¹A. Kar, ¹A.K. Sen, ²R. Gupta
¹Department of Physics, Assam University, Silchar-788011, India
²Inter-University Centre for Astronomy and Astrophysics, Pune-411007, India

New Laboratory phase curves are presented, to examine the effect of porosity on reflectance as a function of phase angle for grain size having dimension about half, twice and those larger than the illuminating wavelength. The experimental setup used for generating reflectance data is a goniometric device developed at the Dept. of Physics, Assam University, Silchar, India. Some of the well-documented samples having different sizes were chosen; alumina, olivine, basalt, rutile, chromite and iron. The sample surfaces were prepared with different porosities, in order to simulate natural regolith surface as much as possible. The wavelength of observation is 632.8 nm. A model based on the Radiative Transfer Equation is presented here to analyze and model the laboratory data. In the present modelling work, the empirical relation of Hapke, Mie theory and Henyey-Greenstein phase function are used. For particles having dimension about half, twice to the wavelength, Mie theory is used to calculate single scattering albedo. Although the Mie theory is insufficient for describing the scattering properties of particles larger than the wavelength, for such large particle single scattering albedo (SSA) is estimated through method of best fit. It has been found that, the porosity has a distinguishable effect on reflectance. Also the contribution of multiple scattering function for different porosity is examined. Further the results presented in the current work, demonstrates the light scattering properties of a diverse collections of regolith like samples.

Reference
Kar A, Sen AK, Gupta R (2016) Laboratory photometry of regolith analogues: effect of porosity. Icarus (in Press)
Link to Article [doi:10.1016/j.icarus.2016.05.024]
Copyright Elsevier

¹R.S. Jackson et al. (>10)*
¹University of New Mexico, Albuquerque, NM 87131, USA
*Find the extensive, full author and affiliation list on the publishers website

The ChemCam instrument on the Mars Science Laboratory rover analyzed the rock surface, drill hole walls, tailings, and unprocessed and sieved dump piles to investigate chemical variations with depth in the first two Martian drill holes and possible fractionation or segregation effects of the drilling and sample processing. The drill sites are both in Sheepbed Mudstone, the lowest exposed member of the Yellowknife Bay formation. Yellowknife Bay is composed of detrital basaltic materials in addition to clay minerals and an amorphous component. The drill tailings are a mixture of basaltic sediments and diagenetic material like calcium sulfate veins, while the shots on the drill site surface and walls of the drill holes are closer to those pure end members. The sediment dumped from the Sample Acquisition, Processing, and Handling Subsystem is of similar composition to the tailings; however, due to the specifics of the drilling process the tailings and dump piles come from different depths within the hole. This allows the ChemCam instrument to analyze samples representing the bulk composition from different depths. On the pre-drill surfaces, the Cumberland site has a greater amount of CaO and evidence for calcium sulfate veins, than the John Klein site. However, John Klein has a greater amount of calcium sulfate veins below the surface, as seen in mapping, drill hole wall analysis, and observations in the drill tailings and dump pile. In addition, the Cumberland site does not have any evidence of variations in bulk composition with depth down the drill hole, while the John Klein site has evidence for a greater amount of CaO (calcium sulfates) in the top portion of the hole compared to the middle section of the hole, where the drill sample was collected.

Reference
Jackson RS et al. (2016) ChemCam Investigation of the John Klein and Cumberland Drill Holes and Tailings, Gale Crater, Mars. Icarus (in Press)
Link to Article [doi:10.1016/j.icarus.2016.04.026]
Copyright Elsevier

¹Paul G. Lucey, ²Benjamin T. Greenhagen, ³Eugenie Song, ⁴Jessica A. Arnold, ^1,5Myriam Lemelin, ⁴Kerri Donaldson Hanna, ⁴Neil E. Bowles, ⁶Timothy D. Glotch, ⁷David A. Paige
¹Hawaii Institute of Geophysics and Planetology, University of Hawaii at Manoa, 1680 East West Road, Honolulu 96822, HI USA
²Johns Hopkins University Applied Physics Laboratory, 11101 Johns Hopkins Rd. Laurel 20723, MD USA
³Jet Propulsion Laboratory, 4800 Oak Grove Drive Pasadena Mail Stop 264-623, CA 91109 USA
⁴Clarendon Laboratory, University of Oxford, Parks Road, Oxford OX1 3PU, United Kingdom
⁵Department of Geology and Geophysics, University of Hawaii at Manoa, 1680 East West Road, Honolulu 96822, HI, USA
⁶Department of Geosciences, Stony Brook University, Stony Brook, NY 11794-2100, USA
⁷Dept. of Earth, Planetary and Space Science, University of California Los Angeles, Los Angeles 90095, CA USA

Multispectral infrared measurements by the Diviner Lunar Radiometer Experiment on the Lunar Renaissance Orbiter enable the characterization of the position of the Christiansen Feature, a thermal infrared spectral feature that laboratory work has shown is proportional to the bulk silica content of lunar surface materials. Diviner measurements show that the position of this feature is also influenced by the changes in optical and physical properties of the lunar surface with exposure to space, the process known as space weathering. Large rayed craters and lunar swirls show corresponding Christiansen Feature anomalies. The space weathering effect is likely due to differences in thermal gradients in the optical surface imposed by the space weathering control of albedo. However, inspected at high resolution, locations with extreme compositions and Christiansen Feature wavelength positions–silica-rich and olivine-rich areas–do not have extreme albedos, and fall off the albedo- Christiansen Feature wavelength position trend occupied by most of the Moon. These areas demonstrate that the Christiansen Feature wavelength position contains compositional information and is not solely dictated by albedo. An optical maturity parameter derived from near-IR measurements is used to partly correct Diviner data for space weathering influences.

Reference
Lucey PG, Greenhagen BT, Song E, Arnold JA, Lemelin M, Donaldson Hanna K, Bowles NE, Glotch TD, Paige DA (2016) Space weathering effects in Diviner Lunar Radiometer multispectral infrared measurements of the lunar Christiansen Feature: Characteristics and mitigation. Icarus (in Press)
Link to Article [doi:10.1016/j.icarus.2016.05.010]
Copyright Elsevier

¹Emily A. Worsham, ¹Katherine R. Bermingham, ¹Richard J. Walker
¹Department of Geology, University of Maryland, College Park, Maryland, 20742 USA

Siderophile trace element abundances and the 187Re-187Os isotopic systematics of the metal phases of 58 IAB complex iron meteorites were determined in order to investigate formation processes and how meteorites within chemical subgroups may be related. Close adherence of 187Re-187Os isotopic data of most IAB iron meteorites to a primordial isochron indicates that the siderophile elements of most members of the complex remained closed to elemental disturbance soon after formation. Minor, presumably late-stage open-system behavior, however, is observed in some members of the sLM, sLH, sHL, and sHH subgroups. The new siderophile element abundance data are consistent with the findings of prior studies suggesting that the IAB subgroups cannot be related to one another by any known crystallization process. Equilibrium crystallization, coupled with crystal segregation, solid-liquid mixing, and subsequent fractional crystallization can account for the siderophile element variations among meteorites within the IAB main group (MG). The data for the sLM subgroup are consistent with equilibrium crystallization, combined with crystal segregation and mixing. By contrast, the limited fractionation of siderophile elements within the sLL subgroup is consistent with metal extraction from a chondritic source with little subsequent processing. The limited data for the other subgroups were insufficient to draw robust conclusions about crystallization processes involved in their formation. Collectively, multiple formational processes are represented in the IAB complex, and modeling results suggest that fractional crystallization within the MG may have been a more significant process than has been previously recognized.

Reference
Worsham EA, Bermingham KR, Walker RJ (2016) Siderophile element systematics of IAB complex iron meteorites: New insights into the formation of an enigmatic group. Geochimica et Cosmochmica Acta (in Press)
Link to Article [doi:10.1016/j.gca.2016.05.019]
Copyright Elsevier

¹Toru Matsumoto et al. (>10)*
¹Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, 3-1-1, Yoshinodai, Chuo-ku, Sagamihara, Kanagawa 252-5210, Japan
*Find the extensive, full author and affiliation list on the publishers website

The morphological properties of 26 regolith particles from asteroid Itokawa were observed using scanning electron microscopes in combination with an investigation of their three-dimensional shapes obtained through X-ray microtomography. Surface observations of a cross section of the LL5 chondrite, and of crystals of olivine and pyroxene, were also performed for comparison. Some Itokawa particles have surfaces corresponding to walls of microdruses in the LL chondrite, where concentric polygonal steps develop and euhedral or subhedral grains exist. These formed through vapor growth owing to thermal annealing, which might have been caused by thermal metamorphism or shock-induced heating in Itokawa’s parent body. Most of the Itokawa particles have more or less fractured surfaces, indicating that they were formed by disaggregation, probably caused by impacts. Itokawa particles with angular and rounded edges observed in computed tomography images are associated with surfaces exhibiting clear and faint structures, respectively. These surfaces can be interpreted by invoking different degrees of abrasion after regolith formation. A possible mechanism for the abrasion process is grain migration caused by impact-driven seismic waves. Space-weathered rims with blisters are distributed heterogeneously across the Itokawa regolith particles. This heterogeneous distribution can be explained by particle motion and fracturing, combined with solar-wind irradiation of the particle surfaces. The regolith activity—including grain motion, fracturing, and abrasion—might effectively act as refreshing process of Itokawa particles against space-weathered rim formation. The space-weathering processes affecting Itokawa would have developed simultaneously with space-weathered rim formation and regolith particle refreshment.

Reference
Matsumoto T et al. (2016) Nanomorphology of Itokawa regolith particles:Application to space-weathering processes affecting the Itokawa asteroid. Geochimica et Cosmochmica Acta (in Press)
Link to Article [doi:10.1016/j.gca.2016.05.011]
Copyright Elsevier

^1,2,3Levke Kööp, ^1,2,3,4Andrew M. Davis, ^5,6Daisuke Nakashima, ^7,8Changkun Park, ⁷Alexander N. Krot, ⁷Kazuhide Nagashima, ⁵Travis J. Tenner, ^1,2,3Philipp R. Heck, ⁵Noriko T. Kita
¹Department of the Geophysical Sciences, The University of Chicago, Chicago, IL, USA
²Chicago Center for Cosmochemistry, The University of Chicago, Chicago, IL, United States
³Robert A. Pritzker Center for Meteoritics and Polar Studies, Field Museum of Natural History, Chicago, IL, USA
⁴Enrico Fermi Institute, The University of Chicago, Chicago, IL, USA
⁵Dept. Geoscience, University of Wisconsin-Madison, WI, USA
⁶Division of Earth and Planetary Materials Science, Tohoku University, Sendai, Japan
⁷HIGP/SOEST University of Hawai‘i at Mānoa, Honolulu, HI, USA
⁸Korea Polar Research Institute, Incheon, Korea

PLACs (platy hibonite crystals) and related hibonite-rich calcium-, aluminum-rich inclusions (CAIs; hereafter collectively referred to as PLAC-like CAIs) have the largest nucleosynthetic isotope anomalies of all materials believed to have formed in the solar system. Most PLAC-like CAIs have low inferred initial 26Al/27Al ratios and could have formed prior to injection or widespread distribution of 26Al in the solar nebula. In this study, we report 26Al-26Mg systematics combined with oxygen, calcium, and titanium isotopic compositions for a large number of newly separated PLAC-like CAIs from the Murchison CM2 chondrite (32 CAIs studied for oxygen, 26 of these also for 26Al-26Mg, calcium and titanium). Our results confirm (1) the large range of nucleosynthetic anomalies in 50Ti and 48Ca (our data range from –70 to 170‰ and –60 to 80‰, respectively), (2) the substantial range of Δ17O values (–28 to –17‰, with Δ17O = δ17O–0.52×δ18O), and (3) general 26Al-depletion in PLAC-like CAIs.

The multielement approach reveals a relationship between Δ17O and the degree of variability in 50Ti and 48Ca: PLAC-like CAIs with the highest Δ17O (∼–17‰) show large positive and negative 50Ti and 48Ca anomalies, while those with the lowest Δ17O (∼–28‰) have small to no anomalies in 50Ti and 48Ca. These observations could suggest a physical link between anomalous 48Ca and 50Ti carriers and an 16O-depleted reservoir. We suggest that the solar nebula was isotopically heterogeneous shortly after collapse of the protosolar molecular cloud, and that the primordial dust reservoir, in which anomalous carrier phases were heterogeneously distributed, was 16O-depleted (Δ17O ⩾ –17‰) relative to the primordial gaseous (CO + H2O) reservoir (Δ17O < –35‰). However, other models such as CO self-shielding in the protoplanetary disk are also considered to explain the link between oxygen and calcium and titanium isotopes in PLAC-like CAIs.

Reference
Kööp L, Davis AM, Nakashima D, Park C,Krot AN, Nagashima K, Tenner TJ, Heck PR, Kita NT (2016) A link between oxygen, calcium and titanium isotopes in 26Al-depleted hibonite-rich CAIs from Murchison and implications for the heterogeneity of dust reservoirs in the solar nebula. Geochimica et Cosmochimica Acta (in Press)
Link to Article [doi:10.1016/j.gca.2016.05.014]
Copyright Elsevier

¹Court, R. W., ¹Tan, J.
¹Impacts and Astromaterials Research Centre, Department of Earth Science and Engineering, Imperial College, London, UK

Ablation of micrometeoroids during atmospheric entry yields volatile gases such as water, carbon dioxide, and sulfur dioxide, capable of altering atmospheric chemistry and hence the climate and habitability of the planetary surface. While laboratory experiments have revealed the yields of these gases during laboratory simulations of ablation, the reactions responsible for the generation of these gases have remained unclear, with a typical assumption being that species simply undergo thermal decomposition without engaging in more complex chemistry. Here, pyrolysis–Fourier transform infrared spectroscopy reveals that mixtures of meteorite-relevant materials undergo secondary reactions during simulated ablation, with organic matter capable of taking part in carbothermic reduction of iron oxides and sulfates, resulting in yields of volatile gases that differ from those predicted by simple thermal decomposition. Sulfates are most susceptible to carbothermic reduction, producing greater yields of sulfur dioxide and carbon dioxide at lower temperatures than would be expected from simple thermal decomposition, even when mixed with meteoritically relevant abundances of low-reactivity Type IV kerogen. Iron oxides were less susceptible, with elevated yields of water, carbon dioxide, and carbon monoxide only occurring when mixed with high abundances of more reactive Type III kerogen. We use these insights to reinterpret previous ablation simulation experiments and to predict the reactions capable of occurring during ablation of carbonaceous micrometeoroids in atmospheres of different compositions.

Reference
Court RW, Tan J (2016) Insights into secondary reactions occurring during atmospheric ablation of micrometeoroids. Meteoritics & Planetary Science (in Press)
Link to Article [doi: 10.1111/maps.12652]
Published by arrangement with John Wiley & Sons

¹Thompson, M. S., ¹Zega, T. J., ¹Becerra, P., ¹Keane, J. T., ¹Byrne, S.
¹Lunar and Planetary Laboratory, University of Arizona, Tucson, Arizona, USA

We report measurements of the oxidation state of Fe nanoparticles within lunar soils that experienced varied degrees of space weathering. We measured >100 particles from immature, submature, and mature lunar samples using electron energy-loss spectroscopy (EELS) coupled to an aberration-corrected transmission electron microscope. The EELS measurements show that the nanoparticles are composed of a mixture of Fe0, Fe2+, and Fe3+ oxidation states, and exhibit a trend of increasing oxidation state with higher maturity. We hypothesize that the oxidation is driven by the diffusion of O atoms to the surface of the Fe nanoparticles from the oxygen-rich matrix that surrounds them. The oxidation state of Fe in the nanoparticles has an effect on modeled reflectance properties of lunar soil. These results are relevant to remote sensing data for the Moon and to the remote determination of relative soil maturities for various regions of the lunar surface.

Reference
Thompson MS, Zega TJ, Becerra P, Keane JT, Byrne S (2016) The oxidation state of nanophase Fe particles in lunar soil: Implications for space weathering. Meteoritics & Planetary Science (in Press)
Link to Article [doi: 10.1111/maps.12646]
Published by arrangement with John Wiley & Sons

¹T. J. Barrett, ¹J. J. Barnes, ^1,2R. Tartèse, ²M. Anand, ¹I. A. Franchi, ¹R. C. Greenwood, ^1,2B. L. A. Charlier, ^1,3M. M. Grady
¹Planetary and Space Sciences, The Open University, Milton Keynes, UK
²Institut de Minéralogie, de Physique des Matériaux et de Cosmochimie, Muséum National d’Histoire Naturelle, Sorbonne Universités, CNRS, UPMC & IRD, Paris, France
³Department of Earth Sciences, Natural History Museum, London, UK

Volatile elements play a key role in the dynamics of planetary evolution. Extensive work has been carried out to determine the abundance, distribution, and source(s) of volatiles in planetary bodies such as the Earth, Moon, and Mars. A recent study showed that the water in apatite from eucrites has similar hydrogen isotopic compositions compared to water in terrestrial rocks and carbonaceous chondrites, suggesting that water accreted very early in the inner solar system given the ancient crystallization ages (~4.5 Ga) of eucrites. Here, the measurements of water (reported as equivalent H2O abundances) and the hydrogen isotopic composition (δD) of apatite from five basaltic eucrites and one cumulate eucrite are reported. Apatite H2O abundances range from ~30 to ~3500 ppm and are associated with a weighted average δD value of −34 ± 67‰. No systematic variations or correlations are observed in H2O abundance or δD value with eucrite geochemical trend or metamorphic grade. These results extend the range of previously published hydrogen isotope data for eucrites and confirm the striking homogeneity in the H-isotopic composition of water in eucrites, which is consistent with a common source for water in the inner solar system.

Reference
Barrett TJ, Barnes JJ, Tartèse R, Anand M, Franchi IA, Greenwood RC, Charlier BLA, Grady MM (2016) The abundance and isotopic composition of water in eucrites. Meteoritics & Planetary Science (in Press)
Link to Article [DOI: 10.1111/maps.12649]
Published by arrangement with John Wiley & Sons

¹Kathryn H. Harriss, ¹M.J.Burchell
¹School of Physical Sciences, University of Kent, Canterbury, Kent, UK

Kuebler et al. (2006) identified variations in olivine Raman spectra based on the composition of individual olivine grains, leading to identification of olivine composition from Raman spectra alone. However, shock on a crystal lattice has since been shown to result in a structural change to the original material, which produces a shift in the Raman spectra of olivine grains compared with the original unshocked olivine (Foster et al. 2013). This suggests that the use of the compositional calculations from the Raman spectra, reported in Kuebler et al. (2006), may provide an incorrect compositional value for material that has experienced shock. Here, we have investigated the effect of impact speed (and hence peak shock pressure) on the shift in the Raman spectra for San Carlos olivine (Fo91) impacting Al foil. Powdered San Carlos olivine (grain size 1–10 μm) was fired at a range of impact speeds from 0.6 to 6.1 km s−1 (peak shock pressures 5–86 GPa) at Al foil to simulate capture over a wide range of peak shock pressures. A permanent change in the Raman spectra was found to be observed only for impact speeds greater than ~5 km s−1. The process that causes the shift is most likely linked to an increase in the peak pressure produced by the impact, but only after a minimum shock pressure associated with the speed at which the effect is first observed (here 65–86 GPa). At speeds around 6 km s−1 (peak shock pressures ~86 GPa), the shift in Raman peak positions is in a similar direction (red shift) to that observed by Foster et al. (2013) but of twice the magnitude.

Reference
Harriss KH, Burchell MJ (2016) A study of the observed shift in the peak position of olivine Raman spectra as a result of shock induced by hypervelocity impacts. Meteoritics & Planetary Science (in Press)
Link to Article [DOI: 10.1111/maps.12660]
Published by arrangement with John Wiley & Sons