The Sudbury-Serenitatis analogy and 'so-called' pristine nonmare rocks

The Serenitatis Basin is the one lunar basin from which we confidently identify a suite of samples as pieces of the impact melt sheet: the distinctive Apollo 17 noritic breccias. Recent studies of the Sudbury Complex indicate that its 'irruptive' is almost entirely of impact-melt origin, making it the closest terrestrial analog to the Serenitatis melt sheet. Any attempt to model the evolution of the Moon's crust should be compatible with the relatively well-understood Sudbury Complex. However, the Sudbury-Moon analogy might be a misleading oversimplification, if applied too rigidly. The cause of evolutionary differences between the Serenitatis impact melt and the Sudbury impact melt is discussed

Mars regolith versus SNC meteorites: Evidence for abundant crustal carbonates

Viking XRF analyses are compared with those for terrestrial and lunar basalt samples, and eucritic meteorites (of possible Mars origin). The comparison indicates depletion of Ca relative to Si in the Mars regolith. It is suggested that carbonate formation during a warmer, wetter epoch early in Mars' history could have been responsible

The Last Stages of Terrestrial Planet Formation: Dynamical Friction and the Late Veneer

The final stage of terrestrial planet formation consists of the cleanup of residual planetesimals after the giant impact phase. Dynamically, a residual planetesimal population is needed to damp the high eccentricities of the terrestrial planets after the giant impact stage. Geochemically, highly siderophile element (HSE) abundance patterns inferred for the terrestrial planets and the Moon suggest that a total of about 0.01 M_Earth of chondritic material was delivered as `late veneer' by planetesimals to the terrestrial planets after the end of giant impacts. Here we combine these two independent lines of evidence for a leftover population of planetesimals and show that: 1) A residual planetesimal population containing 0.01 M_Earth is able to damp the eccentricities of the terrestrial planets after giant impacts to their observed values. 2) At the same time, this planetesimal population can account for the observed relative amounts of late veneer added to the Earth, Moon and Mars provided that the majority of the late veneer was delivered by small planetesimals with radii <10m. These small planetesimal sizes are required to ensure efficient damping of the planetesimal's velocity dispersion by mutual collisions, which in turn ensures that the planets' accretion cross sections are significantly enhanced by gravitational focusing above their geometric values. Specifically we find, in the limit that the relative velocity between the terrestrial planets and the planetesimals is significantly less than the terrestrial planets' escape velocities, that gravitational focusing yields an accretion ratio Earth/Mars~17, which agrees well with the accretion ratio inferred from HSEs of 12-23. For the Earth-Moon system, we find an accretion ratio of ~200, which is consistent with estimates of 150-700 derived from HSE abundances that include the lunar crust as well as mantle component. (Abridged)Comment: accepted for publication in ApJ, 9 pages, 4 figures; minor corrections, additional references adde

Pristine Igneous Rocks and the Early Differentiation of Planetary Materials

Our studies are highly interdisciplinary, but are focused on the processes and products of early planetary and asteroidal differentiation, especially the genesis of the ancient lunar crust. Most of the accessible lunar crust consists of materials hybridized by impact-mixing. Rare pristine (unmixed) samples reflect the original genetic diversity of the early crust. We studied the relative importance of internally generated melt (including the putative magma ocean) versus large impact melts in early lunar magmatism, through both sample analysis and physical modeling. Other topics under investigation included: lunar and SNC (martian?) meteorites; igneous meteorites in general; impact breccias, especially metal-rich Apollo samples and polymict eucrites; effects of regolith/megaregolith insulation on thermal evolution and geochronology; and planetary bulk compositions and origins. We investigated the theoretical petrology of impact melts, especially those formed in large masses, such as the unejected parts of the melts of the largest lunar and terrestrial impact basins. We developed constraints on several key effects that variations in melting/displacement ratio (a strong function of both crater size and planetary g) have on impact melt petrology. Modeling results indicate that the impact melt-derived rock in the sampled, megaregolith part of the Moon is probably material that was ejected from deeper average levels than the non-impact-melted material (fragmental breccias and unbrecciated pristine rocks). In the largest lunar impacts, most of the impact melt is of mantle origin and avoids ejection from the crater, while most of the crust, and virtually all of the impact-melted crust, in the area of the crater is ejected. We investigated numerous extraordinary meteorites and Apollo rocks, emphasizing pristine rocks, siderophile and volatile trace elements, and the identification of primary partial melts, as opposed to partial cumulates. Apollo 15 sample 15434,28 is an extraodinarily large glass spherule, nearly if not entirely free of meteoritic contamination, and provides insight into the diversity of mare basalts in the Hadley-Apennine region. Apollo 14 sample 14434 is in many respects a new rock type, intermediate between nonmare gabbronorites and mare basalts. We helped to both plan and implement a consortium to study the Yamato-793605 SNC/martian meteorite

Geochemical investigation of five lunar meteorites : Implications for the composition, origin and evolution of the lunar crust

The total number of lunar meteorites is now eleven, and the number of distinct finds now stands at eight. We report new data for the bulk compositions of five lunar meteorites, including Y-793274,a unique regolith breccia composed of 2/3 mare material of VLT (very-low-Ti) affinity and 1/3 highlands material compositionally similar to Apollo 16 regolith. Mixing-deconvolution of the bulk-rock composition shows that for any reasonable assumed highlands-component composition, the TiO_2 content of the mare component must be <1.2 wt%. A unique clast from the Y-791197 highlands regolith breccia is also probably of VLT-mare affinity, based on its REE pattern, Sc/Sm, Mn/Sm, and Eu/Al ratios, and the low Ti/(Ti+Cr) ratio of its pyroxene. VLT affinities have previously been inferred for numerous mare clasts observed in thin section studies of highlands meteorites, and three other recently-discovered lunar meteorites have either VLT or borderline VLT/"normal" mare compositions. Apparently, the abundance of VLT-basaltic matter in the lunar crust is greater than previously supposed; and the fundamental dichotomy of lunar magmatism into distinct nonmare and mare styles may have been less abrupt than commonly envisaged. Alternatively, the prevalence of VLT varieties of mare basalt among the lunar meteorites might be a sign of source-crater pairing. However, it seems probable that at least three, and more likely five or more, separate craters are represented; and at least one of the sources is probably on the farside. In terms of major-element composition, the four highlands regolith breccia meteorites resemble the Apollo 16 regolith. The Si content and Ca/Al ratio of the upper lunar crust appear to be monotonously close to 210mg/g and 0.53 (molar), respectively. However, concentrations of incompatible elements, including K, Th, and U, are far lower in the highlands meteorites than in regolith samples from the central nearside. This trend implies that inferred "lower limits" on the bulk-Moon content of U (and associated refractory lithophile elements) should be relaxed

Yamato-793605, EET79001, and other presumed martian meteorites: Compositional clues to their origins

We report new data on the bulk compositions of several martian meteorites, most notably the Y-793605 peridotite. We find that Y-793605 is remarkably similar in composition to the other shergottitic peridotites. ALH77005 and EET79001. This high degree of compositional uniformity, and analogy to terrestrial ultramafic cumulates, suggest that the three peridotites formed in the comparatively uniform early-middle (and thus deep) portion of a martian layered intrusion. Considering that the process of launching off Mars probably required that a prior impact excavated these originally deep materials to positions closer to the surface, their compositional uniformity seems improbable, unless they were originally launched as a single mass that shortly thereafter underwent a secondary collisional fragmentation in the asteroid belt. We interpret "A" lithology of EET79001 as more likely to be an endogenously produced mixed magma than an impact melt. Our results indicate that Au is only slightly enriched in EET79001-A, compared to EET79001-B. The EET79001-A lithophile element composition can only be roughly approximated by modeling it as a mixture of EET79001-B plus any known variety of shergottitic peridotite, and the Au contents of martian meteorites (in general) are highly diverse. The presence of only two grossly dissimilar rock types (EET79001-B and EET79001-"X") as discernible components in EET79001-A seems an unlikely outcome from the chaotic process of impact mixing. Moreover, in general, it seems improbable that one of only 12 martian meteorites would be an impact melt of relatively young age

Geochemical investigation of two lunar mare meteorites: Yamato-793169: and Asuka-881757

We report new data for the bulk compositions of two meteorites derived from the mare lava plains of the Earth\u27s Moon : Yamato-793169 and Asuka-881757. These meteorites are remarkably similar to one another, and clearly of lunar origin, but not quite like any previously studied lunar rock. In the Ti-based classification system for mare basalts, Y-793169 and A-881757 are both low-Ti. However, viewed in relation to their extraordinarily low mg ratios, their Ti contents are also marginally consistent with kinship to previously-studied VLT (very-low-Ti) mare basalts. In another popular (albeit arbitrary) classification system, A-881757 is low-Al, and Y-793169 is marginally high-Al. Resemblances to VLT mare basalts are also evident from their unfractionated middle-REE/heavy-REE ratios and high Sc/Sm ratios. On a plot of Sc vs. Ti, Y-793169 and A-881757 plot near one another, and well apart from any Apollo/Luna mare basalt. They resemble low-Ti basalts in terms of moderate overall REE contents and V/Sm and Cr/Sm ratios, and they even resemble high-Ti basalts in having high Sc coupled with low V. The high Sc contents probably reflect partial melting of a Sc-rich source (or sources) in the lunar mantle. Neither meteorite shows the enrichments in K and Ge that remain peculiar to mare basalts from Apollo 14,apparently due to assimilation of the K-rich granitic and Ge-rich KREEPy materials that are uncommonly abundant in the Apollo 14 region. A set of remarkable coincidences between Y-793169 and A-881757 (from our data : their bizarrely high Sc despite low Ti, and extraordinarily low mg; other workers have demonstrated similar ages, ^U/^Pb, and times of blast-off from the Moon) suggests that these two meteorites might be paired, in the sense of having left the Moon in a single cratering-ejection event. The geochemical contrasts between Y-793169 and A-881757 and previously available mare basalts show that the full diversity of the lunar maria, and the factors that govern variability among mare basalts, are still poorly understood. The diversity and lack of systematics among lunar mare basalt compositions, reinforced by Y-793169 and A-881757,support the magma ocean cumulate model for genesis of the mare source regions. The relationship between geochemistry and age among mare basalts seems more complex that previously supposed, probably because the compositional stratification that develops in the pile of magma ocean cumulates is too disorderly to conform with any model for depth-time-temperature evolution of the lunar interior

The Yamato-793605 martian meteorite consortium

We describe the consortium that has been organized to coordinate study of the Yamato-793605 shergottitic peridotite, a 16-g meteorite of presumed martian origin. Y-793605 is similar to two other Antarctic martian meteorites, ALH-77005 and LEW88516. One of the most important goals of the consortium is to determine how the cosmic-ray exposure history of Y-793605 compares with those of ALH-77005 and LEW88516. An aphanitic-glassy shock melted lithology that constitutes, as scattered veins and globby enclaves, roughly 30 vol% of the rock, has also been targeted for particularly intense scrutiny. Consortium results reported in the papers that follow confirm strong similarities between Y-793605 and ALH-77005 and LEW88516,in terms of mineralogy and petrology, trace elements, and cosmic-ray exposure. The mineralogical, petrological, and trace-element similarities are so strong, they suggest that Y-793605,ALH-77005 and LEW88516 formed as cumulates (i.e., mainly large poikilitic pyroxenes enclosing cumulus olivines and chromites) from a single parent magma. The igneous crystallization age of Y-793605 has not yet been precisely determined. However, some important consortium investigations are still underway

Intercomparison of Multiple UV-LIF Spectrometers using the Aerosol Challenge Simulator

Measurements of primary biological aerosol particles (PBAPs) have been conducted worldwide using ultraviolet light-induced fluorescence (UV-LIF) spectrometers. However, how these instruments detect and respond to known biological and non-biological particles, and how they compare, remains uncertain due to limited laboratory intercomparisons. Using the Defence Science and Technology Laboratory, Aerosol Challenge Simulator (ACS), controlled concentrations of biological and non-biological aerosol particles, singly or as mixtures, were produced for testing and intercomparison of multiple versions of the Wideband Integrated Bioaerosol Spectrometer (WIBS) and Multiparameter Bioaerosol Spectrometer (MBS). Although the results suggest some challenges in discriminating biological particle types across different versions of the same UV-LIF instrument, a difference in fluorescence intensity between the non-biological and biological samples could be identified for most instruments. While lower concentrations of fluorescent particles were detected by the MBS, the MBS demonstrates the potential to discriminate between pollen and other biological particles. This study presents the first published technical summary and use of the ACS for instrument intercomparisons. Within this work a clear overview of the data pre-processing is also presented, and documentation of instrument version/model numbers is suggested to assess potential instrument variations between different versions of the same instrument. Further laboratory studies sampling different particle types are suggested before use in quantifying impact on ambient classification.Peer reviewe