Lunar Atmosphere Research Articles

Telescope pointing software for slit spectroscopy of the lunar exosphere

Telescope pointing software for slit spectroscopy of the lunar exosphere

Feasibility of quasi-frozen, near-polar and extremely low-altitude lunar orbits

Designing long-duration lunar orbiter missions is challenging due to the Moon's highly nonlinear gravity field and the third-body perturbations induced by the Earth, Sun and other large bodies. The absence of a Lunar atmosphere has offered the possibility for mission designers to search for extremely low-altitude, quasi-stable lunar orbits. In addition to the reduced amount of propellant required for station-keeping maneuvers, these orbits present great opportunities for unique scientific studies such as high resolution imaging and characterization of the polar ice deposits in deep craters. Prior to the GRAIL mission, mission planning for Lunar orbiters had suffered from inaccuracies, mainly due to the lack of an accurate Lunar gravity model, which resulted in severe deviations with respect to the spacecraft's nominal orbit.We study station-keeping feasibility for spacecraft in near-polar and extremely low-altitude, quasi-frozen orbits around the Moon, that are perturbed by a high-fidelity lunar gravity model and third-body effects from the Earth and Sun. For several candidate orbits, we compare the trade-space between mission duration and ΔV budget, considering impulsive maneuvers applied once every ‘N∈{2,6,10,14,18}’ orbits at periselene or aposelene. Additionally, we investigate the propulsive cost for different orbit insertion dates, the location of impulsive corrections for arresting argument of periselene (ω) drift, and controlling periselene altitude.

Searching for Lunar Horizon Glow With the Lunar Orbiter Laser Altimeter

AbstractWe present the results of a 2‐year‐long systematic campaign to monitor the lunar dust exosphere for enhancements in dust concentration at altitudes <20 km both during and outside of major meteor stream periods. We utilize the radiometric capabilities of the Lunar Orbiter Laser Altimeter Laser Ranging telescope onboard the Lunar Reconnaissance Orbiter to search for forward‐scattered sunlight from exospheric dust grains, called lunar horizon glow (LHG). Specifically, we test the hypothesis that major meteor streams can produce LHG similar to what was measured in Apollo 15 coronal photography. Assuming a one‐dimensional exponential vertical dust profile and a dust grain radius r∼0.1μm, we derive an upper limit of ∼10−11 g/cm2 on the overlying column dust mass density near the terminator during stream periods, a limit roughly 10 times lower than inferred from Apollo 15. Recent studies at altitudes ∼1–20 km outside of major meteor streams and at higher altitudes during streams placed limits on the density of similarly sized grains ∼100–1,000 times lower than Apollo 15. Our results show that Apollo 15‐like LHG, if real, is also a rare occurrence at altitudes <20 km during major meteor stream periods. This study is the first to quantitatively constrain the role of meteor streams in producing Apollo 15‐like LHG at altitudes <20 km and small forward‐scattering angles and further narrows the properties of any similar high‐altitude LHG and the conditions under which it can occur.

Detection of impact-produced dust clouds near the lunar terminator

Analysis of observations of unique impact-produced flash near the lunar terminator was performed. Maximal brightness of detected thermal flash is 5.2–5.9m in R band, mass of impacted meteoroid is about 0.18–28 kg. Height of shadow in the place of meteoroid’s impact was only about 1 km, making it possible observations of impact-produced dust particles ejected to the lunar exosphere through their sunlight-scattering response. We detected two (fast and slow) clouds that were expanding with 3 km/s and 0.1 km/s, respectively. The maximal brightness reached by these fast and slow clouds in R band was 6.7m and 10m, respectively. Apparent mass of visible ejecta in fast and slow clouds is constrained to 80–8300 and 6–180 kg, respectively. Fast cloud consists of melt droplets and condensed grains while slow cloud consists of ejected lunar regolith dust particles.

High‐Resolution Potassium Observations of the Lunar Exosphere

AbstractWe observed lunar exospheric potassium D1 (7,698.9646 Å) emissions using a high‐spectral resolution Fabry‐Perot spectrometer in 2014. We present the first potassium line profile measurements, which are representative of the potassium velocity distribution. Inferred temperatures are greater during the waxing gibbous phase, 1920±630 K and lower at waning gibbous phase, 980±200 K. Exosphere models suggest that the measured line widths are a combination of photon‐stimulated desorption and impact vaporization sources. The relative potassium emission intensity decreases by ∼2.5 between lunar phases 80° and 30° and is brightest off the northwest limb near the Aristarchus crater, which is a potassium‐rich surface region. Additionally, the emissions off the northern limb are brighter than the southern limb. The intensity decrease and the greater line width during the waxing gibbous versus the waning gibbous phase suggests a dawn‐dusk asymmetry.

Double-pulse laser synchronization aimed at simultaneous detection of enhanced atomic and molecular signals at low pressure conditions

A double pulse laser system that combines Raman spectroscopy and laser induced plasma spectroscopy (LIPS) in a single unit is demonstrated. The study attempts to improve the atomic signals of LIPS while simultaneously extracting the desired molecular signals from Raman. In low pressure conditions such as the lunar atmosphere, the detection of plasma emission is difficult due to the low electron density and short persistence time causing a rapid plasma expansion. Moreover, in the integration of the spectroscopic system aimed at space exploration, the system size and weight should be minimized. Simultaneous molecular and atomic detection that gave highly resolved spectra of both LIPS and Raman at pressure below 0.07 Torr is achieved. First, a low-energy laser pulse was used to produce a small plasma and to detect the molecular signals through the inelastic scattering effect. Then, the laser pulse at a higher energy is used to produce a larger plasma plume for detecting the atomic signals. Amongst eight rock samples tested, plasma stacking by generating two sequential plasmas produced from the double-pulse laser on Calcite for example enhanced the signal intensities of calcium and oxygen lines by twofold, compared to a conventional LIPS.

The lunar neon exosphere seen in LACE data

Using the LACE data from Apollo 17 we have found measured neon densities consistent with the 20Ne surface number densities reported by Cook et al. (2013) for normal conditions, terminator surface densities of 3 (±1.5) × 103 cm−3. These values are almost an order of magnitude less than those reported by Benna et al. (2015) for CME conditions. Using a Monte Carlo model and assuming the normal solar wind and a photoionization lifetime for Ne of 300 days, our result was more consistent with the Benna (2015) result than our measured result. Two lunations showed an increase in Ne during the night, consistent with the simulation, but two of the lunations showed a decrease in surface number density through the night. We have shown that explaining the Ne distribution is not as simple as assuming dynamic equilibrium with the solar wind and an exosphere accommodated to the local surface temperature.

Lunar soil hydration constrained by exospheric water liberated by meteoroid impacts.

Analyses of samples from the Apollo missions suggest that the Moon formed devoid of native water. However, recent observations by Cassini, Deep Impact, Lunar Prospector and Chandrayaan-1 indicate the existence of an active water cycle on the Moon. Here we report observations of this water cycle, specifically detections of near-surface water released into the lunar exosphere by the Neutral Mass Spectrometer on the Lunar Atmosphere and Dust Environment Explorer. The timing of 29 water releases is associated with the Moon encountering known meteoroid streams. The intensities of these releases reflect the convoluted effects of the flux, velocity and impact location of the parent streams. We propose that four additional detected water releases represent the signature of previously undiscovered meteoroid streams. We show that water release from meteoroid impacts is indicative of a lunar surface that has a desiccated soil layer of several centimetres on top of uniformly hydrated soil. We infer that the Moon is currently in the process of losing water that was either delivered long ago or present at its formation.

Coronagraphic observations of the lunar sodium exosphere January–June, 2017

In order to observe the lunar sodium exosphere out to one-half degree around the Moon, we designed, built and installed a small robotically controlled coronagraph at the Winer Observatory in Sonoita, Arizona. Observations are obtained remotely every available clear night from our home base at Goddard Space Flight Center or from Prescott, Arizona. We employ an Andover temperature-controlled 1.5 Å wide narrow-band filter centered on the sodium D2 line, and a similar 1.5 Å filter centered blueward of the D2 line by 3 Å for continuum observations. Our data encompass lunations in 2015, 2016, and 2017, thus we have a long baseline of sodium exospheric calibrated images. During the course of three years we have refined the observational sequence in many respects. Therefore this paper only presents the results of the spring, 2017, observing season. We present limb profiles from the south pole to the north pole for many lunar phases. Our data do not fit any power of cosine model as a function of lunar phase or with latitude. The extended Na exosphere has a characteristic temperature of about 2250–6750 K, indicative of a partially escaping exosphere. The hot escaping component may be indicative of a mixture of impact vaporization and a sputtered component.

Meteoroids at the Moon: Orbital Properties, Surface Vaporization, and Impact Ejecta Production

AbstractWe use a dynamical model to characterize the monthly and yearly variations of the lunar meteoroid environment for meteoroids originating from short and long‐period comets and the main‐belt asteroids. Our results show that if we assume the meteoroid mass flux of 43.3 tons per day at Earth, inferred from previous works, the mass flux of meteoroids impacting the Moon is 30 times smaller, approximately 1.4 tons per day, and shows variations of the order of 10% over a year. The mass flux difference is due to the combined effect of the smaller cross‐section of the Moon (factor of 13.46) and Earth's larger gravitational focusing (factor of 2–2.5). The lunar surface is vaporized by these impactors at an average impact vaporization flux of 11.6 × 10−16 g·cm−2·s−1, providing a significant source for the rarefied lunar exosphere. Our model predicts acceptable vaporization rates and reproduces the local time dependence of observations of the dust ejecta cloud, measured by the Lunar Dust Experiment on board NASA's Lunar Atmosphere and Dust Environment (LADEE) satellite. However, the predicted density of the lunar ejecta cloud is four orders of magnitude larger than reported values by LADEE. This discrepancy might be attributed to a much lower yield from meteoroid impacts on fluffy lunar regolith and/or a lower detection efficiency of the LADEE dust detector. We suggest an upper limit of 30 cm per million years for the soil gardening rate from small meteoroids.

Coupled DSMC-Monte Carlo radiative transfer modeling of gas dynamics in a transient impact-generated lunar atmosphere

Volatile-rich impacts on nominally airless solar system bodies such as the Moon can give rise to collisional, transient atmospheres, traces of which may be preserved at cold, permanently shadowed regions near the lunar poles. Understanding the physical mechanisms that influence gas dynamics and volatile transport in an impact-generated atmosphere is critical to interpreting the lunar polar volatile record. Here, we investigate the influence of radiative heat transfer on the structure of an impact-generated atmosphere, an issue that previous work has not addressed in detail. We develop a Monte Carlo radiative transfer model for the attenuation and absorption of thermal radiation by a three-dimensional, evolving, non-uniform, rarefied water vapor atmosphere, accounting for spontaneous emission at the molecular level, as well as radiation from the Sun and the lunar surface. This radiative transfer model is coupled to Direct Simulation Monte Carlo (DSMC) simulations of an impact-generated lunar atmosphere. We find that the trapping of heat by atmospheric water vapor causes characteristic atmospheric shock structures to become more diffuse than if the gas were modeled as transparent to radiation. Trapping of thermal radiation also allows infalling vapor to retain more energy, increasing the efficacy of transport of water vapor to the lunar night side by day side winds, and reducing the overall rate of volatile loss in the short-term. Cold trap deposition patterns are also affected. Accounting for radiative heat transfer also has consequences for temperature-sensitive physical and chemical processes in a collisional atmosphere – for instance, dense regions of the modeled impact-generated atmosphere remain sufficiently warm that condensation is unlikely.

Impact Ejecta Plumes at the Moon

AbstractThe Lunar Dust Experiment, on‐board National Aeronautics and Space Administration's Lunar Atmosphere and Dust Environment Explorer, observed significant enhancements in impact rate measurements of lunar ejecta. These enhancements were attributed to the spacecraft crossing dense ejecta plumes generated by well‐timed and well‐placed interplanetary meteoroid impacts on the lunar surface. We have used a Monte Carlo approach to implement an initial speed distribution, derived from globally averaged Lunar Dust Experiment observations, in a 3‐D dynamical model describing the ballistic motion of ejecta particles. By matching this model to the observed enhancements, we constrained the initial ejecta angular distribution of these plumes. Our results indicate that lunar impact ejecta plumes reaching altitudes exceeding 24 km are far narrower than previously thought, with initial opening half angles of 8°±3°, suggesting that the high‐altitude lunar dust cloud may be dominated by reverse plumes.

NEW INSIGHTS INTO SOLAR WIND IMPLANTED VOLATILES FOR LUNAR REGOLITH CHARACTERIZATION: A SIMULATION BASED APPROACH

Abstract. The effect of solar wind implanted volatiles into the top 100 nm of the lunar regolith plays a significant role in quantitatively assessing the lunar surface isotopic composition. In essence, these volatiles can either quickly sputter out of the surface or be retained. The implantation processes exhibit a functional dependency on the surface temperature, ilmenite abundance and the activation energy associated with the optical maturity of the lunar soil. The prime focus of this study is to simulate the implication of these incident volatiles in characterizing the regolith for a better insight into the modeling of lunar exosphere during both Interplanetary Coronal Mass Ejection (ICME) and usual cases. Additionally, the proposed model quantifies the total lunar oxygen repository along with determining the associated textural and frequency domain measures for probable future lunar 3He mining sites. In this 30-day simulation, the particles bombard the reconstructed lunar grid wherein each cell displays varying particle density at a given local time. Moreover, both the activation energy and TiO2 content are assumed to be in a Gaussian distribution having (μ, σ) of (0.96, 0.025) and (12.52, 3.44) respectively. It has been found that the surfaces characterized by high activation energy tend to retain solar wind implants due to the large numbers of crystal defects. However, for H and heavy trace ions, intermediate activation energy range demonstrates diurnal behavior with the diffusive loss at local noon time. The study also finds an intriguing relationship between the lunar O2 and retained H sites (frequency domain). Furthermore, this could be utilized as a generic exospheric modeling paradigm for airless bodies and contribute to the understanding of the physical processes associated with solar astronomy.

High‐Resolution, Ground‐Based Observations of the Lunar Sodium Exosphere During the Lunar Atmosphere and Dust Environment Explorer (LADEE) Mission

AbstractWe present the first comprehensive set of lunar exospheric line width and line width derived effective temperatures as a function of lunar phase (66° waxing phase to 79° waning phase). Data were collected between November 2013 and May 2014 during six observing runs at the National Solar Observatory McMath‐Pierce Solar Telescope by applying high‐resolution Fabry‐Perot spectroscopy (R ~ 180,000) to observe emission from exospheric sodium (5,889.9509 Å, D2 line). The 3‐arc min field of view of the instrument, corresponding to ~336 km at the mean lunar distance (384,400 km), was positioned at several locations off the lunar limb; only equatorial observations taken out to 950 km are presented here. We find the sodium effective temperature distribution to be approximately a symmetric function of lunar phase with respect to full Moon. Within magnetotail passage we find temperatures in the range of 2500–9000 K. For phase angles greater than 40° we find that temperatures flatten out to ~1700 K.

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.

Space weathering on the Moon: Farside-nearside solar wind precipitation asymmetry

The lunar surface is continuously under the impact of solar wind plasma, which breaks the chemical bonds of the surface material resulting in weathering of the surface and a modified chemical composition. Ion impact also sputters the surface material, affecting the composition of the lunar exosphere, and it controls the electrical properties of both the lunar surface and the near surface space.We have studied the lunar farside-nearside (FN) hemispherical asymmetry of the solar wind proton impact on the lunar surface along the orbit of the Moon during fast solar wind conditions. The analysis is based on a 3D hybrid model where ions are accelerated by the macroscopic j × B and pressure gradient forces.The derived proton impact surface map shows that the highest cumulative solar wind proton addition on the lunar surface is located on the farside while the most energetic protons precipitate on the nearside. The total ion impact rate was found to be smallest when the Moon is deep in the magnetotail. The total ion impact rate on the lunar surface varies while the Moon orbits the Earth and these longitudinal variations are caused by the magnetosphere and lunar tidal locking.

Effects of multiscale phase-mixing and interior conductance in the lunar-like pickup ion plasma wake. First results from 3-D hybrid kinetic modeling

The study of multiscale pickup ion phase-mixing in the lunar plasma wake with a hybrid model is the main subject of our investigation in this paper. Photoionization and charge exchange of protons with the lunar exosphere are the ionization processes included in our model. The computational model includes the self-consistent dynamics of the light (H+ or H2+ and He+), and heavy (Na+) pickup ions. The electrons are considered as a fluid. The lunar interior is considered as a weakly conducting body. In this paper we considered for the first time the cumulative effect of heavy neutrals in the lunar exosphere (e.g., Al, Ar), an effect which was simulated with one species of Na+ but with a tenfold increase in total production rates. We find that various species produce various types of plasma tail in the lunar plasma wake. Specifically, Na+ and He+ pickup ions form a cycloid-like tail, whereas the H+ or H2+ pickup ions form a tail with a high density core and saw-like periodic structures in the flank region. The length of these structures varies from 1.5RM to 3.3RM depending on the value of gyroradius for H+ or H2+ pickup ions. The light pickup ions produce more symmetrical jump in the density and magnetic field at the Mach cone which is mainly controlled by the conductivity of the interior, an effect previously unappreciated. Although other pickup ion species had little effect on the nature of the interaction of the Moon with the solar wind, the global structure of the lunar tail in these simulations appeared quite different when the H2+ production rate was high.

Constraining the Ratio of Micrometeoroids From Short‐ and Long‐Period Comets at 1 AU From LADEE Observations of the Lunar Dust Cloud

AbstractWe interpret recent observations of the secondary dust ejecta cloud around the Moon from the Lunar Dust Experiment (LDEX) on board the NASA Lunar Atmosphere and Dust Environment Explorer (LADEE) spacecraft with help from dynamical models of meteoroids. Results suggest that in order to match the spatial structure of observed ejecta profiles, the flux of meteoroids on the Moon must be primarily provided by short‐period comets with an excess ratio of at least 1.3:1 compared to long‐period comets. This ratio increases significantly if the dependence of the ejecta yield on impactor velocity is stronger than generally believed. The model accounts for the orbital geometry of LADEE and shows no indication of a large asymmetry in the meteoroid flux impacting from the Helion and Anti‐Helion directions.

Contributions of solar-wind induced potential sputtering to the lunar surface erosion rate and it’s exosphere

Abstract Sputtering of lunar regolith by solar-wind protons and heavy ions with kinetic energies of about 1 keV/amu is an important erosive process that affects the lunar surface and exosphere. It plays an important role in changing the chemical composition and thickness of the surface layer, and in introducing material into the exosphere. Kinetic sputtering is well modeled and understood, but understanding of mechanisms of potential sputtering has lagged behind. In this study we differentiate the contributions of potential sputtering from the standard (kinetic) sputtering in changing the chemical composition and erosion rate of the lunar surface. Also we study the contribution of potential sputtering in developing the lunar exosphere. Our results show that potential sputtering enhances the total characteristic sputtering erosion rate by about 44%, and reduces sputtering time scales by the same amount. Potential sputtering also introduces more material into the lunar exosphere.

Dusty plasmas in the lunar exosphere: Effects of meteoroids

A possibility of the formation in the lunar exosphere of dust cloud due to meteoroid impacts onto the lunar surface is studied. The main attention is paid to the high altitudes over the lunar surface including the range of the altitudes between 30 and 110 km where the measurements of dust were performed within the NASA LADEE mission. From the viewpoint of the formation of dust cloud at high altitudes over the Moon, the most important zone formed by the meteoroid impact is the zone of melting of substance. Only the droplets originated from this zone have the speeds between the first and second astronautical velocities (for the Moon). Correspondingly, only such droplets can perform finite movement around the Moon. The liquid droplets harden when rising over the lunar surface. Furthermore, they aquire electric charges due to the action, in particular, of the solar wind electrons and ions, as well as of the solar radiation. Thus dusty plasmas exist in the lunar exosphere with the characteristic number density ≲ 10−2 m−3 of dust particles with the sizes from 300 nm to 1 μm which is in accordance with the results of measurements performed by LADEE.