¹Thomas B. McCord,²Jennifer E.C. Scully
¹The Bear Fight Institute, Winthrop WA 98862
²Department of Earth, Planetary, and Space Sciences, University of California, Los Angeles, 595 Charles Young Drive East, Box 951567, Los Angeles, CA 90095-1567

Vesta’s surface composition has been of special interest since early, disk-integrated telescopic spectral observations indicated that it is basaltic, differentiated and similar to the HED (howardite-eucrite-diogenite) class of meteorites. The Dawn mission, orbiting Vesta, provided a large and varied set of unique observations on the detailed mineralogy, molecular and elemental composition, and their distributions in association with surface features and geology. The set of articles contained in this special issue is the first treatment of the entire surface composition of Vesta using the complete Dawn Vesta data set and the calibrations from the entire campaign. Most articles treat a region of Vesta within the context of the entire body, but there are several articles that treat global or technical topics. As a whole, these articles provide a current and comprehensive view of Vesta’s composition using all the relevant data that is available. Vesta’s surface composition is consistent with the upper layer being created by igneous processes, while a more mafic lithology generally associated with a mantle is surprisingly limited. There is evidence of contamination by low velocity infall of several types of objects: dark hydrated/hydroxolated material, and probably Fe/Mg silicates differing from Vesta’s. Isolated blocks of differing compositions, seen especially in crater walls, could indicate incomplete melting and mixing during the differentiation process, and retention of some evidence of the original building blocks of the accreted Vesta. This lead article introduces and provides the context for the following articles, presents a summary of the various findings, and integrates them into overall conclusions.

Reference
McCord TB, Scully JEC (2015) The Composition of Vesta from the Dawn Mission. Geochimica et Cosmochimica Acta (in Press)
Link to Article [http://dx.doi.org/10.1016/j.icarus.2015.03.022]

Copyright Elsevier

¹Timothy J. McCoy,²Andrew W. Beck,³Thomas H. Prettyman,⁴David W. Mittlefehldt
¹Department of Mineral Sciences, National Museum of Natural History, Smithsonian Institution, Washington, DC 20560-0119, USA
²Applied Physics Laboratory, The Johns Hopkins University, 11100 Johns Hopkins Road, Laurel, MD 20723, USA
³Planetary Science Institute, 1700 East Fort Lowell, Suite 106, Tucson, AZ 85719, USA
⁴Astromaterials Research Office, NASA Johnson Space Center, Mail code KR, Houston, TX 77058, USA

More than 200 years after its discovery, asteroid (4) Vesta is thought to be the parent body for the howardite, eucrite and diogenite (HED) meteorites. The Dawn spacecraft spent ∼14 months in orbit around this largest, intact differentiated asteroid to study its internal structure, geology, mineralogy and chemistry. Carrying a suite of instruments that included two framing cameras, a visible-near infrared spectrometer, and a gamma-ray and neutron detector, coupled with radio tracking for gravity, Dawn revealed a geologically and geochemically complex world. A constrained core size of ∼110–130 km radius is consistent with predictions based on differentiation models for the HED meteorite parent body. Hubble Space Telescope observations had already shown that Vesta is scarred by a south polar basin comparable in diameter to that of the asteroid itself. Dawn showed that the south polar Rheasilvia basin dominates the asteroid, with a central uplift that rivals the large shield volcanoes of the Solar System in height. An older basin, Veneneia, partially underlies Rheasilvia. A series of graben-like equatorial and northern troughs were created during these massive impact events 1–2 Ga ago. These events also resurfaced much of the southern hemisphere and exposed deeper-seated diogenitic lithologies. Although the mineralogy and geochemistry vary across the surface for rock-forming elements and minerals, the range is small, suggesting that impact processes have efficiently homogenized the surface of Vesta at scales observed by the instruments on the Dawn spacecraft. The distribution of hydrogen is correlated with surface age, which likely results from the admixture of exogenic carbonaceous chondrites with Vesta’s basaltic surface. Clasts of such material are observed within the surficial howardite meteorites in our collections. Dawn significantly strengthened the link between (4) Vesta and the HED meteorites, but the pervasive mixing, lack of a convincing and widespread detection of olivine, and poorly-constrained lateral and vertical extents of units leaves unanswered the central question of whether Vesta once had a magma ocean. Dawn is continuing its mission to the presumed ice-rich asteroid (1) Ceres.

Reference
McCoy TJ, Beck AW, Prettyman TH, Mittlefehldt DW (2015) Asteroid (4) Vesta II: Exploring a geologically and geochemically complex world with the Dawn Mission. Chemie der Erde (in Press)
Link to Article [http://dx.doi.org/10.1016/j.chemer.2014.12.001]

Copyright Elsevier

¹Yamamoto S. et al.(>10)*
¹Center for Environmental Measurement and Analysis, National Institute for Environmental Studies, Tsukuba, Japan
*Find the extensive, full author and affiliation list on the publishers website

We present details of the global distribution of high-Ca pyroxene (HCP)-rich sites in the lunar highlands based on the global dataset of hyper-spectral reflectance obtained by the SELENE Spectral Profiler. Most HCP-rich sites in the lunar highlands are found at fresh impact craters. In each crater, most of the detection points are distributed on the ejecta, rim, and floor of the impact craters rather than the central peaks, while the central peaks are dominated by purest anorthosite (PAN). This indicates that HCP-rich materials originate from relatively shallower regions of the lunar crust than PAN. In addition, while all ray craters with sizes larger than ~40km possess HCP-rich materials, small fresh craters with sizes less than ~6−−10km do not, indicating that the uppermost mixing layers in the lunar crust are not dominated by HCP. Based on these results, we propose that in the upper lunar crust, a HCP-rich zone overlying the PAN layer exists below the uppermost mixing layer. This HCP-rich zone may originate from interstitial melt during the formation of the flotation anorthositic cumulate, while an impact ejecta origin, impact melt origin, and/or magmatic intrusion into the upper lunar crust may also account for the occurrence of HCP-rich sites in the highlands.

Reference
Yamamoto S et al. (2015) Global occurrence trend of high-Ca pyroxene on lunar highlands and its implications. Journal of Geophysical Research Planets (in Press)
Link to Article [DOI: 10.1002/2014JE004740]

Published by arrangement with John Wiley&Sons

^1,2E.C. Sklute, ¹H.B.Jensen, ¹A.D. Rogers, ¹R.J.
Reeder
¹Stony Brook University, Department of Geosciences, Stony Brook, NY 11794-2100, USA
²Mount Holyoke College, Department of Astronomy, South Hadley, MA

Current or past brine hydrologic activity on Mars may provide suitable conditions for the formation of amorphous ferric sulfates. Once formed, these phases would likely be stable under current Martian conditions, particularly at low- to mid-latitudes. Therefore, we consider amorphous iron sulfates (AIS) as possible components of Martian surface materials. Laboratory AIS were created through multiple synthesis routes, and characterized with total x-ray scattering, thermogravimetric analysis, scanning electron microscopy, visible/near-infrared (VNIR), thermal infrared (TIR), and Mössbauer techniques. We synthesized amorphous ferric sulfates (Fe(III)2(SO4)3•~6-8H2O) from sulfate-saturated fluids via vacuum dehydration or exposure to low relative humidity (<11%). Amorphous ferrous sulfate (Fe(II)SO4•~1H2O) was synthesized via vacuum dehydration of melanterite. All AIS lack structural order beyond 11 Å. The short-range (<5 Å) structural characteristics of amorphous ferric sulfates resemble all crystalline reference compounds; structural characteristics for the amorphous ferrous sulfate are similar to but distinct from both rozenite and szomolnokite. VNIR and TIR spectral data for all AIS display broad, muted features consistent with structural disorder and are spectrally distinct from all crystalline sulfates considered for comparison. Mössbauer spectra are also distinct from crystalline phase spectra available for comparison. AIS should be distinguishable from crystalline sulfates based on the position of their Fe-related absorptions in the visible range and their spectral characteristics in the TIR. In the NIR, bands associated with hydration at ~1.4 and 1.9 µm are significantly broadened, which greatly reduces their detectability in soil mixtures. AIS may contribute to the amorphous fraction of soils measured by the Curiosity rover.

Reference
Sklute EC, Jensen HB, Rogers AD, Reeder RJ (2015) Morphological, Structural, and Spectral Characteristics of Amorphous Iron Sulfates. Journal of Geophysical Research Planets (in Press)
Link to Article [DOI: 10.1002/2014JE004784]

Published by arrangement with John Wiley&Sons

¹Steven J. Jaret, ¹William R. Woerner, ¹Brian L. Phillips, ^2,3Lars Ehm¹Hanna Nekvasil, ^4,5Shawn P. Wright, ¹TimothyD.Glotch
¹Department of Geosciences, State University of New York at Stony Brook, Stony Brook, New York, USA,
²Mineral Physics Institute, State University of New York at Stony Brook, Stony Brook, New York, USA,
³Photon Sciences Directorate, Brookhaven National Laboratory, Upton, New York, USA,
⁴Department of Geosciences, Auburn University, Auburn, Alabama, USA,
⁵Planetary Science Institute, Tucson, Arizona, USA

We present the results of a combined study of shocked labradorite from the Lonar crater, India, using optical microscopy, micro-Raman spectroscopy, nuclear magnetic resonance (NMR) spectroscopy, high-energy X-ray total scattering experiments, and micro-Fourier transform infrared (micro-FTIR) spectroscopy. We show that maskelynite of shock class 2 is structurally more similar to fused glass than to crystalline plagioclase. However, there are slight but significant differences—preservation of original preimpact igneous zoning, anisotropy at infrared wavelengths, X-ray anisotropy, and preservation of some intermediate range order—which are all consistent with a solid-state transformation from plagioclase to maskelynite.

Jaret SJ, Woerner WR, Phillips BL, Ehm L, Nekvasil H, Wright SP, Glotch TD (2015) Maskelynite formation via solid-state transformation: Evidence of infrared and X-ray anisotropy. Journal of Geophysical Research Planets (in Press)
Link to Article [DOI: 10.1002/2014JE004764]

Published by arrangement with John Wiley&Sons

¹James M.T. Lewis, ¹Jonathan S. Watson, ²Jens Najorka, ¹Duy Luong, ¹Mark A. Sephton
¹Impacts and Astromaterials Research Centre, Department of Earth Science and Engineering, Imperial College London, London, United Kingdom.
²Impacts and Astromaterials Research Centre, Department of Mineralogy, Natural History Museum, London, United Kingdom.

We currently do not have a copyright agreement with this publisher and cannot display the abstract here

Reference
Lewis JMT, Watson JS, Najorka J, Luong D, Sephton MA (2015) Sulfate Minerals: A Problem for the Detection of Organic Compounds on Mars? Astrobiology 15(3): 247-258
Link to Article [doi:10.1089/ast.2014.1160]

¹Charles A. Poteet, ¹Douglas C. B. Whittet, ²Bruce T. Draine
¹New York Center for Astrobiology, Department of Physics, Applied Physics and Astronomy, Rensselaer Polytechnic Institute, 110 Eighth Street, Troy, NY 12180, USA
²Princeton University Observatory, Peyton Hall, Princeton, NJ 08544, USA

We investigate the composition of interstellar grains along the line of sight toward ζ Ophiuchi, a well-studied environment near the diffuse-dense cloud transition. A spectral decomposition analysis of the solid-state absorbers is performed using archival spectroscopic observations from the Spitzer Space Telescope and Infrared Space Observatory. We find strong evidence for the presence of sub-micron-sized amorphous silicate grains, principally comprised of olivine-like composition, with no convincing evidence of H2O ice mantles. However, tentative evidence for thick H2O ice mantles on large (a ≈ 2.8 μm) grains is presented. Solid-state abundances of elemental Mg, Si, Fe, and O are inferred from our analysis and compared to standard reference abundances. We find that nearly all of the Mg and Si atoms along the line of sight reside in amorphous silicate grains, while a substantial fraction of the elemental Fe resides in compounds other than silicates. Moreover, we find that the total abundance of elemental O is largely inconsistent with the adopted reference abundances, indicating that as much as ~156 ppm of interstellar O is missing along the line of sight. After taking into account additional limits on the abundance of elemental O in other O-bearing solids, we conclude that any missing reservoir of elemental O must reside on large grains that are nearly opaque to infrared radiation.

Reference
Poteet PA, Whittet DCB, Draine BT (2015) The Composition of Interstellar Grains toward ζ Ophiuchi: Constraining the Elemental Budget near the Diffuse-dense Cloud Transition. Astrophysical Journal 801, 110.
Link to Article [doi:10.1088/0004-637X/801/2/110]

¹Nicole G. Lunning,¹Harry Y. McSween Jr.,²Travis J. Tenner,³Noriko T. Kita,⁴Robert J. Bodnar
¹Department of Earth and Planetary Sciences and Planetary Geosciences Institute, University of Tennessee, Knoxville, TN 37996, USA
²Department of Geosciences, University of Wisconsin, Madison, WI 53706, USA
³Department of Geosciences, Virginia Tech, Blacksburg, VA 24061, USA

A number of meteorites contain evidence that rocky bodies formed and differentiated early in our solar system’s history, and similar bodies likely contributed material to form the planets. These differentiated rocky bodies are expected to have mantles dominated by Mg-rich olivine, but direct evidence for such mantles beyond our own planet has been elusive. Here, we identify olivine fragments (Mg# = 80–92) in howardite meteorites. These Mg-rich olivine fragments do not correspond to an established lithology in the howardite–eucrite–diogenite (HED) meteorites, which are thought to be from the asteroid 4 Vesta; their occurrence in howardite breccias, combined with diagnostic oxygen three-isotope signatures and minor element chemistry, indicates they are vestan. The major element chemistry of these Mg-rich olivines suggests that they formed as mantle residues, in crustal layered intrusions, or in Mg-rich basalts. The trace element chemistry of these Mg-rich olivines supports an origin as mantle samples, but other formation scenarios could be possible. Interpreted as mantle samples, the range of Mg-rich olivine compositions indicates that Vesta’s structure differs from that predicted by conventional models: Vesta has a chemically heterogeneous mantle that feeds serial magmatism. The range of olivine major element chemistries is consistent with models of an incompletely melted mantle such as in the model proposed by Wilson and Keil (2013) rather than a whole-mantle magma ocean for Vesta. Trace element chemistries of Mg-rich pyroxenes (Mg# = 85–92) provide support that some of these pyroxenes may represent initial fractional crystallization of mantle partial melts.

Reference
Lunning NG,McSween Jr. HY,Tenner TJ, Kita NT, Bodnar RJ (2015) Olivine and pyroxene from the mantle of asteroid 4 Vesta. Earth and Planetary Science Letters 418, 126–135
Link to Article [doi:10.1016/j.epsl.2015.02.043]

Copyright Elsevier

^1,2Haolan Tang, ¹Nicolas Dauphas
¹Origins Lab, Department of the Geophysical Sciences and Enrico Fermi Institute, The University of Chicago, 5734 South Ellis Avenue, Chicago IL 60637, USA
²Ion Probe Group, Department of earth and Space Sciences, University of California, Los Angeles, 595 Charles E. Young Drive East, Los Angeles, CA 90095, US

Iron-60 (t1/2 = 2.62 Myr) is a short-lived nuclide that can help constrain the astrophysical context of Solar System formation and date early Solar System events. A high abundance of 60Fe(60Fe/56Fe ≈ 4 × 10−7) was reported by in situ techniques in some chondrules from the LL3.00 Semarkona meteorite, which was taken as evidence that a supernova exploded in the vicinity of the birthplace of the Sun. However, our previous multi-collector inductively coupled plasma mass spectrometry (MC-ICPMS) measurements of a wide range of meteoritic materials, including chondrules, showed that 60Fe was present in the early Solar System at a much lower level (60Fe/56Fe ≈ 10−8). The reason for the discrepancy is unknown but only two Semarkona chondrules were measured by MC-ICPMS and these had Fe/Ni ratios below ~2× chondritic. Here, we show that the initial 60Fe/56Fe ratio in Semarkona chondrules with Fe/Ni ratios up to ~24× chondritic is (5.39 ± 3.27) × 10−9. We also establish the initial 60Fe/56Fe ratio at the time of crystallization of the Sahara 99555 angrite, a chronological anchor, to be (1.97 ± 0.77) × 10−9. These results demonstrate that the initial abundance of 60Fe at Solar System birth was low, corresponding to an initial 60Fe/56Fe ratio of (1.01 ± 0.27) × 10−8.

Reference
Tang H, Dauphas N (2015) Low 60Fe Abundance in Semarkona and Sahara 99555. Astrophysical Journal 802 22.
Link to Article [doi:10.1088/0004-637X/802/1/22]

¹Lonia R.Friedlander, ¹Timothy D. Glotch, ²David L. Bish, ³M. Darby Dyar, ⁴Thomas G.Sharp, ¹Elizabeth C. Sklute, ⁵Joseph R. Michalski

¹Geosciences Department, Stony Brook University, Stony Brook, NY, 11794-2100 USA
¹Department of Geological Sciences, Indiana University, 1001 East 10th Street, Bloomington,
IN, 47405-1405 USA
³Department of Astronomy, Mount Holyoke College, 50 College Street, South Hadley, MA, 01075 USA
⁴School of Earth and Space Exploration, Arizona State University, PO Box 871404, Tempe, AZ,85287-1404 USA
⁵Planetary Science Institute, 1700 E. Fort Lowell, Tucson, AZ, 85719 USA

Many phyllosilicate deposits remotely detected on Mars occur within bombarded terrains. Shock metamorphism from meteor impacts alters mineral structures, producing changed mineral spectra. Thus, impacts have likely affected the spectra of remotely sensed martian phyllosilicates. We present spectral analysis results for a natural nontronite sample (NAu-1) before and after laboratory-generated impacts over five peak pressures between 10 – 40 GPa. We conducted a suite of spectroscopic analyses to characterize the sample’s impact-induced structural and spectral changes. Nontronite becomes increasingly disordered with increasing peak impact pressure. Every infrared spectroscopic technique used showed evidence of structural changes at shock pressures above ~25 GPa. Reflectance spectroscopy in the visible near-infrared (VNIR) region is primarily sensitive to the vibrations of metal-OH and interlayer H2O groups in the nontronite octahedral sheet. Mid-infrared (MIR) spectroscopic techniques are sensitive to the vibrations of silicon and oxygen in the nontronite tetrahedral sheet. Because the tetrahedral and octahedral sheets of nontronite deform differently, impact-driven structural deformation may contribute to differences in phyllosilicate detection between remote sensing techniques sensitive to different parts of the nontronite structure. Observed spectroscopic changes also indicated that the sample’s octahedral and tetrahedral sheets were structurally deformed, but not completely dehydroxylated. This finding is an important distinction from previous studies of thermally altered phyllosilicates in which dehydroxylation follows dehydration in a step-wise progression preceding structural deformation. Impact-alteration may thus complicate mineral-specific identifications based on the location of OH-group bands in remotely detected spectra. This is a key implication for martian remote sensing arising from our results.

Reference
Friedlander R, Glotch TD, Bish DL, Dyar MD, Sharp TG, Sklute EC, Michalski JR (2015) Structural and spectroscopic changes to natural nontronite induced by experimental impacts between 10 and 40 GPa. Journal of Geophysical Research, Planets (in Press)
Link to Article [DOI: 10.1002/2014JE004638]

Published by arrangement with John Wiley&Sons