Lunar Ejecta Research Articles

Lunar ejecta origin of near-Earth asteroid Kamo’oalewa is compatible with rare orbital pathways

Near-Earth asteroid, Kamo’oalewa (469219), is one of a small number of known quasi-satellites of Earth; it transitions between quasi-satellite and horseshoe orbital states on centennial timescales, maintaining this dynamics over megayears. The similarity of its reflectance spectrum to lunar silicates and its Earth-like orbit both suggest that it originated from the lunar surface. Here we carry out numerical simulations of the dynamical evolution of particles launched from different locations on the lunar surface with a range of ejection velocities in order to assess the hypothesis that Kamo‘oalewa originated as a debris-fragment from a meteoroidal impact with the lunar surface. As these ejecta escape the Earth-Moon environment, they face a dynamical barrier for entry into Earth’s co-orbital space. However, a small fraction of launch conditions yields outcomes that are compatible with Kamo‘oalewa’s orbit. The most favored conditions are launch velocities slightly above the escape velocity from the trailing lunar hemisphere.

Evidence of shallow basaltic lava layers in Von Kármán crater from Yutu-2 Lunar Penetrating Radar

Yutu-2 – the rover from the Chang’E-4 mission – is the longest operational Lunar rover, and the first rover to land on the far side of the Moon. It is the second planetary rover to be equipped with ground-penetrating radar (GPR), one of the few in-situ geophysical techniques used in planetary exploration. Since landing in 2019, Yutu-2 has travelled more than 1 kilometre in the Von Kármán (VK) crater, and has been able to investigate the dielectric properties of the shallow Lunar ejecta using its on-board high frequency GPR channels. In this paper, we use advanced signal processing and frequency attributes to infer a detailed dielectric structure of the first ≈30 metres of the subsurface, providing valuable information on the ilmenite content of the landing site. Both the dielectric properties and the ilmenite content suggest a shallow sequence of Imbrian basaltic layers overlaying a low-ilmenite ejecta blanket.

Mini-moons from horseshoes: A physical characterization of 2022 NX1 with OSIRIS at the 10.4 m Gran Telescopio Canarias

Context. The near-Earth orbital space is shared by natural objects and space debris that can be temporarily captured in geocentric orbits. Short-term natural satellites are often called mini-moons. Reflectance spectroscopy can determine the true nature of transient satellites because the spectral signatures of spacecraft materials and near-Earth asteroids (NEAs) are different. The recently discovered object 2022 NX1 follows an Earth-like orbit that turns it into a recurrent but ephemeral Earth companion. It has been suggested that 2022 NX1 could have an artificial origin or be lunar ejecta. Aims. Here, we use reflectance spectroscopy and N-body simulations to determine the nature and actual origin of 2022 NX1. Methods. We carried out an observational study of 2022 NX1, using the OSIRIS camera spectrograph at the 10.4 m Gran Telescopio Canarias, to derive its spectral class. N-body simulations were also performed to investigate how it reached NEA space. Results. The reflectance spectrum of 2022 NX1 is neither compatible with an artificial origin nor lunar ejecta; it is also different from the V type of the only other mini-moon with available spectroscopy, 2020 CD3. The visible spectrum of 2022 NX1 is consistent with that of a K-type asteroid, although it could also be classified as an Xk type. Considering typical values of the similar albedo of both K-type and Xk-type asteroids and its absolute magnitude, 2022 NX1 may have a size range of 5 to 15 m. We confirm that 2022 NX1 inhabits the rim of Earth’s co-orbital space, the 1:1 mean-motion resonance, and experiences recurrent co-orbital engagements of the horseshoe-type and mini-moon events. Conclusions. The discovery of 2022 NX1 confirms that mini-moons can be larger than a few meters and also that they belong to a heterogeneous population in terms of surface composition.

Transport and invariant manifolds near L3 in the Earth-Moon Bicircular model

This paper focuses on the role of L3 to organise trajectories for a particle going from Earth to Moon and viceversa, and entering or leaving the Earth-Moon system. As a first model, we have considered the planar Bicircular problem to account for the gravitational effect of the Sun on the particle. The first step has been to compute a family of hyperbolic quasi-periodic orbits near L3. Then, the computation of their stable and unstable manifolds provides connections between Earth and Moon, and also generates trajectories that enter and leave the Earth-Moon system. Finally, by means of numerical simulations based on the JPL ephemeris we show that these connections can guide the journey of lunar ejecta towards the Earth.

The Moon's farside shallow subsurface structure unveiled by Chang'E-4 Lunar Penetrating Radar.

On 3 January 2019, China's Chang'E-4 (CE-4) successfully landed on the eastern floor of Von Kármán crater within the South Pole-Aitken Basin, becoming the first spacecraft in history to land on the Moon's farside. Here, we report the observations made by the Lunar Penetrating Radar (LPR) onboard the Yutu-2 rover during the first two lunar days. We found a signal penetration at the CE-4 landing site that is much greater than that at the CE-3 site. The CE-4 LPR images provide clear information about the structure of the subsurface, which is primarily made of low-loss, highly porous, granular materials with embedded boulders of different sizes; the images also indicate that the top of the mare basal layer should be deeper than 40 m. These results represent the first high-resolution image of a lunar ejecta sequence ever produced and the first direct measurement of its thickness and internal architecture.

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

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

Impact Ejecta Plumes at the Moon

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

Impact Ejecta and Gardening in the Lunar Polar Regions

AbstractThe 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.

Modeling the detection of impact ejecta on the lunar surface

The lunar surface is continuously exposed to the micrometeoroid environment. Hypervelocity impacts of interplanetary dust particles with speeds around 17kms−1 generate secondary ejecta on the lunar surface. A dust detector placed on the moon is capable of characterizing the secondary ejecta population. The purpose of this paper is to study the speed and trajectory information of ejecta by impact simulations and its implications for the location of a dust sensor on the surface. AUTODYN15.0/2D software was used to simulate the velocity and angular distributions of ejecta created by the primary impacts of interplanetary dust particles. We considered projectiles with sizes of 10μm spheres in diameter with speeds of 17kms−1. We used impact angles of 15°, 30°, 45°, 60°, 75°, and 90° with respect to the surface. A significant percentage of the impact ejecta are created in the early-time stage of the impact process. This population can be captured by a sensor placed on the lunar surface (e.g. Lunar Ejecta and Meteorites (LEAM) experiment) or by a sensor mounted directly on a lander (e.g. Lunar Dust eXplorer (LDX)). The secondary ejecta population above the lunar surface is considered to explain the results of the LEAM experiment. A sensor directly placed on the surface like LEAM is not very well suited to measure the high-speed ejecta component – a sensor located at a few meters height (e.g. on top of a lunar lander) would measure higher fluxes.

A permanent, asymmetric dust cloud around the Moon.

Interplanetary dust particles hit the surfaces of airless bodies in the Solar System, generating charged and neutral gas clouds, as well as secondary ejecta dust particles. Gravitationally bound ejecta clouds that form dust exospheres were recognized by in situ dust instruments around the icy moons of Jupiter and Saturn, but have hitherto not been observed near bodies with refractory regolith surfaces. High-altitude Apollo 15 and 17 observations of a 'horizon glow' indicated a putative population of high-density small dust particles near the lunar terminators, although later orbital observations yielded upper limits on the abundance of such particles that were a factor of about 10(4) lower than that necessary to produce the Apollo results. Here we report observations of a permanent, asymmetric dust cloud around the Moon, caused by impacts of high-speed cometary dust particles on eccentric orbits, as opposed to particles of asteroidal origin following near-circular paths striking the Moon at lower speeds. The density of the lunar ejecta cloud increases during the annual meteor showers, especially the Geminids, because the lunar surface is exposed to the same stream of interplanetary dust particles. We expect all airless planetary objects to be immersed in similar tenuous clouds of dust.

Maskelynite in asteroidal, lunar and planetary basaltic meteorites: An indicator of shock pressure during impact ejection from their parent bodies

Maskelynite is a diaplectic glass that forms from plagioclase at shock pressures of ∼20–30GPa, depending on the Ca concentration. The proportion of maskelynite-rich samples in a basaltic meteorite group correlates with the parent-body escape velocity and serves as a shock indicator of launching conditions. For eucrites (basalts widely presumed to be from Vesta; vesc=0.36kms−1), ∼5% of the samples are maskelynite rich. For the Moon (vesc=2.38kms−1), ∼30% of basaltic meteorites are maskelynite rich. For Mars (vesc=5.03kms−1), ∼93% of basaltic meteorites are maskelynite rich. In contrast, literature data show that maskelynite is rare (∼1%) among mare basalts and basaltic fragments in Apollo 11, 12, 15 and 17 soils (which were never ejected from the Moon). Angrites are unbrecciated basaltic meteorites that are maskelynite free; they were ejected at low-to-moderate shock pressures from an asteroid smaller than Vesta.Because most impacts that eject materials from a large (⩾100km) parent body are barely energetic enough to do that, a collision that has little more than the threshold energy required to eject a sample from Vesta will not be able to eject identical samples from the Moon or Mars. There must have been relatively few impacts, if any, that launched eucrites off their parent body that also imparted shock pressures of ∼20–30GPa in the ejected rocks. More-energetic impacts were required to launch basalts off the Moon and Mars. On average, Vesta ejecta were subjected to lower shock pressures than lunar ejecta, and lunar ejecta were subjected to lower shock pressures than martian ejecta.H and LL ordinary chondrites have low percentages of shock-stage S5 maskelynite-bearing samples (∼1% and ∼4%, respectively), probably reflecting shock processes experienced by these rocks on their parent asteroids. In contrast, L chondrites have a relatively high proportion of samples containing maskelynite (∼11%), most likely a result of catastrophic parent-body disruption 470Ma ago.

Constraints on the recent rate of lunar ejecta breakdown and implications for crater ages

We present a new empirical constraint on the rate of breakdown of large ejecta blocks on the Moon based on observations from the Lunar Reconnaissance Orbiter (LRO) Diviner thermal radiometer. We find that the rockiness of fresh crater ejecta can be quantified using the Diviner-derived rock abundance data set, and we present a strong inverse correlation between the 95 th percentile value of the ejecta rock abundance (RA 95/5 ) and crater age. For nine craters with published model ages derived from crater counts on their continuous ejecta, RA 95/5 decreases with crater age, as (age [m.y.]) −0.46 . This result implies shorter rock survival times than predicted based on downward extrapolation of 100 m crater size-frequency distributions, and represents a new empirical constraint on the rate of comminution of large rocks not previously analyzed experimentally or through direct observation. In addition, our result provides a new method for dating young lunar craters.

A new look at Apollo 17 LEAM data: Nighttime dust activity in 1976

One of the unresolved enigmas from the Apollo era is the existence and characteristics of highly electrically charged dust floating above the lunar surface. Potential evidence for this hypothesized phenomenon came from the Lunar Ejecta and Meteorites (LEAM) experiment on Apollo 17. The LEAM instrument consisted of three sets of multi-coincidence dust sensors facing different directions. Recently, new arguments were raised (O'Brien, 2011) that the signals recorded by LEAM may be caused by interferences from heater current switching, which occurred most frequently near sunrise and sunset. In order to shed light on this controversy a new look into the LEAM data was initiated within the Colorado Center for Lunar and Dust and Atmospheric Studies (CCLDAS) team of NASA's Lunar Science Institute (NLSI). The purpose of this analysis is to verify the earlier analysis by Berg et al. (1975), and to find evidence for impacts of interplanetary meteoroids in the LEAM data available to us. A second goal is to find in the LEAM house keeping data evidence for excessive power switching and correlated signals in the LEAM science data. The original analysis by Berg et al. (1975) covered LEAM data during 22 lunations (~22 months) in 1973 and 1974. This data set is no longer available. For the present study, we had access to LEAM data for only about 5 lunations (140 days) in 1976. We analyzed the housekeeping data and observed excessive heating from about 24h after sunrise until about 24h before sunset. We defined sunrise and sunset when the LEAM temperature measurement reached −20°C above which significant solar heating was apparent. For about 9 days around lunar noon the temperatures were so high that LEAM was switched off. During the times of excessive heating LEAM became very noisy. We limit our current analysis to about 24h before sunset to about 24h after sunrise when the LEAM temperatures were moderate <60°C. This carefully analyzed data set of 74.6 days constitutes about 75% of the periods when LEAM was switched on in 1976. We did not find a systematic correlation between the infrequent heater switches and the occurrence of signals. During the lunar night the temperature was quite stable at approx. −25°C. One TOF dust impact event and 19 potential dust events were recorded by all three sensors during the periods when the instrument was at moderate temperatures. This corresponds to an average event rate of 0.25/day. While nine events are compatible with a random occurrence the other 10 events occurred in three statistically significant bursts within about 1h or less after another. Two bursts occurred within the middle of lunar night and one burst of three events was recorded by the West sensor just an hour before sunrise. The background rates are compatible with impact rates recorded by the dust instruments onboard the Pioneer 8 and 9 spacecraft in interplanetary space. Based on our definition of sunrise and sunset, the 1976 LEAM data do not indicate strongly enhanced dust activity at the terminator.

The lunar dust environment

Each year the Moon is bombarded by about 10 6 kg of interplanetary micrometeoroids of cometary and asteroidal origin. Most of these projectiles range from 10 nm to about 1 mm in size and impact the Moon at 10–72 km/s speed. They excavate lunar soil about 1000 times their own mass. These impacts leave a crater record on the surface from which the micrometeoroid size distribution has been deciphered. Much of the excavated mass returns to the lunar surface and blankets the lunar crust with a highly pulverized and “impact gardened” regolith of about 10 m thickness. Micron and sub-micron sized secondary particles that are ejected at speeds up to the escape speed of 2300 m/s form a perpetual dust cloud around the Moon and, upon re-impact, leave a record in the microcrater distribution. Such tenuous clouds have been observed by the Galileo spacecraft around all lunar-sized Galilean satellites at Jupiter. The highly sensitive Lunar Dust Experiment (LDEX) onboard the LADEE mission will shed new light on the lunar dust environment. LADEE is expected to be launched in early 2013. Another dust related phenomenon is the possible electrostatic mobilization of lunar dust. Images taken by the television cameras on Surveyors 5, 6, and 7 showed a distinct glow just above the lunar horizon referred to as horizon glow (HG). This light was interpreted to be forward-scattered sunlight from a cloud of dust particles above the surface near the terminator. A photometer onboard the Lunokhod-2 rover also reported excess brightness, most likely due to HG. From the lunar orbit during sunrise the Apollo astronauts reported bright streamers high above the lunar surface, which were interpreted as dust phenomena. The Lunar Ejecta and Meteorites (LEAM) Experiment was deployed on the lunar surface by the Apollo 17 astronauts in order to characterize the lunar dust environment. Instead of the expected low impact rate from interplanetary and interstellar dust, LEAM registered hundreds of signals associated with the passage of the terminator, which swamped any signature of primary impactors of interplanetary origin. It was suggested that the LEAM events are consistent with the sunrise/sunset-triggered levitation and transport of charged lunar dust particles. Currently no theoretical model explains the formation of a dust cloud above the lunar surface but recent laboratory experiments indicate that the interaction of dust on the lunar surface with solar UV and plasma is more complex than previously thought.

Lunar Dust Levitation

Observations of a lunar horizon glow by several Surveyor spacecraft on the lunar surface in the 1960s and detections of dust particle impacts by the Apollo 17 Lunar Ejecta and Meteoroid Experiment have been explained as the result of micron-sized charged particles lifting off the surface. The surface of the Moon is exposed to the solar wind and solar UV radiation causing photoemission, so it develops a surface charge and an electric field near the surface. Dust particles injected into this plasma from the lunar regolith, whether from human and mechanical activity or from meteoroid impacts or electrostatic forces, may be stably levitated above the surface and may undergo preferential deposition onto areas of the lunar surface or equipment with different electrical properties. This can lead to a net transport as well as contamination of sensitive equipment. This paper reports on new experimental measurements and numerical simu- lations of the plasma environment above the lunar surface and the related behavior of charged dust.

Asteroids on Earth-like orbits and their origin

The orbit of 1991 VG and a set of other asteroids whose orbits are very similar to that of the Earth have been examined. Its origin has been speculated to be a returning spacecraft, lunar ejecta or a low-inclination Amor- or Apollo-class object. The latter is arguably the more likely source, which has been investigated here. The impact probability for these objects has been calculated, and while it is larger than that of a typical near-Earth asteroid (NEA), it is still less than 1:200 000 over the next 5000 yr. In addition, the probability of an NEA ever ending up on an Earth-like orbit has been obtained from numerical simulations and turned out to be about 1:20 000, making this a rare class of objects. The typical time spent in this state is about 10 000 yr, much less than the typical NEA lifetime of 10 Myr.

Measurement of Ejecta from Normal Incident Hypervelocity Impact on Lunar Regolith Simulant

The National Aeronautics and Space Administration (NASA) continues to make progress toward long-term lunar habitation. Critical to the design of a lunar habitat is an understanding of the lunar surface environment. A subject for further definition is the lunar impact ejecta environment. The document NASA SP-8013 was developed for the Apollo program and is the latest definition of the ejecta environment. There is concern that NASA SP-8013 may over-estimate the lunar ejecta environment. NASA’s Meteoroid Environment Office (MEO) has initiated several tasks to improve the accuracy of our understanding of the lunar surface ejecta environment.

An upgraded high-velocity dust particle accelerator at Concordia College in Moorhead, Minnesota

In 1975, Concordia College in Moorhead, Minnesota acquired a 2 MeV dust particle accelerator from NASA/GSFC that was used to test the lunar ejecta and micrometeorite (LEAM) experiment flown on Apollo 17. This high-speed dust particle accelerator is still fully functional and is currently being upgraded. Improvements to the electronic detection system have been undertaken including a computer-based, data acquisition system and new particle detection sensor electronics. These sensors have additional amplifiers that extend the range of charge detection to 1×10 −12 C allowing for the detection of larger particles. Improvements to the vacuum system have also been made. The accelerator beam line is now pumped with an oil-free, turbomolecular pump reducing possible problems with hydrocarbon contamination. In this work, we describe the facility, and outline some of the recent improvements to the dust particle accelerator and discuss its capabilities and limitations. We also summarize some of the recent experiments conducted using the facility.

An Asteroid in a Earth-Like Orbit

The dynamical evolution of an asteroid with orbital elements strikingly similar to the Earth is analysed. The object, 1991 VG, was discovered by the Spacewatch telescope during a particular encounter with the planet. 1991 VG experienced a temporary satellite capture by the Earth, a phenomena that is recurrent in its dynamical history. The possible origin of this puzzling object is discussed, including the suggestion that 1991 VG could be a piece of lunar ejecta after a great impact.

Orbital Evolution of the Lunar Ejecta

AbstractWe have estimated the flux of lunar ejecta near the Earth, e.g., the cumulative flux of ejecta larger than 10-13g is 10-10particles cm-2s-1. In addition, our numerical simulations for the orbital evolution of lunar ejecta lead to the orbital characterization of these ejecta. Since our calculations suggest that the incident direction of lunar ejecta onto the Earth is nearly isotropic, it is expected that the dynamical properties of lunar ejecta make it possible to distinguish them from other IDPs.