Regolith Layer Thickness Research Articles

Kinetic Particle Simulations of Plasma Charging at Lunar Craters Under Severe Conditions

This paper presents fully kinetic particle simulations of plasma charging at lunar craters with the presence of lunar lander modules using the recently developed Parallel Immersed-Finite-Element Particle-in-Cell (PIFE-PIC) code. The computation model explicitly includes the lunar regolith layer on top of the lunar bedrock, taking into account the regolith layer thickness and permittivity as well as the lunar lander module in the simulation domain, resolving a nontrivial surface terrain or lunar lander configuration. Simulations were carried out to study the lunar surface and lunar lander module charging near craters at the lunar terminator region under mean and severe plasma environments. The lunar module’s position is also investigated to see its effect on the plasma charging relative to the craters. Differential surface charging was clearly resolved by the simulations. For the charging of a lunar lander module made of conducting materials, the results show a near-uniform potential close to that of its surrounding environment and moderate levels of local electric fields. Additionally, the risks associated with charging and discharging increase significantly under a more severe plasma charging environment as shown in the severe plasma environment cases.

Comparison of Dielectric Properties and Structure of Lunar Regolith at Chang'e‐3 and Chang'e‐4 Landing Sites Revealed by Ground‐Penetrating Radar

AbstractOn 3 January 2019, the Chang'e‐4 (CE‐4) touched down on the Von Karman crater located inside the South Pole‐Aitken Basin, providing for the first time the opportunity for in situ measurements of the lunar regolith at the farside of the Moon. The CE‐4 ground penetrating radar reveals that fine‐grained regolith, coarse impact ejecta, and fractured bedrocks lie beneath the exploration path of the Yutu‐2 rover. The variations of regolith permittivity with depth and the radargrams indicate that the CE‐4 site has a fine‐grained regolith layer thickness of 11.1 m, which is about 1.3–3 times higher than the in situ measurement results at the Apollo and Chang'e‐3 (CE‐3) sites except for Apollo 16, possibly due to a faster weathering rate of ejecta deposits compared with coherent basalt substrates. The penetration depth of CE‐4 is about 2.85 times (in terms of round‐way delay) deeper than CE‐3, probably due to the differences in abundances of ilmenite and rocks in the regolith.

Immersed Finite Element Particle-in-Cell Simulations of Plasma Charging at the Lunar Terminator

A fully kinetic particle-in-cell model combined with a nonhomogeneous interface immersed finite element field solver is presented for simulations of the plasma charging at the lunar terminator. This model explicitly includes the lunar regolith layer and the bedrock in the simulation domain, taking into account of regolith layer thickness and permittivity, and is capable of resolving a nontrivial surface terrain or spacecraft configuration. Simulations are presented to study surface charging and lunar outpost charging at the lunar terminator region. The results suggest that substantial differential charging can develop, even under a very moderate plasma charging environment.

Analysis of the possibilities to determine lunar soil characteristics with the help of bistatic radar

An opportunity of determining the lunar soil characteristics via bistatic radar involving high-power ground-based transmitters operating in meter and decameter wave ranges and receivers located aboard Moon satellite is analyzed. Direct and inverse problems concerning stratified soil sounding to a large depth, as well as algorithms for determining the regolith layer thickness, permittivity, radio wave absorption, and regolith and bedrock densities, are examined.

Determination of the Lunar Ground Characteristics Using Bistatic Radar

At present, an investigation of the lunar ground at great depths is of paramount importance. This investigation can be carried out using decameter and meter waves. This article aims to analyze the variations of the reflectioncoefficient at decametric, meter and decimeteric bands. A possibility of determination of lunar ground characteristics by bistatic radar using powerful ground-based transmitters at VHF and UHF bands and a receiver aboard a Moon’s satellite is analysed. Appropriate algorithms are considered for determinationof the regolith layer thickness, dielectric permittivity, loss tangent, and density of the regolith and bedrocks. Expected results of measurements have been presented for a two-layer model of lunar ground, consisting of an upper layer with the loose porous rocks (regolith), and the rocks situated more deeply. Revealed regularities are a basis for determining the distribution of the permittivity in subsurface layer.

Simulation of radar echoes from Mars' surface/subsurface and inversion of surface media parameters

A two-layer model of Mars' regolith/bedrock media with a cratered rough surface/subsurface is presented for radar echo simulation of planetary exploration research. The numerical approach of geometric ray tracing for the scattering of rough surfaces, which is digitized by the triangulated network, is applied to the calculation of the scattering and imaging simulation of radar range echoes. Numerical simulations of a cratered rough surface generated by the Monte Carlo method are used to analyze the functional dependence of radar range echoes at 1–50 MHz center frequencies upon the surface/subsurface feature and the parameters of the layering media, that is, layer depth and dielectric properties. The radar range echoes from two areas of the real Mars surface, which is described by digital elevation model data with a resolution of 1 m × 1 m and a vertical error of less than 1 m, are also simulated and analyzed. Based on these simulations, this study presents a numerical imaging test of radar sounder at center frequencies 1–50 MHz for exploration of different dielectric regolith and bedrock media. The channel 50 MHz with high resolution might be an optimal frequency. Finally, inversion of the dielectric constants of the two-layer media and the regolith layer thickness are developed.

Inversion of lunar regolith layer thickness with CELMS data using BPNN method

Inversion of the lunar regolith layer thickness is one of the scientific objectives of current Moon research. In this paper, the global lunar regolith layer thickness is inversed with the back propagation neural network (BPNN) technique. First, the radiative transfer simulation is employed to study the relationship between the lunar regolith layer thickness d and the observed brightness temperature TB׳s. The simulation results show that the parameters such as the surface roughness σ, slope θs and the (FeO+TiO2) abundance S have strong influence on the observed TB׳s. Therefore, TB׳s, σ, θs and S are selected as the inputs of the BPNN network. Next, the four-layer BPNN network with seven-dimension input and two hidden layers is constructed by taking nonlinearity into account with sigmoid functions. Then, BPNN network is trained with the corresponding parameters collected in Apollo landing sites. To tackle issues introduced by the small number of the training samples, the six-dimension similarity degree is introduced to indicate similarities of the inversion results to the correspondent training samples. Thus, the output lunar regolith layer thickness is defined as the sum of the product of the similarity degree and the thickness at the corresponding landing site. Once training phase finishes, the lunar regolith layer thickness can be inversed speedily with the four-channel TB׳s concluded from the CELMS data, σ and θs estimated from LOLA data and S derived from Clementine UV/vis data. the inversed thickness agrees well with the values estimated by ground-based radar data in low latitude regions. The results indicate that the thickness in the maria varies from about 0.5m to 12m, and the mean is about 6.52m; while the thickness in highlands is a bit thicker than the previous estimation, where the thickness varies widely from 10m to 31.5m, and the mean thickness is about 16.8m. In addition, the relation between the ages, the (FeO+TiO2) abundance and the inversed regolith layer thicknesses in the nine main maria indicates that the regolith layer thickness is directly related to its age if the basalt is of the same kind. Furthermore, the correlation between the inversed thickness and the seven input parameters along the Moon Equator indicates that the surface roughness has the largest impact on the inversed thickness, followed by the CELMS data in 3GHz and the slope.

The modeling analysis of microwave emission from stratified media of non-uniform lunar cratered terrain surface for Chinese Chang-E 1 observation

In China’s first lunar exploration project, Chang-E 1, the multi-channel (3.0, 7.8, 19.35, 37 GHz) microwave radiometers were aboard the satellite, with the purpose of measuring microwave brightness temperature from lunar surface and surveying the global distribution of lunar regolith layer thickness, and global evaluation of 3 He content. To analyze the modeling of microwave radiative transfer from three-layered media of lunar surface, some factors, such as the cratered lunar surface roughness, scattering of regolith particulate medium with temperature profile, are discussed. Based on the statistics of the lunar cratered terrain and using Monte Carlo (MC) method, the cratered lunar surfaces are numerically generated. The triangulated network is utilized to divide the undulated lunar surface into discrete triangle meshes with the size 10 m as a digital surface topography. The reflectivities of each plane mesh are calculated, and the average reflectivity for all MC-realized lunar surfaces is obtained. It is found that under the spatial resolution of 30 km×30 km of Chang-E 1 radiometer observation, the lunar surface can be well modeled as a flat surface. It makes the predominance of the parameters, such as the regolith layer thickness and stratified structures, to be studied. Using the radiative transfer equation of stratified media with dense scatterers, the scattering coefficient of the regolith particulate medium is found negligible, and the emission is mainly governed by the absorptive property of the medium. Brightness temperature of multi-layered media, i.e. lunar soil, regolith layer with temperature profile and underlying rock media, are derived and calculated, and relevant main factors to affect the modeling and emission simulation are analyzed.

Simultaneous Observation of Lunar Radar Sounder and Laser Altimeter of Kaguya for Lunar Regolith Layer Thickness Estimate

Simultaneous observations of Lunar Radar Sounder (LRS) and Laser ALTimeter (LALT) of Kaguya, a Japanese lunar exploration project, were carried out for the purpose of mapping regolith layer thickness of the Moon. Nadir surface echo of a high-frequency (5 MHz) pulse of LRS interferes the shallow (<; 10 m) subsurface echo from the bottom of the regolith layer, which subsequently makes the apparent surface be detected at a range deviated from the actual surface range, while the actual surface range is optically detected by LALT. Regolith layer thickness information is retrieved from this range difference after an inversion process. So far, four major maria on the near side of the Moon (Maria Tranquillitatis, Serenitatis, Imbrium, and Oceanus Procellarum) have been investigated, and the mean regolith layer thicknesses of the four maria were found to be about the same, ranging from 6.3 to 6.9 m. However, spatial distribution of areal regolith thickness appears different in eastern maria from western maria, which implies a difference of the growth history of the regolith layer.

The Modeling Analysis of Microwave Emission From Stratified Media of Nonuniform Lunar Cratered Terrain Surface for Chinese Chang-E 1 Observation

In China's first lunar exploration project, Chang-E 1, the multichannel (3.0, 7.8, 19.35, 37 GHz) microwave radiometers were aboard the satellite, with the purpose of measuring microwave brightness temperature from the lunar surface and surveying the global distribution of lunar regolith layer thickness, and global evaluation of 3He content. To analyze the modeling of microwave radiative transfer from three-layered media of the lunar surface, some factors, such as the cratered lunar surface roughness and scattering of regolith particulate medium with temperature profile, are discussed. The three-layer model makes the predominance of the parameters, such as the regolith layer thickness and stratified structures, to be studied.

Analysis of microwave brightness temperature of lunar surface and inversion of regolith layer thickness: Primary results of Chang-E 1 multi-channel radiometer observation

In China’s first lunar exploration project, Chang-E 1 (CE-1), a multi-channel microwave radiometer was aboard the satellite, with the purpose of measuring microwave brightness temperature from lunar surface and surveying the global distribution of lunar regolith layer thickness. In this paper, the primary 621 tracks of swath data measured by Chang-E 1 microwave radiometer from November 2007 to February 2008 are collected and analyzed. Using nearest neighbor interpolation based on the sun incidence angle in observations, global distributions of microwave brightness temperature from lunar surface at lunar daytime and nighttime are constructed. Using the three-layer model (the top dust-soil, regolith and underlying rock media) for microwave thermal emission of lunar surface, the measurements of brightness temperature and dependence upon latitude, frequency and FeO+TiO2 content, etc. are discussed. On the basis of the ground measurements at Apollo landing sites, the observed brightness temperature at these locations are validated and calibrated by numerical three-layer modeling. Using the empirical dependence of physical temperature upon the latitude verified by the measurements at Apollo landing sites, the global distribution of regolith layer thickness is then inverted from the brightness temperature data of CE-1 at 3 GHz channel. Those inversions at Apollo landing sites are compared with the Apollo in situ measurements. Finally, the statistical property of regolith thickness distribution is analyzed and discussed.

A primary analysis of microwave brightness temperature of lunar surface from Chang-E 1 multi-channel radiometer observation and inversion of regolith layer thickness

In China’s first lunar exploration project, Chang-E 1 (CE-1), a multi-channel microwave radiometer was aboard the satellite, with the purpose of measuring microwave brightness temperature (Tb) from lunar surface and surveying the global distribution of lunar regolith layer thickness. In this paper, the primary 621 tracks of swath data measured by CE-1 microwave radiometer from November 2007 to February 2008 are collected and analyzed. Using the nearest neighbor interpolation to collect the Tb data under the same Sun illumination, global distributions of microwave brightness temperature from lunar surface at lunar daytime and nighttime are constructed. Based on the three-layer media modeling (the top dust-soil, regolith and underlying rock media) for microwave thermal emission of lunar surface, the CE-1 measured Tb and its dependence upon latitude, frequency and FeO + TiO 2 content, etc. are discussed. The CE-1 Tb data at Apollo landing sites are especially chosen for validation and calibration on the basis of available ground measurements. Using the empirical dependence of physical temperature upon the latitude verified by the CE-1 multi-channel Tb data at Apollo landing sites, the global distribution of regolith layer thickness is further inverted from the CE-1 brightness temperature data at 3 GHz channel. Those inversions at Apollo landing sites and the characteristics of regolith layer thickness for lunar maria are well compared with the Apollo in situ measurements and the regolith thickness derived from the Earth-based radar data. Finally, the statistical distribution of regolith thickness is analyzed and discussed.

An inversion approach for lunar regolith layer thickness using optical albedo data and microwave emission simulation

An approach to inversion of the lunar regolith layer thickness by using multi-channel brightness temperature observation in passive microwave remote sensing is developed. To first make simulation of brightness temperature from the lunar layered media, the lunar regolith layer thickness ( d ) is proposed being constructed by available lunar DEM (digital elevation mapping) and on site measurements. The physical temperature distribution ( T ) over the lunar surface is also empirically assumed as a monotonic function of the latitude. Optical albedo of the lunar nearside from the telescopic observation is employed to construct the spatial distribution of the FeO + TiO 2 content ( S ) in the lunar regolith layer. A statistic relationship between the DEM and S of the lunar nearside is further extended to construction of S of the lunar farside. Thus, the dielectric permittivity ( ε ) of global lunar regolith layer can then be determined. Based on all these conditions ( d , T , ε ) , brightness temperature of the lunar regolith layer in passive microwave remote sensing, which is planned for China's Chang-E lunar project, is numerically simulated by a parallel layering model using the strong fluctuation theory of random media. Then, taking these simulations with random noise as observations, an inversion method of the lunar regolith layer thickness is developed by using three- or two-channels brightness temperatures. When the S is low, and the four channels brightness temperatures in China's Chang-E project are well distinguishable, the regolith layer thickness and physical temperature of the underlying lunar rock media can be inverted by the three-channels approach. When the S becomes high that the brightness temperature at high frequency channels such as 19.35, 37 GHz are saturated, the regolith layer thickness is alternatively inverted only by the two-channels approach. Numerical simulation and inversion approach in this paper make an evaluation of the performance for lunar passive microwave remote sensing, and for future data calibration and validation.

Quantitative estimation of helium-3 spatial distribution in the lunar regolith layer

3He (helium-3) in the lunar regolith implanted by the solar wind is one of the most valuable resources because of its potential as a fusion fuel. The abundance of 3He in the lunar regolith is related to solar wind flux, lunar surface maturity and TiO 2 content, etc. A model of solar wind flux, which takes account of variations due to shielding of the nearside when the Moon is in the Earth's magnetotail, is used to present a global distribution of relative solar wind flux over the lunar surface. Using Clementine UV/VIS multispectral data, the global distribution of lunar surface optical maturity (OMAT) and the TiO 2 content in the lunar regolith are calculated. Based on Apollo regolith samples, a linear relation between 3He abundance and normalized solar wind flux, optical maturity, and TiO 2 content is presented. To simulate the brightness temperature of the lunar surface, which is the mission of the Chinese Chang-E project's multichannel radiometers, a global distribution of regolith layer thickness is first empirically constructed from lunar digital elevation mapping (DEM). Then an inversion approach is presented to retrieve the global regolith layer thickness. It finally yields the total amount of 3He per unit area in the lunar regolith layer, which is related to the regolith layer thickness, solar wind flux, optical maturity and TiO 2 content, etc. The global inventory of 3He is estimated as 6.50 × 10 8 kg , where 3.72 × 10 8 kg is for the lunar nearside and 2.78 × 10 8 kg is for the lunar farside.

Comment on “Regolith Layer Thickness Mapping of the Moon by Radar and Optical Data,” by Y. G. Shkuratov and N. V. Bondarenko

In a recent Icarus article, Shkuratov and Bondarenko (2001) propose a model for the depth of the lunar regolith based on 70-cm radar observations and near-infrared spectroscopy. This model treats the regolith as a single homogeneous layer of lossy dust, and ignores scattering by buried rocks. There is also an implicit assumption that the regolith substrate is everywhere of uniform morphology. Radar scattering from this substrate is treated using averaged Fresnel coefficients, with little explanation of how such a model might produce the observed lunar polarization properties. Taken together, these issues weaken the validity of the published regolith depth map.

Reply to the Comment by B. Campbell on the Paper “Regolith Layer Thickness Mapping of the Moon by Radar and Optical Data”

High-resolution spectral and spatial data analyses of crater Theophilus revealed the presence of hydroxyl-bearing exposures of possible magmatic origin on its central peak in association with olivine-, spinel- and crystalline plagioclase-bearing lithologies. Hydroxyl-bearing lithologies at Theophilus central peak are characterized primarily by a prominent absorption feature near 2800 nm, the strength of which varies from ∼2% to 6% in some localized zones within the central peak. The strong and sharp 2800-nm hydroxyl feature having localized occurrences on the central peak of crater Theophilus is inconsistent with a surficial origin. We suggest that the observed hydration at Theophilus central peak could possibly be associated with a sub-crustal magmatic source that was subsequently tapped and exposed by Theophilus impact event.

Regolith Layer Thickness Mapping of the Moon by Radar and Optical Data

Mapping of regolith layer thickness for the lunar nearside is carried out with Arecibo radar data at 70 cm wavelength (T. W. Thompson 1987, Earth Moon Planets 37, 59–70) and distributions of iron and titanium content derived from Earth-based optical data (Yu. Shkuratov et al. 1999, Icarus 137, 222–234). For the mapping a new simple model of radio wave multiple scattering in the regolith layer was used. A comparison of this map with independent estimates of regolith layer thickness for landing sites and other areas of the lunar surface showed a good correlation. It was found that regional variations of thickness are almost the same for maria and highlands, though the average thicknesses are different (5 and 12 m, respectively). A relatively thin regolith layer (4 m) covers a portion of Mare Serenitatis, Mare Tranquillitatis, and Mare Humorum, while the thickest regolith layer occurs in Mare Nectaris (9 m). A thin regolith layer is a characteristic of the cryptomare Schiller-Schickard. Very small regolith thickness corresponds to the crater floors. In this case, the model cannot yield reliable estimates of thickness; it predicts only a characteristic distance between rock inclusions buried in regolith. A thick highland regolith occurs at the southeast portion of the lunar disk and the highland to the north of Mare Imbrium and Sinus Iridium. For the lunar nearside, the regolith thickness generally correlates with the surface age: the greater the age, the thicker the regolith. The results are consistent with a higher rate of regolith growth for times earlier than 3.5 byr ago, when meteorite flux was much higher.