Emmanuel Jacquet

Canadian Institute for Theoretical Astrophysics, University of Toronto, 60 St Georges Street, Toronto, ON M5S 3H8, Canada

Primitive meteorites are dominated by millimeter-size silicate spherules called chondrules. The nature of the high-temperature events that produced them in the early solar system remains enigmatic. Beside their thermal history, one important clue is provided by their size which shows remarkably little variation (less than a factor of 6 for the mean chondrule radius of most chondrites) despite the extensive range of ages and heliocentric distances sampled. It is however unclear whether chondrule size is due to the chondrule melting process itself, or has been simply inherited from the precursor material, or yet results from some sorting process. I examine these different possibilities in terms of their analytical size predictions. Unless the chondrule-forming “window” was very narrow, radial sorting can be excluded as size-determining processes because of the large variations it would predict. Molten planetesimal collision or impact melting models, which derive chondrules from the fragmentation of larger melt bodies, would likewise predict too much size variability by themselves; more generally any size modification during chondrule formation is limited in extent by evidence from compound chondrules and the considerable compositional variability of chondrules. Turbulent concentration would predict a low size variability but lack of evidence of any accretion bias in carbonaceous chondrites may be difficult to reconcile with any form of local sorting upon agglomeration. Growth by sticking (especially if bouncing-limited) of aggregates as chondrule precursors would yield limited variations of their final radius in space and time, and would be consistent with the relatively similar size of other chondrite components such as refractory inclusions. This suggests that the chondrule-melting process(es) simply melted such nebular aggregates with little modification of mass.

Reference
Jacquet E (in press) The quasi-universality of chondrule size as a constraint for chondrule formation models. Icarus
[doi:10.1016/j.icarus.2014.01.012]
Copyright Elsevier

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Colin M. Dundas¹, Shane Byrne², Alfred S. McEwen², Michael T. Mellon³, Megan R. Kennedy⁴, Ingrid J. Daubar², Lee Saper⁴

¹Astrogeology Science Center, U. S. Geological Survey, Flagstaff, Arizona, USA
²Lunar and Planetary Laboratory, University of Arizona, Tucson, Arizona, USA
³Southwest Research Institute, Boulder, Colorado, USA
⁴Malin Space Science Systems, San Diego, California, USA

Twenty small new impact craters or clusters have been observed to excavate bright material inferred to be ice at mid-latitudes and high latitudes on Mars. In the northern hemisphere, the craters are widely distributed geographically and occur at latitudes as low as 39°N. Stability modeling suggests that this ice distribution requires a long-term average atmospheric water vapor content around 25 precipitable micrometers, more than double the present value, which is consistent with the expected effect of recent orbital variations. Alternatively, near-surface humidity could be higher than expected for current column abundances if water vapor is not well mixed with atmospheric CO₂, or the vapor pressure at the ice table could be lower due to salts. Ice in and around the craters remains visibly bright for months to years, indicating that it is clean ice rather than ice-cemented regolith. Although some clean ice may be produced by the impact process, it is likely that the original ground ice was excess ice (exceeding dry soil pore space) in many cases. Observations of the craters suggest small-scale heterogeneities in this excess ice. The origin of such ice is uncertain. Ice lens formation by migration of thin films of liquid is most consistent with local heterogeneity in ice content and common surface boulders, but in some cases, nearby thermokarst landforms suggest large amounts of excess ice that may be best explained by a degraded ice sheet.

Reference
Dundas CM, Byrne S, McEwen AS, Mellon MT, Kennedy MR, Daubar IJ and Saper L (in press) HiRISE observations of new impact craters exposing Martian ground ice. Journal of Geophysical Research: Planets
[doi:10.1002/2013JE004482]
Published by arrangement with John Wiley & Sons

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Kirsten L. Siebach and John P. Grotzinger

Geological and Planetary Sciences Division, California Institute of Technology, Pasadena, California, USA

While the presence of water on the surface of early Mars is now well known, the volume, distribution, duration, and timing of the liquid water have proven difficult to determine. This study makes use of a distinctive boxwork-rich sedimentary layer on Mount Sharp to map fluid-based cementation from orbital imagery and estimate the minimum volume of water present when this sedimentary interval was formed. The boxwork structures on Mount Sharp are decameter-scale light-toned polygonal ridges that are unique compared to previous observations of Martian fractured terrain because they are parallel-sided ridges with dark central linear depressions. This texture and the sedimentary setting strongly imply that the ridges are early diagenetic features formed in the subsurface phreatic groundwater zone. High-resolution orbital imagery was used to map the volume of light-toned cemented ridges. Based on the cemented volume, a minimum of 5.25 × 105 m³ of cement was deposited within the fractures. Using a brine composition based on observations of other Martian cements and modeling the degree of evaporation, each volume of cement requires 800–6700 pore volumes of water, so the mapped boxwork ridge cements require a minimum of 0.43 km³ of water. This is a significant amount of groundwater that must have been present at the −3620 m level, 1050 m above the current floor of Gale Crater, providing both a new constraint on the possible origins of Mount Sharp and a possible future science target for the Curiosity rover where large volumes of water were present, and early mineralization could have preserved a once-habitable environment.

Reference
Siebach KL and Grotzinger JP (in press) Volumetric estimates of ancient water on Mount Sharp based on boxwork deposits, Gale Crater, Mars. Journal of Geophysical Research: Planets
[doi:10.1002/2013JE004508]
Published by arrangement with John Wiley & Sons

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Miriam Sharp^a, Iva Gerasimenko^a, Lorne C. Loudin^b, Jingao Liu^a,c, Odette B. James^d, Igor S. Puchtel^a and Richard J. Walker^a

^aDepartment of Geology, University of Maryland, College Park, MD 20742, USA
^bDepartment of Earth Sciences, University of New Hampshire, Durham, NH 03824, USA
^cDepartment of Earth and Atmospheric Sciences, University of Alberta, Edmonton, AB T6G2E3, Canada
^dEmeritus U.S.Geological Survey, Reston, VA 20192, USA

Concentrations of the highly siderophile elements (HSE) Re, Os, Ir, Ru, Pt, and Pd and ¹⁸⁷Os/¹⁸⁸Os isotopic compositions are reported for seven Apollo 17 impact melt rocks. These data are used to examine the dominant chemical signature of the impactor that formed the melts. Six of the samples (72355, 72435, 72535, 76035, 76055, and 76135) have poikilitic textures; one sample (73235) has an aphanitic texture. Data for the samples define linear correlations when Ir is plotted versus other HSE concentrations, with y-intercepts indistinguishable from zero for most HSE in most rocks. Scatter about some of the trends, and occasional trends with positive y-intercepts, indicate either mixing of additional components that are heterogeneously distributed within several rocks, or modest fractionation of some HSE by volatilization, crystal fractionation, or other processes, during formation and evolution of the melt sheet. There is no statistical difference between the aphanitic and poikilitic samples in terms of HSE ratios after visible granulitic clasts were removed from aphanite 73235. Hence, earlier speculations that the two types of impact melt rocks at this site may have been generated by different impactors are not supported by our data.
Most Apollo 17 samples examined here and in prior studies are characterized by very similar HSE signatures, consistent with a common impactor. These samples are characterized by elevated Ru/Ir, Pd/Ir, and Re/Os, relative to most chondrites. Collectively, the data indicate that the impactor was characterized by the following HSE ratios (2σ): Re/Ir 0.093±0.020, Os/Ir 1.03±0.28, Ru/Ir 1.87±0.30, Pt/Ir 2.36±0.31, Pd/Ir 1.85±0.41, and present-day ¹⁸⁷Os/¹⁸⁸Os of 0.1322±0.0013. The results most likely mean that the impactor was a body with a bulk composition that was just outside the range of meteoritic compositions currently sampled on Earth.

Reference
Sharp M, Gerasimenko I, Loudin LC, Liu J, James OB, Puchtel IS and Walker RJ (in press) Characterization of the Dominant Impactor Signature for Apollo 17 Impact Melt Rocks. Geochimica et Cosmochimica Acta
[doi:10.1016/j.gca.2014.01.014]
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David Polishook^a, Nicholas Moskovitz^a, Richard P. Binzel^a, Francesca E. DeMeo^a,b, David Vokrouhlický^c, Jindřich Žižka^c, Dagmara Oszkiewicz^d

^aDepartment of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
^bHarvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
^cInstitute of Astronomy, Charles University, Prague, V Holešovičkách 8, CZ – 18000 Prague 8, Czech Republic
^dAstronomical Observatory Institute, Faculty of Physics, A. Mickiewicz University, Słoneczna 36, 60-286, Poznań, Poland

The rotational-fission of a “rubble-pile” structured asteroid can result in an “asteroid pair” – two un-bound asteroids sharing nearly identical heliocentric orbits. Models suggest that this mechanism exposes material from below the progenitor surface that previously had never have been exposed to the weathering conditions of space. Therefore, the surfaces of asteroid pairs offer the opportunity to observe non-weathered “fresh” spectra.
Here we report near-infrared spectroscopic observations of 31 asteroids in pairs. In order to search for spectral indications of fresh surfaces we analyze their spectral slopes, parameters of their 1μm absorption band and taxonomic classification. Additionally, through backward dynamical integration we estimate the time elapsed since the disintegration of the pairs’ progenitors.
Analyzing the 19 ordinary chondrite-like (S-complex) objects in our sample, we find two Q-type asteroids (19289 and 54827) that are the first of their kind to be observed in the main-belt of asteroids over the full visible and near-infrared range. This solidly demonstrates that the Q-type taxonomy is not limited to the NEA population.
The pairs in our sample present a range of fresh and weathered surfaces with no clear evidence for a correlation with the ages of the pairs. However, our sample includes “old” pairs (2×10⁶ ⩾ age ⩾ 1×10⁶ years) that present relatively low, meteoritic-like spectral slopes (<0.2% per μm). This illustrates a timescale of at least ∼2 million years before an object develops high spectral slope that is typical for S-type asteroids.
We discuss three mechanisms that explain the existence of weathered pairs with young dynamical ages and find that the “secondary fission” model (Jacobson and Scheeres 2011) is the most robust with our observations. In this mechanism an additional and subsequent fission of the secondary component contributes the lion share of fresh material that re-settles on the primary’s surface and recoats it with fresh material. If the secondary breaks loose from the vicinity of the primary before its “secondary fission”, this main source of fresh dust is avoided. We prefer this secondary fission model since i) the secondary members in our sample present “fresh” parameters that tend to be “fresher” than their weathered primaries; ii) most of the fresh pairs in our sample have low size ratios between the secondary and the primary; iii) 33% of the primaries in our sample are fresh, similar to the prediction set by the secondary fission model (Jacobson and Scheeres 2011); iv) known satellites orbit two of the pairs in our sample with low size ratio (D₂/D₁) and fresh surface; v) there is no correlation between the weathering state and the primary shape as predicted by other models.

Reference
Polishook D, Moskovitz N, Binzel RP, DeMeo FE, Vokrouhlický D, Žižka J and Oszkiewicz D (in press) Observations of “Fresh” and Weathered Surfaces on Asteroid Pairs and Their Implications on the Rotational-Fission Mechanism. Icarus
[doi:10.1016/j.icarus.2014.01.014]
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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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