R.K. Mishra and J.N. Goswami

Physical Research Laboratory, Navrangpura, Ahmedabad, 380009;India

The short-lived now-extinct nuclide ⁶⁰Fe, present in the early Solar System, is a unique product of stellar nucleosynthesis. Even though the first hint for its presence in the early Solar System was obtained more than two decades back, a robust value for Solar System Initial (SSI) ⁶⁰Fe/⁵⁶Fe is yet to be established. A combined study of ²⁶Al-²⁶Mg and ⁶⁰Fe-⁶⁰Ni isotope systematics in chondrules from unequilibrated ordinary chondrites of low petrologic type, Semarkona (LL3.0), LEW 86134 (L3.0), and Y 791324 (L3.1), has been conducted to infer the value of SSI ⁶⁰Fe/⁵⁶Fe. Seven of the analyzed chondrules host resolved radiogenic excess in both⁶⁰Ni and ²⁶Mg resulting from in situ decay of the short-lived nuclides ⁶⁰Fe and ²⁶Al, respectively. The initial²⁶Al/²⁷Al values for these chondrules range from (6.9± 5.8)× 10^-6 to (3.01±1.78) ×10^-5 that suggest their formation between 2.1 to 0.6 Ma after CAIs. The initial ⁶⁰Fe/⁵⁶Fe at the time of formation of these chondrules ranges from (3.2±1.3) ×10^-7 to (1.12±0.39) ×10^-6 and show a good correlation with their initial ²⁶Al/²⁷Al values suggesting co-injection of the two short-lived nuclides, ⁶⁰Fe and ²⁶Al, into the protosolar cloud from the same stellar source. Considering ²⁶Al as a reliable early Solar System chronometer, this data set yield a SSI⁶⁰Fe/⁵⁶Fe value of (7.0±1.2) ×10^-7, if we adopt a half-life value of 2.6 Ma for ⁶⁰Fe reported in a recent study. Model stellar nucleosynthesis yields suggest that both a high mass (5-6.5 M_⊙) Asymptotic Giant Branch (AGB) star or a supernova (SN) could be the source of ⁶⁰Fe and ²⁶Al present in the early solar system. A high mass (∼25M_⊙) SN appears more plausible because of the much higher probability of its close association with the protosolar molecular cloud than a high mass AGB star. Such a SN can also account for SSI abundance of²⁶Al and its correlated presence with ⁶⁰Fe in chondrules.

Reference
Mishra RK and Goswami JN (in press) Fe-Ni and Al-Mg isotope records in UOC chondrules: Plausible stellar source of 60Fe and other short-lived nuclides in the early Solar System. Geochimica et Cosmochimica Acta
[doi:10.1016/j.gca.2014.01.011]
Copyright Elsevier

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E. G. Rivera-Valentin^1,2 and A. C. Barr^1,2

¹Department of Geological Sciences, Brown University, 324 Brook Street, Box 1846, Providence, RI 02912, USA
²Center for Lunar Origin and Evolution, Southwest Research Institute, Boulder, CO 80302, USA

Late accretion of a “veneer” of compositionally diverse planetesimals may introduce chemical heterogeneity in the mantles of the terrestrial planets. The size of the late veneer objects is an important control on the angular momenta, eccentricities, and inclinations of the terrestrial planets, but current estimates range from meter-scale bodies to objects with diameters of thousands of kilometers. We use a three-dimensional global Monte Carlo model of impact cratering, excavation, and ejecta blanket formation to show that evidence of mantle heterogeneity can be preserved within ejecta blankets of mantle-exhuming impacts on terrestrial planets. Compositionally distinct provinces implanted at the time of the late veneer are most likely to be preserved in bodies whose subsequent geodynamical evolution is limited. Mercury may have avoided intensive mixing by solid-state convection during much of its history. Its subsequent bombardment may have then excavated evidence of primordial mantle heterogeneity introduced by the late veneer. Simple geometric arguments can predict the amount of mantle material in the ejecta blanket of mantle-exhuming impacts, and deviations in composition relative to geometric predictions can constrain the length-scale of chemical heterogeneities in the subsurface. A marked change in the relationship between mantle and ejecta composition occurs when chemically distinct provinces are ~250 km in diameter; thus, evidence of bombardment by thousand-kilometer-sized objects should be readily apparent from the variation in compositions of ejecta blankets in Mercury’s ancient cratered terrains.

Reference
Rivera-Valentin EG and Barr AC (2014) Estimating the Size of Late Veneer Impactors from Impact-induced Mixing on Mercury. The Astrophysical Journal – Letters 782:L8.
[doi:10.1088/2041-8205/782/1/L8]

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Roger R. Fu^a and Linda T. Elkins-Tanton^b

^aDepartment of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA, USA
^bDepartment of Terrestrial Magnetism, Carnegie Institution for Science, Washington, DC, USA

High abundances of short-lived radiogenic isotopes in the early solar system led to interior melting and differentiation on many of the first planetesimals. Petrologic, isotopic, and paleomagnetic evidence suggests that some differentiated planetesimals retained primitive chondritic material. The preservation of a cold chondritic lid depends on whether deep melts are able to ascend and breach the chondritic crust. We evaluate the likelihood of melt ascent on a range of chondritic parent bodies. We find that, due to the efficient ascent of free volatiles in the gas and supercritical fluid phases at temperatures still below the solidus for silicates and metals, mobile silicate melts on planetesimals were likely volatile-depleted. By calculating the densities of such melts, we show that silicate melts likely breached crusts of enstatite chondrite compositions but did not ascend in the CV and CM parent bodies. Ordinary chondrite melts represent an intermediate case. These predictions are consistent with paleomagnetic results from CV and CM chondrites as well as spectral observations of large E-type asteroids.

Reference
Fua RR and Elkins-Tanton LT (2014) The fate of magmas in planetesimals and the retention of primitive chondritic crusts. Earth and Planetary Science Letters 390:128–137.
[doi:10.1016/j.epsl.2013.12.047]
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A. Izidoro^1,2,3, N. Haghighipour^4,5, O. C. Winter¹ and M. Tsuchida⁶

¹UNESP, Univ. Estadual Paulista – Grupo de Dinâmica Orbital & Planetologia, Guaratinguetá, CEP 12.516-410, São Paulo, Brazil
²Capes Foundation, Ministry of Education of Brazil, Brasília/DF 70040-020, Brazil
³University of Nice-Sophia Antipolis, CNRS, Observatoire de la Côte d’Azur, Laboratoire Lagrange, BP 4229, F-06304 Nice Cedex 4, France
⁴Institute for Astronomy and NASA Astrobiology Institute, University of Hawaii-Manoa, Honolulu, HI 96822, USA
⁵Institute for Astronomy and Astrophysics, University of Tübingen, D-72076 Tübingen, Germany
⁶UNESP, Univ. Estadual Paulista, DCCE-IBILCE, São José do Rio Preto, CEP 15.054-000, São Paulo, Brazil

Models of terrestrial planet formation for our solar system have been successful in producing planets with masses and orbits similar to those of Venus and Earth. However, these models have generally failed to produce Mars-sized objects around 1.5 AU. The body that is usually formed around Mars’ semimajor axis is, in general, much more massive than Mars. Only when Jupiter and Saturn are assumed to have initially very eccentric orbits (e ~0.1), which seems fairly unlikely for the solar system, or alternately, if the protoplanetary disk is truncated at 1.0 AU, simulations have been able to produce Mars-like bodies in the correct location. In this paper, we examine an alternative scenario for the formation of Mars in which a local depletion in the density of the protosolar nebula results in a non-uniform formation of planetary embryos and ultimately the formation of Mars-sized planets around 1.5 AU. We have carried out extensive numerical simulations of the formation of terrestrial planets in such a disk for different scales of the local density depletion, and for different orbital configurations of the giant planets. Our simulations point to the possibility of the formation of Mars-sized bodies around 1.5 AU, specifically when the scale of the disk local mass-depletion is moderately high (50%-75%) and Jupiter and Saturn are initially in their current orbits. In these systems, Mars-analogs are formed from the protoplanetary materials that originate in the regions of disk interior or exterior to the local mass-depletion. Results also indicate that Earth-sized planets can form around 1 AU with a substantial amount of water accreted via primitive water-rich planetesimals and planetary embryos. We present the results of our study and discuss their implications for the formation of terrestrial planets in our solar system.

Reference
Izidoro A, Haghighipour N, Winter OC and Tsuchida M (2014) Terrestrial Planet Formation in a Protoplanetary Disk with a Local Mass Depletion: A Successful Scenario for the Formation of Mars. The Astrophysical Journal 782:31.
[doi:10.1088/0004-637X/782/1/31]

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

Opportunity has investigated in detail rocks on the rim of the Noachian age Endeavour crater, where orbital spectral reflectance signatures indicate the presence of Fe⁺³-rich smectites. The signatures are associated with fine-grained, layered rocks containing spherules of diagenetic or impact origin. The layered rocks are overlain by breccias, and both units are cut by calcium sulfate veins precipitated from fluids that circulated after the Endeavour impact. Compositional data for fractures in the layered rocks suggest formation of Al-rich smectites by aqueous leaching. Evidence is thus preserved for water-rock interactions before and after the impact, with aqueous environments of slightly acidic to circum-neutral pH that would have been more favorable for prebiotic chemistry and microorganisms than those recorded by younger sulfate-rich rocks at Meridiani Planum.

Reference
Arvidson et al. (2014) Ancient Aqueous Environments at Endeavour Crater, Mars. Science vol. 343 no. 6169.
[doi:10.1126/science.1248097]
Reprinted with permission from AAAS

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Farley et al. (>10)*
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We determined radiogenic and cosmogenic noble gases in a mudstone on the floor of Gale Crater. A K-Ar age of 4.21 ± 0.35 billion years represents a mixture of detrital and authigenic components and confirms the expected antiquity of rocks comprising the crater rim. Cosmic-ray–produced ³He,²¹Ne, and ³⁶Ar yield concordant surface exposure ages of 78 ± 30 million years. Surface exposure occurred mainly in the present geomorphic setting rather than during primary erosion and transport. Our observations are consistent with mudstone deposition shortly after the Gale impact or possibly in a later event of rapid erosion and deposition. The mudstone remained buried until recent exposure by wind-driven scarp retreat. Sedimentary rocks exposed by this mechanism may thus offer the best potential for organic biomarker preservation against destruction by cosmic radiation.

Reference
Farley et al. (2014) In Situ Radiometric and Exposure Age Dating of the Martian Surface. Science vol. 343 no. 6169.
[doi:10.1126/science.1247166]
Reprinted with permission from AAAS

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Ming et al. (>10)*
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H₂O, CO₂, SO₂, O₂, H₂, H₂S, HCl, chlorinated hydrocarbons, NO, and other trace gases were evolved during pyrolysis of two mudstone samples acquired by the Curiosity rover at Yellowknife Bay within Gale crater, Mars. H₂O/OH-bearing phases included 2:1 phyllosilicate(s), bassanite, akaganeite, and amorphous materials. Thermal decomposition of carbonates and combustion of organic materials are candidate sources for the CO₂. Concurrent evolution of O₂ and chlorinated hydrocarbons suggests the presence of oxychlorine phase(s). Sulfides are likely sources for sulfur-bearing species. Higher abundances of chlorinated hydrocarbons in the mudstone compared with Rocknest windblown materials previously analyzed by Curiosity suggest that indigenous martian or meteoritic organic carbon sources may be preserved in the mudstone; however, the carbon source for the chlorinated hydrocarbons is not definitively of martian origin.

Reference
Ming et al. (2014) Volatile and Organic Compositions of Sedimentary Rocks in Yellowknife Bay, Gale Crater, Mars. Science vol. 343 no. 6169.
[doi:10.1126/science.1245267]
Reprinted with permission from AAAS

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Hassler et al. (>10)*
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The Radiation Assessment Detector (RAD) on the Mars Science Laboratory’s Curiosity rover began making detailed measurements of the cosmic ray and energetic particle radiation environment on the surface of Mars on 7 August 2012. We report and discuss measurements of the absorbed dose and dose equivalent from galactic cosmic rays and solar energetic particles on the martian surface for ~300 days of observations during the current solar maximum. These measurements provide insight into the radiation hazards associated with a human mission to the surface of Mars and provide an anchor point with which to model the subsurface radiation environment, with implications for microbial survival times of any possible extant or past life, as well as for the preservation of potential organic biosignatures of the ancient martian environment.

Reference
Hassler et al. (2014) Mars’ Surface Radiation Environment Measured with the Mars Science Laboratory’s Curiosity Rover. Science vol. 343 no. 6169.
[doi:10.1126/science.1244797]
Reprinted with permission from AAAS

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McLennan et al. (>10)*
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Sedimentary rocks examined by the Curiosity rover at Yellowknife Bay, Mars, were derived from sources that evolved from an approximately average martian crustal composition to one influenced by alkaline basalts. No evidence of chemical weathering is preserved, indicating arid, possibly cold, paleoclimates and rapid erosion and deposition. The absence of predicted geochemical variations indicates that magnetite and phyllosilicates formed by diagenesis under low-temperature, circumneutral pH, rock-dominated aqueous conditions. Analyses of diagenetic features (including concretions, raised ridges, and fractures) at high spatial resolution indicate that they are composed of iron- and halogen-rich components, magnesium-iron-chlorine–rich components, and hydrated calcium sulfates, respectively. Composition of a cross-cutting dike-like feature is consistent with sedimentary intrusion. The geochemistry of these sedimentary rocks provides further evidence for diverse depositional and diagenetic sedimentary environments during the early history of Mars.

Reference
McLennan et al. (2014) Elemental Geochemistry of Sedimentary Rocks at Yellowknife Bay, Gale Crater, Mars. Science vol. 343 no. 6169.
[doi:10.1126/science.1244734]
Reprinted with permission from AAAS

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Vaniman et al. (>10)*
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Sedimentary rocks at Yellowknife Bay (Gale crater) on Mars include mudstone sampled by the Curiosity rover. The samples, John Klein and Cumberland, contain detrital basaltic minerals, calcium sulfates, iron oxide or hydroxides, iron sulfides, amorphous material, and trioctahedral smectites. The John Klein smectite has basal spacing of ~10 angstroms, indicating little interlayer hydration. The Cumberland smectite has basal spacing at both ~13.2 and ~10 angstroms. The larger spacing suggests a partially chloritized interlayer or interlayer magnesium or calcium facilitating H2O retention. Basaltic minerals in the mudstone are similar to those in nearby eolian deposits. However, the mudstone has far less Fe-forsterite, possibly lost with formation of smectite plus magnetite. Late Noachian/Early Hesperian or younger age indicates that clay mineral formation on Mars extended beyond Noachian time.

Reference
Vaniman et al. (2014) Mineralogy of a Mudstone at Yellowknife Bay, Gale Crater, Mars. Science vol. 343 no. 6169.
[doi:10.1126/science.1243480]
Reprinted with permission from AAAS

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