Regolith Transport Research Articles

Geomorphic Evidence for the Presence of Ice Deposits in the Permanently Shadowed Regions of Scott‐E Crater on the Moon

AbstractThe abundance and distribution of ice on the Moon are still unknown. Here, we examine the connections between geomorphic characteristics and the potential presence of lunar ice in permanently shadowed regions of the Scott‐E crater in the lunar south polar region. We first quantify surface characteristics, such as roughness, based on high‐resolution optical images and topography data and then model spatial variations of water ice stability using a high‐resolution thermal model. We find that the surface roughness in regions where thermally stable surface ice is predicted is significantly lower than in adjacent regions where subsurface ice or no ice is predicted. This correspondence may result from processes such as preferential cold‐trapping in depressions or high albedo areas, or enhanced regolith transport in the ice‐rich regolith.

Aubrite Pesyanoe: Clues to composition and evolution of the enstatite achondrite parent body

AbstractThe Pesyanoe aubrite is an essentially polymict regolith breccia comprised by fragments of different highly magnesian pyroxenitic lithologies: albite; anorthoclase and labradorite‐bearing pyroxenites; diopside and magnesian augite pyroxenites; roedderite‐ and forsterite‐bearing pyroxenites; and impact glasses; porphyritic and melt matrix breccia fragments; FeO‐rich chondritic inclusions; and exotic oxidized clasts. The parent magma of Pesyanoe probably was carbon saturated, as suggested by pyroxenite fragments containing igneous‐textured carbon phases, possibly graphite. The composition of feldspar and trapped melt inclusions in enstatite indicates occurrence of at least three metaluminous melt sources with different (K + Na)/Al and K/(K + Na) atomic ratios on the Pesyanoe parent body and has records of K and Na loss from the melt, possibly due to evaporation from the parent body surface. The roedderite‐ and forsterite‐bearing rocks probably crystallized from a peralkaline melt. We propose that peralkaline melt could be formed from a metaluminous melt(‐s) due to gravitational segregation of djerfisherite‐bearing metal‐sulfide liquid in the lower horizon of the magma chamber and following oxidation of the magma. This should lead to enrichment of silicate melt in K2O and Na2O and increasing of (K + Na)/Al > 1, allowing forsterite and roedderite to crystallize. Rocks enriched in K and containing rare K‐bearing minerals were found among both magmatic and melt rocks. This may imply an insignificant role of regolith transport in the process of the breccia’s formation and, therefore, an origin of all of the breccia components from a local region of the Pesyanoe parent body, probably from a single complex igneous massif.

Advective Fluxes in the Martian Regolith as a Mechanism Driving Methane and Other Trace Gas Emissions to the Atmosphere

AbstractAdvective fluxes influence methane and CO2 soil emissions into the atmosphere on Earth and may drive trace gas emissions in the Mars atmosphere. However, their relevance in the Martian regolith has not been evaluated to date. Our regolith transport simulations show that advective fluxes can be relevant under Martian conditions and may drive the methane abundance detected by Mars Science Laboratory. Trace gas emissions would be highest in regions where winds interact with topography. Emissions in these regions may be further enhanced by time‐varying pressure fields produced by diurnal thermal tides and atmospheric turbulence. Trace gases such as methane should be emitted or produced from the first layers of regolith, or quickly transported to this region from a deeper reservoir through fractured media.

Climate preconditions the Critical Zone: Elucidating the role of subsurface fractures in the evolution of asymmetric topography

Although it has been long understood that rock properties strongly modulate the chemical and physical transformation of rock to regolith, recent studies highlight the central role of mechanical fracturing in the shallow subsurface. Competing hypotheses for how fractures co-evolve with surface processes suggest either that topographic stresses enhance or dampen the potential for fracture propagation, or alternatively suggest that fracture growth depends on the coupling between subsurface hydrology and weathering reactions. Both end-member models produce predictable and systematic spatial patterns of subsurface fracture distributions.To elucidate the processes responsible for the development of asymmetric hillslope topography and critical zone structure, we combine direct observations of subsurface fracture distributions in boreholes, indirect measures of subsurface rock properties inferred from seismic refraction surveys, and geochemical variations from subsurface samples at a small, shale-underlain catchment in central Pennsylvania. Our results reveal that seismic P-wave velocity profiles are consistent with subsurface fracture densities and zones of chemical depletion observed in boreholes, but that these distributions co-vary with hillslope aspect. Simple models of frost cracking show that, while modern climates are not conducive to subsurface segregation ice growth, asymmetric fracture distributions could have been achieved by subtle differences in aspect-related microclimates driving freeze-thaw during past periglacial climates. The spatial co-variance among fracture density, hillslope gradient, regolith depth, and regolith transport efficiency is consistent with chemo-mechanical damage arising from microclimatic conditions governing the long-term architecture of this watershed. Thus, damage driven by climate could explain the asymmetric topography observed today.

The Dynamical Complexity of Surface Mass Shedding from a Top-shaped Asteroid Near the Critical Spin Limit

Abstract The regolith transport near the surface of an asteroid is inherently sensitive to the local topography. In this paper, conditions of surface mass shedding and the subsequent evolution of the shedding material are studied for the primary of 65803 Didymos, serving as a representative for a large group of top-shaped asteroids that rotate near their critical spin limits. We considered the influences of an asymmetric shape and a non-spherical gravity, and demonstrate that these asymmetries play a significant role in the shedding process as well as in the subsequent orbital motion. The mass shedding conditions are given as a function of the geological coordinates, and show a clear-cut dependency on the local topographic features. We find that at different stages of the Yarkovsky–O’Keefe–Radzievskii–Paddack spin-up, the bulged areas exhibit a uniform superior advantage of enabling mass shedding over the depressed areas. “Dead zones” free from mass shedding are found around the polar sites. Numerical simulations show that the orbital motion of the shedding material experiences a drastic change as the spin rate is approaching the critical limit. The “mass leaking” effect is reinforced as the spin rate increases; the lower spin rates correspond to a higher capability of trapping the lofted particles in the vicinity of the asteroid, which statistically improves the probability of collisional growth in orbit. We also find that the topological transition of the equilibrium point can in practice lead to rapid clearance of the shedding material and transport of their orbits to larger distances from the surface.

Heterogeneous impact transport on the Moon

AbstractImpact cratering is the dominant process for transporting material on the Moon's surface. An impact transports both proximal material (continuous ejecta) locally and distal ejecta (crater rays) to much larger distances. Quantifying the relative importance of locally derived material versus distal material requires understandings of lunar regolith evolution and the mixing of materials across the lunar surface. The Moon has distinctive albedo units of darker mare basalt and brighter highland materials, and the contacts between these units are ideal settings to examine this question. Information on the amount of material transported across these contacts comes from both the sample collection and remote sensing data, though earlier interpretations of these observations are contradictory. The relatively narrow (~4–5 km wide) mixing zone at mare/highland contacts had been interpreted as consistent with most material having been locally derived from underneath mare plains. However, even far from these contacts where the mare is thick, highland material is abundant in some soil samples (>20%), requiring transport of highland material over great distances. Any model of impact transport on the Moon needs to be consistent with both the observed width of mare/highland contacts and the commonality of nonmare material in mare soil samples far from any contact. In this study, using a three‐dimensional regolith transport model, we match these constraints and demonstrate that both local and distal material transports are important at the lunar surface. Furthermore, the nature of the distal material transport mechanism in discrete crater rays can result in substantial heterogeneity of surface materials.

Weathering-limited hillslope evolution in carbonate landscapes

Understanding topographic evolution requires integrating elementary processes acting at the hillslope scale into the long-wavelength framework of landscape dynamics. Recent progress has been made in the quantification of denudation of eroding landscapes and its links with topography. Despite these advances, data is still sparse in car-bonate terrain, which covers a significant part of the Earth's surface. In this study, we measured both long-term denudation rates using in situ-produced 36 Cl concentrations in bedrock and regolith clasts and surface convexity at 12 sites along ridges of the Luberon carbonate range in Provence, Southeastern France. Starting from ∼30 mm/ka for the lowering of the summit plateau surface, denudation linearly increases with increasing hilltop convexity up to ∼70 mm/ka, as predicted by diffusive mass transport theory. Beyond this point denuda-tion rates appear to be insensitive to the increase in hilltop convexity. We interpret this constant denudation as indicating a transition from a regime where hillslope evolution is primarily controlled by diffusive downslope regolith transport, toward a situation in which denuda-tion is limited by the rate at which physical and chemical weathering processes can produce clasts and lower the hilltop. Such an abrupt transition into a weathering-limited dynamics may prevent hillslope 1 denudation from balancing the rate of base level fall imposed by the river network and could potentially explain the development of high local relief in many Mediterranean carbonate landscapes.

Aspect-dependent variations in regolith creep revealed by meteoric 10Be

Although variations in insolation and emergent feedbacks among soil moisture, vegetation, and soil cohesion are commonly invoked to explain topographic asymmetry that depends on aspect, few studies have directly quantified the efficiency of regolith transport along hillslopes of opposing aspect. We utilize meteoric 10 Be concentrations in regolith (n = 74) to determine mass flux along equatorial-facing and polar-facing hillslopes in three forested upland watersheds in and adjacent to the Susquehanna Shale Hills Critical Zone Observatory in central Pennsylvania (USA). In combination with regolith depth measurements and high-resolution topography, these fluxes allow us to evaluate transport rate laws and the efficiency of regolith creep. Concentrations of meteoric 10 Be in regolith along six separate transects imply that regolith flux is similar along all hillslopes, despite differences in topographic gradient and regolith thickness. Comparison of flux with regolith depth and topographic gradient reveals that transport depends on regolith depth, and that regolith creep is twice as efficient along low-gradient, south-facing slopes with thin regolith as compared to steep, north-facing slopes mantled with thicker regolith. We suggest that the observed topographic asymmetry in these watersheds has evolved over geologic time as a result of differences in the frequency of freeze-thaw events between hillslopes of opposing aspect.

Rock damage and regolith transport by frost: an example of climate modulation of the geomorphology of the critical zone

ABSTRACTIn this article we craft process‐specific algorithms that capture climate control of hillslope evolution in order to elucidate the legacy of past climate on present critical zone architecture and topography. Models of hillslope evolution traditionally comprise rock detachment into the mobile layer, mobile regolith transport, and a channel incision or aggradation boundary condition. We extend this system into the deep critical zone by considering a weathering damage zone below the mobile regolith in which rock strength is diminished; the degree of damage conditions the rate of mobile regolith production. We first discuss generic damage profiles in which appropriate length and damage scales govern profile shapes, and examine their dependence upon exhumation rate. We then introduce climate control through the example of rock damage by frost‐generated crack growth. We augment existing frost cracking models by incorporating damage rate limitations for long transport distances for water to the freezing front. Finally we link the frost cracking damage model, a mobile regolith production rule in which rock entrainment is conditioned by the damage state of the rock, and a frost creep transport model, to examine the evolution of an interfluve under oscillating climate. Aspect‐related differences in mean annual surface temperatures result in differences in bedrock damage rate and mobile regolith transport efficiency, which in turn lead to asymmetries in critical zone architecture and hillslope form (divide migration). In a quasi‐steady state hillslope, the lowering rate is uniform, and the damage profile is better developed on north‐facing slopes where the frost damage process is most intense. Because the residence times of mobile regolith and weathered bedrock in such landscapes are on the order of 10 to 100 ka, climate cycles over similar timescales result in modulation of transport and damage efficiencies. These lead to temporal variation in mobile regolith thickness, and to corresponding changes in sediment delivery to bounding streams. Copyright © 2012 John Wiley & Sons, Ltd.

Regolith transport quantified by braking block, McMurdo Dry Valleys, Antarctica

The McMurdo Dry Valleys of Antarctica are today a hyper-arid, polar desert. Prior work has identified several in situ volcanic ashes (6–11Myr old) resting on the surface regolith that suggests the persistent stability of the regolith surfaces and climate. However, our field observations of characteristic regolith bulges above and cavities below boulders that are lodged in the hillslope (braking blocks) contradict the apparent preservation of the regolith surfaces. To quantify the regolith mobility we modeled the downslope regolith transport around large boulders in the Dry Valleys using a finite difference regolith-transport model. To guide our modeling effort, we surveyed the detailed topography around one large boulder in the field. Model results fit the observed topography well and allow for the calculation of the minimum volume of regolith per unit width of slope that was mobilized (>4m3/1m width). To assess the general applicability of the braking block analysis on random boulders on hillslope we surveyed the topographic characteristics adjacent to 997 boulders on 10 separate hillslopes. The bulge-cavity development appears to be sensitive to: 1) the adjacent obstructions that restrict the free movement of the regolith around the given boulder, and 2) the inherent surface roughness. Even though, theoretically boulders of all sizes should have a cavity-bulge pair surrounding them, the signal to noise ratio prevented us from extracting such measurements reliably on boulders whose diameter was smaller than about 1m, on boulders that sat on bedrock covered by a thin veneer of regolith, and on boulders that were part of an actively aggrading talus. The limiting minimum topographic diffusivity was found to be 10−6m2/yr. Over time periods of millions of years this is enough to rework the surfaces and highlights the intriguing contradiction between the preserved ashes and the observed regolith transport.

Exploring weathering and regolith transport controls on Critical Zone development with models and natural experiments

The architecture of the Critical Zone, including mobile regolith thickness and depth to the weathering front, is first order controlled by advance of a weathering front at depth and transport of sediment at the surface. Differences in conditions imposed by slope aspect in the Gordon Gulch catchment of the Boulder Creek Critical Zone Observatory present a natural experiment to explore these interactions. The weathering front is deeper and saprolite more decayed on north-facing than on south-facing slopes. Simple numerical models of weathering front advance, mobile regolith production, and regolith transport are used to test how weathering and erosion rates interact in the evolution of weathered profiles. As the processes which attempt are being made to mimic are directly tied to climate variables such as mean annual temperature, the role of Quaternary climate variation in governing the evolution of Critical Zone architecture can be explored with greater confidence.

Global survey of color variations on 433 Eros: Implications for regolith processes and asteroid environments

High-resolution imaging acquired with the Near Earth Asteroid Rendezvous Shoemaker (NEAR Shoemaker) spacecraft is used to elucidate the spectral properties and spatial distribution of color units on Asteroid 433 Eros. Previous workers mapped four distinct types of color units on the surface ( bright streaks, dark soils, ponded materials, average regolith). These units exhibit albedo and color boundaries but there is no evidence to indicate they represent distinct rock types. Rather the units are thought to show evidence of complex regolith transport and sorting processes. Here we report the results of a comprehensive study of all viable color MultiSpectral Imager (MSI) data to identify and characterize the distribution and nature of color units across the whole asteroid. Due to a spacecraft upset that resulted in contamination of the MSI optics, color images are affected with a scattered light problem that hampers interpretation of subtle color contrasts, even after a rigorous remediation. To constrain interpretations of the MSI color data we characterize this residual scattered light and demonstrate how complete correction would affect derived color ratios. Results of our comprehensive study are consistent with previous mapping—confirming that bright streaks, average regolith and dark soils fall on a mixing line, consistent with space weathering effects. We find that the ponded deposits do not fall on this putative mixing line. The color and reflectance of the ponded deposits are consistent with some combination of compositional, grain size and maturity variations from the average regolith. Additionally we show that spectral separation of the four units on ratio plots would only increase with full removal of residual scattered light, especially for features that are small in terms of pixels. Global analysis of the Eros color units illustrates complex regolith processes and grain sorting that may hold clues to understanding space weathering processes and the link between asteroids and meteorites.

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.