Lunar Atmosphere Research Articles

Activity of the 2013 Geminid meteoroid stream at the Moon.

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.

Evidence for a Localized Source of the Argon in the Lunar Exosphere

AbstractWe perform the first tests of various proposed explanations for observed features of the Moon's argon exosphere, including models of the following: spatially varying surface interactions; a source that reflects the lunar near‐surface potassium distribution; and temporally varying cold trap areas. Measurements from the Lunar Atmosphere and Dust Environment Explorer (LADEE) and the Lunar Atmosphere Composition Experiment (LACE) are used to test whether these models can reproduce the data. The spatially varying surface interactions hypothesized in previous work cannot reproduce the persistent argon enhancement observed over the western maria. They also fail to match the observed local time of the near‐sunrise peak in argon density, which is the same for the highland and mare regions and is well reproduced by simple surface interactions with a ubiquitous desorption energy of 28 kJ mol−1. A localized source can explain the observations, with a trade‐off between an unexpectedly localized source or an unexpectedly brief lifetime of argon atoms in the exosphere. To match the observations, a point‐like source requires source and loss rates of ∼1.9 × 1021 atoms s−1. A more diffuse source, weighted by the near‐surface potassium, requires much higher rates of ∼1.1 × 1022 atoms s−1, corresponding to a mean lifetime of just 1.4 lunar days. We do not address the mechanism for producing a localized source, but demonstrate that this appears to be the only model that can reproduce the observations. Large, seasonally varying cold traps could explain the long‐term fluctuation in the global argon density observed by LADEE, but not that by LACE.

Lunar volcanism produced a transient atmosphere around the ancient Moon

Studies of the lunar atmosphere have shown it to be a stable, low-density surface boundary exosphere for the last 3 billion years. However, substantial volcanic activity on the Moon prior to 3 Ga may have released sufficient volatiles to form a transient, more prominent atmosphere. Here, we calculate the volume of mare basalt emplaced as a function of time, then estimate the corresponding production of volatiles released during the mare basalt-forming eruptions. Results indicate that during peak mare emplacement and volatile release ∼3.5 Ga, the maximum atmospheric pressure at the lunar surface could have reached ∼1 kPa, or ∼1.5 times higher than Mars' current atmospheric surface pressure. This lunar atmosphere may have taken ∼70 million years to fully dissipate. Most of the volatiles released by mare basalts would have been lost to space, but some may have been sequestered in permanently shadowed regions on the lunar surface. If only 0.1% of the mare water vented during these eruptions remains in the polar regions of the Moon, volcanically-derived volatiles could account for all hydrogen deposits – suspected to be water – currently observed in the Moon's permanently shadowed regions. Future missions to such locations may encounter evidence of not only asteroidal, cometary, and solar wind-derived volatiles, but also volatiles vented from the interior of the Moon.

Volatile element loss during planetary magma ocean phases

Moderately volatile elements (MVE) are key tracers of volatile depletion in planetary bodies. Zinc is an especially useful MVE because of its generally elevated abundances in planetary basalts, relative to other MVE, and limited evidence for mass-dependent isotopic fractionation under high-temperature igneous processes. Compared with terrestrial basalts, which have δ66Zn values (per mille deviation of the 66Zn/64Zn ratio from the JMC-Lyon standard) similar to some chondrite meteorites (∼+0.3‰), lunar mare basalts yield a mean δ66Zn value of +1.4 ± 0.5‰ (2 st. dev.). Furthermore, mare basalts have average Zn concentrations ∼50 times lower than in typical terrestrial basaltic rocks. Late-stage lunar magmatic products, including ferroan anorthosite, Mg- and Alkali-suite rocks have even higher δ66Zn values (+3 to +6‰). Differences in Zn abundance and isotopic compositions between lunar and terrestrial rocks have previously been interpreted to reflect evaporative loss of Zn, either during the Earth–Moon forming Giant Impact, or in a lunar magma ocean (LMO) phase. To explore the mechanisms and processes under which volatile element loss may have occurred during a LMO phase, we developed models of Zn isotopic fractionation that are generally applicable to planetary magma oceans. Our objective was to identify conditions that would yield a δ66Zn signature of ∼+1.4‰ within the lunar mantle. For the sake of simplicity, we neglect possible Zn isotopic fractionation during the Giant Impact, and assumed a starting composition equal to the composition of the present-day terrestrial mantle, assuming both the Earth and Moon had zinc ‘consanguinity’ following their formation. We developed two models: the first simulates evaporative fractionation of Zn only prior to LMO mixing and crystallization; the second simulates continued evaporative fractionation of Zn that persists until ∼75% LMO crystallization. The first model yields a relatively homogenous bulk solid LMO δ66Zn value, while the second results in a stratification of δ66Zn values within the LMO sequence. Loss and/or isolation mechanisms for volatiles are critical to these models; hydrodynamic escape was not a dominant process, but loss of a nascent lunar atmosphere or separation of condensates into a proto-lunar crust are possible mechanisms by which volatiles could be separated from the lunar interior. The results do not preclude models that suggest a lunar volatile depletion episode related to the Giant Impact. Conversely, LMO models for volatile loss do not require loss of volatiles prior to lunar formation. Outgassing during planetary magma ocean phases likely played a profound role in setting the volatile inventories of planets, particularly for low mass bodies that experienced the greatest volatile loss. In turn, our results suggest that the initial compositions of planets that accreted from smaller, highly differentiated planetesimals were likely to be severely volatile depleted.

A model of the primordial lunar atmosphere

We create the first quantitative model for the early lunar atmosphere, coupled with a magma ocean crystallization model. Immediately after formation, the moon's surface was subject to a radiative environment that included contributions from the early Sun, a post-impact Earth that radiated like a mid-type M dwarf star, and a cooling global magma ocean. This radiative environment resulted in a largely Earth-side atmosphere on the Moon, ranging from ∼104 to ∼102 pascals, composed of heavy volatiles (Na and SiO). This atmosphere persisted through lid formation and was additionally characterized by supersonic winds that transported significant quantities of moderate volatiles and likely generated magma ocean waves. The existence of this atmosphere may have influenced the distribution of some moderate volatiles and created temperature asymmetries which influenced ocean flow and cooling. Such asymmetries may characterize young, tidally locked rocky bodies with global magma oceans and subject to intense irradiation.

Photolysis of metal oxides as a source of atoms in planetary exospheres

The cross sections of photolysis of LiO, NaO, KO, MgO, and CaO molecules have been calculated by the use of quantum chemistry methods. The maximal values for photolysis cross sections of alkali metal monoxides have the order of 10−17 cm2, and for alkaline earth metal monoxides these values are less on 1–2 orders of the magnitude. The lifetimes of photolysis at 1 astronomical unit are estimated as 5, 3, 60, 70, and 3,000 s for LiO, NaO, KO, MgO, and CaO, respectively. Typical kinetic energies of main peaks of photolysis-generated metal atoms are determined. Impact-produced LiO, NaO, KO, and MgO molecules are destroyed in the lunar and Hermean exospheres almost completely during the first ballistic flight while CaO molecule is more stable against destruction by photolysis. Photolysis-generated metal atoms in planetary exospheres can be detected by performing high-resolution spectral observations of velocity distribution of exospheric metal atoms.

Upper limit of helium-4 in the sunlit lunar exosphere during magnetotail passage under low solar wind condition: Result from CHACE aboard MIP in Chandrayaan-1

The outcome of efforts to detect He-4 in the sunlit lunar exosphere as evinced by the CHACE mass spectrometer aboard the Moon Impact Probe in Chandrayaan-1 is reported. The in situ observations by CHACE were carried out in the lunar dayside, covering a broad range of lunar latitudes, when the Moon was on the verge of exiting the Earth’s magnetotail. A combination of daytime He depletion and decrease during magnetotail passage of the Moon, along with low flux of alpha particles in the solar wind at the time of CHACE observations present a case when the He abundance in the Moon had hit one of its lowest values. CHACE, thus had the opportunity to explore the lunar exosphere in an extreme combination of the factors that control the lunar He abundance. Based on the observations and instrument sensitivity, an upper limit of ∼ 8.0 × 102 cm−3 for the surface density of lunar He-4 in the sunlit hemisphere is proposed. This result is expected to provide realistic constraints to the lunar He exosphere models under similar extreme conditions.

Absence of a detectable lunar nanodust exosphere during a search with LRO's LAMP UV imaging spectrograph

AbstractThe Lyman‐Alpha Mapping Project (LAMP) UV spectrograph on board the Lunar Reconnaissance Orbiter (LRO) performed a campaign to observe the Moon's nanodust exosphere, evidence for which was provided by the Lunar Atmosphere and Dust Environment Explorer (LADEE) Ultraviolet and Visible Spectrometer (UVS) during the 2014 Quadrantid meteoroid stream. These LADEE/UVS observations were consistent with a nanodust exosphere modulated by meteoroid impacts. LRO performed off‐nadir maneuvers around the peak of the 2016 Quadrantids, in order to reproduce, as closely as possible, the active meteoroid environment and observing geometry of LADEE/UVS. We analyzed LAMP spectra to search for sunlight backscattering from nanodust. No brightness enhancement attributable to dust, of any size, was observed. We determine an upper limit for dust column concentration of ~105 cm−2 for grains of radius ~25 nm, and an upper limit for dust column mass of ~10−11 g cm−2, nearly independent of grain size for radii <100 nm.

Surface charging of a crater near lunar terminator

Past lunar missions have shown the presence of dust particles in the lunar exosphere. These particles originate from lunar surface and are due to the charging of lunar surface by the solar wind and solar UV flux. Near the lunar terminator region, the low conductivity of the surface and small scale variations in surface topology could cause the surface to charge to different surface potentials. This paper simulates the variation of surface potential for a crater located in the lunar terminator regions using Spacecraft Plasma Interaction Software (SPIS). SPIS employs particle in cell method to simulate the motion of solar wind particles and photoelectrons. Lunar crater has been found to create mini-wake which affects both electron and ion density and causes small scale potential differences. Simulation results show potential difference of 300 V between sunlit area and shadowed area which creates suitable condition for dust levitation to occur.

The effects of spacecraft charging and outgassing on the LADEE ion measurements

AbstractAbnormal ion signals can be usually seen in the Lunar Atmosphere and Dust Environment Explorer (LADEE) mission, including a suddenly enhanced current observed by the Lunar Dust Experiment (LDEX) near the sunlight‐shadow boundary and an unexpected water ion measured by the neutral mass spectrometer (NMS), with their magnitudes insensitive to the convection electric field of solar wind but dependent on the SW density and the elapsed time of LADEE mission. By analyzing both the LDEX measurements and the NMS measurements, we find that the current enhancement can be caused by a negatively charged spacecraft in the shadow region while the significant water ions should be some artificial ions from spacecraft outgassing. The artificial water ions show a peak near 8:00 LT that may be related to a sunlight‐controlled surface outgassing. In addition, the H2O flux can be enhanced near the end of the mission when the spacecraft has a lower altitude. It is found that the H2O enhancement is actually caused by an exosphere‐contributed return flux, rather than a real water exosphere.

Laboratory experiments to investigate sublimation rates of water ice in nighttime lunar regolith

The existence of water ice on the lunar surface has been a long-standing topic with implications for both lunar science and in-situ resource utilization (ISRU). Cold traps on the lunar surface may have conditions necessary to retain water ice, but no laboratory experiments have been conducted to verify modeling results. We present an experiment testing the ability to thermally control bulk samples of lunar regolith simulant mixed with water ice under vacuum in an effort to constrain sublimation rates. The simulant used was JSC-1A lunar regolith simulant developed by NASA's Johnson Space Center. Samples with varying ratios of water ice and JSC-1A regolith simulant, totally about 1kg, were placed under vacuum and cooled to 100K to simulate conditions in lunar cold traps. The resulting sublimation of water ice over an approximately five-day period was measured by comparing the mass of the samples before and after the experimental run. Our results indicate that water ice in lunar cold traps is stable on timescales comparable to the lunar night, and should continue to be studied as possible resources for future utilization. This experiment also gauges the efficacy of the synthetic lunar atmosphere mission (SLAM) as a low-cost water resupply mission to lunar outposts.

ARTEMIS observations of the solar wind proton scattering function from lunar crustal magnetic anomalies.

Despite their small scales, lunar crustal magnetic fields are routinely associated with observations of reflected and/or backstreaming populations of solar wind protons. Solar wind proton reflection locally reduces the rate of space weathering of the lunar regolith, depresses local sputtering rates of neutrals into the lunar exosphere, and can trigger electromagnetic waves and small-scale collisionless shocks in the near-lunar space plasma environment. Thus, knowledge of both the magnitude and scattering function of solar wind protons from magnetic anomalies is crucial in understanding a wide variety of planetary phenomena at the Moon. We have compiled 5.5 years of ARTEMIS (Acceleration, Reconnection, Turbulence and Electrodynamics of the Moon's Interaction with the Sun) observations of reflected protons at the Moon and used a Liouville tracing method to ascertain each proton's reflection location and scattering angles. We find that solar wind proton reflection is largely correlated with crustal magnetic field strength, with anomalies such as South Pole/Aitken Basin (SPA), Mare Marginis, and Gerasimovich reflecting on average 5-12% of the solar wind flux while the unmagnetized surface reflects between 0.1 and 1% in charged form. We present the scattering function of solar wind protons off of the SPA anomaly, showing that the scattering transitions from isotropic at low solar zenith angles to strongly forward scattering at solar zenith angles near 90°. Such scattering is consistent with simulations that have suggested electrostatic fields as the primary mechanism for solar wind proton reflection from crustal magnetic anomalies.

Evidence for detection of energetic neutral atoms by LADEE

The integrated current of the Lunar Dust Experiment (LDEX) instrument aboard the Lunar Atmosphere and Dust Environment Explorer (LADEE) includes sources of current in addition to small (<0.3μm) dust grains. LDEX current correlates strongly with solar wind density and flux, though solar wind ions should be excluded from the signal. Energetic neutral atoms (ENAs), produced by solar wind that is neutralized and reflected from the moon's surface, might contribute to LDEX current. To investigate this possibility, we developed a geometrical model for ENA detection. We find high correlation between predicted ENA flux and LDEX current. Based on the inverse dependence of ENA detection sensitivity on solar wind speed, we exclude sputtering from the spherical surface inside the instrument as the mechanism for ENA detection. Additionally, we observe a drop in LDEX current that is consistent with a drop in the predicted ENA flux associated with a lunar magnetic anomaly (LMA) centered at -178°E and 6°N selenographic longitude and latitude, respectively. This provides additional confirmation that ENAs are the main contributor to the LDEX current rather than ions, because the ENA flux decreases in LMA regions and reflected hydrogen ion flux increases in LMA regions.

Theoretical time variability of mobile water on the Moon and its geographic pattern

A lunar water exosphere, if it currently exists, would lead to diurnal variations in surface water concentration. Here we present model calculations that reveal the specific geographic pattern of such diurnal variations as well as the amplitude of these variations relative to an assumed supply rate. Surface temperatures are derived from Diviner data and hence include topographic temperature heterogeneity. In the polar regions, diurnal variation can be one order of magnitude larger than at the equator and two hundred times larger than the monthly globally average water production density. Even so, the absolute size of any diurnal changes in surface concentration is likely too small to be measurable. An episodic source, that delivers the same mass over a sol (lunation) as a steady source, leads to comparable surface concentrations, but to a much denser exosphere with a half-life of a few Earth hours. Such a transient exosphere may already have been detected.

Meteoroid impacts and dust particles in near-surface lunar exosphere

It is shown that for consideration of dust particle release from the lunar surface one has to take into account (among other effects) both adhesion and meteoroid impacts. The effect of surface roughness on the adhesion intensity on the Moon is discussed. The rate of meteoroid impacts with the lunar surface per unit area is determined. The strength of the regolith due to the adhesion effect is estimated. The processes occurring when a high-speed meteoroid impacts with the lunar surface are described. In particular, the characteristic parameters of zones of evaporation of the substance, its melting, destruction of particles constituting lunar regolith, their irreversible deformations, and elastic deformation of the regolith substance are found. A possibility of the rise of micrometer-sized dust particles above the lunar surface is shown. It is demonstrated that most of the particles rising over lunar surface due to the meteoroid impact originates from the elastic deformation zone. The number of dust particles raised over the lunar surface as result of meteoroid impacts is calculated. The size-distribution function of particles released from the lunar surface due to meteoroid impacts is determined. It is noted that micrometeoroid impacts can result in rise of dust particles of the size of a few μm up to an altitude of about 30 cm that explains the effect of “horizon glow” observed by Surveyor lunar lander.

THE IMPACT EJECTA ENVIRONMENT OF NEAR EARTH ASTEROIDS

ABSTRACT Impact ejecta production is a ubiquitous process that occurs on all airless bodies throughout the solar system. Unlike the Moon, which retains a large fraction of its ejecta, asteroids primarily shed their ejecta into the interplanetary dust population. These grains carry valuable information about the chemical compositions of their parent bodies that can be measured via in situ dust detection. Here, we use recent Lunar Atmosphere and Dust Environment Explorer/Lunar Dust Experiment measurements of the lunar dust cloud to calculate the dust ejecta distribution for any airless body near 1 au. We expect this dust distribution to be highly asymmetric, due to non-isotropic impacting fluxes. We predict that flybys near these asteroids would collect many times more dust impacts by transiting the apex side of the body compared to its anti-apex side. While these results are valid for bodies at 1 au, they can be used to qualitatively infer the ejecta environment for all solar-orbiting airless bodies.

Exploring strong lunar ionosphere successfully with the service module of Chinese circumlunar return and reentry spacecraft

The existence of lunar ionosphere has been under debate for a long time. In Apollo 14 mission, the electron density detected by the charged particle lunar environment experiment (CPLEE) was 104 el/cm3 at several hundred meters high during lunar day time. In Luna-19/22 mission, the electron density profiles were detect and the peak densities were about 103 el/cm3. In the last decade, European mission SMART-1 and Japanese mission SELENE also performed radio occultation experiment for lunar ionosphere. The results of these missions don’t well-matched. In order to explore the lunar ionosphere, a very low frequency radio astronomical payload has been suggested to be sent to the surface of lunar far-side by the Chinese Chang’E-4 lunar lander mission in 2019. The payload will record the Type II solar burst, which may cover the frequency of electro-magnetic wave as low as several dozen kilo-Herz. The possible lunar ionosphere above the payload with certain electron density may truncate or block the solar burst signal as corresponding plasma frequency. To estimate the possible truncate frequencies for these observations by the new kind of payload, and to determine the lunar ionospheric distribution, an lunar radio occultation experiment with the service module of Chinese circumlunar return and reentry spacecraft has been performing. The circumlunar return and reentry spacecraft is a Chinese precursor mission for the Chinese lunar sample return mission. It was launched on 23 October 2014. After the return and reentry experiments, the service module went back into a Lunar Orbit on 11 January 2015 to image the target landing zone for the Chinese lunar sample return mission which has not yet been disclosed. During this period, the radio occultation experiments have been performed to detect the lunar ionosphere. The service module provides a stable and reliable frequency source, whose short-term stability is n ´ 10–9, for both X-band and S-band signal. The signals transmitted from the spacecraft in S and X band passed through lunar ionosphere, interplanetary plasma, Earth ionosphere and atmosphere, finally received by the ground tracking stations. According to the coherent ratio of the S/X signal, we convert the phase information of S-band signal to the frequency of X-band signal and calculate the difference of these two signal. Then, the extrapolation algorithm was used here to deduct the interference error of the earth ionosphere and the interplanetary plasma. Based on the above work, the electron column concentrations of lunar ionosphere was explored preliminary. The maximums of electron column concentrations are between 0.4 ´ 1016–0.5 ´ 1016 el/m2, are two times of the maximum result from Luna 19/22, are 1–2 orders higher than the SELENE result, but well-matched with the result from CPLEE. These results show that the lunar ionosphere is clearly exist and much stronger than we expected. The result here gives a positive support and some dynamical constrains for the scientific objective of the very low frequency radio astronomical payload onboard the Chang’E-4 lander mission. But it also raises a new question that the characteristics and formation mechanism of a stronger lunar ionosphere is remain unknown. More observations will be performed for further scientific targets.

Space Electromagnetic and Plasma Sensor (SEAPS): A Laboratory Prototype for a Space Payload

Space electromagnetic and plasma sensor is a proposed space payload consisting of an electric field vector sensor—a tri-axial arrangement of monopoles in a mutually orthogonal configuration—and a magnetic field vector sensor, which is a tri-axial arrangement of mutually perpendicular loop antennas. Both operate between 0.5 and 20 MHz in order to acquire the complete 3D polarization matrix. The antennas, which are meant to be electrically short, are matched with a high input impedance amplifier, followed by gain blocks, filters and a six-channel data acquisition system. Some of the proposed goals of the project include study of the low frequency Sun, plasma interactions between the solar wind and Earth’s magnetosphere, lunar atmosphere, and radio emission from other planets within the solar system. This article describes the construction of a laboratory prototype and preliminary calibration results.

Methane in the lunar exosphere: Implications for solar wind carbon escape

AbstractA positive identification of methane in the lunar exosphere has been made in data from the neutral mass spectrometer on the Lunar Atmosphere and Dust Environment Explorer (LADEE) spacecraft. Like argon‐40, methane is adsorbed on the lunar surface during nighttime. However, higher activation energies for methane delay its desorption at sunrise by about an hour local time, creating a postsunrise bulge with peak concentration of approximately 400–450 molecules cm−3 at a reference altitude of 12 km, which is just above the highest topographic feature on the Moon. The rate of escape of carbon as methane derived from the LADEE data is estimated to be in the range 1.5–4.5 × 1021 s−1. A lower bound for solar carbon escape derived separately from Apollo sample analyses is 3.4 × 1021 s−1.

ARTEMIS observations of terrestrial ionospheric molecular ion outflow at the Moon

AbstractThe Acceleration, Reconnection, Turbulence, and Electrodynamics of the Moon's Interaction with the Sun (ARTEMIS) spacecraft observes outflowing molecular ionospheric ions at lunar distances in the terrestrial magnetotail. The heavy ion fluxes are observed during geomagnetically disturbed times and consist of mainly molecular species ( , NO+, and , approximately masses 28–32 amu) on the order of 105–106 cm−2 s−1 at nearly identical velocities as concurrently present protons. By performing backward particle tracing in time‐dependent electromagnetic fields from the magnetohydrodynamic Open Global Geospace Circulation Model of the terrestrial magnetosphere, we show that the ions escape the inner magnetosphere through magnetopause shadowing near noon and are subsequently accelerated to common velocities down the low‐latitude boundary layer to lunar distances. At the Moon, the observed molecular ion outflow can sputter significant fluxes of neutral species into the lunar exosphere while also delivering nitrogen and oxygen to the lunar volatile inventory.