David A. Williams^a, R. Aileen Yingst^b and W. Brent Garry^c

^aSchool of Earth & Space Exploration, Arizona State University, Tempe, Arizona 85287-1404.
^bPlanetary Science Institute, Tucson, Arizona
^cNASA Goddard Spaceflight Center, Greenbelt, Maryland

The purpose of this paper is to introduce the Geologic Mapping of Vesta Special Issue/Section of Icarus, which includes several papers containing geologic maps of the surface of Vesta made to support data analysis conducted by the Dawn Science Team during the Vesta Encounter (July 2011-September 2012). In this paper we briefly discuss pre-Dawn knowledge of Vesta, provide the goals of our geologic mapping campaign, discuss the methodologies and materials used for geologic mapping, review the global geologic context of Vesta, discuss the challenges of mapping the geology of Vesta as a small airless body, and describe the content of the papers in this Special Issue/Section. We conclude with a discussion of lessons learned from our quadrangle-based mapping effort and provide recommendations for conducting mapping campaigns as part of planetary spacecraft nominal missions.

Reference
Williams DA, Yingst RA and Garry WB (in press) Introduction: The Geologic Mapping of Vesta. Icarus
[doi:10.1016/j.icarus.2014.03.001]
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Cyrena Anne Goodrich^a,b, George E. Harlow^c, James A. Van Orman^d, Stephen R. Sutton^e, Michael J. Jercinovic^b, Takashi Mikouchi^f

^aPlanetary Science Institute, 1700 E. Ft. Lowell, Suite 106, Tucson, AZ 85719 USA
^bDepartment of Geosciences, University of Massachusetts, 611 North Pleasant Street, Amherst, MA 01003 USA
^cAmerican Museum of Natural History, Department of Earth and Planetary Sciences, Central Park West at 79[th] Street, New York, NY 10024 USA
^dDept. of Earth, Environmental and Planetary Sciences, Case Western Reserve University, Cleveland, OH 44120 USA
^eDept. of Geophysical Sciences and Center for Advanced Radiation Sources, University of Chicago, Chicago, IL 60637 USA
^fDepartment of Earth and Planetary Sciences, University of Tokyo, Tokyo 113-0033 Japan

Ureilites are ultramafic achondrites thought to be residues of partial melting on a carbon-rich asteroid. They show a trend of FeO-variation (olivine Fo from ~74 to 95) that suggests variation in oxidation state. Whether this variation was established during high-temperature igneous processing on the ureilite parent body (UPB), or preserved from nebular precursors, is a subject of debate. The behavior of chromium in ureilites offers a way to assess redox conditions during their formation and address this issue, independent of Fo. We conducted a petrographic and mineral compositional study of occurrences of chromite (Cr-rich spinel) in ureilites, aimed at determining the origin of the chromite in each occurrence and using primary occurrences to constrain models of ureilite petrogenesis. Chromite was studied in LEW 88774 (Fo 74.2), NWA 766 (Fo 76.7), NWA 3109 (Fo 76.3), HaH 064 (Fo 77.5), LAP 03587 (Fo 74.9), CMS 04048 (Fo 76.4), LAP 02382 (Fo 78.6) and EET 96328 (Fo 85.2).
Chromite occurs in LEW 88774 (~5 vol.%), NWA 766 (<1 vol.%), NWA 3109 (<1 vol.%) and HaH 064 (<1 vol.%) as subhedral to anhedral grains comparable in size (~30 μm to 1 mm) and/or textural setting to the major silicates (olivine and pyroxenes[s]) in each rock, indicating that it is a primary phase. The most FeO-rich chromites in these sample (rare grain cores or chadocrysts in silicates) are the most primitive compositions preserved (fe# = 0.55-0.6; Cr# varying from 0.65 to 0.72 among samples). They record olivine-chromite equilibration temperatures of ~1040-1050°C, reflecting subsolidus Fe/Mg reequilibration during slow cooling from ~1200-1300°C. All other chromite in these samples is reduced. Three types of zones are observed. 1) Inclusion-free interior zones showing reduction of FeO (fe# ~0.4→0.28); 2) Outer zones showing further reduction of FeO (fe# ~0.28→0.15) and containing abundant laths of eskolaite-corundum (Cr₂O₃-Al₂O₃); 3) Outermost zones showing extreme reduction of both FeO (fe# <0.15) and Cr₂O₃ (Cr# as low as 0.2). The grains are surrounded by rims of Si-Al-rich glass, graphite, Fe,Cr-carbides ([Fe,Cr]₃C and [Fe,Cr]₇C₃), Cr-rich sulfides (daubréelite and brezinaite) and Cr-rich symplectic bands on adjacent silicates. Chromite is inferred to have been reduced by graphite, forming eskolaite-corundum and carbides as byproducts, during impact excavation. This event involved initial elevation of T (to 1300-1400°C), followed by rapid decompression and drop in T (to <700°C) at 1-20°C/hr. The kinetics of reduction of chromite is consistent with this scenario. The reduction was facilitated by silicate melt surrounding the chromites, which was partly generated by shock-melting of pyroxenes. Symplectic bands, consisting of fine-scale intergrowths of Ca-pyroxene, chromite and glass, formed by reaction between the Cr-enriched melt and adjacent silicates.
Early chromite also occurs in a melt inclusion in olivine in HaH 064 and in a metallic spherule in olivine in LAP 02382. LAP 03587 and CMS 04048 contain ⩽μm-sized chromite+pyroxene symplectic exsolutions in olivine, indicating high Cr valence in the primary olivine. EET 96328 contains a round grain of chromite that could be a late-crystallizing phase. Tiny chromite grains in melt inclusions in EET 96328 formed in late, closed-system reactions.
For 7 of the 8 ureilites we conclude that the relatively oxidizing conditions evidenced by the presence of primary or early chromite pertain to the period of high-T igneous processing. The observation that such conditions are recorded almost exclusively in low-Fo samples supports the interpretation that the ureilite FeO-variation was established during igneous processing on the UPB.

Reference
Goodrich CA, Harlow GE, Van Orman JA, Sutton SR, Jercinovic MJ and Mikouchi T (in press) Petrology of Chromite in Ureilites: Deconvolution of Primary Oxidation States and Secondary Reduction Processes. Geochimica et Cosmochimica Acta
[doi:10.1016/j.gca.2014.02.028]
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J.-P. Williams^a, A.V. Pathare^b and O. Aharonson^c

^aDept. Earth and Space Sciences, University of California, Los Angeles, CA 90095, USA
^bPlanetary Science Institute, Tucson, AZ 85719, USA
^cHelen Kimmel Center for Planetary Science, Weizmann Institute Of Science, Rehovot, 76100 Israel

We model the primary crater production of small (D < 100 m) primary craters on Mars and the Moon using the observed annual flux of terrestrial fireballs. From the size-frequency distribution (SFD) of meteor diameters, with appropriate velocity distributions for Mars and the Moon, we are able to reproduce martian and lunar crater-count chronometry systems (isochrons) in both slope and magnitude. We include an atmospheric model for Mars that accounts for the deceleration, ablation, and fragmentation of meteors. We find that the details of the atmosphere or the fragmentation of the meteors do not strongly influence our results. The downturn in the crater SFD from atmospheric filtering is predicted to occur at D ~ 10-20 cm, well below the downturn observed in the distribution of fresh craters detected by the Mars Global Surveyor (MGS) Mars Orbiter Camera (MOC) or the Mars Reconnaissance Orbiter (MRO) Context Camera (CTX). Crater counts are conducted on the ejecta blanket of Zunil crater and the interior of Pangboche crater on Mars and North Ray and Cone craters on the Moon. Our model isochrons produce a similar slope and age estimate for the formation of Zunil crater as the Hartmann production function (~1 Ma). We derive an age of 35.1 Ma for Pangboche when accounting for the higher elevation (>20 km higher than Zunil), a factor ~2 younger than estimated using the Hartmann production function which assumes 6 mbar surface pressure. We estimate ages of 52.3 Ma and 23.9 Ma for North Ray and Cone crater respectively, consistent with cosmic ray exposure ages from Apollo samples. Our results indicate that the average cratering rate has been constant on these bodies over these time periods. Since our Monte Carlo simulations demonstrate that the existing crater chronology systems can be applied to date young surfaces using small craters on the Moon and Mars, we conclude that the signal from secondary craters in the isochrons must be relatively small at these locations, as our Monte Carlo model only generates primary craters.

Reference
Williams J-P, Pathare AV and O. Aharonson O (in press) The Production of Small Primary Craters on Mars and the Moon. Icarus
[doi:10.1016/j.icarus.2014.03.011]
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J. Leliwa-Kopystynski^a and M. Arakawa^b

^aUniversity of Warsaw, Institute of Geophysics, 02-093 Warsaw, Pasteura 7, Poland
^bGraduate School of Science, Kobe University, 1-1, Rokkodai-cho, Nada-ku, Kobe 657-8501, Japan

A series of impact experiments onto solid decimeter-sized cylinders made of porous gypsum admixed with approximately one centimeter-sized pebbles have been performed. The target densities and their heterogeneous structures could be representative of those of the asteroids Ida, Eros and many others, because asteroid sub-surface could be the consolidated boulders made by self-compaction and/or by impact compaction. Impact velocities in the experiments ranged from 2.0 km/s to 6.7 km/s (collision velocity in the asteroid main belt is approximately 5 km/s). It was found that the slope of the cumulative number distribution of post-impact fragments strongly depends on the specific energy of the impact. The presence of pebbles strongly influences the impact strength of the target as well as the size distribution of the post-impact fragments. Results of the experiments presented here are aimed at identifying the analogy between the laboratory results and the damage of small asteroids or their catastrophic disruption after impacts.

Reference
Leliwa-Kopystynski J and Arakawa M (in press) Impacts experiments onto heterogeneous targets simulating impact breccia: Implications for impact strength of asteroids and formation of the asteroid families. Icarus
[doi:10.1016/j.icarus.2014.03.012]
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Takeru K. Suzuki and Shu-ichiro Inutsuka

Department of Physics, Nagoya University, Nagoya, Aichi 464-8602, Japan

We report results of three-dimensional magnetohydrodynamical (MHD) simulations of global accretion disks threaded with weak vertical magnetic fields. We perform the simulations in the spherical coordinates with different temperature profiles and accordingly different rotation profiles. In the cases with a spatially constant temperature, because the rotation frequency is vertically constant in the equilibrium condition, general properties of the turbulence excited by magnetorotational instability are quantitatively similar to those obtained in local shearing box simulations. On the other hand, in the cases with a radially variable temperature profile, the vertical differential rotation, which is inevitable in the equilibrium condition, winds up the magnetic field lines in addition to the usual radial differential rotation. As a result, the coherent wound magnetic fields contribute to the Maxwell stress in the surface regions. We obtain nondimensional density and velocity fluctuations ~0.1-0.2 at the midplane. The azimuthal power spectra of the magnetic fields show shallower slopes, ~m⁰ – m^–1, than those of velocity and density. The Poynting flux associated with the MHD turbulence drives intermittent and structured disk winds as well as sound-like waves toward the midplane. The mass accretion mainly occurs near the surfaces, and the gas near the midplane slowly moves outward in the time domain of the present simulations. The vertical magnetic fields are also dragged inward in the surface regions, while they stochastically move outward and inward around the midplane. We also discuss an observational implication of induced spiral structure in the simulated turbulent disks.

Reference
Suzuki TK and Inutsuka S-I (2014) Magnetohydrodynamic Simulations of Global Accretion Disks with Vertical Magnetic Fields. The Astrophysical Journal 784:121.
[doi:10.1088/0004-637X/784/2/121]

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Anthony L. Piro¹ and Ehud Nakar²

¹Theoretical Astrophysics, California Institute of Technology, 1200 E California Boulevard, M/C 350-17, Pasadena, CA 91125, USA
²Raymond and Beverly Sackler School of Physics and Astronomy, Tel Aviv University, Tel Aviv 69978, Israel

Ongoing transient surveys are presenting an unprecedented account of the rising light curves of Type Ia supernovae (SNe Ia). This early emission probes the shallowest layers of the exploding white dwarf (WD), which can provide constraints on the progenitor star and the properties of the explosive burning. We use semianalytic models of radioactively powered rising light curves to analyze these observations. As we have summarized in previous work, the main limiting factor in determining the surface distribution of ⁵⁶Ni is the lack of an unambiguously identified time of explosion, as would be provided by detection of shock breakout or shock-heated cooling. Without this the SN may in principle exhibit a “dark phase” for a few hours to days, where the only emission is from shock-heated cooling that is too dim to be detected. We show that by assuming a theoretically motivated time-dependent velocity evolution, the explosion time can be better constrained, albeit with potential systematic uncertainties. This technique is used to infer the surface ⁵⁶Ni distributions of three recent SNe Ia that were caught especially early in their rise. In all three we find fairly similar ⁵⁶Ni distributions. Observations of SN 2011fe and SN 2012cg probe shallower depths than SN 2009ig, and in these two cases ⁵⁶Ni is present merely ~10^-2 M_☉ from the WDs’ surfaces. The uncertainty in this result is up to an order of magnitude given the difficulty of precisely constraining the explosion time. We also use our conclusions about the explosion times to reassess radius constraints for the progenitor of SN 2011fe, as well as discuss the roughly t² power law that is inferred for many observed rising light curves.

Reference
Piro AL and Nakar E (2014) Constraints on Shallow 56Ni from the Early Light Curves of Type Ia Supernovae. The Astrophysical Journal 784:85.
[doi:10.1088/0004-637X/784/1/85]

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Bin Yang^1,2, Jacqueline Keane¹, Karen Meech^1,3, Tobias Owen³ and Richard Wainscoat³

¹NASA Astrobiology Institute, University of Hawaii, Honolulu, HI 96822, USA
²European Southern Observatory, Santiago, Chile
³Institute for Astronomy, University of Hawaii, Honolulu, HI 96822, USA

The dynamically new comet C/2011 L4 (Pan-STARRS) is one of the brightest comets observed since the great comet C/1995 O1 (Hale-Bopp). Here, we present our multi-wavelength observations of C/2011 L4 during its in-bound passage to the inner solar system. A strong absorption band of water ice at 2.0 μm was detected in the near-infrared spectra, obtained with the 8 m Gemini-North and 3 m Infrared Telescope Facility Telescopes. The companion 1.5 μm band of water ice, however, was not observed. Spectral modeling shows that the absence of the 1.5 μm feature can be explained by the presence of sub-micron-sized fine ice grains. No gas lines (i.e., CN, HCN, or CO) were observed pre-perihelion in either the optical or the submillimeter. We derived 3σ upper limits for the CN and CO production rates. The comet exhibited a very strong continuum in the optical and its slope seemed to become redder as the comet approached the Sun. Our observations suggest that C/2011 L4 is an unusually dust-rich comet with a dust-to-gas mass ratio >4.

Reference
Yang B, Keane J, Meech K, Owen T and Wainscoat R (2014) TMulti-wavelength Observations of Comet C/2011 L4 (Pan-STARRS). The Astrophysical Journal – Letters 784:L23.
[doi:10.1088/2041-8205/784/2/L23]

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Shoshana Z. Weider^a,d, Larry R. Nittler^a,d, Richard D. Starr^b,d, Timothy J. McCoy^c,d and Sean C. Solomon^a,c,d

^aDepartment of Terrestrial Magnetism, Carnegie Institution of Washington, Washington, DC 20015, USA
^bPhysics Department, The Catholic University of America, Washington, DC 20064, USA
^cDepartment of Mineral Sciences, National Museum of Natural History, Smithsonian Institution, Washington, DC 20560, USA
^dLamont-Doherty Earth Observatory, Columbia University, Palisades, NY 10964, USA

We present measurements of Mercury’s surface composition from the analysis of MESSENGER X-Ray Spectrometer data acquired during 55 large solar flares, which each provide a statistically significant detection of Fe X-ray fluorescence. The Fe/Si data display a clear dependence on phase angle, for which the results are empirically corrected. Mercury’s surface has a low total abundance of Fe, with a mean Fe/Si ratio of ∼0.06 (equivalent to ∼1.5 wt % Fe). The absolute Fe/Si values are subject to a number of systematic uncertainties, including phase-angle correction and possible mineral mixing effects. Individual Fe/Si measurements have an intrinsic error of ∼10%. Observed Fe/Si values display small variations (significant at two standard deviations) from the planetary average value across large regions in Mercury’s southern hemisphere. Larger differences are observed between measured Fe/Si values from more spatially resolved footprints on volcanic smooth plains deposits in the northern hemisphere and from those in surrounding terrains. Fe is most likely contained as a minor component in sulfide phases (e.g., troilite, niningerite, daubréelite) and as Fe metal, rather than within mafic silicates. Variations in surface reflectance (i.e., differences in albedo and spectral slope) across Mercury are unlikely to be caused by variations in the abundance of Fe.

Reference
Weider SZ, Nittler LR, Starr RD, McCoy TJ and Solomon SC (in press) Variations in the abundance of iron on Mercury’s surface from MESSENGER X-Ray Spectrometer observations. Icarus
[doi:10.1016/j.icarus.2014.03.002]
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Mainzer¹ et al. (>10)*
*Find the extensive, full author and affiliation list on the publishers website.

¹Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA

Only a very small fraction of the asteroid population at size scales comparable to the object that exploded over Chelyabinsk, Russia has been discovered to date, and physical properties are poorly characterized. We present previously unreported detections of 105 close approaching near-Earth objects (NEOs) by the Wide-field Infrared Survey Explorer (WISE) mission’s NEOWISE project. These infrared observations constrain physical properties such as diameter and albedo for these objects, many of which are found to be smaller than 100 m. Because these objects are intrinsically faint, they were detected by WISE during very close approaches to the Earth, often at large apparent on-sky velocities. We observe a trend of increasing albedo with decreasing size, but as this sample of NEOs was discovered by visible light surveys, it is likely that selection biases against finding small, dark NEOs influence this finding.

Reference
Mainzer et al. (2014) The Population of Tiny Near-Earth Objects Observed by NEOWISE. The Astrophysical Journal 784:110.
[doi:10.1088/0004-637X/784/2/110]

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

^aa Istituto di Astrofisica e Planetologia Spaziali INAF-IAPS, Via del Fosso del Cavaliere 100, 00133, Rome, Italy

The Dawn spacecraft recently observed the presence of dark material, which in turn proved to be associated with the presence of OH and H-rich material, on the surface of Vesta. The source of this dark material has been almost unanimously identified with the low albedo asteroids, likely analogous to the carbonaceous chondrites found on Earth, that impacted on Vesta over its lifetime. However, it is still a matter of debate whether the delivery of the dark material is associated with a few large impact events, to micrometeorites or to the continuous, secular flux of impactors on Vesta. The “continuous flux” scenario, in particular, predicts that a significant fraction of the exogenous material accreted by Vesta should be due to non-dark impactors likely analogous to ordinary chondrites, which instead represent only a minor contaminant in the Howardite-Eucrite-Diogenite meteorites. In this work, we explored the “continuous flux” scenario and its implications for the composition of the vestan regolith, taking advantage of the data from the Dawn mission and the Howardite-Eucrite-Diogenite meteorites to constrain the contamination history of Vesta. We developed a model for the delivery of the exogenous material to Vesta and verified how the results it supplies are sensitive to the different parameters we consider. We calibrated the flux of impactors predicted by our model with the number of dark craters observed inside the Rheasilvia basin and we tested the assumptions on the impact conditions by studying the formation of Cornelia crater and of its dark deposits with a hydrocode simulation. We used our calibrated model to show that the “stochastic events” scenario and the “micrometeoritic flux” scenario are just natural consequences of the “continuous flux” scenario. We then used the model to estimate the amounts of dark and hydroxylate materials that were delivered on Vesta since the Late Heavy Bombardment and we showed how our results match well with the values estimated by the Dawn mission. We finally used our model to assess the amount of Fe and siderophile elements that the continuous flux of impactors would mix in the vestan regolith: concerning the siderophile elements, we focused our attention on the role of Ni. The results we obtained are in agreement with the data available on the Fe and Ni content of the Howardite-Eucrite-Diogenite meteorites and can be used as a reference frame in future studies of the data from the Dawn mission and of the Howardite-Eucrite-Diogenite meteorites. Our model cannot yet provide an answer to the conundrum of the fate of the missing non-carbonaceous contaminants, but we discuss some possible reasons for this discrepancy with the otherwise coherent picture described by our results.

Reference
Turrini et al. (in press) The contamination of the surface of Vesta by impacts and the delivery of the dark material. Icarus
[doi:10.1016/j.icarus.2014.02.021]
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