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In the original hypothesis of ice in lunar polar cold traps, it was presumed that meteoritic water acquired by the lunar surface would be concentrated in cold traps by exospheric lateral transport. That supposition is proven to be false by the absence of evidence of exospheric water in data obtained by the neutral mass spectrometer on the Lunar Atmosphere and Dust Environment Explorer (LADEE) spacecraft. The upper limit for exospheric water at the lunar surface, ∼3 molecules cm−3, is deficient by several orders of magnitude in accounting for transport of the present influx of meteoritic water to cold traps. The proffered process for removal of meteoritic water from the lunar regolith is solar wind sputter. This LADEE result does not rule out the possibly of past impulsive exospheric events like cometary impacts but does place a rigid constraint on the modern day water cycling. Plain Language Summary: Whether vast deposits of water ice have accumulated in lunar polar cold traps may hinge on an unproven hypothesis that water acquired from meteor impacts and possibly other sources is moved to polar cold traps by the dynamic transport process of the lunar exosphere (a rarefied, collisionless atmosphere). Movement of exospheric molecules over the lunar surface is a two‐dimensional random walk process in which the steps are random segments of ballistic trajectories that begin with thermal desorption from soil grains and end with adsorption at distances measured in hundreds of kilometers. Obviously, trajectories that end in cold traps must create ice deposits. However, the upper bound for exospheric water derived here from data collected in 2013–2014 by the neutral mass spectrometer on the Lunar Atmosphere and Dust Environment Explorer spacecraft, about three molecules/cc, pales in comparison to the concentration of ∼15,000 molecules/cc needed to sequester the meteoritic water influx. The only pragmatic conclusion is that the hypothesis for water ice accumulation at the poles due to exospheric transport is false. This conclusion forces the question of the fate of water that accretes on the lunar surface. Key Points: The upper limit for water in the lunar exosphere is ∼3 molecules/ccThe lunar exosphere does not transport meteoritic water to polar cold trapsLunar regolith is arid [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]

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]

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 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]

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]