Relative influence upon microwave emissivity of fine-scale stratigraphy,internal scattering,and dielectric properties

Summary The microwave emissivity of relatively low-loss media such as snow, ice, frozen ground, and lunar soil is strongly influenced by fine-scale layering and by internal scattering. Radiometric data, however, are commonly interpreted using a model of emission from a homogeneous, dielectric halfspace whose emissivity derives exclusively from dielectric properties. Conclusions based upon these simple interpretations can be erroneous. Examples are presented showing that the emission from fresh or hardpacked snow over either frozen or moist soil is governed dominantly by the size distribution of ice grains in the snowpack. Similarly, the thickness of seasonally frozen soil and the concentration of rock clasts in lunar soil noticeably affect, respectively, the emissivities of northern latitude soils in winter and of the lunar regolith. Petrophysical data accumulated in support of the geophysical interpretation of microwave data must include measurements of not only dielectric properties, but also of geometric factors such as finescale layering and size distributions of grains, inclusions, and voids.     

The effect of interstitial gaseous pressure on the thermal conductivity of a simulated Apollo 12 lunar soil sample

The thermal conductivity of a simulated Apollo 12 lunar soil sample was measured with a needle probe under vacuum. The result showed that the sample, with bulk densities of 1.70–1.85 g cm^?3 held in a vertical cylinder (2.54 cm in diameter and 6.99 cm long) has a thermal conductivity ranging from 8.8 to 10.9 mW m^?1 K^?1. This is comparable to the lunar regolith's thermal conductivity as determined in situ. Besides the dense packing of the soil particles, an enhanced intergranular thermal contact, due to the self-compression of the sample, is necessary to raise the sample's thermal conductivity from the level of loose soil (< 5 mW m^?1 K^?1) to that of the lunar regolith deeper than 35 cm (～ 10 mW m^?1 K^?1). A model of the lunar regolith, a thin layer of loose soil resting on a compacted self-compressed substratum, is consistent with the lunar regolith's surface structure as deduced from an observation of the lunar surface's brightness temperature. Martian regolith surface structure is similar, except that its surface layer may be missing in places because of aeolian activity. Measurements of thermal conductivity under simulated martian surface conditions showed that the thermal properties of loose and compacted soils agreed with the two peak values of the martian surface's thermal inertia as observed from “Viking” orbiters, suggesting that drifted loose soil and exposed compacted soil are responsible for the bimodal distribution of the martian surface's thermal inertia near zero elevation. For compacted soil exposed to the martian surface to have the same thermal conductivity as that buried under the surface layer, a cohesion of the soil particles must be assumed.     

A limit on the radiation erosion in lunar surface material

Evidence from Apollo 11 and Apollo 12 lunar samples indicates that particle radiation is not important for the production of grains in the lunar soil greater than 22 microns in size even though sufficiently prolonged irradiations by protons, heavy ions and electrons fracture minerals and glasses like those found on the lunar surface.     

Oxygen isotope measurements of phosphate from fish teeth and bones

In situ measurements of lunar surface brightness temperatures made as a part of the Apollo Lunar Surface Experiments Package at the Apollo 15 Hadley Rille landing site are reported. Data derived from 5 thermocouples of the Heat Flow Experiment, which are lying on or just above the surface, are used to examine the thermal properties of the upper 15 cm of the lunar regolith using eclipse and nighttime cool-down temperatures. Application of finite-difference techniques in modeling the lunar soil shows the thermocouple data are best fit by a model consisting of a low-density and low-thermal conductivity surface layer approximately 2 cm thick overlying a region increasing in conductivity and density with depth. Conductivities on the order of 1 × 10^?5 W/cm-°K are postulated for the upper layer, with conductivity increasing to the order of 1 × 10^?4 W/cm-°K at depths exceeding 20 cm. An increase in mean temperature with depth indicates that the ratio of radiative to conductive transfer at 350°K is 2.7 for at least the upper few centimeters of lunar soil; this value is nearly twice that measured for returned lunar fines. The thermal properties model deduced from Apollo 15 surface temperatures is consistent with earth-based microwave observations if electrical properties measured on returned lunar fines are assumed.     

Surface properties of a North Ray Crater soil Apollo 16

Surface properties of lunar fines sample 67481 have been investigated by measuring the adsorptions of nitrogen (at ?196°C) and water (at 20°C). Characteristics of this sample are similar to those of samples from other locations on the lunar surface and include the more typical alteration reaction with adsorbed water. Although their maturities are markedly different, the surface properties of 67481 are very much like those of the more mature 63341 from the adjacent station 13. These results indicate that the surface properties of lunar soils attain an equilibrium state faster than other properties used to indicate maturity.     

Dielectric properties of the first 100 meters of the Moon

Reviewing 92 measurements of lunar sample dielectric constant versus density at frequencies above 100 kHz, gives the relationK′ = (1.93 ± 0.17)^p by regression analysis, where K′ is the dielectric constant of a soil or solid at a density ofpg/cm³. This formula is the geometric mean between the dielectric constant of vacuum (1) and the zero porosity dielectric constant of lunar material. Similarly, the loss tangent (D) can be described byD = [(0.00053 ± 0.00056) + (0.00025 ± 0.00009)C]p whereD is the loss tangent at densitypg/cm³ withC percent of total FeO + TiO₂ (approximately proportional to ilmenite content). Using the density versus depth relations derived from lunar surface core tubes, and from laboratory studies of lunar soil compression gives a model of the dielectric properties as a function of depth in the lunar regolith. The dielectric constant increases smoothly with depth, as a function of the soil compaction only. The loss tangent, however, is more sensitive to the ilmenite content than it is to density. Neither dielectric constant nor loss tangent varies significantly with the temperature observed in a lunar day.     

Preferential sputtering as a method of producing metallic iron,inducing major element fractionation and trace element enrichment

The mechanism responsible for the formation of finely divided metallic iron in lunar samples has been the subject of considerable debate. A review of the data currently available from laboratory simulation studies would appear to favour an origin involving preferential sputtering of oxygen atoms by the solar wind.Using a model presented here we find that it is thermodynamically predictable that preferential sputtering would lead to the formation of free iron. Likewise titanium should be enriched in particle surfaces due to the formation of Ti₂O₃ from TiO₂. Other major elements will be fractionated according to the ratio of their heat of sublimation to the heat of dissociation of their oxide. The changes induced in the surfaces of fine grains will be of considerable importance when these particles are incorporated into complex grains. Thus, we suggest that preferential sputtering may be implicated in the major element fractionations observed for glassy agglutinates. A 1% component of Fe∮ is able to account easily for the enrichment in the total iron observed.If sufficient exposure time is available preferential sputtering may be able to produce metallic iron-rich layers ca. 40Åthick on the surfaces of lunar grains. Most of this metal would derive from helium sputtering rather than hydrogen bombardment. Provided reduction takes place predominantly in fine particles with a mean grain size <10 μm diameter which are subsequently incorporated into glassy agglutinates, preferential sputtering could account for all the free iron in the lunar soil.For elements of atomic number ca. Z = 30 and above, thermodynamic effects become insignificant compared to momentum transfer considerations. Heavy (Pb, Hg) and medium heavy elements (Rb, Sr) could all be enriched by preferential sputtering. Monovalent elements (e.g. K, Rb) in silicates will have a lower binding energy and knock-on collisions with incoming solar wind atoms could cause their increased abundance in fine fractions of lunar soil.     

Characterization of field scale soil variability using remotely and proximally sensed data and response surface method

Soil salinization of the reclaimed tidelands is problematic. Therefore, there is a need to characterize the spatial variability of soil salinity associated with soil moisture and other soil properties across the reclaimed tidelands. One approach is the use of easily-acquired ancillary data as surrogates for the arduous conventional soil sampling. In a reclaimed coastal tideland in the south of Hangzhou Gulf, backscattering coefficient (σ⁰) from remotely sensed ALOS/PALSAR radar imagery (HH polarization mode) and apparent soil electrical conductivity (ECa) from a proximally sensed EM38 were used to indicate the spatial distribution of soil moisture and salinity, respectively. After that, response surface methodology (RSM) was employed to determine an optimal set of 12 soil samples using spatially referenced σ⁰ and ECa data. Spatial distributions of three soil chemical properties [i.e. soil organic matter (SOM), available nitrogen (AN), and available potassium (AK)] were predicted using inverse distance weighted method based on the 12 samples and were then compared with the predictions generated using 42 samples obtained from a conventional grid sampling scheme. It was concluded that combination of radar imagery and EM induction data can delineate the spatial variability of two key soil properties (i.e. moisture and salinity) across the study area. Besides, RSM-based sampling using radar imagery and EM induction data was highly effective in characterizing the spatial variability of SOM, AN and AK, compared with the conventional grid sampling. This new approach may be used to assist site specific management in precision agriculture.     

The Moon

Recent geochemical and geophysical data from the Moon enable a revision of earlier interpretations regarding lunar origin, structure and bulk composition. Earth and Moon show many similarities among their isotopic compositions, but they have evolved in totally dissimilar ways, probably related to the deficiency of water and volatile elements in the Moon as well as the vast differences in size and internal pressure. Some global geochemical differences from the Earth such as volatile depletion based on K/U ratios have been established. However, all current lunar samples come from differentiated regions, making the establishment of a bulk composition more reliant on bulk geophysical properties or isotopic similarities; it remains unclear how the latter arose or relate to whole Moon composition. The lack of fractionation effects among the refractory and super-refractory elements indicates that the proto-lunar material seems unlikely to have been vaporized while the presence of volatile elements may place lower limits on proto-lunar temperatures. The apparent lack of geochemical evidence of an impacting body enables other possible impactors, such as comets, to be considered. Although the origin of the Moon remains currently unknown, it is generally believed that the Moon originated as the result of a giant impact on the Earth.     

Neutron capture effects in lunar gadolinium and the irradiation histories of some lunar rocks

The Gd isotopic composition in 19 lunar rock and soil samples from three Apollo sites is reported. The analytical techniques and the high precision mass spectrometric measurements are discussed. Enrichments in¹⁵⁸GdO/¹⁵⁷GdO due to neutron capture range up to 0.75%. Integrated ‘thermal’ neutron fluxes derived from the isotopic anomalies of Gd are compared with spallation Kr data from aliquot samples to construct a model which gives both average cosmic-ray irradiation depths and effective neutron exposure ages (T_n) for some rocks. In the case of rock 12053, this yields an average sample location of ∼300 g/cm² below the lunar surface and an effective irradiation age of ∼230 my, compared to 99 my obtained by the⁸¹Kr-Kr method. Rock 14310 is the first lunar sample where Kr anomalies due to resonance neutron capture in Br are observed. A⁸¹Kr-Kr exposure age of 262 ± 7 my is calculated for this rock.     

A novel method for simultaneous analysis of particle size and mineralogy for Chang’E-5 lunar soil with minimum sample consumption

Science China Earth Sciences - The successful return of lunar soil samples from the northern Oceanus Procellarum by the Chang’E 5 (CE-5) mission has provided unprecedented ground-truth...     

SmNd constraints on early lunar differentiation and the evolution of KREEP

The Sm-Nd systematics of lunar KREEP basalt 15386 reflects two chronologically distinct events in the development of the incompatible element-rich materials of the moon. The measured Sm-Nd mineral isochron of 15386 indicates an age of 3.85 ± 0.08 AE which is consistent with the reported Rb-Sr and³⁹Ar-⁴⁰Ar ages of many other KREEP-rich samples. This age is interpreted as the time at which 15386 crystallized from a liquid on or near the lunar surface. The frequent occurrence of this age for KREEP-dominated samples, as well as the restricted location of KREEP near major lunar near-side impact basins, suggests that the eruption of these incompatible element-rich liquids was related to deep impact events during the postulated final bombardment phase of the surface of the moon. However, the lower than chrondritic initial¹⁴³Nd/¹⁴⁴Nd of 15386 and the essentially identical Sm-Nd evolution of other KREEP-rich samples require that the light REE enrichment which characterizes KREEP was established considerably before 3.85 AE. Within the limits imposed by model assumptions in the various radiometric systems, it is concluded that the extremely narrow spread of Sm-Nd model ages for these samples around 4.36 AE, and the compatibility of this age with that indicated by the U-Pb and Rb-Sr systems, indicate that the source of later KREEP volcanism was produced in the closing stages of an early global scale lunar differentiation episode.     

Effect of soil stabilization on audible band railway ground vibration

Although railway-generated ground vibrations usually have greater energy levels at lower frequencies, vibrations in the audible range above 20 Hz can nevertheless be relevant for secondary noise problems in buildings. One countermeasure is soil stabilization under the track embankment. While effective at low frequencies, a potential side effect is amplification in some audible bands. Presented here are both experimental and theoretical assessments of the countermeasure in the audible bands. The main innovation is the treatment of an infinite periodic track–ground system, using a transfer matrix approach with a repeating element including the rail, pad, sleeper, and an underlying half-space (ballast and soil). Excitation in this band is attributed to rail and wheel roughness. The model makes successful predictions when the half-space properties are allowed to be frequency-dependent such that the dispersion of the surface wave matches that in the actual layered earth (including ballast and underlying soil layers). The field measurements are also unique in that both before and after evaluation of the countermeasure was possible.     

Early lunar differentiation: 4.42-AE-old plagioclase clasts in Apollo 16 breccia 67435

We report on a⁴⁰Ar-³⁹Ar study of the Apollo 16 breccia 67435 and present ages of five samples representing matrix, lithic clasts and plagioclase clasts. While the matrix age spectrum does not have a well-defined plateau, the two lithic clasts gave plateau ages of 3.96 and 4.04 AE. Since all samples had apparent ages of ～1 AE in the fractions ≤600°C extraction temperature, the breccia might have been assembled in a rather mild process at about that time or even more recently out of material with different metamorphic ages. The two plagioclase samples, of which one was a single 9-mg mineral clast and the other a 15-mg composite of several clasts, also have ages of ～1 AE in the low-temperature release fractions, but are apparently undisturbed by any ～4-AE events since they both have well-defined plateaux at 4.42 AE. The age of these strongly calcic plagioclase clasts, believed to be remnants of the anorthositic lunar crust, establishes a lower age limit to the end of the early lunar differentiation and thus places a strong constraint to the lunar evolution.     

Studies on the thermoluminescence depth profile produced by 600-MeV protons in artificial lunar soil

The thermoluminescence (TL) of various plagioclase feldspars embedded in a thick target of 150 kg of artificial lunar soil was measured after a 600-MeV proton irradiation. No correlation was observed between the parameters of the characteristic feldspar glow peak and the anorthite contents. The relative TL sensitivities of the individual plagioclase variants were measured and found to be practically the same for⁶⁰Co-γ- and 600-MeV proton-irradiated samples.The TL intensity distribution within the target arrangement, converted to a 2π isotropic p-influx, resulted in an approximate TL depth profile of a thermally undisturbed lunar soil bomarded by galactic cosmic protons. The undisturbed TL intensity at a depth of 28 g/cm² (? 17 cm) decreased to 39% at a depth of 106 g/cm² (? 60 cm). For the evaluation of the temperature gradients by TL in lunar samples the experimental data at the sites of Taurus-Littrow and of Hadley-Rille yielded minimum depth intervals for sampling of ～ 20 cm and ～ 40 cm respectively, presuming an error of ± 15% in the TL determination. Certain aspects are seen by using the relation TL intensity/²²Na-activity ratio versus depth (thus representing the total ionization profile) to establish²²Na depth profiles.     

Interaction of Solar-Flare-Accelerated Nuclei with the Solar Photosphere and the Isotopic Composition of the Solar Wind

The nuclear interactions of solar-flare-accelerated protons and ions with the solar atmosphere and the deeper layers of the Sun lead to the formation of several stable and radioactive isotopes. This article examines the GEANT4 depth profiles of ²H, ³H, ³He, ⁶Li, ⁷Li, ¹⁰Be, and ¹⁴C. When accelerated particles pass through a layer of 0.1–2 g cm^–2, ⁶Li, ⁷Li, ¹⁰Be, and ¹⁴C isotopes form in sufficient amounts to explain their anomalous abundances in lunar soil samples. It is assumed that they escape into interplanetary space with coronal mass ejections immediately after the flare.     

Cosmic-ray-induced fission of heavy nuclides: Possible influence on apparent U-fission track ages of extraterrestrial samples

The rates of cosmic-ray-induced fission of U, Th, Bi, Pb, and Au in mineral samples as a function of burial depth in the lunar surface layer are calculated using the available experimental particle flux and cross section data. Theoretical correction factors are given for apparent fission track ages of extraterrestrial samples of different burial depths which were exposed to cosmic rays for various time fractions of their solidification age. Samples having typical lunar heavy element contents can yield apparent fission track ages which are too high by a factor of up to 13 due to cosmic-ray-induced fission. The interference may be neglected, if the ratio of exposure age to solidification age remains 5 × 10⁻³. The calculations show, that the induced fission of Bi, Pb, and Au which are known to have high meteoritic abundances may dominate spontaneous ²³⁸U-fission in long-time exposed meteorites of low U and Th contents.     

Influence of (FeO+TiO2) abundance on the microwave thermal emissions of lunar regolith

One of the essential controls on the microwave thermal emissions (MTE) of the lunar regolith is the abundance of FeO and TiO₂, known as the (FeO+TiO₂) abundance (FTA). In this paper, a radiative transfer simulation is employed first to study the change in the brightness temperature (T_B) with FTA under a range of frequencies and surface temperatures. Then, we analyze the influence of FTA on the MTE of the lunar regolith using microwave sounder (CELMS) data from the Chang’E-2 lunar orbiter, Clementine UV-VIS data, and lunar samples recovered from the Apollo and Surveyor projects. We conclude that: (1) FTA strongly influences the MTE of the lunar regolith, but it is not the decisive control, and (2) FTA decreases slightly with depth. This research plays an essential role in appropriately inverting CELMS data to obtain lunar regolith parameters.     

Sources of inherent infiltration variability in postwildfire soils

An automated disc infiltrometer was developed to improve the measurements of soil hydraulic properties (saturated hydraulic conductivity and sorptivity) of soils affected by wildfire. Guidelines are given for interpreting curves showing cumulative infiltration as a function of time measured by the autodisc. The autodisc was used to measure the variability of these soil hydraulic properties in three different sample sets: (a) a reference soil consisting of a nonrepellent, uniform, fine sand; (b) soils with the same soil textural classification derived from the same bedrock geology but having different initial burn severities; and (c) soils from different bedrock geology but having the same burn severity. The autodisc infiltrometer had greater sampling rates and volume resolution when compared with the visual minidisc infiltrometer from previous studies. There was no statistical difference in the mean values measured using the autodisc and visual minidisc, but the variability of the autodisc measurements was significantly less than the visual minidisc for a given set of samples. The greatest variability of soil hydraulic properties in reference samples with uniform particle size was attributed to different pore geometries (coefficient of variation [COV] = 0.28–0.34). Unburned field samples (same soil type) with heterogeneous particle sizes had greater variability (COV = 0.57–0.78) than the reference samples. However, this basic variability decreased or remained constant in these field samples as burn severity increased. Additional sources of variability (COV = 0.53–1.99) were attributed to multiple layers resulting from ash or sediment deposition. Results indicate that resolving differences in soil hydraulic properties from different sites requires more than the common 10 random samples because of the multiple sources of variability.     

Sm-Nd age of lherzolite 67667: implications for the processes involved in lunar crustal formation

Pristine samples from the lunar highlands potentially offer important information bearing on the nature of early crustal development on all the terrestrial planets. One apparently unique sample of this group of lunar crustal rocks, the feldspathic lherzolite 67667, was studied utilizing the Sm-Nd radiometric system in an attempt to define its age and the implications of that age for the evolution of the lunar highlands. Data for 67667 precisely define an isochron corresponding to an age of 4.18±0.07 AE. The observed lack of disturbance of the Sm-Nd system of this sample may suggest that this time marks its crystallization at shallow depth in the lunar crust. However, the possibility that this age, as well as those of other highland rocks, indicate the time of their impact-induced excavation from regions deep enough in the lunar crust to allow subsolidus isotopic equilibrium to be produced or maintained between their constituent minerals is also considered. Taken together, bulk rock Sm-Nd data for four “high-Mg” rocks, including 67667, indicate that the chemical characteristics of all their source materials were established 4.33±0.08 AE ago and were intimately associated with the parent materials of KREEP. This finding provides more support for the concept of a large-scale differentiation episode early in lunar history. The possible roles of the crystallization of a global magma ocean, endogenous igneous activity, and of planetesimal impact, in producing the observed geochemical and chronological aspects of lunar highland rocks are discussed.