These are the five most popular papers based on the interest (clicks) on this blog during August 2014.

1-Rozitis B, MacLennan E, Emery JP (2014) Cohesive forces prevent the rotational breakup of rubble-pile asteroid (29075) 1950 DA. Nature 512, 174–176
Link to Article [doi:10.1038/nature13632]

2-Westphal AJ et al.(2014) Evidence for interstellar origin of seven dust particles collected by the Stardust spacecraft. Science 345, 6198, 786-791
Link to Article [DOI: 10.1126/science.1252496]

3-Day JMD, Moynier F (2014) Evaporative fractionation of volatile stable isotopes and their bearing on the origin of the Moon. Philosophical Transactions of the Royal Society A 13,372,2024
Link to Article [doi: 10.1098/rsta.2013.0259]

4-Neish CD, Madden J, Carter LM, Hawke BR, Giguere T, Bray VJ, Osinski GR and Cahill JTS (2014) Global distribution of lunar impact melt flows. Icarus (in Press)
Link to Article [DOI: 10.1016/j.icarus.2014.05.049]

5-Lugaro M, Heger A, Osrin D, Goriely S, Zuber K, Karakas AI, Gibson BK, Doherty CL, Lattanzio JC, Ott U (2014) Stellar origin of the 182Hf cosmochronometer and the presolar history of solar system matter. Science 345:6197, 650-653.
Link to Article [DOI: 10.1126/science.1253338]

¹R.R. Fu, ¹E.A. Lima, ¹B.P. Weiss
¹Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA, USA

Magnetic fields in the solar nebula may have played a central role in mass and angular momentum transport in the protosolar disk and facilitated the accretion of the first planetesimals. Thought to be key evidence for this hypothesis is the high unblocking-temperature, randomly oriented magnetization in chondrules in the Allende CV carbonaceous chondrite. However, it has recently been realized that most of the ferromagnetic minerals in Allende are products of secondary processes on the parent planetesimal. Here we reevaluate the pre-accretional magnetism hypothesis for Allende using new paleomagnetic analyses of chondrules including the first measurements of mutually oriented subsamples from within individual chondrules. We confirm that Allende chondrules carry a high-temperature component of magnetization that is randomly oriented among chondrules. However, we find that subsamples of individual chondrules are also non-unidirectionally magnetized. Therefore, the high-temperature magnetization in Allende chondrules is not a record of nebular magnetic fields and is instead best explained by remagnetization during metasomatism in a

Reference
Fu RR, Lima, EA, Weiss BP (2014) No nebular magnetization in the Allende CV carbonaceous chondrite. Earth and Planetary Science Letters 404, 54–66
Link to Article [DOI: 10.1016/j.epsl.2014.07.014]

Copyright Elsevier

^1,2Thomas S. Kruijer, ¹Thorsten Kleine, ¹Mario Fischer-Gödde, ³Christoph Burkhardt, ²Rainer Wieler
¹Institut für Planetologie, Westfälische Wilhelms-Universität Münster, Wilhelm Klemm-Strasse 10, 48149 Münster, Germany
²ETH Zürich, Institute of Geochemistry and Petrology, Clausiusstrasse 25, 8092 Zürich, Switzerland
³Origins Laboratory, Department of the Geophysical Sciences, The University of Chicago, 5734 South Ellis Avenue, Chicago, IL

Ca–Al-rich inclusions (CAI) are the oldest dated objects formed in the solar system and are pivotal reference points in early solar system chronology. Knowledge of their initial 182Hf/180Hf and 182W/184W is essential, not only for obtaining precise Hf–W ages relative to the start of the solar system, but also to assess the distribution of short-lived radionuclides in the early solar nebula. However, the interpretation of Hf–W data for CAI is complicated by nucleosynthetic W isotope variations. To explore their extent and nature, and to better quantify the initial Hf and W isotope compositions of the solar system, we obtained Hf–W data for several fine- and coarse-grained CAI from three CV3 chondrites. The fine-grained CAI exhibit large and variable anomalies in ε 183W (εiWεiW equals 0.01% deviation from terrestrial values), extending to much larger anomalies than previously observed in CAI, and reflecting variable abundances of s – and r -process W isotopes. Conversely, the coarse-grained (mostly type B) inclusions show only small (if any) nucleosynthetic W isotope anomalies. The investigated CAI define a precise correlation between initial ε 182W and ε 183W, providing a direct empirical means to correct the ε 182W of any CAI for nucleosynthetic isotope anomalies using their measured ε 183W. After correction for nucleosynthetic W isotope variations, the CAI data define an initial 182Hf/180Hf of (1.018±0.043)×10−4(1.018±0.043)×10−4 and an initial ε 182W of −3.49±0.07−3.49±0.07. The Hf–W formation intervals of the angrites D’Orbigny and Sahara 99555 relative to this CAI initial is 4.8±0.6 Ma4.8±0.6 Ma, in good agreement with Al–Mg ages of these two angrites. This renders a grossly heterogeneous distribution of 26Al in the inner solar system unlikely, at least in the region were CAI and angrites formed.

Reference
Kruijer TS, Thorsten Kleine T, Mario Fischer-Gödde M, Christoph Burkhardt C, Wieler R (2014) Nucleosynthetic W isotope anomalies and the Hf–W chronometry of Ca–Al-rich inclusions. Earth and Planetary Science Letters 403,317–327.
Link to Article [DOI: 10.1016/j.epsl.2014.07.003]

Copyright Elsevier

¹Raúl O.C. Fonseca,²Guilherme Mallmann, ³Peter Sprung, ¹Johanna E. Sommer, ¹Alexander Heuser, ¹Iris M. Speelmanns, ¹Henrik Blanchard

¹Steinmann-Institut, Universität Bonn, 53115 Bonn, Germany
²School of Earth Sciences, The University of Queensland, Brisbane QLD 4072, Australia
³Institut für Planetologie, Westfälische Wilhelms-Universität Münster, 48149 Münster, Germany

The timing of core formation is essential for understanding the early differentiation history of the Earth and the Moon. Because Hf is lithophile and W is siderophile during metal–silicate segregation, the decay of 182Hf to 182W (half-life of 9 Ma) has proven to be a useful chronometer of core–mantle differentiation events. A key parameter for the interpretation of 182Hf/182W data is the Hf/W ratio of the primitive (i.e. undepleted) mantle. Since W is incompatible during mantle melting, its ratio relative to U and other similarly incompatible elements in basalts (e.g. Th, La) may be used as proxies for their mantle sources. However, the assumption that W and U are equally incompatible may be flawed for petrological systems that equilibrated over a large range of oxygen fugacity (fO2). Although W is typically perceived as being homovalent, evidence suggests that U is heterovalent over the range of fO2 inferred for the silicate mantles of the Earth and the Moon.

Here we report new partitioning data for W, U, high-field-strength elements (HFSE), and Th between clinopyroxene, orthopyroxene, olivine, plagioclase and silicate melt. In agreement with previous studies, we show that these elements behave as homovalent elements at fO2 characteristic of Earth’s upper mantle. However, both W and U become more compatible at low fO2, indicating a change in their redox state, with W becoming more compatible at progressively lower fO2. This result for W is particularly unexpected, because this element was thought to be hexavalent even at very low fO2. The much higher compatibility of W4+ (the species inferred here at low fO2) relative to W6+ means that even a small fraction of W4+ will have a significant effect on the overall compatibility of W. Our results imply that over the range of reducing conditions in which lunar differentiation is thought to have taken place (i.e. ∼IW-2 to IW-0.5), W is likely to become fractionated from U. When our partitioning data are applied to model the fractional crystallization of a lunar magma ocean, lunar trends for U/W, Hf/W and Th/W are well reproduced. The result of this model carries with it the implication that the Hf/W of the bulk silicate fractions that comprise the Earth and the Moon are virtually identical.

Reference
Fonseca ROC, Mallmann G, Sprung P, Sommer JE, Heuser A, Speelmanns IM, Blanchard H (2014) Redox controls on tungsten and uranium crystal/silicate melt partitioning and implications for the U/W and Th/W ratio of the lunar mantle. Earth and Planetary Science Letters 404, 1-13
Link to Article [DOI: 10.1016/j.epsl.2014.07.015]

Copyright Elsevier

¹Martin R. Lee, ¹Paula Lindgren, ¹Mahmood R. Sofe
¹School of Geographical and Earth Sciences, University of Glasgow, Glasgow G12 8QQ, U.K

Carbonate minerals in CM carbonaceous chondrite meteorites, along with the silicates and sulphides with which they are intergrown, provide a detailed record of the nature and evolution of parent body porosity and permeability, and the chemical composition, temperature and longevity of aqueous solutions. Fourteen meteorites were studied that range in petrologic subtype from mildly aqueously altered CM2.5 to completely hydrated CM2.0. All of them contain calcite, whereas aragonite occurs only in the CM2.5-CM2.2 meteorites and dolomite in the CM2.2-CM2.0. All of the aragonite crystals, and most of the calcite and dolomite grains, formed during early stages of parent body aqueous alteration by cementation of pores produced by the melting of tens of micrometre size particles of H2O-rich ice. Aragonite was the first carbonate to precipitate in the CM2.5 to CM2.2 meteorites, and grew from magnesium-rich solutions. In the least altered of these meteorites the aragonite crystals formed in clusters owing to physical restriction of aqueous fluids within the low permeability matrix. The strong correlation between the petrologic subtype of a meteorite, the abundance of its aragonite crystals and the proportion of them that have preserved crystal faces, is because aragonite was dissolved in the more altered meteorites on account of their higher permeability, and/or greater longevity of the aqueous solutions. Dolomite and breunnerite formed instead of aragonite in some of the CM2.1 and CM2.2 meteorites owing to higher parent body temperatures. The pore spaces that remained after precipitation of aragonite, dolomite and breunnerite cements were occluded by calcite. Following completion of cementation, the carbonates were partially replaced by phyllosilicates and sulphides. Calcite in the CM2.5-CM2.2 meteorites was replaced by Fe-rich serpentine and tochilinite, followed by Mg-rich serpentine. In the CM2.1 and CM2.0 meteorites dolomite, breunnerite and calcite were replaced by Fe-rich serpentine and Fe-Ni sulphide, again followed by Mg-rich serpentine. The difference between meteorites in the mineralogy of their replacive sulphides may again reflect greater temperatures in the parent body regions from where the more highly altered CMs were formed. This transition from Fe-rich to Mg-rich carbonate replacement products mirrors the chemical evolution of parent body solutions in response to consumption of Fe-rich primary minerals followed by the more resistant Mg-rich anhydrous silicates. Almost all of the CMs examined contain a second generation of calcite that formed after the sulphides and phyllosilicates and by replacement of remaining anhydrous silicates and dolomite (dedolomitization). The Ca and CO2 required for this replacive calcite is likely to have been sourced by dissolution of earlier formed carbonates, and ions may have been transported over metre-plus distances through high permeability conduits that were created by impact fracturing.

Reference
Lee MR, Lindgren P, Sofe MR (2014) Aragonite, breunnerite, calcite and dolomite in the CM carbonaceous chondrites: High fidelity recorders of progressive parent body aqueous Alteration. Geochimica et Cosmochimica Acta (in Press)
Link to Article [DOI: 10.1016/j.gca.2014.08.019]

Copyright Elsevier

¹Huan Y. A. Meng, ²Kate Y. L. Su, ^1,2George H. Rieke, ³David J. Stevenson, ^4,5Peter Plavchan, ^2,6,7Wiphu Rujopakarn, ⁸Carey M. Lisse, ⁹Saran Poshyachinda, ¹⁰Daniel E. Reichart

¹Lunar and Planetary Laboratory and Department of Planetary Sciences, University of Arizona, 1629 East University Boulevard, Tucson, AZ 85721, USA.
²Steward Observatory and Department of Astronomy, University of Arizona, 933 North Cherry Avenue, Tucson, AZ 85721, USA.
³Division of Geological and Planetary Sciences, California Institute of Technology, MC 170-25, 1200 East California Boulevard, Pasadena, CA 91125, USA.
⁴NASA Exoplanet Science Institute, California Institute of Technology, MC 100-22, 770 South Wilson Avenue, Pasadena, CA 91125, USA.
⁵Missouri State University, 901 South National Avenue, Springfield, MO 65897, USA.
⁶Department of Physics, Faculty of Science, Chulalongkorn University, 254 Phayathai Road, Pathumwan, Bangkok 10330, Thailand.
⁷Kavli Institute for the Physics and Mathematics of the Universe (WPI), Todai Institute for Advanced Study, University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, 277-8583, Japan.
⁸Space Department, Applied Physics Laboratory, Johns Hopkins University, 11100 Johns Hopkins Road, Laurel, MD 20723, USA.
⁹National Astronomical Research Institute of Thailand (Public Organization), Ministry of Science and Technology, 191 Siriphanich Building, Huay Kaew Road, Muang District, Chiang Mai 50200, Thailand.
¹⁰Department of Physics and Astronomy, Campus Box 3255, University of North Carolina at Chapel Hill, Chapel Hill, NC 27599, USA.

The final assembly of terrestrial planets occurs via massive collisions, which can launch copious clouds of dust that are warmed by the star and glow in the infrared. We report the real-time detection of a debris-producing impact in the terrestrial planet zone around a 35-million-year-old solar-analog star. We observed a substantial brightening of the debris disk at a wavelength of 3 to 5 micrometers, followed by a decay over a year, with quasi-periodic modulations of the disk flux. The behavior is consistent with the occurrence of a violent impact that produced vapor out of which a thick cloud of silicate spherules condensed that were then ground into dust by collisions. These results demonstrate how the time domain can become a new dimension for the study of terrestrial planet formation.

Reference
Meng HYA, Su, KYL, Rieke GH, Stevenson DJ, Plavchan P, Rujopakarn W, Lisse CM, Poshyachinda S, Reichart DE (2014) Large impacts around a solar-analog star in the era of terrestrial planet Formation. Science 345, 6200, 1032-1035
Link to Article [DOI: 10.1126/science.1255153]

Reprinted with permission from AAAS

¹Joshua T.S. Cahill, ²Justin J. Hagerty, ¹David J. Lawrence, ¹Rachel L. Klima, ¹David T. Blewett
¹Johns Hopkins Applied Physics Laboratory, Laurel, MD
²U. S. Geological Survey, Astrogeology Science Center, Flagstaff, AZ

The Moon has areas of magnetized crust (“magnetic anomalies”), the origins of which are poorly constrained. A magnetic anomaly near the northern rim of South Pole-Aitken (SPA) basin was recently postulated to originate from remnant metallic iron emplaced by the SPA basin-forming impactor. Here, we remotely examine the regolith of this SPA magnetic anomaly with a combination of Clementine and Lunar Prospector derived iron maps for any evidence of enhanced metallic iron content. We find that these data sets do not definitively detect the hypothesized remnant metallic iron within the upper tens of centimeters of the lunar regolith.

Reference
Cahill JTS, Hagerty JJ, Lawrence DJ, Klima RL, Blewett DT (2014) Surveying the South Pole-Aitken basin magnetic anomaly for remnant impactor metallic iron. Icarus (in Press)
Link to Article [DOI: 10.1016/j.icarus.2014.08.035]

Copyright Elsevier

¹Elizabeth A. Frank, ²Bradley S. Meyer, ^1,3,4Stephen J. Mojzsis

¹Department of Geological Sciences, NASA Lunar Science Institute & Center for Lunar Origin and Evolution (CLOE), University of Colorado, 2200 Colorado Avenue, UCB 399, Boulder, Colorado 80309-0399 USA
²Department of Physics and Astronomy, Clemson University, Clemson, South Carolina
³Laboratoire de Géologie de Lyon, École Normale Supérieure de Lyon and Université Claude Bernard Lyon 1, CNRS UMR 5276, 2 rue Raphaël Dubois, 69622 Villeurbanne, France
⁴Hungarian Academy of Sciences, Institute for Geological and Geochemical Research, 45 Budaörsi ut, H-1112 Budapest, Hungary

Discoveries of rocky worlds around other stars have inspired diverse geophysical models of their plausible structures and tectonic regimes. Severe limitations of observable properties require many inexact assumptions about key geophysical characteristics of these planets. We present the output of an analytical galactic chemical evolution (GCE) model that quantitatively constrains one of those key properties: radiogenic heating. Earth’s radiogenic heat generation has evolved since its formation, and the same will apply to exoplanets. We have fit simulations of the chemical evolution of the interstellar medium in the solar annulus to the chemistry of our solar system at the time of its formation and then applied the carbonaceous chondrite/Earth’s mantle ratio to determine the chemical composition of what we term “cosmochemically Earth-like” exoplanets. Through this approach, predictions of exoplanet radiogenic heat productions as a function of age have been derived. The results show that the later a planet forms in galactic history, the less radiogenic heat it begins with; however, due to radioactive decay, today, old planets have lower heat outputs per unit mass than newly formed worlds. The long half-life of 232Th allows it to continue providing a small amount of heat in even the most ancient planets, while 40K dominates heating in young worlds. Through constraining the age-dependent heat production in exoplanets, we can infer that younger, hotter rocky planets are more likely to be geologically active and therefore able to sustain the crustal recycling (e.g. plate tectonics) that may be a requirement for long-term biosphere habitability. In the search for Earth-like planets, the focus should be made on stars within a billion years or so of the Sun’s age.

Reference
Frank EA, Meyer BS, Mojzsis SJ (2014) A radiogenic heating evolution model for cosmochemically Earth-like exoplanets. Icarus (in Press)
Link to Article [DOI: 10.1016/j.icarus.2014.08.031]

Copyright Elsevier

^1,2D.L. Nuding, ³E.G. Rivera-Valentin, ^1,4R.D. Davisa, ^1,4R.V. Gough, ⁵V.F. Chevrier, ^1,4M.A. Tolbert
¹Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder CO 80309
²Department of Atmospheric and Oceanic Sciences, University of Colorado, Boulder CO 80309
³Department of Geological Sciences, Brown University, Providence RI 02912
⁴Department of Chemistry and Biochemistry, University of Colorado, Boulder CO 80309
⁵W. M. Keck Laboratory for Space and Planetary Simulation, Arkansas Center for Space and Planetary Science, University of Arkansas, Fayetteville, AR 72701

Calcium perchlorate (Ca(ClO4)2) is a highly deliquescent salt that may exist on the surface of present-day Mars; however, its water uptake properties have not been well characterized at temperatures and relative humidity conditions relevant to Mars. Here, we quantify the deliquescent relative humidity (DRH) and efflorescent relative humidity (ERH) of Ca(ClO4)2 as a function of temperature (223 K to 273 K) to elucidate its behavior on the surface of Mars. A Raman microscope equipped with an environmental cell was used to simulate Mars relevant temperature and relative humidity conditions and monitor deliquescence (solid to aqueous) and efflorescence (aqueous to solid) phase transitions of Ca(ClO4)2. Deliquescence and efflorescence were monitored visually using optical images and spectroscopically using Raman microscopy. We find that there is a wide range of deliquescence RH values between 5% and 55% RH. This range is due to the formation of hydrates in different temperatures regimes, with the higher DRH values occurring at the lowest temperatures. Experimental deliquescence results were compared to a thermodynamic model for three hydration states of Ca(ClO4)2. The model predicts that the higher hydration states deliquesce at a higher RH than the lower hydration states. Calcium perchlorate was found to supersaturate, with lower ERH values than DRH values. The ERH results were less dependent on temperature with an average 15 ±± 4%, but values as low as 3 ±± 2% were measured at 273 K. Levitation experiments were performed on single particles of Ca(ClO4)2 and Mg(ClO4)2 at 298 K. While efflorescence was observed around 15% RH for Mg(ClO4)2, the efflorescence of Ca(ClO4)2 was not observed, even when exposed to 1% RH at 298 K. Additionally, a 17-hour experiment was conducted to simulate a martian subsurface diurnal cycle. This demonstrated Ca(ClO4)2 aqueous solutions can persist without efflorescing for the majority of a martian sol, up to 17 hours under Mars temperature heating rates and RH conditions. We find that Ca(ClO4)2 aqueous solutions could persist for most of the martian sol under present-day conditions. The aqueous phase stability and metastability quantified for Ca(ClO4)2 under Mars relevant temperature and relative humidity conditions has important implications for the water cycle and the stability of liquid water on present day Mars.

Reference
Nuding DL, Rivera-Valentin EG, Davis RD, Gough RV, Chevrier VF, Tolbert MA (2014) Deliquescence and Efflorescence of Calcium Perchlorate: An Investigation of Stable Aqueous Solutions Relevant to Mars. Icarus (in Press)
Link to Article [DOI: 10.1016/j.icarus.2014.08.036]

Copyright Elsevier

¹S. Schröder et al. (>10)*
¹Institut de Recherche en Astrophysique et Planétologie (IRAP), CNRS/Université de Toulouse, UPS-OMP, BP 44346, 31028 Toulouse, France
*Find the extensive, full author and affiliation list on the publishers Website

One of the main advantages of ChemCam’s LIBS (Laser-Induced Breakdown Spectroscopy) instrument onboard the Curiosity rover is its potential to detect light elements such as hydrogen at fine scales, which has never been achieved on Mars. Hydrogen lines are detected in most of the data obtained within the first 320 sols of the mission at Gale Crater, Mars. This work is a description of the hydrogen signal and its variability in the ChemCam LIBS spectra; it discusses the challenges of qualitative and quantitative analysis. Data acquisition and processing steps are investigated and optimized for the detection of hydrogen on Mars. Subtraction of an appropriate dark spectrum and the deconvolution of the superimposed emission of carbon from the low-pressure CO2-dominated atmosphere are particularly important. Because the intensities of hydrogen are also affected by matrix effects, the hydrogen signal was investigated within groups of targets sharing common chemical features and similar matrices. The different groups cover a variety of rock and soil compositions encountered along the traverse (calcium sulfate veins, mafic soils, felsic, Mg-rich and Fe-rich rocks) including data from both drill holes and their tailings. Almost all these targets were found to be hydrated to variable extents. Soils have systematically higher hydrogen signals than rocks and pebbles, probably as a result of their alteration. The results from rocks suggest that various alteration processes leading to their hydration have taken place, which is consistent with the fluvial lacustrine context, the diagenetic features, and the mineralogy observed by Curiosity in Yellowknife Bay.

Reference
Schröder S et al. (2014) Hydrogen detection with ChemCam at Gale crater. Icarus (in Press)
Link to Article [DOI: 10.1016/j.icarus.2014.08.029]

Copyright Elsevier