Tobias W. A. Müller and Wilhelm Kley

Institut für Astronomie & Astrophysik, Universität Tübingen, Auf der Morgenstelle 10, 72076 Tübingen, Germany

Context. Transitional disks are protoplanetary disks around young stars that display inner holes in the dust distribution within a few au that are accompanied by some gas accretion onto the central star. These cavities could possibly be created by the presence of one or more massive planets that opened a large gap or even cleared the whole inner region.
Aims. If the gap is created by planets and gas is still present in it, then there should be a flow of gas past the planet into the inner region. It is our goal to study in detail the mass accretion rate into this planet-created gap in transitional disks and in particular the dependency on the planet’s mass and the thermodynamic properties of the disk.
Methods. We performed 2D hydrodynamical simulations using the grid-based FARGO code for disks with embedded planets. We added radiative cooling from the disk surfaces, radiative diffusion in the disk midplane, and stellar irradiation to the energy equation to have more realistic models.
Results. The mass flow rate into the gap region depends, for given disk thermodynamics, non-monotonically on the mass of the planet. Generally, more massive planets open wider and deeper gaps which would tend to reduce the mass accretion into the inner cavity. However, for larger mass planets the outer disk becomes eccentric and the mass flow rate is enhanced over the low mass cases. As a result, for the isothermal disks the mass flow is always comparable to the expected mass flow of unperturbed disks Ṁ_d, while for more realistic radiative disks the mass flow is very small for low mass planets (≤4 M_jup) and about 50% of Ṁ_d for larger planet masses. The critical planet mass that allows the disk to become eccentric is much larger for radiative disks than for purely isothermal cases.
Conclusions. Massive embedded planets can reduce the mass flow across the gap considerably, to values of about an order of magnitude smaller than the standard disk accretion rate, and can be responsible for opening large cavities. The remaining mass flow into the central cavity is in good agreement with the observations.

Reference
Müller TWA and Kley W (2013) Modelling accretion in transitional disks. Astronomy & Astrophysics 560:A40.
[doi:10.1051/0004-6361/201322503]
Reproduced with permission © ESO

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Olga P. Popova¹ (>>10)*
*Find the extensive, full author and affiliation list on the publishers website.

¹Institute for Dynamics of Geospheres of the Russian Academy of Sciences, Leninsky Prospect 38, Building 1, Moscow, 119334, Russia.

The asteroid impact near the Russian city of Chelyabinsk on 15 February 2013 was the largest airburst on Earth since the 1908 Tunguska event, causing a natural disaster in an area with a population exceeding one million. Because it occurred in an era with modern consumer electronics, field sensors, and laboratory techniques, unprecedented measurements were made of the impact event and the meteoroid that caused it. Here, we document the account of what happened, as understood now, using comprehensive data obtained from astronomy, planetary science, geophysics, meteorology, meteoritics, and cosmochemistry and from social science surveys. A good understanding of the Chelyabinsk incident provides an opportunity to calibrate the event, with implications for the study of near-Earth objects and developing hazard mitigation strategies for planetary protection.

Reference
Popova OP et al. (2013) Chelyabinsk Airburst, Damage Assessment, Meteorite Recovery, and Characterization. Science 342:1069-1073
[doi:10.1126/science.1242642]
Reprinted with permission from AAAS

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Clark R. Chapman

Department of Space Studies, Southwest Research Institute, 1050 Walnut Street, Suite 300, Boulder, CO 80302, USA.

An asteroid impact on Earth about 65 million years ago caused a mass extinction, opening an opportunity for mammals and, eventually, human beings to evolve. We could suffer the dinosaurs’ fate this century, but chances are extremely tiny. More realistic, though less catastrophic, threats come from much more numerous but much smaller near-Earth asteroids (NEAs). NEA impacts with the potential to kill millions of people, like the very largest floods, earthquakes, and hurricanes, occur far less than 1% as often as such natural terrestrial calamities. Indeed, truly dangerous NEA impacts, discounting mere meteorites that puncture roofs, occur so rarely that none have been reliably observed until this year. On page 1069, Popova et al. (1) describe the impact and atmospheric explosion of a 20-m-wide NEA over the Russian city of Chelyabinsk (population 1.2 million) on 15 February 2013, the first NEA impact disaster in modern history.

Reference
Chapman CR (2013) Calibrating Asteroid Impact. Science 342:1051-1052
[doi:10.1126/science.1246250]
Reprinted with permission from AAAS

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Jane Qiu and Richard Stone

Featuring the first lunar rover in 40 years, Chang’e-3 is seen as an important milestone on China’s quest to send a crewed mission to the moon by 2030. Its premier scientific instrument is a wide-angle extreme ultraviolet camera that will continuously observe Earth’s plasmasphere and the tail of comet ISON.

Reference
Qiu J and Stone R (2013) Chinese Mission Ushers in New Era of Lunar Exploration. Science 342:1026
[doi:10.1126/science.342.6162.1026]
Reprinted with permission from AAAS

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McSween¹ et al. (>>10)*
*Find the extensive, full author and affiliation list on the publishers website.

¹Planetary Geoscience Institute and Department of Earth & Planetary Sciences, University of Tennessee, Knoxville, Tennessee, USA

The Dawn mission has provided new evidence strengthening the identification of asteroid Vesta as the parent body of the howardite, eucrite, and diogenite (HED) meteorites. The evidence includes Vesta’s petrologic complexity, detailed spectroscopic characteristics, unique space weathering, diagnostic geochemical abundances and neutron absorption characteristics, chronology of surface units and impact history, occurrence of exogenous carbonaceous chondritic materials in the regolith, and dimensions of the core, all of which are consistent with HED observations and constraints. Global mapping of the distributions of HED lithologies by Dawn cameras and spectrometers provides the missing geologic context for these meteorites, thereby allowing tests of petrogenetic models and increasing their scientific value.

Reference
McSween HY et al. (in press) Dawn; the Vesta–HED connection; and the geologic context for eucrites, diogenites, and howardites. Meteoritics & Planetary Science
[doi:10.1111/maps.12108]
Published by arrangement with John Wiley & Sons

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David W. Mittlefehldt^1,*, Jason S. Herrin^2,‡, Julie E. Quinn^2,§, Stanley A. Mertzman³, Julia A. Cartwright^4,¶, Karen R. Mertzman³, Zhan X. Peng²

¹KR/Astromaterials Research Office, Astromaterials Research and Exploration Sciences Directorate, NASA/Johnson Space Center, Houston, Texas, USA
²Science Analysis and Research Development, Engineering and Science Contract Group, Houston, Texas, USA
³Department of Geosciences, Franklin & Marshall College, Lancaster, Pennsylvania, USA
⁴Max-Planck-Institut für Chemie, Mainz, Germany
^‡Facility for Analysis, Characterization, Testing and Simulation, School of Materials Science and Engineering, and Earth Observatory of Singapore, Nanyang Technological University, Singapore
^§PANalytical, Inc., Westborough, Massachusetts, USA
^¶Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California, USA

We have done petrologic and compositional studies on a suite of polymict eucrites and howardites to better understand regolith processes on their parent asteroid, which we accept is (4) Vesta. Taking into account noble gas results from companion studies, we interpret five howardites to represent breccias assembled from the true regolith: Elephant Moraine (EET) 87513, Grosvenor Mountains (GRO) 95535, GRO 95602, Lewis Cliff (LEW) 85313, and Meteorite Hills (MET) 00423. We suggest that EET 87503 is paired with EET 87513, and thus is also regolithic. Pecora Escarpment (PCA) 02066 is dominated by melt-matrix clasts, which may have been formed from true regolith by impact melting. These meteorites display a range in eucrite:diogenite mixing ratio from 55:45 to 76:24. There is no correlation between degree of regolith character and Ni content. The Ni contents of howardite, eucrite, and diogenites (HEDs) are mostly controlled by the distribution of coarse chondritic clasts and metal grains, which in some cases resulted from individual, low-velocity accretion events, rather than extensive regolith gardening. Trace element compositions indicate that the mafic component of HED polymict breccias is mostly basalt similar to main-group eucrites; Stannern-trend basaltic debris is less common. Pyroxene compositions show that some trace element-rich howardites contain abundant debris from evolved basalts, and that cumulate gabbro debris is present in some breccias. The scale of heterogeneity varies considerably; regolithic howardite EET 87513 is more homogeneous than fragmental howardite Queen Alexandra Range (QUE) 97001. Individual samples of a given howardite can have different compositions even at roughly 5 g masses, indicating that obtaining representative meteorite compositions requires multiple or large samples.

Reference
Mittlefehldt DW, Herrin JS, Quinn JE, Mertzman SA, Cartwright JA, Mertzman KR and Peng ZX (in press) Composition and petrology of HED polymict breccias: The regolith of (4) Vesta. Meteoritics & Planetary Science
[doi:10.1111/maps.12182]
Published by arrangement with John Wiley & Sons

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Makiko K. Haba^a,b, Akira Yamaguchi^a, Kenji Horie^a, Hiroshi Hidaka^c

^aNational Institute of Polar Research, Tachikawa, Tokyo 190-8518, Japan
^bGeochemical Research Center, University of Tokyo, Bunkyo-ku, Tokyo 113-0033, Japan
^cDepartment of Earth and Planetary Systems Science, Hiroshima University, Higashi Hiroshima, Hiroshima 739-8526, Japan

The major and trace elements in zircons obtained from basaltic eucrites Yamato [Y]-75011, Y-792510, Y-82082, Asuka [A]-881467, Juvinas, and Stannern were determined using an electron microprobe analyzer and a sensitive high-resolution ion microprobe, and the formation of zircons on the parent body was discussed. The maximum sizes of zircon grains in each sample are likely to be related to its metamorphic grade. The least-metamorphosed basaltic eucrite Y-75011 contains zircons with sizes of a few μm. Conversely, the highly metamorphosed basaltic eucrites Y-792510 and A-881467 include zircons larger than 20 μm. The relationship between the maximum grain size and metamorphic grade suggests that the formation process of zircons is related to metamorphic events. The Zr/Hf ratios of the zircons found in this study show a wide variation (31.6–71.7), as compared to the chondritic values (Zr/Hf = 32.8–34.3) and bulk basaltic eucrites (34.0–34.7). The Zr/Hf ratios of the zircons from Y-792510 and A-881467 are relatively constant, whereas those of the zircons from Y-82082, Juvinas, and Stannern show large variations. The rare earth element (REE) content in the zircons from Y-792510 and A-881467 is given by La=0.1×CI and Lu = 1000 × CI. On the other hand, the zircons from Stannern show higher REE content (La=0.1–1×CI and ) than those from Y-792510 and A-881467. The most reliable REE data of a large zircon from A-881467 show no Ce anomaly and a distinct negative Eu anomaly. Therefore, it is presumed that the zircons formed under reducing condition in which Ce³⁺ was stable. The melt compositions coexisting with the studied zircons suggest that the zircons from highly metamorphosed eucrites might react with the REE-enriched melt derived from partial melting of the mesostasis region during metamorphism.

Reference
Haba MK, Yamaguchi A, Horie K and Hidaka H (2013) Major and trace elements of zircons from basaltic eucrites: Implications for the formation of zircons on the eucrite parent body. Earth and Planetary Science Letters 387:10–21
[doi:10.1016/j.epsl.2013.11.006]
Copyright Elsevier

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Vadym Kaydash^a, Yuriy Shkuratov^a and Gorden Videen^b

^aAstronomical Institute of Kharkov V.N. Karazin National University, Sumskaya 35, Kharkov 61022, Ukraine
^bSpace Science Institute, 4750 Walnut St. Suite 205, Boulder CO 80301, USA

Images acquired by the Narrow Angle Camera of the Lunar Reconnaissance Orbiter allow phase-ratio imagery of young lunar craters surrounded by dark halos. Such imaging is a new optical remote-sensing technique that is sensitive to the degree of surface roughness. We apply the phase-ratio technique to LRO images of young, dark-halo craters near the crater Denning and in the Balmer basin, in addition to craters created by the impacts of the Ranger-6 spacecraft and Saturn-5 sections of Apollo-13 and Apollo-17. We suggest an alternative explanation of the dark halos and rays seen near the craters at large phase angles. Phase-ratio imaging suggests that these features result from higher surface roughness. Thus, the interpretation of dark crater halos and rays as a composition/maturity variance should be used with caution. The composition and structure factors can be effectively discriminated only using images acquired in a wide range of phase angles including small angles.

Reference
Kaydash V, Yuriy Shkuratov Y and Videen G (in press) Dark halos and rays of young lunar craters: a new insight into interpretation. Icarus
[doi:10.1016/j.icarus.2013.11.025]
Copyright Elsevier

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

^aDepartment of Earth, Ocean and Atmospheric Science, and National High Magnetic Field Laboratory, Florida State University, Tallahassee, Florida 32310, USA

We still seek a copyright agreement with Nature to display abstracts of their cosmochemistry related publications.

Reference
Humayun M et al. (2013) Origin and age of the earliest Martian crust from meteorite NWA 7533. Nature 503:513–516.
[doi:10.1038/nature12764]

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McSween HY

Department of Earth and Planetary Sciences, University of Tennessee, Knoxville, Tennessee 37996–1410, USA.

We still seek a copyright agreement with Nature to display abstracts of their cosmochemistry related publications.

Reference
McSween HY (2013) Planetary science: A chunk of ancient Mars. Nature 503:473–474.
[doi:10.1038/nature12836]

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