Meter Of Regolith Research Articles

Rover-Mounted Radar Observation of Discrete Layers Within the Top 4 Meters of Regolith at the Chang’E-3 Landing Site, the Moon

Rover-Mounted Radar Observation of Discrete Layers Within the Top 4 Meters of Regolith at the Chang’E-3 Landing Site, the Moon

A possible perchlorate-enabled mechanism for forming thick near surface excess ice layers; in the Amazonian regolith of Mars

We use the freezing point depressing magnesium and calcium perchlorates in Martian regolith to redistribute ground ice by residual liquid water migration following the initial emplacement of ground ice by vapour deposition. This residual liquid water is moved by forces generated by periodic surface temperatures that decay with depth in conjunction with the geothermal vertical temperature gradient. We examine the period means of the bulk water speeds with depth and the mean divergence of the bulk water speeds, which relates to the rate of change in ice content in the regolith. Silt and clay rich regoliths behave differently. In silty regolith, for the short 1.88 a period and for longer 50 ka (precession) and 120 ka (obliquity) temperature cycles, there is a mean movement of liquid perchlorate aqueous solution that results in formation of near surface excess ice layers. The excess ice formed by the seasonal 1.88a period is confined at high latitudes to the upper meter of regolith. For the longer periods, there is a well-defined surface temperature region where near surface thick (≥40 m) excess ice layers form; (from 192 K to 210 K). For mean surface temperatures <192 K no near surface thick excess ice layers formed, but deeper layers are predicted and followed. The formation of excess ice layers in silt near the surface is controlled by the relationship between the temperature cycles, geothermal gradient and the eutectic temperature of the perchlorate (eg. ~198 K for Ca and ~ 205 K for Mg perchlorate). At a given depth if the periodic temperature is below the eutectic then there is nearly no liquid water left and what there is has much higher viscosity. The sudden change in the liquid water amount and viscosity with temperature generates net average water speeds in silts that are two orders larger than in clays.

Asteroid 16 Psyche: Shape, Features, and Global Map

Abstract We develop a shape model of asteroid 16 Psyche using observations acquired in a wide range of wavelengths: Arecibo S-band delay-Doppler imaging, Atacama Large Millimeter Array (ALMA) plane-of-sky imaging, adaptive optics (AO) images from Keck and the Very Large Telescope (VLT), and a recent stellar occultation. Our shape model has dimensions 278 (−4/+8 km) × 238(−4/+6 km) × 171 km (−1/+5 km), an effective spherical diameter D eff = 222-1/+4 km, and a spin axis (ecliptic lon, lat) of (36°, −8°) ± 2°. We survey all the features previously reported to exist, tentatively identify several new features, and produce a global map of Psyche. Using 30 calibrated radar echoes, we find Psyche’s overall radar albedo to be 0.34 ± 0.08 suggesting that the upper meter of regolith has a significant metal (i.e., Fe–Ni) content. We find four regions of enhanced or complex radar albedo, one of which correlates well with a previously identified feature on Psyche, and all of which appear to correlate with patches of relatively high optical albedo. Based on these findings, we cannot rule out a model of Psyche as a remnant core, but our preferred interpretation is that Psyche is a differentiated world with a regolith composition analogous to enstatite or CH/CB chondrites and peppered with localized regions of high metal concentrations. The most credible formation mechanism for these regions is ferrovolcanism as proposed by Johnson et al. (2020).

The Lunar Polar Hydrogen Mapper CubeSat Mission

The Lunar Polar Hydrogen Mapper (LunaH-Map) mission will map the distribution of hydrogen around the lunar South Pole using a miniature neutron spectrometer. The mission builds upon a decade of lunar science, which has revealed both regional and more localized enrichments of water ice near the lunar poles. Localized enrichments are primarily within permanently shadowed regions (PSRs) and craters throughout the South Pole. The spatial extent of these regions is often below the resolution of previous neutron instruments that have flown on lunar missions. The neutron leakage spectrum from planetary surfaces is primarily sensitive to hydrogen abundance in the top meter of regolith, however, for neutron spectrometers with omnidirectional sensitivity, the spatial resolution is limited by the spacecraft orbital altitude above the surface. A low altitude measurement from a distance on the same scale of the PSRs could spatially isolate and constrain the hydrogen enrichments both within and around within those regions. A small spacecraft mission is ideally suited to acquire the low-altitude measurements required to localize hydrogen enrichments using neutron spectroscopy at the lunar South Pole. LunaH-Map will use a solid iodine ion propulsion system, X-Band radio communications through the NASA Deep Space Network, star tracker, Command & Data Handling System, and EPS systems from Blue Canyon Technologies, solar arrays from MMA Designs, LLC, mission design and navigation by KinetX. Spacecraft systems design, integration, qualification, test, and mission operations are performed by Arizona State University, AZ Space Technologies and Qwaltec.

Incipient chemical weathering at bedrock fracture interfaces in a tropical critical zone system, Puerto Rico

Abstract The processes that control chemical weathering of bedrock in the deep critical zone at a mm-scale are still poorly understood, but may produce 100 s of meters of regolith and substantial fluxes of silicate weathering products and thus may be important for modeling long-term, global CO2. Weathering controls are also difficult to ascertain, as laboratory determined dissolution rates tend to be 2–5 orders of magnitude faster than field determined dissolution rates. This study aims to establish (i) the incipient processes that control the chemical weathering of the Bisley bedrock and (ii) why weathering rates calculated for the watershed may differ from laboratory rates (iii) why rates may differ across different scales of measurement. We analyzed mineralogy, elemental chemistry, and porosity in thin sections of rock obtained from drilled boreholes using Scanning Electron Microscopy (SEM) with energy dispersive spectrometry, electron probe microanalysis, and synchrotron-based Micro X-ray Fluorescence (µXRF) and X-ray Absorption Near Edge Structure (XANES). Weathering ages were determined from U-series isotope analysis. Mineral specific dissolution rates were calculated from solid-state mineralogical gradients and weathering ages. Mineralogical and elemental transects across thin sections and SEM images indicate that trace pyrite is the first mineral to dissolve. Micro-XRF mapping at 2 µm resolution revealed sulfate in pore space adjacent to dissolving pyrite, indicating that the incipient reaction is oxidative. The oxidative dissolution of pyrite produces a low pH microenvironment that aids the dissolution of pyroxene and chlorite. The rate-limiting step of weathering advance, and therefore the creation of the critical zone in the Bisley watershed, is pyrite oxidation, despite the low abundance (∼0.5 vol%) of pyrite in the parent rock. The naturally determined dissolution rates presented here either approach, converge with, or in some cases exceed, rates from the literature that have been experimentally determined. The U-series weathering age data on the mm-scale integrates the weathering advance rate over the ∼4.2 ± 0.3 kyrs that the weathering rind took to form. The weathering advance rate calculated at a watershed scale (from stream chemistry data) represents a contemporary weathering advance rate, which compares well with that calculated for the weathering rind, suggesting that the Bisley watershed has been weathering at steady-state for the last ∼4 kyrs.

Lunar crater ejecta: Physical properties revealed by radar and thermal infrared observations

We investigate the physical properties, and changes through time, of lunar impact ejecta using radar and thermal infrared data. We use data from two instruments on the Lunar Reconnaissance Orbiter (LRO) – the Diviner thermal radiometer and the Miniature Radio Frequency (Mini-RF) radar instrument – together with Earth-based radar observations. We use this multiwavelength intercomparison to constrain block sizes and to distinguish surface from buried rocks in proximal ejecta deposits. We find that radar-detectable rocks buried within the upper meter of regolith can remain undisturbed by surface processes such as micrometeorite bombardment for >3Gyr. We also investigate the thermophysical properties of radar-dark haloes, comprised of fine-grained, rock-poor ejecta distal to the blocky proximal ejecta. Using Diviner data, we confirm that the halo material is depleted in surface rocks, but show that it is otherwise thermophysically indistinct from background regolith. We also find that radar-dark haloes, like the blocky ejecta, remain visible in radar observations for craters with ages >3Ga, indicating that regolith overturn processes cannot replenish their block populations on that timescale.

Moonshine: Diurnally varying hydration through natural distillation on the Moon, detected by the Lunar Exploration Neutron Detector (LEND)

The Lunar Exploration Neutron Detector (LEND), on the polar-orbiting Lunar Reconnaissance Orbiter (LRO) spacecraft, has detected suppression in the Moon’s naturally-occurring epithermal neutron leakage flux that is consistent with the presence of diurnally varying quantities of hydrogen in the regolith near the equator. Peak hydrogen concentration (neutron flux suppression) is on the dayside of the dawn terminator and diminishes through the dawn-to-noon sector. The minimum concentration of hydrogen is in the late afternoon and dusk sector. The chemical form of hydrogen is not determinable from these measurements, but other remote sensing methods and anticipated elemental availability suggest water molecules or hydroxyl ions. Signal-to-noise ratio at maximum contrast is 5.6σ in each of two detector systems. Volatiles are deduced to collect in or on the cold nightside surface and distill out of the regolith after dawn as rotation exposes the surface to sunlight. Liberated volatiles migrate away from the warm subsolar region toward the nearby cold nightside surface beyond the terminator, resulting in maximum concentration at the dawn terminator. The peak concentration within the upper ∼1m of regolith is estimated to be 0.0125±0.0022 weight-percent water-equivalent hydrogen (wt% WEH) at dawn, yielding an accumulation of 190±30ml recoverable water per square meter of regolith at each dawn. Volatile transport over the lunar surface in opposition to the Moon’s rotation exposes molecules to solar ultraviolet radiation. The short lifetime against photolysis and permanent loss of hydrogen from the Moon requires a resupply rate that greatly exceeds anticipated delivery of hydrogen by solar wind implantation or by meteoroid impacts, suggesting that the surface inventory must be continually resupplied by release from a deep volatile inventory in the Moon. The natural distillation of water from the regolith by sunlight and its capture on the cold night surface may provide energy-efficient access to volatiles for in situ resource utilization (ISRU) by direct capture before volatiles can enter the surface, eliminating the need to actively mine regolith for volatile resource recovery.

Initiation and growth of martian ice lenses

Water ice in the upper meters of the martian regolith is a major volatile reservoir. Although the geographic extent, burial depth, and thermal stability of this shallow ice are well understood, its origin, history, and stratigraphy are not. Over the past decade, a growing body of observational evidence has indicated that shallow ground ice exceeds the pore volume of its host soil over large regions of both martian hemispheres. This is confounding, given that (1) the physical theory that accurately predicts the location of ground ice also assumes that ice should be pore-filling in the upper meter of regolith, and (2) the Phoenix spacecraft uncovered far more pore-filling ice than excess ice at its landing site in the northern hemisphere. The development of ice lenses by low-temperature in situ segregation – analogous to the processes that generate frost heave on Earth – has been hypothesized to explain shallow excess ice on Mars. We have developed a numerical model of ice lens initiation and growth in the martian environment, and used it to test this hypothesis for the first time. We carried out a large suite of numerical simulations in order to place quantitative constraints on the timing and location of ice lens initiation, and on the magnitude of ice lens growth in a variety of host soils. We find that ice lens initiation is a ubiquitous process in the martian high latitudes, but the ultimate magnitude of lens growth, or frost heave, is sensitive to the properties of the host soil. Depending on the specific properties of martian soils, in situ segregation may be a very slow process sufficient to explain the excess ice observed in the Dodo–Goldilocks trench at the Phoenix landing site, but without regionally significant effects. Alternatively, if clay-sized particles or perchlorate salts are present, in situ segregation may be a vigorous process that has significantly affected the stratigraphy of ground ice in the upper meter of regolith throughout the high latitudes.

An upper limit for ice in Shackleton crater as revealed by LRO Mini‐RF orbital radar

Although diverse measurements have indicated H+, OH−, or H2O species in the lunar polar regions, pinpointing its location, form, and abundance in specific reservoirs has proven elusive. Here we report on the first orbital radar measurements of Shackleton crater near the lunar south pole. Mini‐RF observations indicate a patchy, heterogeneous enhancement in CPR (circular polarization ratio) on the crater walls whose strength decreases with depth toward the crater floor, a result that is most consistent with a roughness effect due to less mature regolith present on the crater wall slopes. However, the results do not rule out a modest ice contribution, and an upper limit of ∼5–10 wt% H2O ice (up to 30 vol.%) present in the uppermost meter of regolith is also consistent with the observations.

Surface morphology of domes in the Marius Hills and Mons Rümker regions of the Moon from Earth‐based radar data

Clusters and coalesced complexes of domes represent a relatively rare landform within the extensive, low‐relief, lunar maria. While they are typically spectroscopically similar to mare basalts emplaced by high‐volume, low‐viscosity eruptions, the large‐scale morphology of the domes is more consistent with slower emplacement and/or more viscous magma. We use new Earth‐based radar data to study the decimeter‐ to meter‐scale rock abundance, at the surface and buried within the probing range of the radar signal, of domes in the Marius Hills and Mons Rümker regions. Domes of the Marius Hills are characterized by high circular polarization ratio (CPR) values at 12.6‐cm and 70‐cm wavelength, similar to the near‐rim ejecta deposits of young lunar craters like Tycho, areas of large ejecta fragments near the rim of Meteor Crater, and blocky lavas such as SP flow in Arizona. We infer that a very rugged flow surface morphology, associated with the original dome‐forming eruption rather than postemplacement modification, exists beneath at most a few meters of regolith. Magmas with greater silica content than sampled lunar basalts are unlikely, so the blocky structure of the dome‐forming lava must arise from some combination of compositional change, effusion rate, and/or cooling effects not typically observed in basaltic landforms on the Earth. Much of the 80‐km diameter Mons Rümker has low backscatter and CPR at 12.6‐cm wavelength relative to nearby mare terrain. At 70‐cm wavelength, the northern and most of the southern parts of the complex are radar dark, but there is a radar bright feature that extends from the central region eastward into Oceanus Procellarum. We conclude that much of Mons Rümker is mantled by rock‐poor, fine‐grained pyroclastic material; the radar bright feature in the 70‐cm map is detectable owing to some combination of greater roughness (like the Marius domes) and thinner mantling cover.

Ejecta exchange and satellite color evolution in the Pluto system, with implications for KBOs and asteroids with satellites

In this Note, I present first-order scaling calculations to examine the efficacy of impacts by Kuiper Belt debris in causing regolith exchange between objects in the Pluto system. It is found that ejecta can escape Nix and Hydra with sufficient velocity to reach one another, as well as Charon, and even Pluto. The degree of ejecta exchanged between Nix and Hydra is sufficient to cover these bodies with much more material than is required for photometrically change. In specific, Nix and Hydra may have exchanged as up to 10s of meters of regolith, and may have covered Charon to depths up to 14 cm with their ejecta. Pluto is likely unaffected by most Nix and Hydra ejecta by virtue of a combination of dynamical shielding from Charon and Pluto's own annual atmospheric frost deposition cycle. As a result of ejecta exchange between Nix, Hydra, and Charon, these bodies are expected to evolve their colors, albedos, and other photometric properties to be self similar. These are testable predictions of this model, as is the prediction that Nix and Hydra will have diameters near 50 km, owing to having a Charon-like albedo induced by ejecta exchange. As I discuss, this ejecta exchange process can also be effective in many KBOs and asteroids with satellites, and may be the reason that very many KBO and asteroid satellite systems have like colors.

Sublimation rate of ice under simulated Mars conditions and the effect of layers of mock regolith JSC Mars‐1

We have studied the sublimation of ice buried beneath ≤200 mm of JSC Mars‐1 model regolith under simulated Mars conditions. As expected, even thin layers of regolith cause large decreases in sublimation rate, up to one order of magnitude at 50 mm. When the depth of the regolith was 50 to 200 mm we detected water desorbing from the overlying layers for which we were able to determine a desorption coefficient of 1.45 ± 0.5 × 10−3 h−1. After allowing for the effect of desorption, we found that the diffusion coefficient for water vapor through our regolith is 1.74 ± 0.70 × 10−4 m2 s−1, in excellent agreement with theoretical values. We thus find that a 1‐m thick layer of ice buried below a meter of regolith resembling JSC Mars‐1 on Mars at 235 K would last ∼800 years.

Delineating a shallow fault zone and dipping bedrock strata using multichannal analysis of surface waves with a land streamer

The multichannel analysis of surface waves (MASW) seismic method was used to delineate a fault zone and gently dipping sedimentary bedrock at a site overlain by several meters of regolith. Seismic data were collected rapidly and inexpensively using a towed 30-channel land streamer and a rubberband-accelerated weight-drop seismic source. Data processed using the MASW method imaged the subsurface to a depth of about [Formula: see text] and allowed detection of the overburden, gross bedding features, and fault zone. The fault zone was characterized by a lower shear-wave velocity [Formula: see text] than the competent bedrock, consistent with a large-scale fault, secondary fractures, and in-situ weathering. The MASW 2D [Formula: see text] section was further interpreted to identify dipping beds consistent with local geologic mapping. Mapping of shallow-fault zones and dipping sedimentary rock substantially extends the applications of the MASW method.

Burial rate of Mercury's polar volatile deposits

Delay Doppler images of Mercury's polar regions showing enhanced radar backscatter have been interpreted as evidence for water ice in the polar craters and are consistent with pure water ice covered by about 20 cm of regolith. In this paper we assume that the volatile at the Hermean poles is water ice, and that it was initially deposited at one time in a clean layer. We constrain the time of such an impact by determining the rate of regolith emplacement. The rate of burial of ice deposits in polar craters on Mercury is estimated to be 0.43 cm/Myr. If the ice deposits in Hermean polar craters are both clean and buried by 20 cm of regolith, they must be relatively recently emplaced, &lt;50 Myr ago. The predicted H concentration in the uppermost meter of regolith is calculated as a function of both time since emplacement of the ice and initial thickness of the ice layer. The combination of neutron data and radar data can provide additional insight into the size and age of ice deposits at Mercury.

Collisional evolution of the asteroid belt

A new synthesis of asteroid collisional evolution is motivated by the question of whether most asteroids larger than ∼1 km size are strengthless gravitational aggregates (rubble piles). NEAR found Eros not to be a rubble pile, but a shattered collisional fragment, with a through-going fracture system, and an average of about 20 m regolith cover. Of four asteroids visited by spacecraft, none appears likely to be a rubble pile, except perhaps Mathilde. Nevertheless, current understanding of asteroid collisions and size-dependent strength, and the observed distribution of rotation rates versus size, have led to a theoretical consensus that many or most asteroids larger than 1 km should be rubble piles. Is Eros, the best-observed asteroid, highly unusual because it is not a rubble pile? Is Mathilde, if it is a rubble pile, like most asteroids? What would be expected for the small asteroid Itokawa, the MUSES-C sample return target? An asteroid size distribution is synthesized from the Minor Planet Center listing and results of the Sloan Digital Sky Survey, an Infrared Space Observatory survey, the Small Main-belt Asteroid Spectroscopic Survey and the Infrared Astronomical Satellite survey. A new picture emerges of asteroid collisional evolution, in which the well-known Dohnanyi result, that the size distribution tends toward a self-similar form with a 2.5-index power law, is overturned because of scale-dependent collision physics. Survival of a basaltic crust on Vesta can be accommodated, together with formation of many exposed metal cores. The lifetimes against destruction are estimated as 3 Gyr at the size of Eros, 10 Gyr at ten times that size, and 40 Gyr at the size of Vesta. Eros as a shattered collisional fragment is not highly unusual. The new picture reveals the new possibility of a transition size in the collisional state, where asteroids below 5 km size would be primarily collisional breakup fragments whereas much larger asteroids are mostly eroded or shattered survivors of collisions. In this case, well-defined families would be found in asteroids larger than about 5 km size, but for smaller asteroids, families may no longer be readily separated from a background population. Moreover, the measured boulder size distribution on Eros is re-interpreted as a sample of impactor size distributions in the asteroid belt. The regolith on Eros may result largely from the last giant impact, and the same may be true of Itokawa, in which case about a meter of regolith would be expected there. Even a small asteroid like Itokawa may be a shattered object with regolith cover.

Impacts on Asteroids: Fragmentation, Regolith Transport, and Disruption

We use a numerical hydrocode model to examine the outcomes of various size impacts into targets the sizes of Asteroids 951 Gaspra and 243 Ida, which were imaged by the Galileo spacecraft. A shock wave fractures the asteroid in advance of crater excavation flow; thus, for impactors larger than 100 m impacting at 5.3 km s −1, tensile strength is unimportant in these bodies, whether they are initially intact or are “rubble piles.” Because of the shock-induced fracture, impact results are controlled by gravity. Therefore these asteroids are much more resistant to catastrophic disruption than predicted by previous estimates, which had assumed that strength was controlling these processes for rock targets. The rubble and regolith produced by this fracture can be “jolted” by the impact, redistributing surface material and globally erasing craters. The crater ejecta can produce tens of meters of regolith per large event, likely consisting of 100-m-size boulders mixed with smaller particles. The response of kilometer-size asteroids to impacts is qualitatively different from that of few-centimeter targets in terrestrial experiments, making prediction based on such experiments difficult. The compositional distribution of delivered meteorites depends on the outcomes of such asteroid impacts.

Geographic variations in the thermal and diffusive stability of ground ice on Mars

To investigate the stability of ground ice within the top several meters of the Martian regolith, time‐dependent models of the thermal and diffusive behavior of the regolith have been developed. The geographic distribution of thermal inertia and albedo as well as the latitudinal variation in insolation have been included in calculations of surface and subsurface temperatures between ±60° latitude. Ground ice was found to be stable where the annual mean surface and subsurface temperatures were below the atmospheric frost point. This generally occurs poleward of the mid‐latitudes. The latitude poleward of which ground ice is stable varies by about 20° to 30° from one longitude to another. Geographic variations in thermal inertia and albedo are the primary factors controlling regional variations in ice stability. Calculations of temperatures at high and low obliquity suggest that ground ice would be stable globally at high obliquity and would not be stable between ±60° latitude at low obliquity. Thermally driven diffusion of atmospheric water vapor within the regolith was modeled accounting for both ordinary molecular and Knudsen transport and equilibrium between ice, vapor, and adsorbed phases. Atmospheric water vapor was found to be able to supply the top few meters of the regolith with ice in regions where the annual mean surface temperature was below the atmospheric frost point. Ice was found to begin condensing in as short as 1000 Martian years. Rapid accumulation of ice in the pore space of the upper layers of the regolith acted to choke transport to lower layers and slow the diffusion process. Even so, after 105 Martian years, in some cases, as much as 30% to 40% of the available pore space accumulated ice. The total amount of subsurface ice ranged from a few to more than 25 g/cm2 within the top few meters in regions of stability. Ground ice was found to form below a depth where the annual average vapor pressure over ice was equal to the annual average atmospheric vapor pressure near the surface. Atmospheric vapor was not found to accumulate as ice below a depth where the seasonal temperature oscillations gave way to the geothermal gradient. The time scales for condensation of ground ice were found to be comparable to that of orbital oscillations suggesting that the present geographic distribution of ground ice may depend on the orbital history of Mars.

Apollo 15 measurement of lunar surface brightness temperatures thermal conductivity of the upper 1 1/2 meters of regolith

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.