Search

We investigate the dynamics of a water pumping mechanism driven by temperature variations in the lunar subsurface. The thermal environment at three polar sites and three sites in the Clavius region was simulated, taking into account local terrain and scattered radiation. A separate heat and mass transfer model was used to simulate depth‐dependent, temperature‐dependent, and pressure‐dependent properties of the lunar subsurface. The results suggest that diurnally varying heat fluxes create suitable conditions for water migration at many sites across the lunar surface. Enabled by a constant supply to the lunar surface, water molecules typically migrate a few centimeters deep via the formation of distinct concentration peaks, and a downward flux driven by the repeated desorption and resorption. With a constant supply rate of 10−15 kg/(m2 s), the quantity of adsorbed water stored at Ga timescales reaches values on the order of 10−10 to 10−7 mol/m2 at the investigated sites. Based on our results, we present a new relation for the water migration depth that takes into account the ratios of surface temperatures. The results of a sensitivity analysis show that the desorption activation energy is a dominant factor for the quantity and depth of water migration. In addition to long‐term accumulation of subsurface adsorbate, the model shows that temporarily captured water at shallower depths can be released during the lunar day at quantities of up to several µg m−2. This, as well as exposure of shallow water disturbed by impacts, may be a relevant source for surface water in illuminated areas. Plain Language Summary: Water bound to the lunar soil is released by the daily temperature cycle on the Moon. A portion of it is subsequently lost to space, but some of it is transported to deeper layers in the lunar soil. How much water will be transported within the soil depends on the temperature distribution, timescales, and how strongly the water bonds with the soil. Using a simulation model for the heat exchange at the lunar surface and the heat and mass transfer within the lunar soil, we show how pronounced this phenomenon is at six selected sites in the lunar south polar region and the Clavius region. We find that the quantities of accumulated subsurface water are generally low, on the order of several micrograms per unit area, and tend to be higher at the polar sites. Subsurface water can be found at depths of only a few centimeters and the depth of the peak concentration can be determined using a newly established relation for surface temperature ratios. Such shallow accumulated water is also partly released during some lunar days and could in principle be observed during day at the surface, even at warmer mid‐latitudes, such as the Clavius region. Key Points: Our model combines 3D lunar surface temperatures and a detailed parametrization of heat and mass transfer properties of the subsurfaceWater typically accumulates at depths <10 cm with a distinct peak and migration depth can be estimated based on surface temperature ratiosWater can be temporarily trapped at shallow depth even at mid‐latitudes, where it is partially released to the exosphere during day [ABSTRACT FROM AUTHOR]

The lunar mascon basins are characterized by high gravity anomalies and thin crust. Study of the density structure beneath the mascon basins can help to understand the origin of these gravity anomalies and infer their formation and evolution. We propose an efficient forward gravity method based on the 3‐D Gauss‐Legendre quadrature (GLQ) and Fast Fourier Transforms combined with the adaptive discretization strategy to ensure high accuracy. The numerical example demonstrates that computational efficiency is increased by about three orders of magnitude compared to the traditional 3D GLQ method. We employ this forward method in a 3‐D inversion of the gravity data derived from the lunar gravity model GL1500E. The inverted results show prominent high‐density structures (namely, mascons) representing significant mantle uplift and thinned crust beneath most impact basins. Marked low‐density rings surround the mascons. Most of the corresponding low‐density anomalies extend from the near‐surface to the Moho, indicating a thick, low‐density crust. Our density model is consistent with the formation processes of mascons that an impact causes collapse of the transient crater and then the shocked mantle drives mantle flow. The low‐density rings surrounding the mascons may stem from the crust thickening following an impact and extensive fracturing of the crustal column. Plain Language Summary: Mascons (i.e., mass concentration) have been found beneath most of the lunar impact basins. Knowledge of their density distribution is vital for understanding of their structure and evolution. The mass excesses in some impact basins extend from the base of the lunar crust to about 60 km depth and correspond to a density anomaly of up to 400 kg/m3. These mass concentrations are usually surrounded by a ring of low‐density crust. These features are consistent with the hypothesis that the mascons were formed after an asteroid impact and the upwelling mantle material filling the impact crater. This high‐density material produces the mass excess under the impact basins, while the broken crustal material forms the thick low‐density crust around the mascons. Key Points: A highly efficient forward modeling method for gravity calculations is proposed and used in inversionProminent high‐density structures represent significant mantle uplift and thinned crust beneath most of the impact basinsNegative density anomalies around the mascons form outer rings which may be associated with crustal thickening [ABSTRACT FROM AUTHOR]

China's Chang'e-6 mission aims to land in the South Pole-Aitken Basin (SPA), the Moon's largest and oldest impact crater, to collect rocks and soil samples. Scientists hope that these samples will provide insights into the timing and nature of the SPA impact, which is still a topic of debate. The samples may also shed light on the differences between the Moon's near side and far side, as well as the formation of the Moon itself. Chang'e-6 is part of China's successful lunar exploration program and is expected to share samples internationally. [Extracted from the article]

Under lunar polar cold traps, volatile molecules within porous regolith may experience temperature and depth dependent slow mobility. Many degraded lunar craters exhibit thick regolith fill based on models of topographic diffusion and observations of fresh and degraded craters. Regolith has a low thermal conductivity relative to megaregolith and may act as a blanket for internal lunar heat flow, leading to increased temperatures at depth. We develop 2D thermal models of fresh and regolith-filled lunar craters over depths of meters to hundreds of meters below the surface. We find that the base of the stability and slow mobility zones migrate upward with regolith fill, which leads to temperatures that may increase the sublimation rate of volatiles at depth. For a notional cold trap crater 1.6 km in diameter and 3.6 billion years old, topographic diffusion fills it with approximately 90 m of regolith, and the regolith fill's blanketing effect causes the 110 K isotherm to shift about 180 m upward. This places it approximately 25 m below the current cold trap surface and well above the initial crater floor. The slow water ice mobility zone below the 110 K isotherms also shifts upward with regolith fill, potentially increasing volatile concentrations at shallower depths. These secondary volatile concentrations may be targets for sampling and testing hypotheses of volatile system processes. In addition, remobilized volatile concentrations may be a resource for future In-Situ Resource Utilization (ISRU) applications. The thick regolith fill in degraded craters and volatile remobilization potential in lunar subsurface cold traps have implications for future exploration instruments, sampling, and ISRU architectures. • Subsurface lunar polar cold traps impact volatile mobility. • Many lunar craters show enhanced regolith thickness. • Low thermal conductivity regolith can act as a thermal blanket. • Planar 2D thermal models reveal stability zone migration with regolith fill. • Remobilized subsurface volatile concentration potential and ISRU implications. [ABSTRACT FROM AUTHOR]

The lunar surface is constantly bombarded by the solar wind, photons, and meteoroids, which can liberate Na atoms from the regolith. These atoms are subsequently accelerated by solar photon pressure to form a long comet‐like tail opposite the sun. Near new moon, these atoms encounter the Earth's gravity and are "focused" into a beam of enhanced density. This beam appears as the ∼3° diameter Sodium Moon Spot (SMS). Data from the all sky imager at the El Leoncito Observatory have been analyzed for changes in the SMS shape and brightness. New geometry‐based relationships have been found that affect the SMS brightness. The SMS is brighter when the Moon is north of the ecliptic at new moon; the SMS is brighter when new moon occurs near perigee; and the SMS peaks in brightness ∼5 h after new moon. After removing these effects, the data were analyzed for long term and seasonal patterns that could be attributed to changes in source mechanisms. No correlation was found between the SMS brightness and the 11‐year solar‐cycle, the proton or the He++ flow pressure, the density, the speed or the plasma temperature of the solar wind, but an annual pattern was found. A ∼0.83 correlation (Pearson's "r") was found between the SMS brightness and a 4‐year average of sporadic meteor rates at Earth, suggesting a cause‐and‐effect. The new insights gained from this long‐term study put new constraints on the variability of the potential sources of the Na atoms escaping from the Moon. Plain Language Summary: Once a month, at the time of new moon a small, diffuse spot of light appears in the sky opposite the Sun. This spot is about five times the diameter of the full moon, and is 50 times fainter than can be seen with the unaided eye. The spot is reflected light from millions of sodium atoms that two days earlier were on the surface of the Moon. These atoms were liberated from the Moon's surface by meteoroid impacts, the solar wind and photons. Once free of the lunar surface, the atoms are pushed by light pressure into a long comet‐like tail. When this very diffuse cloud of sodium atoms encounters the Earth's gravity, it is "focused" into a region of enhanced density becoming visible to sensitive cameras equipped with filters tuned to the orange light emitted by sodium near 589.3 nm. All‐Sky‐Imager data were analyzed to see how the brightness of this spot varies over time, with a goal of finding a correlation with one (or more) of the above mentioned source processes. A correlation was found between the brightness of the sodium spot and the meteor rate observed at Earth using meteor radar data. Key Points: Brightness of the Sodium Moon Spot (SMS) is dominated by the Earth‐Moon‐Sun geometryThe SMS brightness appears to be correlated with the sporadic source of meteorsThe SMS brightness appears not to be correlated with the density, the speed, the pressure, or the temperature of the solar wind [ABSTRACT FROM AUTHOR]

The evolution and compositional structure of the lunar mantle has been extensively modeled but insufficiently constrained by observations. Here, we identify and characterize mantle materials exposed by the Moon's largest impact basin to better understand the composition, stratigraphy, and evolution of the upper mantle. The vast South Pole-Aitken Basin (SPA) exhibits a broad, crescent-shaped thorium and potassium distribution. These incompatible elements are predicted to be concentrated in the dregs of the lunar magma ocean during end-stage crystallization. Through consideration of basin formation models convolved with subsequent geologic evolution, we demonstrate that the distribution and implied stratigraphy of Th- and K-bearing materials across SPA are consistent with an upper mantle ejecta origin. The most pristine exposures of these materials are confined to northwest SPA and also exhibit elevated Ti and Fe (relative to the farside highlands) in association with a gabbronoritic mineralogy. This is consistent with latestage magma ocean assemblages predicted by petrologic models. In contrast, SPA impact melt derived from greater depths is associated with a low-Ca pyroxene-dominated assemblage. Together, these compositional patterns are evidence for a stratified ancient upper mantle. Importantly, the incompatible-element-enriched, ilmenite-bearing ferroan gabbronoritic cumulates evidently had not participated in gravitational overturn at the time of SPA formation. Contrary to recent hypotheses invoking nearside sequestration of incompatible elements to explain hemispherical differences in crustal building and volcanic resurfacing, it follows that incompatible elements were globally distributed in the magma ocean at the time of SPA formation. [ABSTRACT FROM AUTHOR]

Lunar horizon glows observed by the Apollo missions suggested a dense dust exosphere near the lunar terminator. But later missions failed to see such a high‐density dust exosphere. Why the Apollo missions could observe so large number of dust grains remains a mystery. For the first time, we report five dust enhancement events observed by the Lunar Dust Experiment on board Lunar Atmosphere and Dust Environment Explorer mission, which happen near a twilight crater with dust densities comparable to the Apollo measurements. Moreover, the dust densities are larger on the downstream side of the crater and favor a higher solar wind temperature, consistent with an electrostatic dust lofting from the negatively charged crater floor. We also check the Apollo observations and find similar twilight craters, suggesting that the so‐called dust exosphere is not a global phenomenon but just a local electrified dust fountain near twilight craters. Plain Language Summary: With in situ dust measurements, we find that a shadowed crater near the terminator can dramatically change the surface electrical environment and bring a dense dust cloud surrounding the crater, which should be carefully assessed by engineers for future lunar explorations. Moreover, our findings have the general implications in studying the dust environment near large topographic features (mountains and deep craters) of all kinds of airless bodies. Key Points: We find five dust enhancements events near twilight craters, with the in situ dust measurements of the LADEE missionThe dust enhancement events can be caused by an electrostatic dust lofting from the leeward wall of a twilight craterThe lunar horizon glow observed in the Apollo missions can be also related to a twilight crater [ABSTRACT FROM AUTHOR]

The article discusses that the study of the Apollo lunar samples reveal that the Moon generated its own global magnetosphere. Topics discussed include Solar storms strip a planet's atmosphere over time, and only a strong magnetosphere would be able to provide maximum protection; and present simplified magnetic dipole field modeling confined within a paraboloidal-shaped magnetopause to show how the expected Earth-Moon coupled magnetospheres provide a substantial buffer.

A transient lunar atmosphere formed during a peak period of volcanic outgassing and lasting up to about ~70 Ma was recently proposed. We utilize forward‐modeling of individual lunar basaltic eruptions and the observed geologic record to predict eruption frequency, magma volumes, and rates of volcanic volatile release. Typical lunar mare basalt eruptions have volumes of ~102–103 km3, last less than a year, and have a rapidly decreasing volatile release rate. The total volume of lunar mare basalts erupted is small, and the repose period between individual eruptions is predicted to range from 20,000 to 60,000 years. Only under very exceptional circumstances could sufficient volatiles be released in a single eruption to create a transient atmosphere with a pressure as large as ~0.5 Pa. The frequency of eruptions was likely too low to sustain any such atmosphere for more than a few thousand years. Transient, volcanically induced atmospheres were probably inefficient sources for volatile delivery to permanently shadowed lunar polar regions. Plain Language Summary: Could gas emitted from volcanic eruptions during the most intense and voluminous period of lunar mare volcanism produce a temporary lunar atmosphere? Could the presence of such an atmosphere enable volatiles to reach the cold traps in the permanently shadowed regions at the lunar poles? We use information from lunar geology and sample analyses to predict the number of eruptions with time, the volume of individual eruptions, the rates of volcanic gas release during each eruption, and the time between eruptions. We find that only under rare circumstances could a single eruption or two eruptions closely spaced in time release enough gas to create a transient atmosphere with a pressure as large as ~0.5 Pa. Furthermore, it is difficult to sustain such an atmosphere for more than a few thousand years. These results suggest that volcanically produced atmospheres are inefficient source mechanisms for delivery of volatiles to form deposits in permanently shadowed polar regions of the Moon; this favors volatile‐rich impactors as the major source of polar ice. Key Points: A transient lunar atmosphere from peak volcanic degassing lasting up to ~70 Ma was recently proposed as a source of lunar polar volatilesWe forward‐model individual eruption volume, degassing patterns, and duration of periods between eruptions (repose periods)Transient, volcanically induced atmospheres are inefficient sources for volatile delivery to permanently shadowed lunar polar regions [ABSTRACT FROM AUTHOR]

Global exosphere models of alkali gases surrounding Mercury and the Moon assume that the primary effect of the porous soil is to reduce the effective desorption rates. We demonstrate with a kinetic simulation that, following adsorption, the complicated structure of soils has two additional effects on the fate of previously released alkali atoms: (1) trapping of free atoms at lunar temperatures by microscopic shadows and inward diffusion, which becomes the primary sink mechanism, and (2) high‐energy barriers for thermal desorption compared to what would be retrieved from experiments on thin films or compacted pellets, especially when surface diffusion of adsorbates is considered. Lunar soils retain one fifth to two thirds of recycled adsorbates, depending on the assumed adsorbate mobility, photodesorption cross section, and soil thermal gradient. A transition from a retentive surface to full outgassing at T > 500 K will produce complex feedback mechanisms of alkali circulation at Mercury. Plain Language Summary: This paper seeks to understand physical processes that occur on mineral surfaces throughout the Solar System and which affect the atmospheres of Mercury and the Moon. When meteoroids and the solar wind impact the surface, they vaporize sodium and potassium atoms, some of which cannot escape the body's gravity and can populate the atmosphere with successive bounces until they are lost to space via ionization, react with the surface, or penetrate into the soil. We quantify the competition between evaporation and diffusion within the top 1 mm of the soil, which supports the atmosphere reservoir, by following test particle trajectories in soils simulated with a sphere packing code. The simulations provide empirically constrained insights about the longevity of these free atoms. These findings provide context for understanding observations from ground‐based telescopes, MESSENGER and LADEE observations, and laboratory experiments and pave the way for higher‐fidelity models of exospheres for these species. Key Points: We modeled processes occurring on very different timescales in the soil of Mercury and the MoonMicroscopic shadows trap up to half of the deposited sodium and potassium atoms at lunar temperaturesDesorption of previously trapped atoms at higher temperatures could induce additional feedback in Mercury sodium circulation models [ABSTRACT FROM AUTHOR]

Conditions on Saturn's moon Titan suggest that dust devils, which are convective, dust‐laden plumes, may be active. Although the exact nature of dust on Titan is unclear, previous observations confirm an active aeolian cycle, and dust devils may play an important role in Titan's aeolian cycle, possibly contributing to regional transport of dust and even production of sand grains. The Dragonfly mission to Titan will document dust devil and convective vortex activity and thereby provide a new window into these features, and our analysis shows that associated winds are likely to be modest and pose no hazard to the mission. Plain Language Summary: Saturn's moon Titan may host active dust devils, small dust‐laden plumes, which could significantly contribute to transport of dust in that moon's atmosphere. Although the exact nature of dust on Titan is unclear, previous observations confirm that there is actively blowing dust on that world. If dust devils are active on Titan's surface, NASA's upcoming Dragonfly mission is likely to encounter them, but dust devils on Titan are unlikely to pose a hazard to the mission. Key Points: As probed by Huygens, the meteorological conditions near the surface of Saturn's moon Titan appear conducive to dust devil formationDust devils may contribute significantly to Titan's aeolian cycleIf dust devils are active on Titan, NASA's upcoming Dragonfly mission is likely to encounter them [ABSTRACT FROM AUTHOR]

The Dual Frequency Radio Science experiment aboard Chandrayaan-2 uses the communication channel between orbiter and ground in radio occultation mode to study the temporal evolution of electron density in the lunar ionosphere. It consists of a highly stable 20 MHz evacuated miniaturized crystal oscillator source, having a stability of the order of 10-11, which generates two coherent signals at the X (8496 MHz) and S (2240 MHz) bands of radio frequencies. The coherent radio signals, transmitted simultaneously from the satellite and received at the ground-based deep station network receivers would be used to study temporal and spatial variations in the lunar ionosphere. The major science objectives of the experiments include: (i) to study variations in the ionosphere/atmosphere of the Moon; (ii) to explore if the ionosphere of the Moon is omnipresent or has episodic appearances, and (iii) to confirm the source of ions in the lunar ionosphere - whether dusty or molecular. [ABSTRACT FROM AUTHOR]

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 Δ V 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. • Analytical insights on ω variation drawn and specific trends for polar orbits reported. • Six extremely low-altitude (Periselene alt. ~ 20 km), quasi-frozen polar orbits are presented. • Investigation on optimal gravity of the Moon's fidelity model is conducted. • High-fidelity GRAIL model and third-body effects from Earth and Sun are considered. • Station-keeping analyses performed for different orbit insertion dates and maneuver points. [ABSTRACT FROM AUTHOR]

We consider the features of the mechanisms of the electric charging of dust particles, emergence of electric fields, and discharges in the atmospheres of planets and near the surface of the Moon. We find a significant difference in the nature of the influence of electric fields (discharges) on the vortex motions of two-phase (gas and dust) in the atmospheres of the planets and single-phase (dust) near the surface of the Moon. [ABSTRACT FROM AUTHOR]

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. Unlabelled Image • Double-pulse lasing with inter-delay time is applied to Raman-LIPS detection. • Previous attempts on simultaneous Raman-LIPS have never achieved signal enhancement. • Simultaneous molecular and atomic signals on eight rock samples were detected. • Improved sensitivity and signal enhancement were achieved. [ABSTRACT FROM AUTHOR]

We 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. Plain Language Summary: Potassium is one of the constituents in the rarefied but measurable lunar atmosphere. Although not the most abundant element, it can be observed spectroscopically from Earth and can be used to understand processes that are responsible for sustaining the lunar atmosphere. We present the first atmospheric potassium data set of ground‐based observations to cover several months and the first potassium line profile measurements. We analyze potassium in the Moon's atmosphere in terms of temperature and intensity as it relates to the lunar phase. The potassium temperature during waxing phases is approximately 2 times hotter than the temperature during the waning phases, creating a noticeable asymmetric pattern. The potassium intensity gradually decreases as the Moon gets brighter until the emission signal is lost in scattered moonlight then increases again with time after full Moon. Data near surface regions known to be rich in potassium have higher intensities in the atmosphere than at other locations. Studying temperature and intensity patterns combined with future modelling gives insight to the processes regulating the Moon's atmosphere. Key Points: The average potassium effective temperature is 1920 +/‐ 630 K and 980 +/‐ 200 K for waxing and waning gibbous phases, respectivelyThe relative intensity curve suggests that the source rate decreases more rapidly with local time for waxing than waning phasesThe relative intensity is affected by location, being brightest above potassium‐rich KREEP soils [ABSTRACT FROM AUTHOR]

The 21 August 2017 total solar eclipse was the first total eclipse on the mainland of the United States since 1979. The Atmospheric Responses of 2017 Total Solar Eclipse (ARTSE2017) project was created to observe the response of the atmosphere to the shadow of the moon. During the eclipse, 10 sites launched radiosondes in a very rapid, serial weather balloon deployment along the totality path, and high-resolution mesoscale meteorological network (mesonet) data were collected in three states. Here, we focus on the results obtained from the radiosonde field campaign in Fort Laramie, Wyoming, and the New York State Mesonet (NYSM). In Fort Laramie, 36 people from 13 institutions flew 19 radiosondes and launched 5 large balloons carrying video payloads before, during, and after the eclipse while continuously recording surface weather data. Preliminary analysis of the radiosonde data provided inconclusive evidence of eclipse-driven gravity waves but showed that the short duration of darkness during totality was enough to alter boundary layer (BL) height, the lowest layer of the atmosphere, substantially. The statewide impact of the partial eclipse in New York State (NYS) was observed for solar radiation, surface temperature, surface wind, and surface-layer lapse rate using NYSM data. Importantly, the radiosonde and mesonet data collected during the eclipse will be available for public access. ARTSE2017 also focused on education, including students from all demographics (undergraduate and K–12) and the general public. Finally, we summarize goals accomplished from leveraging resources for education, research, and workforce development on undergraduate students from a variety of fields. [ABSTRACT FROM AUTHOR]

The lunar history of water deposition, loss, and transport postaccretion has become an important consideration in relation to the possibility of a human outpost on the Moon. Very recent work has shown that a secondary primordial atmosphere of up to 10 mbar could have been emplaced ∼3.5×109 years ago due to volcanic outgassing from the maria. Using a zero‐dimensional chemistry model, we demonstrate the temperature dependence of the resulting major atmospheric components (CO or CO2). We use a three‐dimensional general circulation model to test the viability of such an atmosphere and derive its climatological characteristics. Based on these results, we then conjecture on its capability to transport volatiles outgassed from the maria to the permanently shadowed regions at the poles. Our preliminary results demonstrate that atmospheres as low as 1 mbar are viable and that permanent cold trapping of volatiles is only possible at the poles. Key Points: We confirm the viability of transient maria outgassed atmosphere for pressures from 1 to 10 mbarThree‐dimensional simulations demonstrate where volatile deposition may occurAtmospheric escape and outgassing chemistry play important roles [ABSTRACT FROM AUTHOR]

In this study, Global Ionosphere Specification (GIS) based on Gauss‐Markov Kalman filter assimilation of slant total electron content observed from ground‐based global positioning system receivers and space‐based radio occultation instrumentations is applied to investigate the ionospheric day‐to‐day tidal variability during the 2009 stratospheric sudden warming (SSW) period. Including the improved daily three‐dimensional global electron density distribution from GIS enables us to retrieve the daily solar tidal solution by using least squares tidal analysis. We find prominent reductions followed by enhancements in the amplitude of the solar semidiurnal migrating tide (SW2) after the peak warming, with recurrent phase variations occurring at low magnetic latitudes over a period of about 15 days. This is close to the beating period (15.13 day) between SW2 and lunar semidiurnal (M2), thus suggesting the existence of strong M2, and our results demonstrate that the intensification of M2 exists only during the SSW period. Additionally, M2 acts as the key contributor to make the semidiurnal ionospheric perturbations shift toward later local times. Our tidal analyses of daily GIS thus provide evidence for the combined impact of amplitudes and phases of the SW2 and M2 in producing semidiurnal variations in ionosphere during the 2009 SSW. Plain Language Summary: Large‐scale meteorological disturbances like stratospheric sudden warmings (SSWs) are often of interest for the investigation of a variety of mechanisms and processes that link different regions of the Earth's atmosphere across a wide range of altitudes and latitudes. Earlier studies have shown large and long‐lasting anomalies caused by SSWs in the Earth's daytime ionosphere. In this study, we show that the typical semidiurnal (12‐hr) ionospheric responses in connection with SSWs are caused by the combined effect of solar and lunar semidiurnal migrating tides during the 2009 SSW. The 15‐day oscillation of the semidiurnal variations is indeed a result from the beating between solar and lunar semidiurnal migrating tides, suggesting the importance of strong enhancements in the lunar semidiurnal migrating tide during SSWs. These results improve our understanding of the reasons for day‐to‐day variations in the ionosphere and the role of upward propagation of solar and gravitational lunar‐induced tides from lower altitudes during large‐scale meteorological disturbances. Key Points: Investigation of daily ionospheric tidal signature during a major stratospheric sudden warming (SSW)Solar and lunar migrating tides are both crucial to the ionospheric responses to the SSWHarmonic beating between solar and lunar migrating tides reflects on the semidiurnal ionospheric variation [ABSTRACT FROM AUTHOR]

Abstract The first multi-instrument nocturnal aerosol optical depth (AOD) intercomparison campaign was held at the high-mountain Izaña Observatory (Tenerife, Spain) in June 2017, involving 2-min synchronous measurements from two different types of lunar photometers (Cimel CE318-T and Moon Precision Filter Radiometer, LunarPFR) and one stellar photometer. The Robotic Lunar Observatory (ROLO) model developed by the U.S. Geological Survey (USGS) was compared with the open-access ROLO Implementation for Moon photometry Observation (RIMO) model. Results showed rather small differences at Izaña over a 2-month time period covering June and July, 2017 (± 0.01 in terms of AOD calculated by means of a day/night/day coherence test analysis and ± 2% in terms of lunar irradiance). The RIMO model has been used in this field campaign to retrieve AOD from lunar photometric measurements. No evidence of significant differences with the Moon's phase angle was found when comparing raw signals of the six Cimel photometers involved in this field campaign. The raw signal comparison of the participating lunar photometers (Cimel and LunarPFR) performed at coincident wavelengths showed consistent measurements and AOD differences within their combined uncertainties at 870 nm and 675 nm. Slightly larger AOD deviations were observed at 500 nm, pointing to some unexpected instrumental variations during the measurement period. Lunar irradiances retrieved using RIMO for phase angles varying between 0° and 75° (full Moon to near quarter Moon) were compared to the irradiance variations retrieved by Cimel and LunarPFR photometers. Our results showed a relative agreement within ± 3.5% between the RIMO model and the photometer-based lunar irradiances. The AOD retrieved by performing a Langley-plot calibration each night showed a remarkable agreement (better than 0.01) between the lunar photometers. However, when applying the Lunar-Langley calibration using RIMO, AOD differences of up to 0.015 (0.040 for 500 nm) were found, with differences increasing with the Moon's phase angle. These differences are thought to be partly due to the uncertainties in the irradiance models, as well as instrumental deficiencies yet to be fully understood. High AOD variability in stellar measurements was detected during the campaign. Nevertheless, the observed AOD differences in the Cimel/stellar comparison were within the expected combined uncertainties of these two photometric techniques. Our results indicate that lunar photometry is a more reliable technique, especially for low aerosol loading conditions. The uncertainty analysis performed in this paper shows that the combined standard AOD uncertainty in lunar photometry is dependent on the calibration technique (up to 0.014 for Langley-plot with illumination-based correction, 0.012–0.022 for Lunar-Langley calibration, and up to 0.1 for the Sun-Moon Gain Factor method). This analysis also corroborates that the uncertainty of the lunar irradiance model used for AOD calculation is within the 5–10% expected range. This campaign has allowed us to quantify the important technical difficulties that still exist when routinely monitoring aerosol optical properties at night-time. The small AOD differences observed between the three types of photometers involved in the campaign are only detectable under pristine sky conditions such as those found in this field campaign. Longer campaigns are necessary to understand the observed discrepancies between instruments as well as to provide more conclusive results about the uncertainty involved in the lunar irradiance models. Highlights • The first attempt to compare instruments with nocturnal remote sensing capabilities. • Moon photometry has better capabilities to measure low aerosol contents. • Some instrumental differences detected between the two lunar photometers. • Small AOD differences only detectable in such pristine conditions were measured. • A freely-available lunar irradiance model has been released. [ABSTRACT FROM AUTHOR]

Abstract Impact craters are essential and dominant features of the lunar surface. Under the presence of a very thin atmosphere, the degradation of the lunar craters takes place possibly due to gravity, slope processes, or solar wind weathering. Hence, these craters act as tools to understand the properties and processes of the lunar surface. Our analysis is based on the inferences from the craters located in the western hemisphere of the moon. For a long time, the depth to diameter (d/D) ratio and slope of the craters have been used to characterize the craters and to analyze crater degradation processes. However, the d/D ratio and slope are dependent on the size of the crater and therefore cannot be directly generalized for craters of varying sizes. We have thus applied new parameters, namely, slope range and max slope along the wall which along with the d/D ratio and slope can help distinguish the degradation classes of the craters at different ranges of the crater diameter. We have also introduced the topographic roughness index (TRI) near the crater rim which can be used to quantify the roughness of crater rim and to further distinguish the crater degradation classes. Further, our study examines the role of crater shape, i.e., flat-floored or round-floored craters and regional topography on crater degradation. Our study also shows that out of all the round-floored craters in our study area, more craters are in degraded class (class B and class C) than in preserved class (class A). Highlights • New parameters for studying crater morphology are introduced and their relation with crater degradation classes is analyzed. • Depth-diameter ratio of craters is size dependent and this alone may not be sufficient to understand the crater degradation. • An approach is presented to derive a boundary between large craters and small craters using a mathematical error function. • Degradation rates for small and large craters are different. Multiple parameters can better distinguish degradation classes. • Regional topography has shown strong role in crater degradation. [ABSTRACT FROM AUTHOR]

Abstract The microwave radiation passively emitted from the Moon is valuable for investigating the physical properties of the lunar regolith. Microwave maps, which can reveal subsurface characteristics at depths of centimeters to meters, give clues to understanding the evolution of the lunar surface. China's Chang'E-2 (CE-2) spacecraft carried a microwave radiometer (MRM) and conducted passive microwave remote sensing of the Moon at 3, 7.8, 19.35 and 37 GHz (10, 3.84, 1.55 and 0.81 cm wavelengths) during 2010–2011. Brightness temperature (TB) data derived from the MRM orbital observations cover the entire Moon, and the time coverage is over a complete lunation. In this paper, we use spherical harmonic fits to model the TB variation as functions of local time and latitude. Using these fits, we normalized the day- and nighttime TB data measured at various local times into noon-time and midnight conditions. The spatial sampling of the normalized TB maps is ∼6 km/pixel, much higher than ground-based observations and higher resolution than the MRM maps derived from Chang'E-1. By removing the variation in TB related to local time and latitude, we produced a series of microwave maps that reveal differences in the properties of the regolith across the lunar surface. The microwave behavior of various lunar surface features can be explained primarily by differences in the microwave loss tangent of the materials. For example, we find that the high-Ti maria have high daytime TB and exhibit lower nighttime TB than the other maria as a result of the high dielectric loss tangent of ilmenite. The ilmenite-rich materials are less transparent to microwaves, and hence emissions emerge from closer to the surface, which is heated strongly in the day and cools quickly at night. Most fresh craters correspond to low-TB spots in the nighttime microwave maps, in contrast to their behavior as hot spots in nighttime infrared data, suggesting that the higher loss tangent of rocks (compared to soils) suppresses microwave emission at night. The Aristarchus plateau has a lower loss tangent relative to its surroundings because it is blanketed by fine-grained, rock-poor pyroclastic material. The pyroclastic blanket is more transparent to microwaves than the nearby maria, causing the plateau to have relatively cool daytime TB and warm nighttime TB as a consequence of microwave emissions that originate at greater depths The Orientale basin is another prominent anomaly in the microwave maps, indicating that Orientale materials differ in thermophysical and/or dielectric properties from typical highland lithologies. Brightness temperature differences among prominent rayed craters indicate that the very youngest crater rays (e.g., Giordano Bruno) have lower thermal inertia than older but still optically immature rays (e.g., Tycho). [ABSTRACT FROM AUTHOR]

Again; he proceeds, whatever may be said of the reason assigned by Dr. Halley for the dim light of the sun before his immersion; namely, the vapors in the moon's atmosphere, collected during the long night; yet it would seem to be very certain, that the temperature of the western edge, which is just then passing from night to day, must be decidedly lower than that of the eastern edge, which has been exposed to the sun's light for many days together. He expresses these laws in the following terms: - L aw I. On the near approach of the moon, a minute beam of light is, in some cases, bent into a physical curve, the direction of 232 CELEBRATION OF THE HUNDREDTH ANNIVERSARY OF THE APS which is concave toward the surface of the moon, in the region in which the beam escapes from her influence. I. If a minute beam from a star, which is about to he occulted, pass near to that region of the moon's limb at which the distortion outward may prevail, the rays of light will be more and more bent as in fig. 1, until the flexure arrives at its maximum, the moon's edge having advanced to M, or very near it. In other cases, or in the neighborhood of other portions of the moon's limb, a minute beam of light is sometimes bent into a physical curve, the direction of which is convex toward the surface of the moon, in the region in which the beam escapes from her influence. [Extracted from the article]

Abstract It is widely accepted that the Apollo 17 orange glass beads are of volcanic origin. The degree of degassing of the glass beads depends on their cooling rates, so the estimation of volatile contents in the parental magmas of these lunar pyroclastic glasses also depends on the cooling rates. The cooling rate can be estimated using the calorimetric properties of the glass across the glass transition. In this study, a series of heat capacity measurements were carried out on hand-picked lunar volcanic orange glass beads during several cycles of heating to temperatures above their glass transition using a differential scanning calorimeter. The cooling rate of orange glass beads (sample 74220,867) was calculated to be 101 K/min using the correlation between glass cooling rates and fictive temperatures estimated from their heat capacity–temperature paths. This cooling rate is close to the lower end of the range that best fits the diffusion profiles of several volatile species in the glass beads, and at the upper end of the cooling-rate range recorded in glasses quenched subaerially on Earth. The cooling rate is likely to be controlled by the cooling environment (cooling medium) such that the lunar volcanic glass beads could have been cooled in a gaseous medium released from volcanic eruptions on the Moon, but not during "free flight" in vacuum. The existence of a gas medium suggests, in turn, that there may have been at least a short-lived or episodic atmosphere on the early Moon at around 3.5 Ga. Highlights • We measured heat capacities of lunar volcanic glass beads. • A cooling rate of 101 K/min was estimated for the Apollo 17 orange glass beads. • Cooling rate is close to those derived from volatile diffusions in previous studies. • Lunar orange glass beads were likely cooled in a gaseous medium of volcanic origin. • Fire fountaining could have contributed to a transient or early lunar atmosphere. [ABSTRACT FROM AUTHOR]

Abstract The far side of the lunar hemisphere is dominated by a large number of impact craters having a smaller diameter and few Mare basins compared to the near side of the Moon. This study presents a detailed analyses of gravity anomalies derived from Gravity Recovery and Interior Laboratory (GRAIL) mission and topography over the central equatorial region of Lunar far side covering the major impact basins such as Dirichlet-Jackson, Korolev, Hertzsprung and adjoining regions of the highlands, to understand sub-surface structure. Bouguer gravity anomaly map of the region is prepared using the density of 2.7 g/cc, which is estimated using fractals based approach. The spectral analyses using Maximum Entropy Method of calculated Bouguer anomaly and topography provided an estimated of Effective Elastic Thickness (Te) as 19 km and also a cut-off wavelength of 270 km for separating Regional and residual Bouguer gravity anomalies. Further a crustal thickness of this region of Moon is estimated by inversion of regional Bouguer gravity anomalies, which varies from 20 to 71 km. An estimated crustal thickness of Dirichlet Jackson and Hertzpsrung, Korolev basins varies between 39 and 65 km, 20–65 km and 39–64 km, respectively. The residual gravity anomaly map shows symmetric gravity highs and lows in Dirichlet-Jackson, Hertzsprung and Korolev basins. These gravity highs can be attributed to high-density mare basalts within inner rims of these basins. The observed nature of gravity anomalies and crustal thickness map can be attributed to the buried impact crater under this region. Highlights • Bouguer anomaly map is prepared using Bouguer crustal density determined through the Fractal approach. • Based on spectral approaches, EET is estimated and Regional and Residual gravity anomaly maps are prepared. • Crustal thickness map is derived from inversion of Regional Bouguer gravity anomalies. [ABSTRACT FROM AUTHOR]

It is known that the center of mass (CM) of the Moon does not coincide with its geometrical center of figure (CF), and that the CF-CMline deviates to the Southeast of the direction toward the center of the Earth. An investigation of this phenomenon, which has remained incompletely understood, has been carried out in two stages. One mechanism can explain part of the eastward shift of the lunar CM as being due to tidal evolution of the lunar orbit. A second mechanism is considered here, which relates this shift of the lunar CM with evolution of the shape of the Moon. A differential equation describing the shift of the lunar CMto the East in the course of the physically inevitable rounding of its shape as it moves away from the Earth is derived and solved. This mechanism not only explains the eastward shift of the lunar CM, but also predicts that the oblateness of the Moon could have been appreciable at earlier epochs, reaching values ε ≈ 0.31. The theory of figures of equilibrium in a tidal gravitational field is used to determine how close to the Earth the Moon could have formed. [ABSTRACT FROM AUTHOR]

We utilize measurements of electron plasma frequency oscillations made by the two‐probe Acceleration, Reconnection, Turbulence, and Electrodynamics of Moon's Interaction with the Sun mission to investigate the charged particle density in the lunar environment as the Moon passes through the Earth's geomagnetic tail. We find that the Moon possesses a tenuous ionosphere with an average density of ~0.1–0.3 cm−3, present at least 50% of the time in the geomagnetic tail, primarily confined to within a few thousand kilometers of the dayside of the Moon. The day‐night asymmetry and dawn‐dusk symmetry of the observed plasma suggests that photoionization of a neutral exosphere with dawn‐dusk symmetry produces the majority of the lunar‐derived plasma. The lunar plasma density commonly exceeds the ambient plasma density in the tail, allowing the presence of the lunar ionosphere to appreciably perturb the local plasma environment. Plain Language Summary: Though usually considered an airless body, the Moon actually possesses a very tenuous atmosphere. Ionization of this atmosphere, primarily by sunlight, creates a lunar ionosphere roughly one million times more tenuous than that of the Earth. At most times, this ionosphere does not noticeably affect the surrounding environment. However, when the Moon passes through the near vacuum of the geomagnetic tail of the Earth each lunar month around full Moon, its presence becomes more important. In this unique environment, the lunar‐derived charged particle density becomes comparable to the ambient value, and the presence of the lunar ionosphere can appreciably affect its surroundings. We utilize measurements from lunar orbit to investigate the density, structure, and dynamics of the faint lunar ionosphere and its interaction with the local environment of the geomagnetic tail. Key Points: When the Moon passes through the geomagnetic tail, a tenuous ionosphere exists at least 50% of the time above the dayside lunar surfaceThe day‐night asymmetry and dawn‐dusk symmetry of the observed ionosphere favors formation by photoionization of a symmetric exosphereThe lunar‐derived plasma density commonly exceeds the ambient density in the tail, allowing lunar plasma to locally perturb the environment [ABSTRACT FROM AUTHOR]

Mapping of lunar nighttime surface temperatures has revealed anomalously low nighttime temperatures around recently formed impact craters on the Moon. The thermophysically distinct "cold spots" provide a way of identifying the most recently formed impact craters. Over 2,000 cold spot source craters were measured with diameters ranging from 43 m to 2.3 km. Comparison of the crater size‐frequency distribution with crater chronology models and crater counts of superposed craters on the ejecta of the largest cold spot craters constrains the retention time of the cold spots to no more than ~0.5–1.0 Myr with smaller cold spots possibly retained for only few hundred kyr. This would suggest a relatively rapid impact gardening rate with regolith overturn depths exceeding ~5 cm over this time scale. We observe a longitudinal heterogeneity in the cold spot distribution that reflects the Moon's synchronous rotation with a higher density of cold spots at the apex of motion. The magnitude of the asymmetry indicates the craters formed from a population of objects with low mean encounter velocities ~8.4 km/s. The larger cold spots (D > 800 m) do not follow this trend, and are concentrated on the trailing farside. This could result from a shorter retention time for larger cold spots on the leading hemisphere due to the greater number of smaller, superposed impacts. Alternatively, the abundance of large cold spots on the trailing farside resulted from a swarm of 100‐m‐scale impactors striking the Moon within the last ~0.5 Myr. Plain Language Summary: Impact craters on the Moon modify the surfaces around them, resulting in patches of colder nighttime surface temperatures. These "cold spots" fade over time. Using the cold spots as markers to identify the most recent impacts that have occurred on the Moon, we measured the diameters of all the craters with cold spots. Comparing the population of these craters with the expected impact rate, we estimate that the cold spots fade over a few hundred thousand years. The cold spots are also concentrated on the western hemisphere due to the Moon's synchronous rotation where the western half of the Moon always faces toward the direction of motion of the Moon orbiting the Earth. This suggests that a relatively slow population of objects impacted the Moon in the last few hundred thousand years. The largest cold spots, however, are concentrated on the trailing hemisphere. This could result either from more small impacts on the leading hemisphere destroying larger cold spots and leaving a higher number of large cold spots on the trailing hemisphere, or a swarm of 100‐m‐sized objects colliding on the trailing side during this time period. Key Points: We measure diameters of craters associated with cold spots. Their size‐frequency distribution indicates cold spots survive a few hundred kyrThe distribution of cold spots reflects the Moon's synchronous rotation with cold spots focused on the apex of motionThe largest cold spots with source craters larger than 800 m are concentrated on the trailing side of the moon [ABSTRACT FROM AUTHOR]

We 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. Plain Language Summary: High spectral resolution observations of optical line emissions are used to investigate the morphology and dynamics of the lunar sodium exosphere. These observations were obtained from the National Solar Observatory McMath‐Pierce Solar Telescope, coincident with the Lunar Atmosphere and Dust Environment Explorer mission. The equatorial data presented here are the first comprehensive set of direct sodium emission line profile observations of the lunar exosphere. These observations will help constrain atmospheric and surface process modeling, and help quantify lunar exospheric source and escape mechanisms. Studying the morphology of the lunar exosphere with altitude and local time provides a useful laboratory for testing space weather effects at Earth and theoretical models of other bodies with similar exospheres (e.g., Mercury). Key Points: Comprehensive sets of direct lunar exospheric line width observations are presented as a function of lunar phaseExospheric temperatures are highest near full Moon and decrease toward large phase anglesPhoton stimulated desorption dominates source mechanisms at subsolar points, with contributions by other processes near terminators [ABSTRACT FROM AUTHOR]

In this study we describe the distribution of iron on the Moon as obtained by the Kaguya high energy resolution gamma-ray spectrometer (KGRS). We achieved for the first time the identification of iron based on the fast neutron flux obtained by the KGRS. The iron distribution obtained by KGRS is compared to that of the Lunar Prospector Gamma-Ray Spectrometer (LP GRS), showing that the FeO distributions observed by KGRS and LP GRS, in general, are in good agreement. Furthermore, we compare the iron content data obtained by KGRS and LP GRS to spectral reflectance measurements of the Clementine, Kaguya and Chandrayaan-1 spacecraft as well as those inferred from returned samples. We found differences in FeO concentration and distribution in areas of moderate abundance (6–15 wt%) of the South Pole-Aitken basin, Mare Orientale, and around the crater Tycho crater. It implies that high concentrations of FeO at Mare Ingenii in the South Pole-Aitken basin and Mare Orientale are due to the presence of mare basalts, whereas the enriched FeO content in the central depression of the South Pole-Aitken basin and the Tycho crater indicates the presence of mafic materials such as impact melt breccia. [ABSTRACT FROM AUTHOR]

The Apollo Program launched numerous missions to the Moon, Earth’s nearest and only natural satellite. NASA is now planning new Moon missions as a first step toward human exploration of Mars and other planets. However, the Moon has an extreme environment for humans. In-situ resource utilization (ISRU) construction must be used on the Moon to build habitable structures. Previous studies on polymeric lunar concrete investigated top-down heating for stabilizing the surface. This study investigates bottom-up heating with manufacturing temperatures as low as 200 °C in a vacuum chamber that simulates the lunar environment. A maximum compressive strength of 5.7 MPa is attained; this is suitable for constructing habitable structures. Furthermore, the bottom-up heating approach achieves solidification two times faster than does the top-down heating approach. [ABSTRACT FROM AUTHOR]

Abstract: Lunar basalt 15016 (~3.3 Ga) is among the most vesicular (50% by volume) basalts recovered by the Apollo missions. We investigated the possible occurrence of indigenous lunar nitrogen and noble gases trapped in vesicles within basalt 15016, by crushing several cm‐sized chips. Matrix/mineral gases were also extracted from crush residues by fusion with a CO2 laser. No magmatic/primordial component could be identified; all isotope compositions, including those of vesicles, pointed to a cosmogenic origin. We found that vesicles contained ~0.2%, ~0.02%, ~0.002%, and ~0.02% of the total amount of cosmogenic 21Ne, 38Ar, 83Kr, and 126Xe, respectively, produced over the basalt's 300 Myr of exposure. Diffusion/recoil of cosmogenic isotopes from the basaltic matrix/minerals to intergrain joints and vesicles is discussed. The enhanced proportion of cosmogenic Xe isotopes relative to Kr detected in vesicles could be the result of kinetic fractionation, through which preferential retention of Xe isotopes over Kr within vesicles might have occurred during diffusion from the vesicle volume to the outer space through microleaks. This study suggests that cosmogenic loss, known to be significant for 3He and 21Ne, and to a lesser extent for 36Ar (Signer et al. ), also occurs to a negligible extent for the heaviest noble gases Kr and Xe. [ABSTRACT FROM AUTHOR]

The important aspect in the study of the structure of the interiors of planets is the question of the presence and state of core inside them. While for the Earth this task was solved long ago, the question of whether the core of the Moon is in a liquid or solid state up to the present is debatable up to present. If the core of the Moon is liquid, then the velocity of longitudinal waves in it should be lower than in the surrounding mantle. If the core is solid, then most likely, the velocity of longitudinal waves in it is higher than in the mantle. Numerical calculations of the wave field allow us to identify the criteria for drawing conclusions about the state of the lunar core. In this paper we consider the problem of constructing an analytical solution for wave fields in a layered sphere of arbitrary radius. A stable analytic solution is obtained for the wave fields of longitudinal waves in a three-layer sphere. Calculations of the total wave fields and rays for simplified models of the Earth and the Moon with real parameters are presented. The analytical solution and the ray pattern showed that the low-velocity cores of the Earth and the Moon possess the properties of a collecting lens. This leads to the emergence of a wave field focusing area. As a result, focused waves of considerable amplitude appear on the surface of the Earth and the Moon. In the Earth case, they appear before the first PKP -wave arrival. These are so-called “precursors”, which continue in the subsequent arrivals of waves. At the same time, for the simplified model of the Earth, the maximum amplitude growth is observed in the 147-degree region. For the Moon model, the maximum amplitude growth is around 180°. [ABSTRACT FROM AUTHOR]

We analyze available remote-sensing data of the crater Alphonsus, focusing on the analysis of the crater's dark mantle deposits (DMDs), which includes images from NASA Clementine and Lunar Reconnaissance Orbiter (LRO), Japanese Selene (Kaguya), and Indian Chandrayaan-1 missions. The Alphonsus DMDs are gentle-sloped flat hills with typical heights of several meters, which are presented with pyroclastic materials. Our determination of the absolute ages of the Alphonsus DMDs by the technique of crater size-frequency distributions shows that they are ∼200–400 m.y. old. However, being geologically young, the Alphonsus DMDs are not seen in OMAT maps. The DMDs have noticeably lower content of TiO 2 (2–3%) than the mare regions to the west (>4%). The assessment of total pyroxene shows it has a higher abundance in the DMDs, although LRO Diviner measurements of the Chirstiansen feature suggest, rather, a high abundance of olivine. The DMDs pyroclastic material has no signs of OH/H 2 O compounds. We may suggest that this characteristic of the DMDs either relates to their impact reworking and loss of the OH/H 2 O compounds or to the non-water volatiles as the driving agent of the pyroclastic activity. The compositional assessments of the DMDs may be flawed from contamination with the surrounding material due to horizontal and vertical transportation due to impacts. This effect probably can be observed in LROC NAC images of high resolution. A very dark material outcropping on the slopes of the vent depression is seen due to renovation of the regolith on the steep walls of the depression. Thus, at smaller phase angles, the pyroclastic material is dark and at larger phase angles it appears almost like the surrounding material. This means that the phase dependence of the outcropping dark material is shallow; i.e. the dark surface is smoother than its surroundings. This may suggest venting of gases resulting in fluidization of the granular pyroclastic material of the deposit. [ABSTRACT FROM AUTHOR]

The aim of this work is to check whether the technique of estimation of age of small lunar craters based on spatial density of rock boulders on their rims described in Basilevsky et al. (2013, 2015b) and Li et al. (2017) for the craters < 1 km in diameter is applicable to the larger craters. The work presents the rock counts on the rims of four craters having diameters 1000, 1100, 1240 and 1400 m located in Mare Nectaris. These craters are secondaries of the primary crater Tycho, whose age was found to be 109 ± 4 Ma (Stoffler and Ryder, 2001) so this may be taken as the age of the four craters, too. Using the dependence of the rock spatial densities at the crater rims on the crater age for the case of mare craters (Li et al., 2017) our measured rock densities correspond to ages from ∼100 to 130 Ma. These estimates are reasonably close to the given age of the primary crater Tycho. This, in turn, suggests that this technique of crater age estimation is applicable to craters up to ∼1.5 km in diameter. For the four considered craters we also measured their depth/diameter ratios and the maximum angles of the crater inner slopes. For the considered craters it was found that with increasing crater diameter, the depth/diameter ratios and maximum angles of internal slopes increase, but the values of these parameters for specific craters may deviate significantly from the general trends. The deviations probably result from some dissimilarities in the primary crater geometries, that may be due to crater to crater differences in characteristics of impactors (e.g., in their bulk densities) and/or differences in the mechanical properties of the target. It may be possible to find secondaries of crater Tycho in the South pole area and, if so, they may be studied to check the specifics and rates of the rock boulder degradation in the lunar polar environment. [ABSTRACT FROM AUTHOR]

The Lunar Dust Experiment (LDEX) onboard the Lunar Atmosphere and Dust Environment Explorer mission orbited the Moon from 2013 October to 2014 April and detected impact ejecta generated by the continual bombardment of meteoroids to the lunar surface. While the Moon transited the Geminid meteoroid stream, LDEX observed a large enhancement in the lunar impact ejecta cloud, particularly above the portion of lunar surface normal to the Geminids radiant. Here, we present the LDEX measurements during the Geminids, using the surface density of impact ejecta at the Moon as a proxy for meteoroid activity. We find two peaks during the Geminids, a smaller peak at solar longitude λ☉ = 261. ° 3 ± 0.°12 followed by a larger peak at λ☉ = 262.°2 ± 0.°12, with a surface density ratio of 2.6 between the two. Both peaks coincide with radar observations of shallower mass indices than most of the Geminids, suggesting an enhancement of larger particles during the two peaks. The total duration of the 2013 Geminid meteoroid shower at the Moon measured by LDEX is Δλ☉ = 1.°7 for activity >10 per cent of the peak value, corresponding to a width of 1.9 × 106 km normal to the Geminids velocity vector. The timing of the main observed peak matches ground-based visual observations of meteors with magnitude of -1 to -3 and suggests LDEX is detecting ejecta from primary impactors with radii ~2 mm to 2 cm during this time. [ABSTRACT FROM AUTHOR]