Julien Faure¹, Sébastien Fromang¹ and Henrik Latter²

¹Laboratoire AIM, CEA/DSM – CNRS – Université Paris 7, Irfu/Service d’Astrophysique, CEA-Saclay, 91191 Gif-sur-Yvette, France
²Department of Applied Mathematics and Theoretical Physics, University of Cambridge, Centre for Mathematical Sciences, Wilberforce Road, Cambridge CB3 0WA, UK

Context. In protoplanetary disks, the inner boundary between the turbulent and laminar regions could be a promising site for planet formation, thanks to the trapping of solids at the boundary itself or in vortices generated by the Rossby wave instability. At the interface, the disk thermodynamics and the turbulent dynamics are entwined because of the importance of turbulent dissipation and thermal ionization. Numerical models of the boundary, however, have neglected the thermodynamics, and thus miss a part of the physics.
Aims. The aim of this paper is to numerically investigate the interplay between thermodynamics and dynamics in the inner regions of protoplanetary disks by properly accounting for turbulent heating and the dependence of the resistivity on the local temperature.
Methods. Using the Godunov code RAMSES, we performed a series of 3D global numerical simulations of protoplanetary disks in the cylindrical limit, including turbulent heating and a simple prescription for radiative cooling.
Results. We find that waves excited by the turbulence significantly heat the dead zone, and we subsequently provide a simple theoretical framework for estimating the wave heating and consequent temperature profile. In addition, our simulations reveal that the dead-zone inner edge can propagate outward into the dead zone, before stalling at a critical radius that can be estimated from a mean-field model. The engine driving the propagation is in fact density wave heating close to the interface. A pressure maximum appears at the interface in all simulations, and we note the emergence of the Rossby wave instability in simulations with extended azimuth.
Conclusions. Our simulations illustrate the complex interplay between thermodynamics and turbulent dynamics in the inner regions of protoplanetary disks. They also reveal how important activity at the dead-zone interface can be for the dead-zone thermodynamic structure.

Reference
Faure J, Fromang S and Latter H (2014) Thermodynamics of the dead-zone inner edge in protoplanetary disks. Astronomy & Astrophysics 564:A22.
[doi:10.1051/0004-6361/201321911]
Reproduced with permission © ESO

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Yuri I. Fujii¹, Satoshi Okuzumi^1,2, Takayuki Tanigawa³ and Shu-ichiro Inutsuka¹

¹Department of Physics, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, Aichi 464-8602, Japan
²Department of Earth and Planetary Sciences, Tokyo Institute of Technology, Meguro-ku, Tokyo 152-8551, Japan
³Institute of Low Temperature Science, Hokkaido University, Sapporo 066-0819, Japan

We examine whether the magnetorotational instability (MRI) can serve as a mechanism of angular momentum transport in circumplanetary disks. For the MRI to operate the ionization degree must be sufficiently high and the magnetic pressure must be sufficiently lower than the gas pressure. We calculate the spatial distribution of the ionization degree and search for the MRI-active region where the two criteria are met. We find that there can be thin active layers at the disk surface depending on the model parameters, however, we find hardly any region which can sustain well-developed MRI turbulence; when the magnetic field is enhanced by MRI turbulence at the disk surface layer, a magnetically dominated atmosphere encroaches on a lower altitude and a region of well-developed MRI turbulence becomes smaller. We conclude that if there are no angular momentum transfer mechanisms other than MRI in gravitationally stable circumplanetary disks, gas is likely to pile up until disks become gravitationally unstable, and massive disks may survive for a long time.

Reference
Fujii YI, Okuzumi S, Tanigawa T and Shu-ichiro Inutsuka S-I (2014) On the Viability of the Magnetorotational Instability in Circumplanetary Disks. The Astrophysical Journal 785:101.
[doi:10.1088/0004-637X/785/2/101]

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Richard A. Kerr

Orbiting Saturn, Cassini had “tasted” the plumes of salty water that spew from Enceladus’s south polar region, so scientists had growing confidence that the plumes tap briny liquid water tens of kilometers beneath the surface. Now, analyses of Cassini measurements of undulations in Enceladus’s gravity field point to a 10-kilometer-thick layer of water beneath the south polar region, if not the entire moon.

Reference
Kerr RA (2014) Cassini Plumbs the Depths of the Enceladus Sea. Science 344:17.
[doi:10.1126/science.344.6179.17]
Reprinted with permission from AAAS

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L. Iess¹ et al. (>10)*
*Find the extensive, full author and affiliation list on the publishers website.

¹Dipartimento di Ingegneria Meccanica e Aerospaziale, Sapienza Università di Roma, via Eudossiana 18, 00184 Rome, Italy.

The small and active Saturnian moon Enceladus is one of the primary targets of the Cassini mission. We determined the quadrupole gravity field of Enceladus and its hemispherical asymmetry using Doppler data from three spacecraft flybys. Our results indicate the presence of a negative mass anomaly in the south-polar region, largely compensated by a positive subsurface anomaly compatible with the presence of a regional subsurface sea at depths of 30 to 40 kilometers and extending up to south latitudes of about 50°. The estimated values for the largest quadrupole harmonic coefficients (10⁶J₂ = 5435.2 ± 34.9, 10⁶C₂₂ = 1549.8 ± 15.6, 1σ) and their ratio (J₂/C₂₂ = 3.51 ± 0.05) indicate that the body deviates mildly from hydrostatic equilibrium. The moment of inertia is around 0.335MR², where M is the mass and R is the radius, suggesting a differentiated body with a low-density core.

Reference
Iess et al. (2014) The Gravity Field and Interior Structure of Enceladus. Science 344:78.
[doi:10.1126/science.1250551]
Reprinted with permission from AAAS

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

^aInstitut de Planétologie et d’Astrophysique de Grenoble (IPAG), Université Grenoble Alpes / CNRS-INSU, UMR 5274, Grenoble F-38041, France

Insoluble organic matter (IOM) extracted from primitive chondrites is a polyaromatic solid with a structure and composition resembling that of terrestrial kerogens. A survey of its composition and structure has been carried out on a series of 27 CR, CM, CI and ungrouped C2 carbonaceous chondrites (Tagish Lake, Bells, Essebi, Acfer 094) using infrared and multi-wavelength Raman micro-spectroscopy (244, 514 and 785 nm laser excitations). The results show that chondritic IOM from PCA 91008 (CM2), WIS 91600 (CM2), QUE 93005 (CM2), Tagish Lake (C2 ungrouped) and possibly Cold Bokkeveld (CM2) has been subjected to the past action of short duration thermal metamorphism, presumably triggered by impacts. The IOM in most of the CM chondrites that experienced moderate to heavy aqueous alteration may have been slightly modified by collision-induced heating. However, even IOM from chondrites that escaped significant thermal metamorphism displays Raman characteristics consistent with a formation by thermal processing, either in the protosolar disk or in the parent body. An alternative energetic process to thermal heating is ion irradiation. After thoroughly analyzing both these scenarii, no conclusion can be drawn as to which is the most plausible mechanism nor whether the heating process took place prior or after accretion. The results show for the first time that the width of the G band in spectra collected with a 514 nm excitation correlates with the O/C atomic ratio, suggesting a major role of oxygen in the cross-linking of polyaromatic units.

Reference
Quirico et al. (in press) Origin of insoluble organic matter in type 1 and 2 chondrites: new clues, new questions. Geochimica et Cosmochimica Acta
[doi:10.1016/j.gca.2014.03.025]
Copyright Elsevier

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Jon M. Friedrich^a,b, Michael K. Weisberg^b,c and Mark L. Rivers^d

^aDepartment of Chemistry, Fordham University, Bronx, NY 10458, USA
^bDepartment of Earth and Planetary Sciences, American Museum of Natural History, New York, NY 10024, USA
^cDepartment of Physical Sciences, Kingsborough College and Graduate School of the City University of New York, Brooklyn, NY 11235, USA
^dConsortium for Advanced Radiation Sources, University of Chicago, Argonne, IL 60439, USA

The major geologic process that has shaped the asteroids and led to development of their regoliths is impact. Petrofabrics in ordinary chondrites are undoubtedly the result of impact events on their asteroidal parent bodies and the foliation present in a chondrite serves as an enduring record of the magnitude of the most intense compacting event experienced by the material. An overwhelming majority of chondrites have an internally consistent petrofabric contained within the spatial dimensions of the entire rock, including across clasts or different petrographic domains. This indicates that the magnitude of the most recent impact to have affected the assembled chondrite was significant enough to impart a foliation across all lithologies. Information of any previous impacts is largely lost because of the consistent, realigned foliations. We present X-ray microtomography derived 3D petrofabric intensity and orientation data for three lithologies in the NWA 7298 breccia. The internally inconsistent petrofabrics among differing lithologies indicate that the magnitude of the final impact event was smaller than previous ones. This latter case preserves fabric information recorded during previous impacts and allows a more complete interpretation of the impact history of a local region of the asteroidal parent. We used our data to infer the sequence and intensity of distinct impact events affecting the NWA 7298 parent asteroid. We suggest a near-surface impact debris zone on the H chondrite parent asteroid as an origin for NWA 7298. These observations yield new opportunities for investigating and interpreting the dynamic collisional evolution of asteroids.

Reference
Friedrich JM, Weisberg MK and Rivers ML (2014) Multiple impact events recorded in the NWA 7298 H chondrite breccia and the dynamical evolution of an ordinary chondrite asteroid. Earth and Planetary Science Letters 394:13.
[doi:10.1016/j.epsl.2014.03.016]
Copyright Elsevier

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Sohsuke Ohno¹ et al. (>10)*
*Find the extensive, full author and affiliation list on the publishers website.

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

Reference
Ohno S (2014) Production of sulphate-rich vapour during the Chicxulub impact and implications for ocean acidification. Nature Geoscience  7:279.
[doi:10.1038/ngeo2095]

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Timothy A. Goudge¹ et al. (>10)*
*Find the extensive, full author and affiliation list on the publishers websi

¹Department of Geological Sciences, Brown University, Providence, Rhode Island, USA

We present new observations of pyroclastic deposits on the surface of Mercury from data acquired during the orbital phase of the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) mission. The global analysis of pyroclastic deposits brings the total number of such identified features from 40 to 51. Some 90% of pyroclastic deposits are found within impact craters. The locations of most pyroclastic deposits appear to be unrelated to regional smooth plains deposits, except some deposits cluster around the margins of smooth plains, similar to the relation between many lunar pyroclastic deposits and lunar maria. A survey of the degradation state of the impact craters that host pyroclastic deposits suggests that pyroclastic activity occurred on Mercury over a prolonged interval. Measurements of surface reflectance by MESSENGER indicate that the pyroclastic deposits are spectrally distinct from their surrounding terrain, with higher reflectance values, redder (i.e., steeper) spectral slopes, and a downturn at wavelengths shorter than ~400 nm (i.e., in the near-ultraviolet region of the spectrum). Three possible causes for these distinctive characteristics include differences in transition metal content, physical properties (e.g., grain size), or degree of space weathering from average surface material on Mercury. The strength of the near-ultraviolet downturn varies among spectra of pyroclastic deposits and is correlated with reflectance at visible wavelengths. We suggest that this interdeposit variability in reflectance spectra is the result of either variable amounts of mixing of the pyroclastic deposits with underlying material or inherent differences in chemical and physical properties among pyroclastic deposits.

Reference
Goudge et al. (in press) Global inventory and characterization of pyroclastic deposits on Mercury: New insights into pyroclastic activity from MESSENGER orbital data. Journal of Geophysical Research: Planets
[doi:10.1002/2013JE004480]
Published by arrangement with John Wiley & Sons

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William B. McKinnon

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

Reference
McKinnon WB (2014) Planetary science: Shrinking wrinkling Mercury. Nature Geoscience 7:251.
[doi:10.1038/ngeo2123]

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Paul K. Byrne, Christian Klimczak, A. M. Celâl Şengör, Sean C. Solomon, Thomas R. Watters & Steven A. Hauck, II

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

Reference
Byrne PK, Klimczak C, Şengör AMC, Solomon SC, Watters TR & Hauck, II SA (2014) Mercury’s global contraction much greater than earlier estimates. Nature Geoscience 7:301.
[doi:10.1038/ngeo2097]

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