. For some decades, the answer to these tasks has been the adoption of multilayer of textile and composite materials, (Abrate, 1998; Hoog, 2006) based on synthetic fibers (e.g., Kevlar®, Dyneema®) that have allowed to reach protection levels previously unimaginable with metallic targets. Nowadays, in the era of nanomaterials, we are raising the bar to atomistic 2D materials, like graphene, coupling high resistance (Lee et al., 2008) and flaw tolerance (Zhang et al., 2012) at the nanoscale, even for possible application to nanoarmours (Pugno et al., 2007; Lee et al., 2012, 2014). Alternatively, the same goal may be pursued through smart structural solutions to be employed even with traditional materials, with all the benefits that this option implies. Nature, having worked over the ages for optimizing defense mechanisms against predators attacks or shock loads, is one of the most inspiring sources: as most remarkable examples we mention the coupled hard-soft layers in the Arapaima gigas fish’s dermis (Yang et al., 2014), the internal undulated walls of the Bombardier Beetle’s (Carabidae, Brachinus) explosion chamber (Lai and Ortiz, 2010), the cross-scale toughening mechanisms in the foam-like structure of dropping fruits (Thielen et al., 2013), dermal armors with scales (Ghosh et al., 2014), and the extreme robustness provided by the spider silk constitutive law (Cranford et al., 2012). On the other side, we could be interested in gaining an efficient strike, like the deadly underwater punch mechanism of the mantis-shrimp (Patek et al., 2005). Upon impact, several complex physical phenomena take place: elastic–plastic deformation and wave propagation, fracture and fragmentation, heat generation (by yielding and friction), changing of material properties due to strain-rate effects up to phase change. Their occurrence and magnitude depend on the impact velocity that may be very low or up to extreme values (>3 km/s for hypervelocity impact), with increasing challenges for armor resistance as well as for its accurate modeling. The theoretical description of the basic aspects of impact mechanics (Stronge, 2000; Goldsmith, 1999, 2001) has reached a level of advanced maturity but it is in a sort of stalemate due to the severe mathematical complexity in representing the above mentioned phenomena, which also mutually interact. With high speed calculators and the development of computational methods (e.g., finite element method, FEM), simulation has become the favorite design tool, allowing optimization studies. Nonetheless, the advent of nanomaterials and bio-inspiration is further questioning the capabilities of these tools and pushing modeling research. The traditional stand-alone experimental approach for armor design according to the philosophy “add material until it stops” it is not viable any more. First, multilayer panels can show crosscurrent behavior in relation to material coupling and interface strength, being even non-optimized for increasing areal density (Signetti and Pugno, 2014, Figure 1). Technological and economic limits in large scale production of nanomaterials, the difficulties in their manipulation or in their structural arrangement into complex bio-inspired structures require a systematic and reliable design process able to provide a tentative target optimum. With mere experiments is nearly impossible to investigate the whole design space for understanding still unexplained mechanism in order to mimic nature and, why not, do even better. Going down to nanoarmours at atomic scale, we enter in a new world with completely unexplored scenarios. Deformation, fracture, contact forces are matter of potentials, electronic interactions, affinity, and reactivity of particles of colliding bodies in relation also to their atomic arrangement. Can we still call it only impact mechanics? Probably not. Some studies have been published about the protection capabilities of graphene nanoarmours via molecular dynamics [see Ozden et al. (2014) and Shang and Wang (2014)]: many considerations can be done with these simulations, which allow the modeling of systems up to 1 M atoms. However, the uncertainty and insensitivity of the behavior at this

DETECTIONS OF TRANS-NEPTUNIAN ICE IN PROTOPLANETARY DISKS

We present Herschel Space Observatory PACS spectra of T Tauri stars, in which we detect amorphous and crystalline water ice features. Using irradiated accretion disk models, we determine the disk structure and ice abundance in each of the systems. Combining a model-independent comparison of the ice feature strength and disk size with a detailed analysis of the model ice location, we estimate that the ice emitting region is at disk radii >30AU, consistent with a proto-Kuiper belt. Vertically, the ice emits most below the photodesorption zone, consistent with Herschel observations of cold water vapor. The presence of crystallized water ice at a disk location a) colder than its crystallization temperature and b) where it should have been re-amorphized in ~1 Myr suggests that localized generation is occurring; the most likely cause appears to be micrometeorite impact or planetesimal collisions. Based on simple tests with UV models and different ice distributions, we suggest that the SED shape from 20 to 50 micron may probe the location of the water ice snow line in the disk upper layers. This project represents one of the first extra-solar probes of the spatial structure of the cometary ice reservoir thought to deliver water to terrestrial planets.

Martian planetary heavy ion sputtering of Phobos

The Martian moons, Phobos and Deimos, have long been suspected to be the sources of tenuous neutral gas tori encircling Mars. While direct outgassing has been ruled out as a strong source, micrometeoroid impact vaporization and charged particle sputtering must operate based on observations at other airless bodies. Previous models have addressed solar wind sputtering of Phobos; however, Phobos and Deimos are also subject to a significant, yet temporally variable, flux of heavy planetary ions escaping from Mars. In this report, we use a combination MHD/test-particle model to calculate the planetary heavy ion flux to Phobos and the ensuing neutral sputtered flux. Depending on ambient solar wind conditions and the location of Phobos, heavy ion sputtering of Phobos generates neutral fluxes up to and exceeding that from solar wind sputtering. We model pickup ions from the Phobos torus itself with applications for observations by the upcoming Mars Atmospheric and Volatile Evolution mission.

Microchemical and structural evidence for space weathering in soils from asteroid Itokawa

Here we report microchemical and microstructural features indicative of space weathering in a particle returned from the surface of asteroid Itokawa by the Hayabusa mission. Space weathering features include partially and completely amorphous rims, chemically and structurally heterogeneous multilayer rims, amorphous surface islands, vesiculated rim textures, and nanophase iron particles. Solar-wind irradiation is likely responsible for the amorphization as well as the associated vesiculation of grain rims. The multilayer rims contain a nanocrystalline outer layer that is underlain by an amorphous inner layer, and both have compositions that are distinct from the underlying, crystalline orthopyroxene grain. The multilayer rim features could be derived from either radiation-induced sputter deposition or vapor deposition from micrometeorite impact events. The amorphous islands on grain surfaces have a distinctive morphology and composition suggesting that they represent surface deposits of melt derived from micrometeorite impact events. These observations indicate that both irradiation damage and micrometeorite impacts play a role in surface modification and space weathering on asteroid Itokawa.

Micrometeoroid impact charge yield for common spacecraft materials

AbstractThe impact ionization charge yield is experimentally measured from four common materials used in space and specifically on the two STEREO spacecraft (germanium‐coated black Kapton, beryllium copper, multilayer insulation, and solar cells). Cosmic dust particle impacts on spacecraft have been detected by electric field and plasma and radio wave instruments. The accurate interpretation of these signals is complicated by many factors, including the details of the spacecraft antenna system, the local spacecraft plasma environment, and our understanding of the physics of the impact process. The most basic quantity, the amount of charge liberated upon impact, is generally considered poorly constrained and is suspected to depend on the target material. Here we show that for common materials used on spacecraft this variability is small for impacts around 10 km/s, and the impact charge yield can be approximated by 80 fC for a 1 pg projectile. At higher speeds (∼50 km/s), variation of up to a factor of 5 is observed. The measured yields in the 10–50 km/s range are compared to measurements and predictions from the literature and are found to be lower than predicted by at least a factor of 12 at 10 km/s and at least a factor of 1.7 at 50 km/s. Impact charge is also found to depend on angle of incidence; the data suggest a maximum at 45°.

Thermal fatigue as the origin of regolith on small asteroids

Space missions and thermal infrared observations have shown that small asteroids (kilometre-sized or smaller) are covered by a layer of centimetre-sized or smaller particles, which constitute the regolith. Regolith generation has traditionally been attributed to the fall back of impact ejecta and by the break-up of boulders by micrometeoroid impact. Laboratory experiments and impact models, however, show that crater ejecta velocities are typically greater than several tens of centimetres per second, which corresponds to the gravitational escape velocity of kilometre-sized asteroids. Therefore, impact debris cannot be the main source of regolith on small asteroids. Here we report that thermal fatigue, a mechanism of rock weathering and fragmentation with no subsequent ejection, is the dominant process governing regolith generation on small asteroids. We find that thermal fragmentation induced by the diurnal temperature variations breaks up rocks larger than a few centimetres more quickly than do micrometeoroid impacts. Because thermal fragmentation is independent of asteroid size, this process can also contribute to regolith production on larger asteroids. Production of fresh regolith originating in thermal fatigue fragmentation may be an important process for the rejuvenation of the surfaces of near-Earth asteroids, and may explain the observed lack of low-perihelion, carbonaceous, near-Earth asteroids.

Cratering studies in Polyvinylidene Fluoride (PVDF) thin films

Thin, permanently polarized Polyvinylidene Fluoride (PVDF) films have been used as dust detectors on a number of missions including the Dust Counter and Mass Analyzer (DUCMA) instrument on Vega 1 and 2 to comet 1P/Halley, the High Rate Detector (HRD) on the Cassini Mission to Saturn, the Student Dust Counter (SDC) on New Horizons to Pluto, the Dust Flux Monitor Instrument (DFMI) on the Stardust mission to comet 81P/Wild 2, the Space Dust (SPADUS) instrument on the Earth orbiting Advanced Research and Global Observation Satellite (ARGOS) and the Cosmic Dust Experiment (CDE) on the Aeronomy of Ice in the Mesosphere (AIM) mission in orbit around the Earth. Due to their low power requirements and light weight, large surface area detectors can be built for observing low dust fluxes. The operation principle behind metal-coated PVDF detectors is that a micrometeorite impact removes a portion of the metal surface layer, exposing the permanently polarized PVDF dielectric underneath. This changes the local electric potential near the crater, and the surface charge of the metal layer, which can be recorded as a transient current. The dimensions of the crater determine the strength of the potential change and thus the signal generated by the PVDF. Currently used scaling laws relating impactor parameters to crater geometry, which are used to predict PVDF response, are suspected to have systematic errors. Work is being undertaken to develop a new crater diameter scaling law using iron particles in PVDF. Cratered samples are analyzed using a 3D reconstruction technique using stereo image pairs taken in a Scanning Electron Microscope (SEM) and cross sections taken in a Focused Ion Beam (FIB). We report on the details of the reconstruction techniques and the initial findings of the crater parameter scaling law study.

Space debris and micrometeoroid impact on spacecraft elements: Experimental simulation

We study experimentally how optical, electrical, and physical properties of spacecraft surface elements (optical glasses, “metal-insulator-metal” structures, and solar arrays) change upon bombardment by high-speed particles of submicron and micron size. For particle acceleration, we use an electrodynamic accelerator.

Modeling MESSENGER observations of calcium in Mercury's exosphere

The Mercury Atmospheric and Surface Composition Spectrometer (MASCS) on the MESSENGER spacecraft has made the first high‐spatial‐resolution observations of exospheric calcium at Mercury. We use a Monte Carlo model of the exosphere to track the trajectories of calcium atoms ejected from the surface until they are photoionized, escape from the system, or stick to the surface. This model permits an exploration of exospheric source processes and interactions among neutral atoms, solar radiation, and the planetary surface. The MASCS data have suggested that a persistent, high‐energy source of calcium that was enhanced in the dawn, equatorial region of Mercury was active during MESSENGER's three flybys of Mercury and during the first seven orbits for which MASCS obtained data. The total Ca source rate from the surface varied between 1.2 × 1023 and 2.6 × 1023 Ca atoms s−1, if its temperature was 50,000 K. The origin of this high‐energy, asymmetric source is unknown, although from this limited data set it does not appear to be consistent with micrometeoroid impact vaporization, ion sputtering, electron‐stimulated desorption, or vaporization at dawn of material trapped on the cold nightside.

Geology, geochemistry, and geophysics of the Moon: Status of current understanding

The Moon is key to understanding both Earth and our Solar System in terms of planetary processes and has been a witness of the Solar System history for more than 4.5Ga. Building on earlier telescopic observations, our knowledge about the Moon was transformed by the wealth of information provided by Apollo and other space missions. These demonstrated the value of the Moon for understanding the fundamental processes that drive planetary formation and evolution. The Moon was understood as an inert body with its geology mainly restricted to impact and volcanism with associated tectonics, and a relative simple composition. Unlike Earth, an absence of plate tectonics has preserved a well-defined accretion and geological evolution record. However recent missions to the Moon show that this traditional view of the lunar surface is certainly an over simplification. For example, although it has long been suspected that ice might be preserved in cold traps at the lunar poles, recent results also indicate the formation and retention of OH− and H2O outside of polar regions. These volatiles are likely to be formed as a result of hydration processes operating at the lunar surface including the production of H2O and OH by solar wind protons interacting with oxygen-rich rock surfaces produced during micrometeorite impact on lunar soil particles. Moreover, on the basis of Lunar Prospector gamma-ray data, the lunar crust and underlying mantle has been found to be divided into distinct terranes that possess unique geochemical, geophysical, and geological characteristics. The concentration of heat producing elements on the nearside hemisphere of the Moon in the Procellarum KREEP Terrane has apparently led to the nearside being more volcanically active than the farside. Recent dating of basalts has shown that lunar volcanism was active for almost 3Ga, starting at about 3.9–4.0Ga and ceasing at ∼1.2Ga. A recent re-processing of the seismic data supports the presence of a partially molten layer at the base of the mantle and shows not only the presence of a 330km liquid core, but also a small solid inner core. Today, the Moon does not have a dynamo-generated magnetic field like that of the Earth. However, remnant magnetization of the lunar crust and the paleomagnetic record of some lunar samples suggest that magnetization was acquired, possibly from an intrinsic magnetic field caused by an early lunar core dynamo. In summary, the Moon is a complex differentiated planetary object and much remains to be explored and discovered, especially regarding the origin of the Moon, the history of the Earth–Moon system, and processes that have operated in the inner Solar System over the last 4.5Ga. Returning to the Moon is therefore the critical next stepping-stone to further exploration and understanding of our planetary neighborhood.

An iron isotope perspective on the origin of the nanophase metallic iron in lunar regolith

The surfaces of the Moon and other airless planetary bodies are constantly weathered by meteorite impacts and sputtering by charged particles. One of the hallmarks of this “space weathering” is the presence of nanophase metallic Fe (npFe0) at the surface of airless bodies. These npFe0 grains alter the surface optical spectra of planetary bodies without an atmosphere and their concentration is used to estimate the degree of maturity of lunar regolith. The origin of npFe0 has been debated between in situ reduction due to the solar wind, and evaporation generated by charged particle sputtering and/or micrometeorite impact followed by re-condensation of metallic Fe. These two mechanisms will impart completely different Fe isotopic fractionation effects on the npFe0. In this study we measure the Fe isotopic composition of npFe0 using a step-by-step surface etching technique on lunar regolith plagioclase. Our results show that npFe0 is highly enriched in the heavy isotopes of Fe (δ56Fe up to 0.71‰) compared to bulk plagioclase and other lunar materials such as regolith and igneous rocks. We suggest that the formation of npFe0 in lunar regolith is responsible for the higher δ56Fe in the lunar regolith compared to lunar igneous rocks. In addition, a thermal escape model shows that the heavy Fe isotopic composition of npFe0 is best explained by the preferential escape of light Fe isotopes to space in the vaporization phase of Fe. The temperature of the vapor can be inferred from our model (2750–3000K), which is compatible with those proposed by previous calculations and experiments. Therefore our results unambiguously support the vapor deposit origin of npFe0, explain the origin of the heavy Fe isotopic composition of the lunar regolith and provide a temperature estimate for the impact event at the origin of the npFe0.

Metallic species, oxygen and silicon in the lunar exosphere: Upper limits and prospects for LADEE measurements

The only species that have been so far detected in the lunar exosphere are Na, K, Ar, and He. However, models for the production and loss of species derived from the lunar regolith through micrometeoroid impact vaporization, sputtering, and photon‐stimulated desorption, predict that a host of other species should exist in the lunar exosphere. Assuming that loss processes are limited to ballistic escape, photoionization, and recycling to the surface, we have computed column abundances and compared them to published upper limits for the Moon. Only for Ca do modeled abundances clearly exceed the available measurements. This result suggests the relevance of some loss processes that were not included in the model, such as the possibility of gas‐to‐solid phase condensation during micrometeoroid impacts or the formation of stable metallic oxides. Our simulations and the recalculation of efficiencies for resonant light scattering show that models for other species studied are not well constrained by existing measurements. This fact underlines the need for improved remote and in situ measurements of the lunar exosphere such as those planned by the Lunar Atmosphere and Dust Environment Explorer (LADEE) spacecraft. Our simulations of the LADEE neutral mass spectrometer and visible/ultraviolet spectrometer indicate that LADEE measurements promise to provide definitive observations or set stringent upper limits for all regolith‐driven exospheric species. We predict that observations by LADEE will constrain assumed model parameters for the exosphere of the Moon.

M3 spectral analysis of lunar swirls and the link between optical maturation and surface hydroxyl formation at magnetic anomalies

We examined the lunar swirls using data from the Moon Mineralogy Mapper (M3). The improved spectral and spatial resolution of M3 over previous spectral imaging data facilitates distinction of subtle spectral differences, and provides new information about the nature of these enigmatic features. We characterized spectral features of the swirls, interswirl regions (dark lanes), and surrounding terrain for each of three focus regions: Reiner Gamma, Gerasimovich, and Mare Ingenii. We used Principle Component Analysis to identify spectrally distinct surfaces at each focus region, and characterize the spectral features that distinguish them. We compared spectra from small, recent impact craters with the mature soils into which they penetrated to examine differences in maturation trends on‐ and off‐swirl. Fresh, on‐swirl crater spectra are higher albedo, exhibit a wider range in albedos and have well‐preserved mafic absorption features compared with fresh off‐swirl craters. Albedoand mafic absorptions are still evident in undisturbed, on‐swirl surface soils, suggesting the maturation process is retarded. The spectral continuum is more concave compared with off‐swirl spectra; a result of the limited spectral reddening being mostly constrained to wavelengths less than ∼1500 nm. Off‐swirl spectra show very little reddening or change in continuum shape across the entire M3 spectral range. Off‐swirl spectra are dark, have attenuated absorption features, and the narrow range in off‐swirl albedos suggests off‐swirl regions mature rapidly. Spectral parameter maps depicting the relative OH surface abundance for each of our three swirl focus regions were created using the depth of the hydroxyl absorption feature at 2.82 μm. For each of the studied regions, the 2.82 μm absorption feature is significantly weaker on‐swirl than off‐swirl, indicating the swirls are depleted in OH relative to their surroundings. The spectral characteristics of the swirls and adjacent terrains from all three focus regions support the hypothesis that the magnetic anomalies deflect solar wind ions away from the swirls and onto off‐swirl surfaces. Nanophase iron (npFe0) is largely responsible for the spectral characteristics we attribute to space weathering and maturation, and is created by vaporization/deposition by micrometeorite impacts and sputtering/reduction by solar wind ions. On the swirls, the decreased proton flux slows the spectral effects of space weathering (relative to nonswirl regions) by limiting the npFe0 production mechanism almost exclusively to micrometeoroid impact vaporization/deposition. Immediately adjacent to the swirls, maturation is accelerated by the increased flux of protons deflected from the swirls.

In Situ Optical Investigations of Hypervelocity Impact Induced Dynamic Fracture

One of the prominent threats in the endeavor to develop next-generation space assets is the risk of space debris impact in earth’s orbit and micrometeoroid impact damage in deep space. To date, there is no study available which concentrates on the analysis of dynamic crack growth from hypervelocity impacts on such structures, resulting in their eventual catastrophic degradation. Experiments conducted using a unique two-stage light-gas gun facility have examined the in situ dynamic fracture of brittle polymers subjected to this high-energy-density event. Optical techniques of caustics and photoelasticity, combined with high-speed photography, analyze crack growth behavior of Mylar and Homalite 100 thin plates after impact at velocities ranging from 3 to 7 km/s (7,000–15,500 mph). Results indicate that even under extreme impact conditions of out-of-plane loading, highly localized heating, and energetic impact phenomena involving plasma formation and ejecta, the dynamic fracture process occurs during a deformation regime dominated by in-plane loading.

Three-dimensional simulations of the lunar sodium exosphere and its tail

[1] The brightness distributions of the lunar sodium exosphere reported by Flynn and Mendillo (1993) and the receding velocities of the lunar sodium tail observed by Mierkiewicz et al. (2006) are reproduced by 3-D Monte Carlo simulations. We consider the effects of two spatially different sodium sources simultaneously: dayside source with the dependency of solar zenith angle and an isotropic source due to micrometeoroid impact. In the simulations, the following effects are taken into account: (1) the gravity of the Moon, the Earth, and the Sun with the orbital motion of the Moon; (2) photoionizations, solar radiation pressure, and returns to the lunar surface; and (3) the shadows of the Earth and Moon. The sodium brightness observed by Flynn and Mendillo (1993) is successfully modeled, from which the most probable source ratio between the isotropic and dayside sources is estimated to be 70%∼80%: 30%∼20%, respectively. The best coma model provides an initial velocity of 2.0 km/s with a narrow Doppler width of 0.2 km/s and a total production rate of 0.7 × 1022/s. On the basis of the best 3-D lunar coma model, we also simulate the receding velocity distribution of lunar sodium tail, and we find satisfactory models for high receding velocity and its wide dispersion observed by Mierkiewicz et al. (2006) considering the effect of variable solar radiation pressure and appropriate ionization times.