Regolith Evolution Research Articles

Geologic context of in situ rocky exposures in Mare Serpentis, Mars: Implications for crust and regolith evolution in the cratered highlands

Global acquisition of infrared spectra and high-resolution visible and infrared imagery has enabled the placement of compositional information within stratigraphic and geologic context. Mare Serpentis, a low albedo region located northwest of Hellas Basin, is rich in spectral and thermophysical diversity and host to numerous isolated exposures of in situ rocky material. Most martian surfaces are dominated by fine-grained particulate materials that bear an uncertain compositional and spatial relationship to their source. Thus location and characterization of in situ rock exposures is important for understanding the origin of highland materials and the processes which have modified those materials. Using spectral, thermophysical and morphologic information, we assess the local and regional stratigraphy of the Mare Serpentis surface in an effort to reconstruct the geologic history of the region. The martian highlands in Mare Serpentis are dominated by two interspersed surface units, which have distinct compositional and thermophysical properties: (1) rock-dominated surfaces relatively enriched in olivine and pyroxene, and depleted in high-silica phases, and (2) sediment or indurated material depleted in olivine and pyroxene, with relatively higher abundance of high-silica phases. This is a major, previously unrecognized trend which appears to be pervasive in the Mare Serpentis region and possibly in other highland areas. The detailed observations have led us to form two hypotheses for the relationship between these two units: either (1) they are related through a widespread mechanical and/or chemical alteration process, where less-mafic plains materials are derived from the mafic bedrock, but have been compositionally altered in the process of regolith formation, or (2) they are stratigraphically distinct units representing separate episodes of upper crust formation. Existing observations suggest that the second scenario is more likely. In this scenario, plains materials represent older, degraded, and possibly altered, “basement” rock, whereas the rocky exposures represent later additions to the crust and are probably volcanic in origin. These hypotheses should be further testable with decimeter-resolution imagery and meter-resolution short wavelength infrared spectra.

Airborne gamma-ray spectrometry to quantify chemical erosion processes

Airborne gamma-ray spectrometry data (uranium, potassium and thorium contents) reveal geochemical heterogeneities within the monolithological Hyrôme watershed (ca. 150 km 2) in the Armorican massif (western France). Our observations and computations provide important constraints on the spatial distribution and the associated magnitudes of chemical erosion processes at the scale of a small watershed. Two distinct, partially preserved, weathering profiles exhibit a strong correlation between regolith evolution and airborne-derived K/Th ratios, suggesting that the variability is linked to supergene processes. Using both airborne data and laboratory measurements on rock samples, the total net export of potassium has been estimated at 422 ± 50 kg/m 2 and the chemical weathering rate of potassium at 17 ± 2 kg/km 2/a.

Rock fragment distributions and regolith evolution in the Ouachita Mountains, Arkansas, USA

AbstractRock fragments in the regolith are a persistent property that reflects the combined influences of geologic controls, erosion, deposition, bioturbation, and weathering. The distribution of rock fragments in regoliths of the Ouachita Mountains, Arkansas, shows that sandstone fragments are common in all layers, even if sandstone is absent in parent material. Shale and sandstone fragments are produced at the bedrock weathering front, but the shale weathers rapidly and intact fragments are rare in the solum. Sandstone is weathered from ridgetop outcrops and transported downslope. Some of these fragments are moved downward, by faunalturbation and by transport into pits associated with rotting tree stumps. Upward movement by treethrow is common, resulting in a net concentration of rocks near the surface. However, the highest fragment concentrations are in the lower regolith, indicating active production at the weathering front. The regolith is a dynamic feature, reflecting the influences of vertical and horizontal processes, of active weathering at the bedrock interface, and of surficial sediment movements. The role of trees in redistributing rock fragments suggests that significant regolith mixing occurs over time scales associated with forest vegetation communities, and that forest soils have likely been extensively mixed within Holocene and historic time. Copyright © 2005 John Wiley & Sons, Ltd.

Regolith history of lunar meteorites

Abstract— The regolith evolution of the lunar meteorites Dhofar (Dho) 081, Northwest Africa (NWA) 032, NWA 482, NWA 773, Sayh al Uhaymir (SaU) 169, and Yamato (Y-) 981031 was investigated by measuring the light noble gases He, Ne, and Ar. The presence of trapped solar neon in Dho 081, NWA 773, and Y-981031 indicates an exposure at the lunar surface. A neon three-isotope diagram for lunar meteorites yields an average solar 20Ne/22Ne ratio of 12.48 ± 0.07 representing a mixture of solar energetic particles neon at a ratio of 11.2 and solar wind neon at a ratio of 13.8. Based on the production rate ratio of 21Ne and 38Ar, the shielding depth in the lunar regolith of NWA 032, NWA 482, SaU 169, and Y-981031 was obtained. The shielding depth of these samples was between 10.5 g/cm2 and >500 g/cm2. Based on spallogenic Kr and Xe, the shielding depth of Dho 081 was estimated to be most likely between 120 and 180 g/cm2. Assuming a mean density of the lunar regolith of 1.8 g/cm3, 10.5 g/cm2 corresponds to a depth of 5.8 cm and 500 g/cm2 to 280 cm below the lunar surface. The range of regolith residence time observed in this study is 100 Ma up to 2070 Ma.

Megaregolith evolution and cratering cataclysm models—Lunar cataclysm as a misconception (28 years later)

Abstract— The hypothesis of a lunar cataclysmic cratering episode between 3.8 and 3.9 Gyr ago lacks proof. Its strongest form proposes no cratering before about 4.0 Gyr, followed by catastrophic formation of most lunar craters and basins in >200 Myr. The premise that “zero impact melts implies zero impacts” is disproved by data from asteroids, on which early collisions clearly occurred, but from which early impact melts are scarce. Plausible cataclysm models imply that any cataclysm should have affected the whole inner solar system, but among available lunar and asteroid impact melt and impact age resetting data, a narrow, strong 3.8–3.9 Gyr spike in ages is seen only in the region sampled by Apollo/Luna. Reported lunar meteorite data do not show the spike. Asteroid data show a broader, milder peak, spreading from about 4.2 to 3.5 Gyr. These data suggest either that the spike in Apollo impact melt ages is associated with unique lunar front side events, or that the lunar meteorites data represent different kinds of events than the Apollo/Luna data. Here, we develop an alternate “megaregolith evolution” hypothesis to explain these data. In this hypothesis, early impact melts are absent not because there were no impacts, but because the high rate of early impacts led to their pulverization. The model estimates survival halflives of most lunar impact melts prior to 4.1 Gyr at >100 Myr. After a certain time, Tcritical ˜4.0 Gyr, impact melts began to survive to the present. The age distribution differences among impact melts and plutonic rocks are controlled by, and hold clues to, the history of regolith evolution and the relative depths of sequestration of impact melts versus plutonic rocks, both among lunar and asteroidal samples. Both the “zero cratering, then cataclysm” hypothesis and the “megaregolith evolution” hypothesis require further testing, especially with lunar meteorite impact melt studies.

The occurrence and significance of biogenic opal in the regolith

Biogenic opal produced by vascular plants, diatoms, and siliceous sponges have been found in soils and terrestrial sediments of all continents except Antarctica since the middle of the 19th century. The opal particles range in size from fine silt to fine sand. Almost all soils contain detectable opal up to levels of 2–3%, and a significant number contain values in excess of 5%. Even higher values have been found from soils and sediments of all continents in a wide range of soil types. The most important factor is poor soil drainage and seasonal to permanent water logging. This encourages the proliferation of silica producing organisms. Such conditions have been found in the soils and aquatic sediments of the monsoonal tropics, tropical rain forests, temperate forests, tropical savanna, tropical islands, semi-arid grasslands and savanna, and temperate woodland and grassland. The presence of a volcanic substrate also appears favourable in some cases, but is not necessary. Biogenic opal preferentially collects in the A horizon of soils and, to a lesser extent, in the B horizon. This preferential distribution facilitates identification of palaeosols in stacked soil sequences. Biogenic opal is also a component of windblown dust, even in arid environments. Biogenic opal is significant to regolith processes in a number of ways. Firstly, as in the case in marine environments, it is likely to be important in silica cycling and storage because of its greater lability compared to quartz. Secondly, dissolution and reprecipitation of opal A as opal CT or micro-quartz may play a role in cementation and silicification of regolith to form silica hardpans and silcrete. Thirdly, the organisms that form biogenic opal can have considerable palaeoenvironmental significance and be valuable in reconstructing regolith evolution. Finally, some forms of biogenic silica, in particular sponge spicules, can present a health hazard. Their high abundance in some soils and sediments needs to be considered when assessing the health implications of airborne dust.

Regolith geology of the Yilgarn Craton, Western Australia: Implications for exploration

This paper presents and reviews the processes responsible for the distribution and formation of regolith and associated landscapes of the Yilgarn Craton and highlights their implications for mineral exploration. It provides an analysis of regolith geology investigations that were conducted in many districts of contrasting contemporary geomorphic and climatic conditions.The Yilgarn Craton is composed of Archaean rocks, predominantly granitoids, that are crossed by north‐northwest‐trending belts of greenstones. It has an arid to humid climate at present. The gently undulating landsurface forms a partial etchplain and the topography is largely related to bedrock lithologies and a complex history of valley development and aggradation. Deep weathering has affected most lithologies and geological provinces across the craton. The depth of the weathered mantle may be as much as 150 m, but it varies considerably and rock outcrop may occur in any part of the landscape. The main factors influencing extent of weather...

Mass Movement and Landform Degradation on the Icy Galilean Satellites: Results of the Galileo Nominal Mission

The Galileo mission has revealed remarkable evidence of mass movement and landform degradation on the icy Galilean satellites of Jupiter. Weakening of surface materials coupled with mass movement reduces the topographic relief of landforms by moving surface materials down-slope. Throughout the Galileo orbiter nominal mission we have studied all known forms of mass movement and landform degradation of the icy galilean satellites, of which Callisto, by far, displays the most degraded surface. Callisto exhibits discrete mass movements that are larger and apparently more common than seen elsewhere. Most degradation on Ganymede appears consistent with sliding or slumping, impact erosion, and regolith evolution. Sliding or slumping is also observed at very small (100 m) scale on Europa. Sputter ablation, while probably playing some role in the evolution of Ganymede's and Callisto's debris layers, appears to be less important than other processes. Sputter ablation might play a significant role on Europa only if that satellite's surface is significantly older than 108 years, far older than crater statistics indicate. Impact erosion and regolith formation on Europa are probably minimal, as implied by the low density of small craters there. Impact erosion and regolith formation may be important on the dark terrains of Ganymede, though some surfaces on this satellite may be modified by sublimation–degradation. While impact erosion and regolith formation are expected to operate with the same vigor on Callisto as on Ganymede, most of the areas examined at high resolution on Callisto have an appearance that implies that some additional process is at work, most likely sublimation-driven landform modification and mass wasting. The extent of surface degradation ascribed to sublimation on the outer two Galilean satellites implies that an ice more volatile than H2O is probably involved.

The Kapoeta howardite: Implications for the regolith evolution of the howardite‐eucrite‐diogenite parent body

Abstract— A large hand sample and numerous polished thin sections, made from the hand sample, of the Kapoeta howardite and its many diverse lithic clasts were studied in detail by optical microscopy and electron microprobe techniques in an attempt to understand the surface processes that operated on the howardite‐eucrite‐diogenite (HED) parent body (most likely the asteroid 4 Vesta). Four unique, unusually large clasts, designated A (mafic breccia), B (granoblastic eucrite), D (howardite) and H (melt‐coated breccia), were selected for detailed study (modal analysis, mineral microprobe analysis, and noble gas measurements). Petrographic studies reveal that Kapoeta consists of a fine‐grained matrix made mostly of minute pyroxene and plagioclase fragments, into which are embedded numerous different lithic and mineral clasts of highly variable sizes. The lithic clasts include pyroxene‐plagioclase (eucrite), orthopyroxenite (diogenite), howardite, impact‐melt, metal‐sulfide‐rich, and carbonaceous chondrite clasts. The howardite clasts include examples of lithic clasts that constitute breccias‐within‐breccias, suggesting that at least two regolith generations are represented in the Kapoeta sample we studied. The clast assemblage suggests that repeated shock lithification was an important process during regolith evolution. Noble gas analyses of clast samples fall into two populations: (a) solar‐gas‐rich clasts H (rim only) and D and (b) clasts A and B, which are essentially free of solar gases. The concentrations of solar noble gases in the two matrix samples differ by a factor of ∼40. It appears that clast D is a true regolith breccia within the Kapoeta howardite (breccia‐within‐breccia), while clast H is a regolith breccia that has been significantly impact reworked. Our data indicate that the Kapoeta howardite is an extraordinarily heterogeneous rock in modal mineral and lithic clast abundances, grain size distributions, solar‐wind noble gas concentrations and cosmic‐ray exposure ages. These results illustrate the repetitive nature of impact comminution and lithification in the regolith of the HED parent body.

The Barringer Award Address Presented 1996 July 25, Berlin, Germany: Impact experiments related to the evolution of planetary regoliths

Abstract— Impact‐induced comminution of planetary surfaces is pervasive throughout the solar system and occurs on submillimeter to global scales, resulting in comminution products that range from fine‐grained surface soils, to massive, polymict ejecta deposits, to collisionally fragmented objects. Within this wide range of comminution products, we define regoliths in a narrow sense as materials that were processed by repetitive impacts to dimensional scales comparable to or smaller than that of component minerals of the progenitor rock(s). In this paper, we summarize a wide variety of impact experiments and other observations that were primarily intended to understand the evolution of regoliths on lunar basalt flows, and we discuss some of their implications for asteroidal surfaces.Cratering experiments in both rock and noncohesive materials, combined with photogeologic observations of the lunar surface, demonstrate that craters <500 m in diameter contribute most to the excavation of local bedrock for subsequent processing by micrometeorites. The overall excavation rate and, thus, growth rate of the debris layer decreases with time, because the increasingly thicker fragmental layer will prevent progressively larger projectiles from reaching bedrock. Typical growth rates for a 5 m thick lunar soil layer are initially (∼≥3 Ga ago) a few mm/Ma and slowed to <1 mm/Ma at present. The coarse‐grained crater ejecta are efficiently comminuted by collisional fragmentation processes, and the mean residence time of a 1 kg rock is typically 10 Ma. The actual comminution of either lithic or monomineralic detritus is highly mineral specific, with feldspar and mesostasis comminuting preferentially over pyroxene and olivine, thus resulting in mechanically fractionated fines, especially at grain sizes <20 μm. Such fractionated fines also participate preferentially in the shock melting of lunar soils, thus giving rise to “agglutinate” melts. As a consequence, agglutinate melts are systematically enriched in feldspar components relative to the bulk composition of their respective host soil(s). Compositionally homogeneous, impact derived glass beads in lunar soils seem to result from micrometeorite impacts on rock surfaces, reflecting lithic regolith components and associated mineral mixtures. Cumulatively, experimental and observational evidence from lunar mare soils suggests that regoliths derive substantially from the comminution of local bedrock; the addition of foreign, exotic components is not necessary to explain the modal and chemical compositions of diverse grain size fractions from typical lunar soils.Regoliths on asteroids are qualitatively different from those of the Moon. The modest impact velocities in the asteroid belt, some 5 km s−1, are barely sufficient to produce impact melts. Also, substantially more crater mass is being displaced on low‐gravity asteroids compared to the Moon; collisional processing of surface boulders should therefore be more prominent in producing comminuted asteroid surfaces. These processes combine into asteroidal surface deposits that have suffered modest levels of shock metamorphism compared to the Moon. Impact melting does not seem to be a significant process under these conditions. However, the role of cometary particles encountering asteroid surfaces at presumably higher velocities has not been addressed in the past. Unfortunately, the asteroidal surface processes that seemingly modify the spectral properties of ordinary chondrites to match telescopically obtained spectra of S‐type asteroids remain poorly understood at present, despite the extensive experimental and theoretical insights summarized in this report and our fairly mature understanding of lunar surface processes and regolith evolution.

An experimental evaluation of mineral‐specific comminution

Abstract— Early “regolith‐evolution” experiments using fragmental, polycrystalline gabbro targets displayed mineral‐specific comminution trends, with feldspar being significantly fractionated into the finest grain sizes. Since planetary regoliths are similar mixtures of lithic and monomineralic detritus, the comminution of monomineralic grains is important in understanding the evolution of such regoliths. Particulate targets of monomineralic feldspar, olivine, pyroxene, and quartz therefore were subjected to at least 25 impacts each to complement the previous gabbro‐based investigations. Stainless‐steel projectiles 3.18 mm in diameter were launched at the targets at nominal velocities of 1.4 km s−1, depositing an average of about 2.6 × 106 ergs per g of target per impact. The quartz and feldspar comminuted most readily, while olivine and pyroxene were the most resistant. In addition, the feldspar and quartz were virtually indistinguishable in terms of any measure of comminution used here. The behavior of the olivine differed somewhat from that of the pyroxene, but the variation between these two minerals was much less than the difference between them and the tectosilicates. The apparent energy required to create new surfaces through comminution was about a factor of two higher for the mafic minerals. Densities of pre‐existing cracks and other crystal defects depend on sample preparation techniques, among other things, and appear to play a notable role in the early evolution of these “regoliths.” It is probable that the history of a particular regolith's parent rocks will exert a comparable influence on the early stages of evolution of a planetary regolith. The trends for the individual targets in these monomineralic series duplicated those exhibited by the gabbro charges, leading to the general conclusion that mineral‐specific comminution will occur during repetitive impact of planetary surfaces, whether they comprise freshly excavated, large blocks, or highly comminuted, clastic fines containing substantial fractions of monomineralic debris. This mineral‐specific behavior will cause small grain sizes in planetary regoliths to be substantially fractionated relative to coarse regolith components, and especially relative to their source rocks.

Meteorite-asteroid spectral comparison: The effects of comminution, melting, and recrystallization

Laboratory results from a simulation of the possible effects of spectral alteration on reflectance of the optical surface of ordinary chondrite parent bodies is presented. Diffuse reflectance spectra from 0.3 to 2.6 μm were obtained for three chondritic meteorites. To simulate possible regolith processes the samples were comminuted to finer grain sizes, and the effect of communution on their reflectance spectra was measured. Following comminution, the samples were melted, recrystallized, recomminuted, and remeasured. These laboratory alterations produced a decrease in absorption band depths at 0.95 μm, and melting and recrystallization produced a significant drop in albedo. A reddening of the continuum, as previously reported for CM meteorites upon comminution and for lunar samples upon vitrification, also occurred for OC meteorites to a minor extent. Thus, although it was found that spectral characteristics could each be significantly changed by these procedures, no set of procedures was able to simultaneously affect all relevant parameters in such a way as to improve the match between ordinary chondritic meteorites and S-class asteroids. These results are consistent with what is known of the physical aspects of asteroid regolith evolution and also with the existence of several currently satisfactory spectral matches between meteorites and asteroid spectral classes.

Regolith evolution in the laboratory: Scaling dissimilar comminution experiments

Abstract— Repeated impacts into fragmental targets simulating unconsolidated debris on planetary surfaces have provided empirical insight into the evolution of planetary regoliths. Quantitative understanding and interpretation of these results, however, are often made difficult by the complex, multivariate nature of the impact process, even under controlled laboratory conditions. The techniques of dimensional analysis have been employed to quantify and examine the relationships between the more important variables in the evolution of these experimental regoliths. Application of this method to the results of ten experimental series shows that the quantity of comminuted target mass is directly proportional to (a) the number of impacts sustained by the target, (b) the diameter of the projectile, (c) the mean size of the crystals composing the target rock, (d) the mean grain‐size of the evolving “regolith,” (e) the total target mass, (f) the impactor density, and (g) the ratio of the impact velocity to the velocity of sound in the target rock. The comminuted mass is inversely proportional to the density of the target rock and the sorting of the “regolith.” Similar results hold for the creation of new surfaces by fracturing, except that the initial surface area possessed by the target takes the place of the total target mass. In addition, the ease with which new surfaces are created is independent of the mean crystal‐size. Although it appears that the ratio between impactor and target densities is an important parameter in both cases, more experiments will be necessary to establish the quantitative contribution of that parameter, as well as those of the total target mass and the effective strengths of the individual minerals composing the target rock.

Exposure history of the regolithic chondrite Fayetteville: I. Solar-gas-rich matrix

Solar gas bearing matrix samples of the H4 chondrite Fayetteville contain up to 45% more cosmogenic 21Ne than some of the solar-gas-free light inclusions. Concentrations of solar and cosmogenic Ne in the matrix samples correlate. The excesses of cosmogenic Ne are comparable to those found in two out of three groups of track-rich grains of Fayetteville studied by Caffee et al. (1986). The excesses can be explained by a GCR-irradiation of a considerable part, if not all, of the matrix grains in their parent body regolith during at least some 20 Ma. We see no need to postulate a T-Tauri type solar cosmic ray irradiation of Fayetteville's regolith. The parent body exposures derived here are considerably longer than the < 1 Ma predicted by the model on the evolution of asteroidal regoliths by housen et al. (1979). The ratio of solar to cosmogenic gas acquired in the regolith points to an orbit of Fayetteville's parent body at some 2–3 AU.

Apollo 16 regolith breccias and soils: recorders of exotic component addition to the Descartes region of the moon

The subdivision of Apollo 16 regolith breccias into ancient ( ∼ 4 b.y.) and relatively younger samples on the basis of trapped 40Ar/ 36Ar ratios [1] makes possible, with the present-day soils as a third sample suite, a petrologic and chemical determination of regolith evolution and exotic component addition at the A-16 site. Petrologic data for the ancient breccias show that the early regolith was composed of fragments of plutonic rocks and impact melt rocks, and minerals and impact glasses derived from these lithologies. The presence of KREEPy glasses in the ancient breccias demonstrates that KREEPy lithologies and impact melts formed early in lunar history. Comparison of the ancient breccias with the young breccias and the soils shows that the mare components, mainly in the form of orange high-TiO 2 glass and green low-TiO 2 glass, were mostly added to the site after formation of the ancient breccias and prior to formation of the young breccias. The young breccias are petrologically similar to the soils. The major change in the regolith since the formation of the young breccias is an increase in maturity—the formation of fused soil particles with prolonged exposure to micrometeorite impacts. By contrast, the breccias formed at times when large impacts and soil mixing were dominant over small impacts and maturation.

Agglutinates as recorders of regolith evolution: Application to the Apollo 17 drill core

We report chemical data (27 major, minor, and trace elements by INAA) for agglutinates from 26 depth intervals of the Apollo 17 deep drill core (sections 70009–70001) and compare their compositions with those of the soils in which they occur. The results show that seven stratigraphic units contain agglutinate populations in which the “average agglutinate” is in equilibrium (E) with the soil in which it resides and thus has a chemical composition that falls within the compositional range for the various size fractions. Four units contain average agglutinate populations with compositions out of equilibrium (O) with the soils in which they reside, and two units contain agglutinate populations that are close to the compositional limits of the soil size fractions and are designated (M) for mixed. The sequence is: (0–22 cm) E; (28–47 cm) O; (58–70 cm) E; (81 cm) O; (90–98 cm) E; (108–118 cm) M; (113–178 cm) E; (189 cm) O; (198–210 cm) E; (220–242 cm) O; (253 cm) E; (258–269 cm) M; (278–286 cm) E. This agglutinate sequence suggests a scenario in which several closely‐spaced depositional events were involved in the formation of the A‐17 drill core rather than a continuous accumulation process. Chemical mixing models indicate that agglutinates from section 70008 (22–48 cm) are enriched in mare components (81%), containing 55% orange glass and 26% ilmenite basalt. Agglutinates from core section 70003 (220–242 cm) did not form from the KREEPy soils that occur in this section but were probably mixed in from stratigraphic horizons above and below this unit. The Apollo 17 drill core soils have complex histories and agglutinates occurring in this stratigraphic sequence are powerful recorders of soil maturation and mixing events.

Regoliths on Small Bodies in the Solar System

A regolith is defined as a layer or mantle of loose, incoherent, rocky material of whatever origin, that nearly everywhere forms the surface of the land and rests on coherent bedrock. The regoliths on many planetary bodies are the result of continual impacts, which transform coherent surfaces into fragmental debris. The present investigation is concerned with the special case of regolith formation and evolution on small objects, such as asteroids and meteorite parent bodies. First order models of regolith evolution on asteroidal surfaces are constructed on the basis of data provided by studies of lunar samples and meteorites. It appears that regolith formation proceeds by deposition of discrete layers of the widely spread ejecta primarily from the larger impacts. Moderate-size (100-300 km diameter) asteroids are covered by modest regoliths of the order of one km in depth. Small rocky asteroids develop negligible regoliths.

Origin of Iron Meteorite Groups IAB and IIICD

Abstract Several low-Ni iron meteorites previously assigned to group IAB are reclassified IIICD on the basis of lower Ge, Ga, W and Ir concentrations and higher As concentrations; the low-Ni extreme of IIICD is now 62 mg/g, that of IAB is 64 mg/g. The resulting fractionation patterns in the two groups are quite similar. It has long been established that, in contrast to the magmatic iron meteorite groups, IAB and IIICD did not form by fractional crystallization of a metallic magma. Other models have been proposed, but all have serious flaws. A new model is proposed involving the formation of each iron in small pools of impact melt on a parent body consisting of material similar to the chondritic inclusions found in some IAB and IIICD irons, but initially unequilibrated. These impact melts ranged in temperatures from ~ 1190 K to ~ 1350 K. The degree of equilibration between melt and unmelted solids ranged from minimal at the lowest temperature to moderate at the highest temperature. The lowest temperature melts were near the cotectic in the Fe-Ni-S system with Ni contents of ~ 12 atom %. Upon cooling, these precipitated metal having ~ 600 mg/g Ni by equilibrium crystallization. The Ni-rich melt resulted from the melting of Ni-rich sulfides and metal in the unequilibrated chondritic parent. Low-Ni irons formed in high temperature melts near the composition of the FeS-Fe eutectic or somewhat more metal rich. We suggest that the decreasing Ge, Ga and refractory abundances with increasing Ni concentration reflect the trapping of these elements in oxide phases in the unequilibrated chondritic material, and that very little entered the Ni-rich melt parental to the Oktibbeha County iron. The remaining elements tended to have element/Ni ratios in the melts that were more or less independent of temperature. The remarkable correlation between I-Xe age of the chondritic inclusions and Ni content of the host metal is explained by a detailed evolution of (mega)regolith in which these groups originated. The most Ni-rich melts could only be generated from an unequilibrated chondrite parent; as the continuing deposition of impact energy produced increasingly higher grades of metamorphism, the maximum Ni content of the impact melts (and their subsequently precipitated metal) gradually decreased.

Asteroidal regoliths

We develop a physical model for the evolution of regoliths on small bodies and apply it to the asteroids and meteorite parent bodies. The model considers global deposition of that fraction of cratering ejecta that is not lost to space. It follows the build up of regolith on a typical region, removed from the larger craters which are the source of most regolith blankets. Later in the evolution, larger craters saturate the surface and are incorporated into the typical region; their net ejection of materials to space causes the elevation of the typical region to decrease and once-buried regolith becomes susceptible to ejection or gardening. The model is applied to cases of both strong, cohesive bodies and to bodies of weak, unconsolidated materials. Evolution of regolith depths and gardening rates are followed until a sufficiently large impact occurs that fractures the entire asteroid. (Larger asteroids are not dispersed, however, and evolve mergaregoliths from multiple generations of surficial regoliths mixed into their interiors.) We find that large, strong asteroids generate surficial regoliths of a few kilometers depth while strong asteroids smaller than 10-km diameter generate negligible regoliths. Our model does not treat large, weak asteroids, because their cratering ejecta fail to surround such bodies; regolith evolution is probably similar to that of the Moon. Small, weak asteroids of 1- to 10-km diameter generate centimeter- to meter-scale regoliths. In all cases studied, blanketing rates exceed excavation rates, so asteroid regoliths are rarely, if ever, gardened and should be very immature measured by lunar standards. They should exhibit many of the characteristics of the brecciated, gas-rich meteorites; intact foreign clasts, relatively low-exposure durations to galactic and solar cosmic rays low solar gas contents, minimal evidence for vitrification and agglutinate formation, etc. Both large, strong asteroids and small, weak ones provide regolith environments compatible with those inferred for the parent bodies of brecciated meteorites. But from volumetric calculations, we conclude that most brecciated meteorites formed on the surfaces of, and were recycled through the interiors of, parent bodies at least several tens of kilometers in diameter. The implications of our regolith model are consistent with properties inferred for asteroid regoliths from a variety of astronomical measurements of asteroids, although such data do not constrain regolith properties nearly as strongly as meteoritical evidence Our picture of substantial asteroidal regoliths produced predominantly by blanketing differs from earlier hypotheses that asteroidal regoliths might be thin or absent and that short surface exposure of asteroidal materials is due chiefly to erosion rather than blanketing.

Lunar evolution: is there a global radioactive crust on the Moon?

Chemical and isotopic analyses of various grainsize fractions of lunar soils show the presence of an ‘ exotic component’ in practically all lunar soils. The patterns of enrichments in the grain-size fractions and the Sr-isotopic data show that the regolith evolution displays the combined effects comminution of local rock types and addition of the exotic component. The chemical characteristics of this exotic component as deduced from the chemical and isotopic data in soils from Apollo 11, 12, 15 and 16 uniformly point to compositions similar to the material from Fra Mauro region collected in the Apollo 14 mission. There is a strong correlation between the amount of exotic component in a soil and its distance from the Fra Mauro region. It is suggested that the exotic component represents trace element enriched material from the Imbrium-Procellarum region, which was surficially deposited during Imbrium excavation and re-exposed from under the mare-lavas in subsequent cratering events. Surficial transport processes have distributed these materials widely over the lunar surface. There appears no need to invoke a global radioactive crust on the Moon nor of * hot spots ’ distributed over the entire surface of the Moon to explain the ubiquitous presence of this component in lunar regolith, nor is there a compelling reason at present to postulate a global melting process for the generation of highly differentiated materials such as ‘kreep’ and the exotic component.