A realistic model of physico-chemical processes during collisions between meteoroids and the Moon considering condensation of refractory elements in the form of minerals and variable adiabatic index during expansion of impact-produced clouds was developed. Quenched chemical composition of impact-produced cloud is estimated. In accordance with this model relative fraction of atoms delivered to the lunar exosphere by impacts of meteoroids is significantly higher than that previously estimated with usage of the model with constant adiabatic index and without considering condensation as a factor affecting on pressure in impact-produced clouds.

We review studies of physical processes associated with the impact of external factors in outer space flows of micrometeoroids and solar radiation on the lunar regolith. Under the influence of these factors, regolith microparticles can detach from the surface and levitate. Near-surface plasma and levitating dust particles form a plasma-dust exosphere of the Moon. Under anthropogenic effects on the lunar environment, charged levitating microparticles can have an extremely negative impact on the engineering systems of lunar landers and on the activity and health of astronauts on the Moon. Based on information gained by automated and manned lunar missions and in laboratory experiments, we discuss modern ideas about physical processes occurring near the Moon's surface. Unsolved problems associated with the plasma-dust exosphere of the Moon are considered, and the principal strategies for their solution are outlined.

The ionization-type cosmic dust detector METEOR-L is being developed for the lunar orbiter Luna-26 and is designed to study the distribution of meteoric bodies in space by mass and velocity, and for long-term monitoring of the dynamic evolution of the dust component in the lunar exosphere. Recent studies of dust clouds around the Moon show a close relationship between the constant and dynamic evolution of the components of the lunar exosphere, the geological history of the formation of the lunar regolith, the processes of formation and accumulation of volatiles in the lunar regolith with the constant impact of such components of the interplanetary medium as interplanetary dust of predominantly cometary origin and meteoroids from the belt asteroids. The cosmic dust detector is capable of registering meteoric particles 0.1-3 mu m in size with a mass of 10(-14)-10(-9) g and speeds from 3 to 35 km s(-1). Tests and calibration at a particle accelerator have confirmed the declared functionality of the detector for detecting cosmic dust particles with parameters characteristic of the lunar exosphere.

We calculated the cross sections of photolysis of OH, LiO, NaO, KO, HCl, LiCl, NaCl, KCl, HF, LiF, NaF, and KF molecules using quantum chemistry methods. The maximal values for photolysis cross sections of alkali metal monoxides are on the order of 10(-18) cm(2). The lifetimes of photolysis for quiet Sun at 1 astronomical unit are estimated as 2.0 x 10(5), 28, 5, 14, 2.1 x 10(5), 225, 42, 52, 2 x 10(6), 35 400, 486, and 30 400 s for OH, LiO, NaO, KO, HCl, LiCl, NaCl, KCl, HF, LiF, NaF, and KF, respectively. We performed a comparison between values of photolysis lifetimes obtained in this work and in previous studies. Based on such a comparison, our estimations of photolysis lifetimes of OH, HCl, and HF have an accuracy of about a factor of 2. We determined typical kinetic energies of main peaks of photolysis-generated metal atoms. Impact-produced LiO, NaO, KO, NaCl, and KCl molecules are destroyed in the lunar and Hermean exospheres almost completely during the first ballistic flight, while other considered molecules are more stable against destruction by photolysis.

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

We use a dynamical model to characterize the monthly and yearly variations of the lunar meteoroid environment for meteoroids originating from short and long-period comets and the main-belt asteroids. Our results show that if we assume the meteoroid mass flux of 43.3tons per day at Earth, inferred from previous works, the mass flux of meteoroids impacting the Moon is 30 times smaller, approximately 1.4tons per day, and shows variations of the order of 10% over a year. The mass flux difference is due to the combined effect of the smaller cross- of the Moon (factor of 13.46) and Earth's larger gravitational focusing (factor of 2-2.5). The lunar surface is vaporized by these impactors at an average impact vaporization flux of 11.6x10(-16)g.cm(-2).s(-1), providing a significant source for the rarefied lunar exosphere. Our model predicts acceptable vaporization rates and reproduces the local time dependence of observations of the dust ejecta cloud, measured by the Lunar Dust Experiment on board NASA's Lunar Atmosphere and Dust Environment (LADEE) satellite. However, the predicted density of the lunar ejecta cloud is four orders of magnitude larger than reported values by LADEE. This discrepancy might be attributed to a much lower yield from meteoroid impacts on fluffy lunar regolith and/or a lower detection efficiency of the LADEE dust detector. We suggest an upper limit of 30cm per million years for the soil gardening rate from small meteoroids. Plain Language Summary The lunar surface is continuously bombarded by small but fast and abundant particles at rates that amount to 1.4tons per day. These particles originate from asteroids and comets and after striking the surface produce a variety of observable phenomena such as a thin atmosphere and a dust cloud engulfing our satellite. Our novel model describes for the first time in detail the directions and velocities of particles impacting the Moon, including their variability in time and space. This approach correctly reproduces the shape of the dust cloud that was measured by the Lunar Atmosphere and Dust Environment Explorer mission. However, the density of the dust cloud predicted by this model is thousands of times higher than what was inferred from the measurement. On the other hand, our model provides realistic numbers for the mass of particles delivered daily to the Moon and for the exosphere density, resulting in a disagreement that is yet to be understood.

The Moon is continually bombarded by interplanetary meteoroids. While many of the meteoroid sources are near the ecliptic plane, a significant population of high-inclination meteoroids exists at 1 au that bombards the lunar polar regions. Building on previous measurements of the response of the lunar impact ejecta cloud to known meteoroid sources, we develop an ejecta model for the entire lunar surface by incorporating the high-inclination sources. We find that the polar regions of the Moon experience similar quantities of impact ejecta production as the equator. Due to the enhanced impactor fluxes near the equator and at high latitudes on the dawn side, lunar regolith is preferentially distributed to the mid to high-latitude regions over long timescales, providing a pathway to mix regolith from different regions. We find impact ejecta yields at the Moon to be significantly lower than the Galilean moons, suggesting meteoroids deliver more energy to the local regolith and can be an important driver in the evolution of volatiles near the lunar surface. Additionally, we find that a polar orbiting spacecraft equipped with a dust analyzer can measure appreciable quantities of lunar ejecta near the poles to constrain the water content in the polar regions. Plain Language Summary The Moon is continually impacted by small particles shed primary from comets, which impact the Moon from a variety of organized directions. These impacts kick up a significant amount of the lunar soil above the lunar surface and sustain a permanently present cloud of ejecta around the Moon. Previous work categorized the ejecta cloud in the Moon's equatorial plane. Here we extend that work to understand the ejecta environment in the high-latitude polar regions of the Moon. We find that there are significant quantities of impact ejecta generated in the polar regions. Over long periods of time, lunar material is preferentially distributed to the mid to high-latitude regions, providing a pathway to mix equatorial and polar regolith. Additionally, we find that a polar orbiting spacecraft equipped with a dust analyzer can measure appreciable quantities of lunar ejecta near the poles to constrain the water content in the polar regions.

Context. The Moon has a tenuous exosphere consisting of atoms that are ejected from the surface by energetic processes, including hypervelocity micrometeoritic impacts, photon-stimulated desorption by UV radiation, and ion sputtering. Aims. We calculate the vapor and neutral Na production rates on the Moon caused by impacts of meteoroids in the radius range of 5-100 mu m. We considered a previously published dynamical model to compute the flux of meteoroids at the heliocentric distance of the Moon. Methods. The orbital evolution of dust particles of different sizes is computed with an N-body numerical code. It includes the effects of Poynting-Robertson drag, solar wind drag, and planetary perturbations. The vapor production rate and the number of neutral atoms released in the exosphere of the Moon are computed with a well-established formulation. Results. The result shows that the neutral Na production rate computed following our model is higher than previous estimates. This difference can be due to the dynamical evolution model that we used to compute the flux and also to the mean velocity, which is 15.3 kms(-1) instead of 12.75 km s(-1) as reported in literature. Conclusions. Until now, the micrometeoritic impacts have been considered a negligible source for the release of neutral sodium atoms into the exosphere compared to other mechanisms, but according to our calculations, the contribution may be 8% of the photo-stimulated desorption at the subsolar point, becoming similar in the dawn and dusk regions and dominant on the night side.

After a brief historical review about the Moon sodium exosphere and lunar impacts, the attention is focused on the lack of enhancements of the sodium emissions by meteor showers different from Leonids. In order to contribute to the solution of this problem we perform an order-of-magnitude calculation of the physical conditions of sodium atoms during meteoroid impacts. This calculation suggests that the lack of sodium emission enhancements during different meteor showers could be caused by the different ionization degree of the sodium atoms which, in turn, depends on the meteoroid impact velocity.