Four lunar mare meteorites: Crystallization trends of pyroxenes and spinels

Abstract— We studied crystallization trends of pyroxene and spinel in four Antarctic meteorites known to be derived from mare regions of the Moon: Y-793169 and A-881757 (YA meteorites) are unbrecciated igneous basalts, EET 87521 is a fragmental breccia, and Y-793274 is a regolith breccia. All have relatively low bulkrock TiO₂ content, and the YA meteorites are uncommonly ancient. Our electron probe microanalysis (EPMA) data indicate that the YA meteorites and the dominant mare components of Y-793274 and EET 87521 conform to a general trend for Ti-poor (low-Ti and very low-Ti) mare basalts. Their pyroxenes show a strong correlation between Fe/(Fe + Mg) (Fe#) and Ti/(Ti + Cr) (Ti#), both ratios typically increasing from core to rim. These trends presumably reflect local crystallization differentiation of interstitial melt. Previous studies (M. J. Drake and coworkers) have suggested that the detailed configurations of such Fe# vs. Ti# trends may reflect the bulk TiO₂ contents of the parent magmas (basalts). As a more systematic approach to this problem, we plot bulk-rock TiO₂ as a function of the Fe# = 0.50 intercept of each rock's pyroxene Fe# vs. Ti# trend. We call this intercept the Fe#-normalized Ti#. Based on our data for EET 87521, the YA meteorites, and Apollo 12 basalts 12031 and 12064, plus literature data for several other Ti-poor mare basalts, we find a strong correlation between Fe#-normalized Ti# and the bulk TiO₂ content of the parent basalt. This correlation confirms that fragmental breccia EET 87521 is nearly pure very low-Ti (VLT) basalt and that the YA meteorites, for which bulk-rock TiO₂ results scatter due to unusually coarse grain size (A-881757) or scarcity of available sample (Y-793169), are pieces of an uncommonly Ti-poor, but not quite VLT, variety of low-Ti mare basalt. Extrapolating from this correlation, the dominant mare component of regolith breccia Y-793274 is probably of VLT affinity. Besides the normal mare pyroxene trend of strong correlation between Fe# and Ti#, Y-793274 includes two additional pyroxene compositional trends, both showing a wide range of Ti# despite relatively constant (and low, by mare standards) Fe#. The most magnesian of these trends consists of a single clast with a mode of orthopyroxene + MgO-rich ilmenite. These two trends are of uncertain origin. Possibly one or both represents the highland component of this regolith breccia, although, unlike most highland pyroxenes, these appear relatively unaltered by impact brecciation and metamorphism. Compositions of spinels in the coarse-grained A-881757 show an extraordinary distribution: chromite and ulvöspinel components vary among grains but are nearly constant within grains. Despite its old age and unusually coarse grain sizes, mineralogical evidence (i.e., heterogeneity within both pyroxene and spinel; typical pyroxene exsolution scale very coarse by mare standards but exceeded by the pyroxenes of EET 87521 and Y-793274) indicates that A-881757 was cooled only slightly more slowly than typical mare basalts and may have formed near the center of an uncommonly thick lava flow. Both of the VLT basaltic lunar meteorite breccias, EET 87521 and Y-793274, are composed dominantly of pyroxenes with exsolution coarser than normal for mare basalts. Possibly VLT basalt flows tend to be systematically thicker, and thus more slowly cooled, than more Ti-rich flows.     

Petrogenesis of basaltic shergottite Northwest Africa 8657: Implications for fO2 correlations and element redistribution during shock melting in shergottites

Northwest Africa (NWA) 8657 is an incompatible trace element-enriched, low-Al basaltic shergottite, similar in texture and chemistry to Shergotty, Zagami, and NWA 5298. It is composed of zoned pyroxene, maskelynite, merrillite, and Ti-oxide minerals with minor apatite, silica, and pyrrhotite. Pyroxene grains are characterized by patchy zoning, with pigeonite or augite cores zoned to Fe-rich pigeonite mantles. The cores have rounded morphologies and irregular margins. Combined with the low Ti/Al of the cores, the morphology and chemistry of the pyroxene grains are consistent with initial crystallization at depth (30–70 km) followed by partial resorption en route to the surface. Enriched rare earth element (REE) equilibrium melt compositions and calculated oxygen fugacities (fO₂) conditions for pigeonite cores indicate that the original parent melts were enriched shergottite magmas that staged in chambers at depth within the Martian crust. NWA 8657 does not represent a liquid but rather entrained a proportion of pyroxene crystals from magma chambers where fractional crystallization was occurring at depth. Variation between fO₂ and bulk-rock (La/Yb)_N of the enriched and intermediate shergottites suggests that oxidation conditions and degree of incompatible element enrichment in the source may not be correlated, as thought previously. Shock melt pockets are characterized by an absence of phosphates and oxide minerals. It is likely that these phases were melted during shock. REEs were redistributed during this process into maskelynite and to a lesser extent the shock melt; however, the overall normalized REE profile of the shock melt is like that of the bulk-rock, but at lower absolute concentrations. Overall, shock melting has had a significant effect on the mineralogy of NWA 8657, especially the distribution of phosphates, which may be significant for geochronological applications of this meteorite and other Martian meteorites with extensive shock melt.     

Modification of REE distribution of ordinary chondrites from Atacama (Chile) and Lut (Iran) hot deserts: Insights into the chemical weathering of meteorites

The behavior of rare earth elements (REEs) during hot desert weathering of meteorites is investigated. Ordinary chondrites (OCs) from Atacama (Chile) and Lut (Iran) deserts show different variations in REE composition during this process. Inductively coupled plasma–mass spectrometry (ICP‐MS) data reveal that hot desert OCs tend to show elevated light REE concentrations, relative to OC falls. Chondrites from Atacama are by far the most enriched in REEs and this enrichment is not necessarily related to their degree of weathering. Positive Ce anomaly of fresh chondrites from Atacama and the successive formation of a negative Ce anomaly with the addition of trivalent REEs are similar to the process reported from Antarctic eucrites. In addition to REEs, Sr and Ba also show different concentrations when comparing OCs from different hot deserts. The stability of Atacama surfaces and the associated old terrestrial ages of meteorites from this region give the samples the necessary time to interact with the terrestrial environment and to be chemically modified. Higher REE contents and LREE‐enriched composition are evidence of contamination by terrestrial soil. Despite their low degrees of weathering, special care must be taken into account while working on the REE composition of Atacama meteorites for cosmochemistry applications. In contrast, chondrites from the Lut desert show lower degrees of REE modification, despite significant weathering signed by Sr content. This is explained by the relatively rapid weathering rate of the meteorites occurring in the Lut desert, which hampers the penetration of terrestrial material by forming voluminous Fe oxide/oxyhydroxides shortly after the meteorite fall.     

The distribution of basaltic asteroids in the Main Belt

We present the observational results of a survey designed to target and detect asteroids whose photometric colors are similar to those of Vesta family members and thus may be considered as candidates for having a basaltic composition. Fifty basaltic candidates were selected with orbital elements that lie outside of the Vesta dynamical family. Optical and near-infrared spectra were used to assign a taxonomic type to 11 of the 50 candidates. Ten of these were spectroscopically confirmed as V-type asteroids, suggesting that most of the candidates are basaltic and can be used to constrain the distribution of basaltic material in the Main Belt. Using our catalog of V-type candidates and the success rate of the survey, we calculate unbiased size-frequency and semi-major axis distributions of V-type asteroids. These distributions, in addition to an estimate for the total mass of basaltic material, suggest that Vesta was the predominant contributor to the basaltic asteroid inventory of the Main Belt, however scattered planetesimals from the inner Solar System (a<2.0 AU) and other partially/fully differentiated bodies likely contributed to this inventory. In particular, we infer the presence of basaltic fragments in the vicinity of Asteroid 15 Eunomia, which may be derived from a differentiated parent body in the middle Main Belt (2.5<a<2.8). We find no asteroidal evidence for a large number of previously undiscovered basaltic asteroids, which agrees with previous theories suggesting that basaltic fragments from the ∼100 differentiated parent bodies represented in meteorite collections have been “battered to bits” [Burbine, T.H., Meibom, A., Binzel, R.P., 1996. Meteorit. Planet. Sci. 31, 607-620].     

Postcrystallization metasomatism in shergottites: Evidence from the paired meteorites LAR 06319 and LAR 12011

Apatite is the major volatile‐bearing phase in Martian meteorites, containing structurally bound fluorine, chlorine, and hydroxyl ions. In apatite, F is more compatible than Cl, which in turn is more compatible than OH. During degassing, Cl strongly partitions into the exsolved phase, whereas F remains in the melt. For these reasons, the volatile concentrations within apatite are predictable during magmatic differentiation and degassing. Here, we present compositional data for apatite and merrillite in the paired enriched, olivine‐phyric shergottites LAR 12011 and LAR 06319. In addition, we calculate the relative volatile fugacities of the parental melts at the time of apatite formation. The apatites are dominantly OH‐rich (calculated by stoichiometry) with variable yet high Cl contents. Although several other studies have found evidence for degassing in the late‐stage mineral assemblage of LAR 06319, the apatite evolutionary trends cannot be reconciled with this interpretation. The variable Cl contents and high OH contents measured in apatites are not consistent with fractionation either. Volatile fugacity calculations indicate that water and fluorine activities remain relatively constant, whereas there is a large variation in the chlorine activity. The Martian crust is Cl‐rich indicating that changes in Cl contents in the apatites may be related to an external crustal source. We suggest that the high and variable Cl contents and high OH contents of the apatite are the results of postcrystallization interaction with Cl‐rich, and possibly water‐rich, crustal fluids circulating in the Martian crust.     

A petrologic and trace element study of Dar al Gani 476 and Dar al Gani 489: Twin meteorites with affinities to basaltic and lherzolitic shergottites

Abstract— We present the results of a combined mineralogic‐petrologic and ion microprobe study of two martian meteorites recently recovered in the Lybian Sahara, Dar al Gani 476 (DaG 476) and Dar al Gani 489 (DaG 489). Having resided in a hot desert environment for an extended time, DaG 476 and DaG 489 were subjected to terrestrial weathering that significantly altered their chemical composition. In particular, analyses of some of the silicates show light rare earth element (LREE)‐enrichment resulting from terrestrial alteration. In situ measurement of trace element abundances in minerals allows us to identify areas unaffected by this contamination and, thereby, to infer the petrogenesis of these meteorites. No significant compositional differences between DaG 476 and DaG 489 were found, supporting the hypothesis that they belong to the same fall. These meteorites have characteristics in common with both basaltic and lherzolitic shergottites, possibly suggesting spatial and petrogenetic associations of these two types of lithologies on Mars. However, the compositions of Fe‐Ti oxides and the size of Eu anomalies in the earliest‐formed pyroxenes indicate that the two Saharan meteorites probably experienced more reducing crystallization conditions than other shergottites (with the exception of Queen Alexandra Range (QUE) 94201). As is the case for other shergottites, trace element microdistributions in minerals of the DaG martian meteorites indicate that closed‐system crystal fractionation from a LREE‐depleted parent magma dominated their crystallization history. Furthermore, rare earth element abundances in the orthopyroxene megacrysts are consistent with their origin as xenocrysts rather than phenocrysts.     

Mineralogical characterization of some basaltic asteroids in the neighborhood of (4) Vesta: first results

We present reflectance spectra of 19 V-type asteroids obtained at the 3.6 m Telescopio Nazionale Galileo covering 0.8 to 2.5 μm. For 8 of these asteroids we obtained also visible spectra in the same observational run. The range from 0.8 to 2.5 μm, encompassing the 1 and 2 μm pyroxene features, allows a precise mineralogical characterization of these asteroids. The obtained data suggests the possible coexistence of distinct mineralogical groups among the V-type asteroids, either probing different layers of (4) Vesta or coming from different bodies. No clear correlation was found between mineralogies and the objects being, or not, member of the Vesta dynamical family.     

Cation distribution and exsolution in the Luna-20 pyroxenes as the chronological indicator of their thermal history

During the process of continuous post-crystallization cooling, the degree of phase transformation in pyroxenes (ordering and decomposition of solid solutions) is determined by the total cooling time and by cooling rates at individual stages; this degree can also be applied to the estimation of the sub-solidus cooling time of pyroxenes and of rocks, genetically associated with them (geochronometer). The degree of Fe²⁺-Mg order in distribution between theM ₁ andM ₂ sites observed in orthopyroxenes can be used as the criterion for the sub-solidus cooling rates of orthopyroxenes. A technique for approximate estimation of cooling time (rates) of orthopyroxenes in the 1000–500°C interval is suggested on the basis of an ion exchange reaction kinetic model, with the adoption of a few assumptions. X-ray structural analysis study revealed the character of the Fe²⁺-Mg intersite distribution in the M-511 and M-533 orthopyroxenes from the Luna-20 regolith; the obtainedK _D values make 0.072(T _e ^o ~ 460°C) and 0.06 (T _e ^o ~ 480°C), respectively. The corresponding evaluation of the linear cooling rate in the 1000-500°C range, as inferred by the obtained data, gave the value of 0.2–0.25° h^–1. Two features of the exosolution relate to the sub-solidus cooling rate of clinopyroxenes: the degree of phase separation by composition W_o and the exsolution scale . The monocrystal X-ray diffraction study, the transmission electron microscopy and the X-ray microprobe analysis produced the exsolution parameters for clinopyroxenes 2001-3, 2001-6 and 559 from the Luna-20 regolith: W_o 26%, 28% and 45% respectively; 300 Å, 1000 Å and 60 , respectively. The solvus temperatures (T _e) of the clinopyroxenes 2001-3 and 2001-6 make, respectively, 1100 ± 25°C and 1095 ± 45°C; the spinodal temperatures (T _s) areT _s =T _e – 50°C and represent quenching temperatures of the given pyroxenes in relation to the exsolution process. The cooling time fromT _cryst. toT _s was estimated and the value of the formal linear cooling rate was found to be ~ 90° h^–1 (2001-3) and ~ 8° h^–1 (2001-6). The average value of the pyroxene 559 sub-soldius cooling rate in the 1150–1000°C range was four orders less. The obtained data were discussed in view of the possible genesis of the studied Luna-20 pyroxenes.     

Martian basalt (shergottite) Queen Alexandra Range 94201 and lunar basalt 15555: A tale of two pyroxenes

Abstract— We present here ion microprobe analyses of rare earth and other selected trace and minor elements in pyroxenes of shergottite Queen Alexandra Range 94201 and lunar basalt 15555. Pyroxene zonation patterns record the crystallization histories of these two basaltic samples from Mars and the Moon, respectively, and allow a comparison of mafic melt evolution on these two planetary bodies. Elemental abundances and trends in pyroxenes of these two rocks indicate that their minerals formed by continuous, closed system fractional crystallization of their respective parent melts. This further supports the idea that QUE 94201 closely represents the composition of a true Martian basaltic melt (McSween et al., 1996). The main differences in pyroxene elemental zonation patterns in these two objects are attributed to earlier crystallization of whitlockite in QUE 94201 (i.e., before the Fe-rich pyroxenes) than in 15555 (after the Fe-rich pyroxenes). The size of Eu anomalies in pyroxenes of QUE 94201 is intermediate between that in pyroxenes of 15555 and the other shergottites and may imply that fO₂ conditions during crystallization of this Martian basalt were significantly more reducing than for other shergottites, although not quite as reducing as for lunar basalts. Cerium anomalies appear to be less prevalent in pyroxenes of QUE 94201 than other Antarctic shergottites and could be indicative of lesser degree of weathering in the Antarctic.     

Chemical compositions of martian basalts (shergottites): Some inferences on b; formation,mantle metasomatism,and differentiation in Mars

Abstract— Bulk chemical compositions of the shergottite basalts provide important constraints on magma genesis and mantle processes in Mars. Abundances of many major and trace elements in the shergottites covary in 2 distinct groups: Group 1 (Gl) includes mostly highly incompatible elements (e.g., La, Th), and Group 2 (G2) includes mostly moderately incompatible elements (e.g., Ti, Lu, Al, Hf). Covariations of G2 elements (not necessarily linear) are consistent with partitioning between basalt magma and orthopyroxene + olivine. This fractionation represents partial melting to form the shergottites and their crystallization; the restite minerals cannot include aluminous phase(s), phosphate, ilmenite, zircon, or sulfides. Overall, abundances of Gl elements are decoupled from those of G2. In graphing abundances of a Gl element against those of a G2 element, G1/G2 abundance ratios do not appear to be random but are restricted to 4 values. Shergottites with a given G1/G2 value need not have the same crystallization age and need not fall on a single fractionation trajectory involving compatible elements (e.g., Ti versus Fe*). These observations imply that the G1/G2 families were established before basalt formation and suggest metasomatic enrichment of their source region (major carrier of G2 elements) by a component rich in Gl elements. Group 1 elements were efficiently separated from G2 elements very early in Mars' history. Such efficient fractionation is not consistent with simple petrogenesis; it requires multiple fractionations, “complex” petrogenetic processes, or minerals with unusual geochemistry. The behavior of phosphorus in this early fractionation event is inexplicable by normal petrogenetic processes and minerals. Several explanations are possible, including significant compatibility of P in majoritic garnet and the presence of P‐bearing iron metal (or a phosphide phase) in the residual solid assemblage (carrier of G2 elements). If the latter, Mars' mantle is more oxidized now than during the ancient fractionation event.     

Water distribution in Martian permafrost regions from joint analysis of HEND (Mars Odyssey) and MOLA (Mars Global Surveyor) data

We jointly analyze data from the High-Energy Neutron Detector (HEND) onboard the NASA Mars Odyssey spacecraft and data from the Mars Orbiter Laser Altimeter (MOLA) onboard the Mars Global Surveyor spacecraft. The former instrument measures the content of hydrogen (in the form of H₂O or OH) in the subsurface layer of soil and the latter instrument measures the surface albedo with respect to the flux of solar energy. We have checked the presence of a correlation between these two data sets in various Martian latitude bands. A significant correlation has been found between these data at latitudes poleward of 40° in the northern hemisphere and at latitudes 40°–60° in the southern hemisphere. This correlation is interpreted as evidence for the presence of stable water ice in these regions under a dry layer of soil whose thickness is determined by the condition for equilibrium between the condensation of water from the atmosphere and its sublimation when heated by solar radiation. For these regions, we have derived an empirical relation between the flux of absorbed solar radiation and the thickness of the top dry layer. It allows the burial depth of the water ice table to be predicted with a sub-kilometer resolution based on near-infrared albedo measurements. We have found no correlation in the southern hemisphere at latitudes >60°, although neutron data also suggest that water ice is present in this region under a layer of dry soil. We conclude that the thickness of the dry layer in this region does not correspond to the equilibrium condition between the water ice table and the atmosphere.     

Petrologic insights from the spectra of the unbrecciated eucrites: Implications for Vesta and basaltic asteroids

Abstract– We investigate the relationship between the petrology and visible–near infrared spectra of the unbrecciated eucrites and synthetic pyroxene–plagioclase mixtures to determine how spectra obtained by the Dawn mission could distinguish between several models that have been suggested for the petrogenesis of Vesta’s crust (e.g., partial melting and magma ocean). Here, we study the spectra of petrologically characterized unbrecciated eucrites to establish spectral observables, which can be used to yield mineral abundances and compositions consistent with petrologic observations. No information about plagioclase could be extracted from the eucrite spectra. In contrast, pyroxene dominates the spectra of the eucrites and absorption band modeling provides a good estimate of the relative proportions of low‐ and high‐Ca pyroxene present. Cr is a compatible element in eucrite pyroxene and is enriched in samples from primitive melts. An absorption at 0.6 μm resulting from Cr³⁺ in the pyroxene structure can be used to distinguish these primitive eucrites. The spectral differences present among the eucrites may allow Dawn to distinguish between the two main competing models proposed for the petrogenesis of Vesta (magma ocean and partial melting). These models predict different crustal structures and scales of heterogeneity, which can be observed spectrally. The formation of eucrite Allan Hills (ALH) A81001, which is primitive (Cr‐rich) and relatively unmetamorphosed, is hard to explain in the magma ocean model. It could only have been formed as a quench crust. If the magma ocean model is correct, then ALHA81001‐like material should be abundant on the surface of Vesta and the Vestoids.     

Albedos and size distribution of meteoroids from 0.3 to 4.8 AU

Comparison is made between the run of number density of meteoroids from penetration detectors aboard Helios A (masses below 10^?8 g) and Pioneer 10 (masses near and above 3 × 10^?9 g), the source function of the zodiacal light deduced from photometric observations aboard Helios A and Pioneer 10, counts versus brightness of objects passing by Pioneer 10 from the Sisyphus experiment and the distribution of meteoroids deduced from radar and optical meteors at the Earth. The Sisyphus experiment on Pioneer 10 observed reflecting glints on meteoroids rather than the meteoroids themselves and the counting statistics refer not to the effective radii of the meteoroids but to the effective radii of curvature of the reflecting glints on the meteoroids. The penetration detectors appear to find some increase in number density toward the Sun and a flat distribution outward to 5.2 AU. The overall behavior of the zodiacal light is that the relative distribution over direction is unchanged while the source scattering function diminishes as the inverse 1.4 power of distance from the Sun. The fit to the brightness of the zodiacal light obtained from these statistics can be combined with the mass distribution results from the optical meteors to deduce a mean geometric albedo of meteoroids of 0.006 at 1 AU from the Sun. Combination of the space distribution from radar meteors with the scattering source function of the zodiacal light yields geometric albedos for meteoroids running from 0.07 at 0.1 AU, from the Sun through 0.006 at 1 AU down to about 0.0001 at 3.3 AU which may run flat thence outward. This result is imposed by the indicated modest increase in density of meteoroids very near the Sun, a minimum between the Sun and the Earth near 0.4 AU and rising density outward to somewhere beyond 3.3 AU which is very different from the inverse 1.4 power of the distance shown for scatterers (product of number density and albedo) by the zodiacal light. A check on the distribution at very large sizes is possible if a search is made for fireballs in Jupiter's atmosphere by the Mariner Jupiter Saturn 1977 television cameras during the two encounters with Jupiter in 1979. An easy detection of such activity would put the maximum in the meteoroid distribution out near Jupiter and lend further confirmation to the indicated drop in albedo.     

The size distribution of Jupiter's main ring from Galileo imaging and spectroscopy

Galileo's Solid State Imaging experiment (SSI) obtained 36 visible wavelength images of Jupiter's ring system during the nominal mission (Ockert-Bell et al., 1999, Icarus 138, 188-213) and another 21 during the extended mission. The Near Infrared Mapping Spectrometer (NIMS) recorded an observation of Jupiter's main ring during orbit C3 at wavelengths from 0.7 to 5.2 μm; a second observation was attempted during orbit E4. We analyze the high phase angle NIMS and SSI observations to constrain the size distribution of the main ring's micron-sized dust population. This portion of the population is best constrained at high phase angles, as the light scattering behavior of small dust grains dominates at these geometries and contributions from larger ring particles are negligible. High phase angle images of the main ring obtained by the Voyager spacecraft covered phase angles between 173.8° and 176.9° (Showalter et al., 1987, Icarus 69, 458-498). Galileo images extend this range up to 178.6°. We model the Galileo phase curve and the ring spectra from the C3 NIMS ring observation as the combination of two power law distributions. Our analysis of the main ring phase curve and the NIMS spectra suggests the size distribution of the smallest ring particles is a power law with an index of 2.0±0.3 below a size of ∼15 μm that transitions to a power law with an index of 5.0±1.5 at larger sizes. This combined power law distribution, or “broken power law” distribution, yields a better fit to the NIMS data than do the power law distributions that have previously been fit to the Voyager imaging data (Showalter et al., 1987, Icarus 69, 458-498). The broken power law distribution reconciles the results of Showalter et al. (1987, Icarus 69, 458-498) and McMuldroch et al. (2000, Icarus 146, 1-11), who also analyzed the NIMS data, and can be considered as an obvious extension of a simple power law. This more complex size distribution could indicate that ring particle production rates and/or lifetimes vary with size and may relate to the physical processes that control their evolution. The significant near arm/far arm asymmetry reported elsewhere (see Showalter et al., 1987, Icarus 69, 458-498; Ockert-Bell et al., 1999, Icarus 138, 188-213) persists in the data even after the main ring is isolated in the SSI images. However, the sense of the asymmetry seen in Galileo images differs from that seen in Voyager images. We interpret this asymmetry as a broad-scale, azimuthal brightness variation. No consistent association with the magnetic field of Jupiter has been observed. It is possible that these longitudinal variations may be similar to the random brightness fluctuations observed in Saturn's F ring by Voyager (Smith et al., 1982, Science 215, 504-537) and during the 1995 ring plane crossings (Nicholson et al., 1996, Science 272, 509-515; Bosh and Rivkin, 1996, Science 272, 518-521; Poulet et al., 2000, Icarus 144, 135-148). Stochastic events may thus play a significant role in the evolution of the jovian main ring.     

A derivation of the luminosity function of the Kuiper belt from a broken power-law size distribution

We have derived a model of the Kuiper belt luminosity function exhibited by a broken power-law size distribution. This model allows direct comparison of the observed luminosity function to the underlying size distribution. We discuss the importance of the radial distribution model in determining the break diameter. We determine a best-fit break-diameter of the Kuiper belt size-distribution of 30<Db<90 km via a maximum-likelihood fit of our model to the observed luminosity function. We also confirm that the observed luminosity function for m(R)∼21-28 is consistent with a broken power-law size distribution, and exhibits a break at .     

Hapke mixture modeling applied to VNIR spectra of mafic mineral mixtures and shergottites: Implications for quantitative analysis of satellite data

The mineralogy of Mars is well understood on a qualitative level at a global scale due to satellite data. Quantitative analysis of visible and near-infrared (VNIR) satellite data is a desirable but nontrivial task, due partly to the nonlinearity of VNIR reflectance spectra from the mineral mixtures of the Martian surface. In this study, we investigated the use of the Hapke radiative transfer model to generate linearly mixed single scattering albedo data from nonlinearly mixed VNIR reflectance data and then quantitatively analyzed them using the linear spectral mixture model. Simplifications to the Hapke equation were tested accounting for variables that would be unknown when using satellite data. Mineral mixture spectra from the RELAB spectral library were degraded to test the robustness of the unmixing technique in the face of data that mimic some of the complexities of satellite spectral data collected at Mars. A final test was performed on spectra from shergottite meteorites to assess the technique against real Martian mineral mixtures. The simplified Hapke routine produced robust abundance estimates within 5–10% accuracy when applied to laboratory standard spectra from the synthetic mixtures of igneous minerals in agreement with previous studies. The results of tests involving degraded data to mimic the low spectral contrast of the Martian surface and the lack of a priori knowledge of the constituent mineral spectral endmembers, however, were less encouraging, with errors in abundance estimation greater than 25%. These results cast doubt on the utility of Hapke unmixing for the quantitative analysis of VNIR data of the surface of Mars.     

Complex crater formation: Insights from combining observations of shock pressure distribution with numerical models at the West Clearwater Lake impact structure

Large impact structures have complex morphologies, with zones of structural uplift that can be expressed topographically as central peaks and/or peak rings internal to the crater rim. The formation of these structures requires transient strength reduction in the target material and one of the proposed mechanisms to explain this behavior is acoustic fluidization. Here, samples of shock‐metamorphosed quartz‐bearing lithologies at the West Clearwater Lake impact structure, Canada, are used to estimate the maximum recorded shock pressures in three dimensions across the crater. These measurements demonstrate that the currently observed distribution of shock metamorphism is strongly controlled by the formation of the structural uplift. The distribution of peak shock pressures, together with apparent crater morphology and geological observations, is compared with numerical impact simulations to constrain parameters used in the block‐model implementation of acoustic fluidization. The numerical simulations produce craters that are consistent with morphological and geological observations. The results show that the regeneration of acoustic energy must be an important feature of acoustic fluidization in crater collapse, and should be included in future implementations. Based on the comparison between observational data and impact simulations, we conclude that the West Clearwater Lake structure had an original rim (final crater) diameter of 35–40 km and has since experienced up to ~2 km of differential erosion.     

An improbable concentration of basaltic meteorite falls (HED and mesosiderite) in the mid-20th century

Abstract The fall rate of HED basaltic meteorites (howardites, eucrites, diogenites) has not been constant in the 20th century, while the fall rate of chondrites has been constant within error. Thirteen of the 26 dated HED falls (day of fall known, 1900 through 1989) fell in 1924 through 1939. A fall cluster (not a meteorite stream) like this will occur in less than one in 100 random distributions of fall days. The proportions of HED types in the whole cluster are statistically identical to those of the whole historical record of HED falls, as is the distribution of cosmic ray exposure ages. In a subset of the cluster, 1924 through 1933, eight of those nine HED falls from have exposure ages of 10–20 Ma; this grouping is statistically distinct from that of the historical record. The mesosiderite meteorites share many chemical and isotopic properties with the HEDs but are not from the same parent body. However, the dates of the three mesosiderite falls of the 20th century (all in 1924 through 1939) are a likely sampling of the distribution of HED fall dates; less than one in 200 random distributions of three fall dates would have them all in a given IS year interval of the 20th century. If the concentration of HED and mesosiderite falls in 1924 through 1939 is not a result of chance (odds of less than 1 in 200), it must have had a cause or causes. The cause(s) are not dear but appear(s) to have operated: on parent bodies only of basaltic meteorites; on a number of such parent bodies (mesosiderite and at least one HED); distant from Earth; and so as to produce a duster of only 15 years duration. This duration is much shorter than the expected time scales or orbital evolution of asteroidal fragments.     

The fossilized size distribution of the main asteroid belt

Planet formation models suggest the primordial main belt experienced a short but intense period of collisional evolution shortly after the formation of planetary embryos. This period is believed to have lasted until Jupiter reached its full size, when dynamical processes (e.g., sweeping resonances, excitation via planetary embryos) ejected most planetesimals from the main belt zone. The few planetesimals left behind continued to undergo comminution at a reduced rate until the present day. We investigated how this scenario affects the main belt size distribution over Solar System history using a collisional evolution model (CoEM) that accounts for these events. CoEM does not explicitly include results from dynamical models, but instead treats the unknown size of the primordial main belt and the nature/timing of its dynamical depletion using innovative but approximate methods. Model constraints were provided by the observed size frequency distribution of the asteroid belt, the observed population of asteroid families, the cratered surface of differentiated Asteroid (4) Vesta, and the relatively constant crater production rate of the Earth and Moon over the last 3 Gyr. Using CoEM, we solved for both the shape of the initial main belt size distribution after accretion and the asteroid disruption scaling law . In contrast to previous efforts, we find our derived function is very similar to results produced by numerical hydrocode simulations of asteroid impacts. Our best fit results suggest the asteroid belt experienced as much comminution over its early history as it has since it reached its low-mass state approximately 3.9-4.5 Ga. These results suggest the main belt's wavy-shaped size-frequency distribution is a “fossil” from this violent early epoch. We find that most diameter D?120 km asteroids are primordial, with their physical properties likely determined during the accretion epoch. Conversely, most smaller asteroids are byproducts of fragmentation events. The observed changes in the asteroid spin rate and lightcurve distributions near D∼100-120 km are likely to be a byproduct of this difference. Estimates based on our results imply the primordial main belt population (in the form of D<1000 km bodies) was 150-250 times larger than it is today, in agreement with recent dynamical simulations.