Lyuba V. Moroz^a,b, Larissa V. Starukhina^c, Surya Snata Rout^a,1, Sho Sasaki^d,2, Jörn Helbert^b, Dietmar Baither^e, Addi Bischoff^a and Harald Hiesinger^a

^aWestfälische Wilhelms-Universität Münster, Institut für Planetologie, Wilhelm-Klemm Str. 10, D-48149 Münster, Germany
^bDeutsches Zentrum für Luft- und Raumfahrt e.V. (DLR), Institut für Planetenforschung, Rutherfordstr. 2, D-12489 Berlin, Germany
^cAstronomical Institute of Kharkov National University, Sumska 35, 61022 Kharkov, Ukraine
^dRISE Project, National Astronomical Observatory of Japan, 2-12 Hoshigaoka, Mizugawa, Oshu 023-0861, Iwate, Japan
^eWestfälische Wilhelms-Universität Münster, Institut für Materialphysik, Wilhelm-Klemm Str. 10, D-48149 Münster, Germany
¹Present address: Center for Meteoritics and Polar Studies, The Field Museum of Natural History, 1400 S Lake Shore Dr., Chicago, IL 60605-2496, USA.
²Present address: Osaka University, Toyonaka, Japan.

To investigate effects of micrometeorite bombardment on optical spectra and composition of planetary and asteroid regoliths with low Fe contents, we irradiated samples of a Fe-poor plagioclase feldspar (andesine–labradorite) using a nanosecond pulsed laser. We measured reflectance spectra of irradiated and non-irradiated areas of the samples (pressed pellets) between 0.5 and 18 μm and performed SEM/EDS and TEM studies of the samples. Bulk FeO content of 0.72 wt.% in the samples is comparable, for example, to FeO content in silicates on the surface of Mercury, that was recently mapped by NASA’s MESSENGER mission and will be spectrally mapped by remote sensing instruments MERTIS and SYMBIO-SYS on board the ESA BepiColombo spacecraft. We also employed theoretical spectral modeling to characterize optical alteration caused by formation of nano- and submicrometer Fe⁰ inclusions within space-weathered surface layers and grain rims of a Fe-poor regolith. The laser-irradiated surface layer of plagioclase reveals significant melting, while reflectance spectra show mild darkening and reddening in the visible and near-infrared (VNIR). Our spectral modeling indicates that the optical changes observed in the visible require reduction of bulk FeO (including Fe from mineral impurities found in the sample) and formation of nanophase (np) Fe⁰ within the glassy surface layer. A vapor deposit, if present, is optically too thin to contribute to optical modification of the investigated samples or to cause space weathering-induced optical alteration of Fe-poor regoliths in general. Due to low thickness of vapor deposits, npFe⁰ formation in the latter can cause darkening and reddening only for a regolith with rather high bulk Fe content. Our calculations show that only a fraction of bulk Fe is likely to be converted to npFe⁰ in nanosecond laser irradiation experiments and probably in natural regolith layers modified by space weathering. The previously reported differences in response of different minerals to laser irradiation, and probably to space weathering-induced heating are likely controlled by their differences in electrical conductivities and melting points. For a given mineral grain, its susceptibility to melting/vaporization is also affected by electric conductivities of adjacent grains of other minerals in the regolith. Published nanosecond laser irradiation experiments simulate optical alteration of immature regoliths, since only the uppermost surface layer of an irradiated pellet is subject to heating. According to our calculations, if regolith particles due to impact-induced turnover are mantled with npFe⁰-bearing rims of the same thickness, then even low contents of Fe similar to our sample or Mercury’ surface can cause significant darkening and reddening, provided a melt layer, rather than a thin vapor deposit is involved into npFe⁰ formation. All spectral effects observed in the thermal infrared (TIR) after irradiation of our feldspar sample are likely to be associated with textural changes. We expect that mineralogical interpretation of the BepiColombo MERTIS infrared spectra of Mercury between 7 and 17 μm will be influenced mostly by textural effects (porosity, comminution) and impact glass formation rather than formation of npFe⁰ inclusions.

Reference
Moroz LV, Starukhina LV, Rout SS, Sasaki S, Jörn Helbert J, Baither D, Bischoff B and Hiesinger H (2014) Space weathering of silicate regoliths with various FeO contents: New insights from laser irradiation experiments and theoretical spectral simulations. Icarus 235:187.
[doi:10.1016/j.icarus.2014.03.021]
Copyright Elsevier

Link to Article

S. Bisterzo^1,2, C. Travaglio^1,5, R. Gallino^2,5, M. Wiescher³ and F. Käppeler⁴

¹INAF-Astrophysical Observatory Turin, Turin, Italy
²Department of Physics, University of Turin, Turin, Italy
³Joint Institute for Nuclear Astrophysics (JINA), Department of Physics, University of Notre Dame, Notre Dame IN, USA
⁴Karlsruhe Institute of Technology, Campus Nord, Institut für Kernphysik, Karlsruhe, Germany
⁵B2FH Association-c/o Strada Osservatorio 20, I-10023 Turin, Italy.

We study the s-process abundances (A  90) at the epoch of the solar system formation. Asymptotic giant branch yields are computed with an updated neutron capture network and updated initial solar abundances. We confirm our previous results obtained with a Galactic chemical evolution (GCE) model: (1) as suggested by the s-process spread observed in disk stars and in presolar meteoritic SiC grains, a weighted average of s-process strengths is needed to reproduce the solar s distribution of isotopes with A > 130; and (2) an additional contribution (of about 25%) is required in order to represent the solar s-process abundances of isotopes from A = 90 to 130. Furthermore, we investigate the effect of different internal structures of the ¹³C pocket, which may affect the efficiency of the ¹³C(α, n)¹⁶O reaction, the major neutron source of the s process. First, keeping the same ¹³C profile adopted so far, we modify by a factor of two the mass involved in the pocket; second, we assume a flat ¹³C profile in the pocket, and we test again the effects of the variation of the mass of the pocket. We find that GCE s predictions at the epoch of the solar system formation marginally depend on the size and shape of the ¹³C pocket once a different weighted range of ¹³C-pocket strengths is assumed. We obtain that, independently of the internal structure of the ¹³C pocket, the missing solar system s-process contribution in the range from A = 90 to 130 remains essentially the same.

Reference
Bisterzo S. Travaglio C, Gallino R, Wiescher M and Käppeler F (2014) Galactic Chemical Evolution and Solar s-process Abundances: Dependence on the 13C-pocket Structure. The Astrophysical Journal 787:10.
[doi:10.1088/0004-637X/787/1/10]

Link to Article

Alexander Hubbard^a and Denton S. Ebel^b

^aDepartment of Astrophysics, American Museum of Natural History, New York, NY 10024-5192, USA
^bDepartment of Earth and Planetary Sciences, American Museum of Natural History, New York, NY 10024-5192, USA

The Earth is known to be depleted in volatile lithophile elements in a fashion that defies easy explanation. We resolve this anomaly with a model that combines the porosity of collisionally grown dust grains in protoplanetary disks with heating from FU Orionis events that dramatically raise protoplanetary disk temperatures. The heating from an FU Orionis event alters the aerodynamical properties of the dust while evaporating the volatiles. This causes the dust to settle, abandoning those volatiles. The success of this model in explaining the elemental composition of the Earth is a strong argument in favor of highly porous collisionally grown dust grains in protoplanetary disks outside our Solar System. Further, it demonstrates how thermal (or condensation based) alterations of dust porosity, and hence aerodynamics, can be a strong factor in planet formation, leading to the onset of rapid gravitational instabilities in the dust disk and the subsequent collapse that forms planetesimals.

Reference
Hubbard A and Ebel DS (in press) Protoplanetary dust porosity and FU Orionis Outbursts: Solving the mystery of Earth’s missing volatiles. Icarus
[doi:10.1016/j.icarus.2014.04.015]
Copyright Elsevier

Link to Article

T. Kohout^a et al. (>10)*
*Find the extensive, full author and affiliation list on the publishers website.

^aInstitute of Geology, Academy of Sciences of the Czech Republic, Prague, Czech Republic

Airless planetary bodies are directly exposed to space weathering. The main spectral effects of space weathering are darkening, reduction in intensity of silicate mineral absorption bands, and an increase in the spectral slope towards longer wavelengths (reddening). Production of nanophase metallic iron (npFe⁰) during space weathering plays major role in these spectral changes. A laboratory procedure for the controlled production of npFe⁰ in silicate mineral powders has been developed. The method is based on a two-step thermal treatment of low-iron olivine, first in ambient air and then in hydrogen atmosphere. Through this process, a series of olivine powder samples was prepared with varying amounts of npFe⁰ in the 7-20 nm size range. A logarithmic trend is observed between amount of npFe⁰ and darkening, reduction of 1 μm olivine absorption band, reddening, and 1 μm band width. Olivine with a population of physically larger npFe⁰particles follows spectral trends similar to other samples, except for the reddening trend. This is interpreted as the larger, ∼40-50 nm sized, npFe⁰ particles do not contribute to the spectral slope change as efficiently as the smaller npFe⁰ fraction. A linear trend is observed between the amount of npFe⁰ and 1 μm band center position, most likely caused by Fe²⁺ disassociation from olivine structure into npFe⁰ particles.

Reference
Kohouta T et al. (in press) Space weathering simulations through controlled growth of iron nanoparticles on olivine. Icarus
[doi:10.1016/j.icarus.2014.04.004]
Copyright Elsevier

Link to Article

Hiroshi Hidaka¹ and Shigekazu Yoneda²

¹Department of Earth and Planetary Systems Science, Hiroshima University, Higashi-Hiroshima 739-8526, Japan
²Department of Science and Engineering, National Museum of Nature and Science, Tsukuba 305-0005, Japan

The idea that solar system materials were irradiated by solar cosmic rays from the early Sun has long been suggested, but is still questionable. In this study, Sr, Ba, Ce, Nd, Sm, and Gd isotopic compositions of sequential acid leachates from the Kapoeta meteorite (howardite) were determined to find systematic and correlated variations in their isotopic abundances of proton-rich nuclei, leading to an understanding of the irradiation condition by cosmic rays. Significantly large excesses of proton-rich isotopes (p-isotopes), ⁸⁴Sr,¹³⁰Ba, ¹³²Ba, ¹³⁶Ce, ¹³⁸Ce, and ¹⁴⁴Sm, were observed, particularly in the first chemical separate, which possibly leached out of the very shallow layer within a few ?m from the surface of regolith grains in the sample. The results reveal the production of p-isotopes through the interaction of solar cosmic rays with the superficial region of the regolith grains before the formation of the Kapoeta meteorite parent body, suggesting strong activity in the early Sun

Reference
Hidaka H and Yoneda S (2014) Isotopic Excesses of Proton-rich Nuclei Related to Space Weathering Observed in a Gas-rich Meteorite Kapoeta. The Astrophysical Journal 786:138.
[doi:10.1088/0004-637X/786/2/138]

Link to Article

Silvia Protopapa^a et al. (>10)*
*Find the extensive, full author and affiliation list on the publishers website.

^aDepartment of Astronomy, University of Maryland, College Park, MD, 20742, USA

On November 4th, 2010, the Deep Impact eXtended Investigation (DIXI) successfully encountered comet 103P/Hartley 2, when it was at a heliocentric distance of 1.06 AU. Spatially resolved near-IR spectra of comet Hartley 2 were acquired in the 1.05 – 4.83 μm wavelength range using the HRI-IR spectrometer. We present spectral maps of the inner ∼10 kilometers of the coma collected 7 minutes and 23 minutes after closest approach. The extracted reflectance spectra include well-defined absorption bands near 1.5, 2.0, and 3.0 μm consistent in position, bandwidth, and shape with the presence of water ice grains. Using Hapke’s radiative transfer model, we characterize the type of mixing (areal vs. intimate), relative abundance, grain size, and spatial distribution of water ice and refractories. Our modeling suggests that the dust, which dominates the innermost coma of Hartley 2 and is at a temperature of 300K, is thermally and physically decoupled from the fine-grained water ice particles, which are on the order of 1 μm in size. The strong correlation between the water ice, dust, and CO₂ spatial distribution supports the concept that CO₂ gas drags the water ice and dust grains from the nucleus. Once in the coma, the water ice begins subliming while the dust is in a constant outflow. The derived water ice scale-length is compatible with the lifetimes expected for 1-μm pure water ice grains at 1 AU, if velocities are near 0.5 m/s. Such velocities, about three order of magnitudes lower than the expansion velocities expected for isolated 1-μm water ice particles ( and ), suggest that the observed water ice grains are likely aggregates.

Reference
Protopapa S (in press) Water ice and dust in the innermost coma of Comet 103P/Hartley 2. Icarus
[doi:10.1016/j.icarus.2014.04.008]
Copyright Elsevier

Link to Article

Mahesh Anand

Planetary and Space Sciences, Open University, Milton Keynes MK7 6AA, UK.

The paradigm of a “dry Moon” was recently challenged on the basis of reexamination of lunar samples collected during the Apollo missions, raising the possibility of a volatile-rich lunar interior (1–6). Several of these studies measured appreciable quantities of water (reported as equivalent H, OH, or H₂O) and other volatiles (e.g., Cl, F) in the mineral apatite (4–6), which is ubiquitous in lunar basalts (mare basalts) (see the figure). However, an accurate estimation of the water content of the magmatic liquid from which apatite formed, and ultimately of the mantle source regions of mare basalts, depends on a number of parameters. In cases where apatite in a mare basalt formed through the process of fractional crystallization (when newly formed crystals in a cooling magma are physically separated, preventing any further interaction with the remaining melt), it may not be possible to obtain any reliable estimates of the water contents of the parental magma (and its source region). On page 400 of this issue, Boyce et al. (7) present an elegant numerical model, applicable to mare basalt apatite formed through fractional crystallization, to demonstrate that some of the highest water contents reported for lunar apatite can be reconciled with an original melt containing not much water at all. These new results cast doubt on the utility of apatite volatile abundances in reliably estimating the water content of mare basalt source regions.

Reference
Anand M (2014) Analyzing Moon Rocks. Science 344:365.
[doi:10.1126/science.1253266]
Reprinted with permission from AAAS

Link to Article

J. W. Boyce¹, S. M. Tomlinson¹, F. M. McCubbin², J. P. Greenwood³ and A. H. Treiman⁴

¹Department of Earth, Planetary, and Space Sciences, University of California, Los Angeles, Los Angeles, CA 90095, USA.
²Institute for Meteoritics, Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, NM 87131, USA.
³Department of Earth and Environmental Sciences, Wesleyan University, 265 Church Street, Middletown, CT 06459, USA.
⁴Lunar and Planetary Institute, 3600 Bay Area Boulevard, Houston, TX 77058–1113, USA.

Recent discoveries of water-rich lunar apatite are more consistent with the hydrous magmas of Earth than the otherwise volatile-depleted rocks of the Moon. Paradoxically, this requires H-rich minerals to form in rocks that are otherwise nearly anhydrous. We modeled existing data from the literature, finding that nominally anhydrous minerals do not sufficiently fractionate H from F and Cl to generate H-rich apatite. Hydrous apatites are explained as the products of apatite-induced low magmatic fluorine, which increases the H/F ratio in melt and apatite. Mare basalts may contain hydrogen-rich apatite, but lunar magmas were most likely poor in hydrogen, in agreement with the volatile depletion that is both observed in lunar rocks and required for canonical giant-impact models of the formation of the Moon.

Reference
Boyce JW, Tomlinson SM, McCubbin FM, Greenwood JP and Treiman AH (2014) The Lunar Apatite Paradox. Science 344:400.
[doi:10.1126/science.1250398]
Reprinted with permission from AAAS

Link to Article

Norman H. Sleep¹ and Donald R. Lowe²

¹Department of Geophysics, Stanford University, Stanford, California, USA
²Department of Geological and Environmental Sciences, Stanford University, Stanford, California, USA

Archean asteroid impacts, reflected in the presence of spherule beds in the 3.2–3.5 Ga Barberton greenstone belt (BGB), South Africa, generated extreme seismic waves. Spherule bed S2 provides a field example. It locally lies at the contact between the Onverwacht and Fig Tree Groups in the BGB, which formed as a result of the impact of asteroid (possibly 50 km diameter). Scaling calculations indicate that very strong seismic waves traveled several crater diameters from the impact site, where they widely damaged Onverwacht rocks over much of the BGB. Lithified sediments near the top of the Onverwacht Group failed with opening-mode fractures. The underlying volcanic sequence then failed with normal faults and opening-mode fractures. Surficial unlithified sediments liquefied and behaved as a fluid. These liquefied sediments and some impact-produced spherules-filled near-surface fractures, today represented by swarms of chert dikes. Strong impact-related tsunamis then swept the seafloor. P waves and Rayleigh waves from the impact greatly exceeded the amplitudes of typical earthquake waves. The duration of extreme shaking was also far longer, probably hundreds of seconds, than that from strong earthquakes. Dynamic strains of ~10⁻³ occurred from the surface and downward throughout the lithosphere. Shaking weakened the Onverwacht volcanic edifice and the surface layers locally moved downhill from gravity accommodated by faults and open-mode fractures. Coast-parallel opening-mode fractures on the fore-arc coast of Chile, formed as a result of megathrust events, are the closest modern analogs. It is even conceivable that dynamic stresses throughout the lithosphere initiated subduction beneath the Onverwacht rocks.

Reference
Sleep NH and Lowe DR (in press) Physics of crustal fracturing and chert dike formation triggered by asteroid impact, ∼3.26 Ga, Barberton greenstone belt, South Africa. Geochemistry, Geophysics, Geosystems
[doi:10.1002/2014GC005229.]
Published by arrangement with John Wiley & Sons

Link to Article

Wladimir Neumann^a, Doris Breuer^a and Tilman Spohn^a,b

^aInstitute of Planetary Research, German Aerospace Center (DLR), Rutherfordstraße 2, 12489 Berlin, Germany
^bInstitute of Planetology, Westfälische Wilhelm-University Münster, Wilhelm-Klemm-Str. 10, 48149 Münster, Germany

The Dawn mission confirms earlier predictions that the asteroid 4 Vesta is differentiated with an iron-rich core, a silicate mantle and a basaltic crust, and supports the conjecture of Vesta being the parent body of the HED meteorites. To better understand its early evolution, we perform numerical calculations of the thermo-chemical evolution adopting new data obtained by the Dawn mission such as mass, bulk density and size of the asteroid.
We have expanded the thermo-chemical evolution model of Neumann et al. (2012) that includes accretion, compaction, melting and the associated changes of the material properties and the partitioning of incompatible elements such as the radioactive heat sources, advective heat transport, and differentiation by porous flow, to further consider convection and the associated effective cooling in a potential magma ocean. Depending on the melt fraction, the heat transport by melt segregation is modelled either by assuming melt flow in a porous medium or by simulating vigorous convection and heat flux of a magma ocean with a high effective thermal conductivity.
Our results show that partitioning of ²⁶Al and its transport with the silicate melt is crucial for the formation of a global and deep magma ocean. Due to the enrichment of ²⁶Al in the liquid phase and its accumulation in the sub-surface (for formation times t₀<1.5 Ma), a thin shallow magma ocean with a thickness of 1 to a few tens of km forms – its thickness depends on the viscosity of silicate melt. The lifetime of the shallow magma ocean is O(10⁴)–O(10⁶) years and convection in this layer is accompanied by the extrusion of ²⁶Al at the surface, resulting in the formation of a basaltic crust. The interior differentiates from the outside inwards with a mantle that is depleted in ²⁶Al and core formation is completed within ∼0.3 Ma. The lower mantle experiences a maximal melt fraction of 45% suggesting a harzburgitic to dunitic composition. Our results support the formation of non-cumulate eucrites by the extrusion of early partial melt while cumulate eucrites and diogenites may form from the crystallising shallow magma ocean. Silicate melt is present in the mantle for up to 150 Ma, and convection in a crystallising core proceeds for approximately 100 Ma, supporting the idea of an early magnetic field to explain the remnant magnetisation observed in some HED meteorites.

Reference
Neumann W, Breuer D and Spohn T (2014) Differentiation of Vesta: Implications for a shallow magma ocean. Earth and Planetary Science Letters 395:267.
[doi:10.1016/j.epsl.2014.03.033]
Copyright Elsevier

Link to Article