Richard A. Kerr

The simple organic compounds discovered by the Curiosity Mars rover either came with the tons of never-alive cosmic debris that sifts onto every planetary body or are something far more exciting: remains of martian life from eons ago, when a habitable lake graced the rover’s landing site. Researchers could learn more about the complex organic molecules that yielded these first finds if Curiosity’s drilling strikes a richer vein of organics in the coming months or if a more sophisticated analytical technique is employed.

Reference
Kerr RA (2014) Search for Martian Life Clears Another Hurdle. Science 343:1419.
[doi:10.1126/science.343.6178.1419]
Reprinted with permission from AAAS

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Eleanor R. Mare¹, Andrew G. Tomkins¹ and Belinda M. Godel²

¹School of Geosciences, Monash University, Melbourne, Australia
²CSIRO Earth Science and Resource Engineering, Australian Resources Research Centre, Kensington, Western Australia, Australia

Ordinary chondrite meteorites contain silicates, Fe,Ni-metal grains, and troilite (FeS). Conjoined metal-troilite grains would be the first phase to melt during radiogenic heating in the parent body, if temperatures reached over approximately 910–960 °C (the Fe,Ni-FeS eutectic). On the basis of two-pyroxene thermometry of 13 ordinary chondrites, we argue that peak temperatures in some type 6 chondrites exceeded the Fe,Ni-FeS eutectic and thus conjoined metal-troilite grains would have begun to melt. Melting reactions consume energy, so thermal models were constructed to investigate the effect of melting on the thermal history of the H, L, and LL parent asteroids. We constrained the models by finding the proportions of conjoined metal-troilite grains in ordinary chondrites using high-resolution X-ray computed tomography. The models show that metal-troilite melting causes thermal buffering and inhibits the onset of silicate melting. Compared with models that ignore the effect of melting, our models predict longer cooling histories for the asteroids and accretion times that are earlier by 61, 124, or 113 kyr for the H, L, and LL asteroids, respectively. Because the Ni/Fe ratio of the metal and the bulk troilite/metal ratio is higher in L and LL chondrites than H chondrites, thermal buffering has the greatest effect in models for the L and LL chondrite parent bodies, and least effect for the H chondrite parent. Metal-troilite melting is also relevant to models of primitive achondrite parent bodies, particularly those that underwent only low degrees of silicate partial melting. Thermal models can predict proportions of petrologic types formed within an asteroid, but are systematically different from the statistics of meteorite collections. A sampling bias is interpreted to explain these differences.

Reference
Mare ER, Tomkins AG and Godel BM (in press) Restriction of parent body heating by metal-troilite melting: Thermal models for the ordinary chondrites. Meteoritics & Planetary Science
[doi:10.1111/maps.12280]
Published by arrangement with John Wiley & Sons

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Alan W. Harris and Line Drube

German Aerospace Center (DLR) Institute of Planetary Research, Rutherfordstrasse 2, D-12489 Berlin, Germany

The metal content of asteroids is of great interest, not only for theories of their origins and the evolution of the solar system but, in the case of near-Earth objects (NEOs), also for impact mitigation planning and endeavors in the field of planetary resources. However, since the reflection spectra of metallic asteroids are largely featureless, it is difficult to identify them and relatively few are known. We show how data from the Wide-field Infrared Survey Explorer (WISE)/NEOWISE thermal-infrared survey and similar surveys, fitted with a simple thermal model, can reveal objects likely to be metal rich. We provide a list of candidate metal-rich NEOs. Our results imply that future infrared surveys with the appropriate instrumentation could discover many more metal-rich asteroids, providing valuable data for assessment of the impact hazard and the potential of NEOs as reservoirs of vital materials for future interplanetary space activities and, eventually perhaps, for use on Earth.

Reference
Harris AW and Drube L (2014) How to find metal-rich asteroids. The Astrophysical Journal – Letters 785:L4.
[doi:10.1088/2041-8205/785/1/L4]

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W. D. Pesnell¹ and P. Bryans²

¹NASA Goddard Space Flight Center, Code 671, Greenbelt, MD 20771, USA
²ADNET Systems Inc., NASA Goddard Space Flight Center, Code 671, Greenbelt, MD 20771, USA

Recent improvements in solar observations have greatly progressed the study of sungrazing comets. They can now be imaged along the entirety of their perihelion passage through the solar atmosphere, revealing details of their composition and structure not measurable through previous observations in the less volatile region of the orbit further from the solar surface. Such comets are also unique probes of the solar atmosphere. The debris deposited by sungrazers is rapidly ionized and subsequently influenced by the ambient magnetic field. Measuring the spectral signature of the deposited material highlights the topology of the magnetic field and can reveal plasma parameters such as the electron temperature and density. Recovering these variables from the observable data requires a model of the interaction of the cometary species with the atmosphere through which they pass. The present paper offers such a model by considering the time-dependent chemistry of sublimated cometary species as they interact with the solar radiation field and coronal plasma. We expand on a previous simplified model by considering the fully time-dependent solutions of the emitting species’ densities. To compare with observations, we consider a spherically symmetric expansion of the sublimated material into the corona and convert the time-dependent ion densities to radial profiles. Using emissivities from the CHIANTI database and plasma parameters derived from a magnetohydrodynamic simulation leads to a spatially dependent emission spectrum that can be directly compared with observations. We find our simulated spectra to be consistent with observation.

Reference
Pesnell WD and Bryans P (2014) The Time-dependent Chemistry of Cometary Debris in the Solar Corona. The Astrophysical Journal 785:50.
[doi:10.1088/0004-637X/785/1/50]

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