Time filter

Source Type

Turrini D.,National institute for astrophysics | Combe J.-P.,Bear Fight Institute | McCord T.B.,Bear Fight Institute | Oklay N.,Max Planck Institute for Solar System Research | And 9 more authors.
Icarus | Year: 2014

The Dawn spacecraft recently observed the presence of dark material, which in turn proved to be associated with the presence of OH and H-rich material, on the surface of Vesta. The source of this dark material has been almost unanimously identified with the low albedo asteroids, likely analogous to the carbonaceous chondrites found on Earth, that impacted on Vesta over its lifetime. However, it is still a matter of debate whether the delivery of the dark material is associated with a few large impact events, to micrometeorites or to the continuous, secular flux of impactors on Vesta. The "continuous flux" scenario, in particular, predicts that a significant fraction of the exogenous material accreted by Vesta should be due to non-dark impactors likely analogous to ordinary chondrites, which instead represent only a minor contaminant in the Howardite-Eucrite-Diogenite meteorites. In this work, we explored the "continuous flux" scenario and its implications for the composition of the vestan regolith, taking advantage of the data from the Dawn mission and the Howardite-Eucrite-Diogenite meteorites to constrain the contamination history of Vesta. We developed a model for the delivery of the exogenous material to Vesta and verified how the results it supplies are sensitive to the different parameters we consider. We calibrated the flux of impactors predicted by our model with the number of dark craters observed inside the Rheasilvia basin and we tested the assumptions on the impact conditions by studying the formation of Cornelia crater and of its dark deposits with a hydrocode simulation. We used our calibrated model to show that the "stochastic events" scenario and the "micrometeoritic flux" scenario are just natural consequences of the "continuous flux" scenario. We then used the model to estimate the amounts of dark and hydroxylate materials that were delivered on Vesta since the Late Heavy Bombardment and we showed how our results match well with the values estimated by the Dawn mission. We finally used our model to assess the amount of Fe and siderophile elements that the continuous flux of impactors would mix in the vestan regolith: concerning the siderophile elements, we focused our attention on the role of Ni. The results we obtained are in agreement with the data available on the Fe and Ni content of the Howardite-Eucrite-Diogenite meteorites and can be used as a reference frame in future studies of the data from the Dawn mission and of the Howardite-Eucrite-Diogenite meteorites. Our model cannot yet provide an answer to the conundrum of the fate of the missing non-carbonaceous contaminants, but we discuss some possible reasons for this discrepancy with the otherwise coherent picture described by our results. © 2014 Elsevier Inc. All rights reserved.

Consolmagno G.J.,Specola Vaticana | Golabek G.J.,ETH Zurich | Golabek G.J.,University of Bayreuth | Turrini D.,National institute for astrophysics | And 4 more authors.
Icarus | Year: 2015

It is difficult to find a Vesta model of iron core, pyroxene and olivine-rich mantle, and HED crust that can match the joint constraints of (a) Vesta's density and core size as reported by the Dawn spacecraft team; (b) the chemical trends of the HED meteorites, including the depletion of sodium, the FeO abundance, and the trace element enrichments; and (c) the absence of exposed mantle material on Vesta's surface, among Vestoid asteroids, or in our collection of basaltic meteorites. These conclusions are based entirely on mass-balance and density arguments, independent of any particular formation scenario for the HED meteorites themselves. We suggest that Vesta either formed from source material with non-chondritic composition or underwent after its formation a radical physical alteration, possibly caused by collisional processes, that affected its global composition and interior structure. © 2015 Elsevier Inc.

Russell S.S.,Natural History Museum in London | Joy K.H.,University of Manchester | Jeffries T.E.,Natural History Museum in London | Consolmagno G.J.,Specola Vaticana | Kearsley A.,Natural History Museum in London
Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences | Year: 2014

The lunar magma ocean model is a well-established theory of the early evolution of the Moon. By this model, the Moon was initially largely molten and the anorthositic crust that now covers much of the lunar surface directly crystallized from this enormous magma source. We are undertaking a study of the geochemical characteristics of anorthosites from lunar meteorites to test this model. Rare earth and other element abundances have been measured in situ in relict anorthosite clasts from two feldspathic lunar meteorites: Dhofar 908 and Dhofar 081. The rare earth elements were present in abundances of approximately 0.1 to approximately 10× chondritic (CI) abundance. Every plagioclase exhibited a positive Eu-anomaly, with Eu abundances of up to approximately 20 × CI. Calculations of the melt in equilibrium with anorthite show that it apparently crystallized from a magma that was unfractionated with respect to rare earth elements and ranged in abundance from 8 to 80 × CI. Comparisons of our data with other lunar meteorites and Apollo samples suggest that there is notable heterogeneity in the trace element abundances of lunar anorthosites, suggesting these samples did not all crystallize from a common magma source. Compositional and isotopic data from other authors also suggest that lunar anorthosites are chemically heterogeneous and have a wide range of ages. These observations may support other models of crust formation on the Moon or suggest that there are complexities in the lunar magma ocean scenario to allow for multiple generations of anorthosite formation. © 2014 The Author(s) Published by the Royal Society.

Turrini D.,National institute for astrophysics | Turrini D.,University of Atacama | Svetsov V.,Institute for Dynamics of Geospheres | Consolmagno G.,Specola Vaticana | And 2 more authors.
Icarus | Year: 2016

The survival of asteroid Vesta during the violent early history of the Solar System is a pivotal constraint on theories of planetary formation. Particularly important from this perspective is the amount of olivine excavated from the vestan mantle by impacts, as this constrains both the interior structure of Vesta and the number of major impacts the asteroid suffered during its life. The NASA Dawn mission revealed that olivine is present on Vesta's surface in limited quantities, concentrated in small patches at a handful of sites not associated with the two large impact basins Rheasilvia and Veneneia. The first detections were interpreted as the result of the excavation of endogenous olivine, even if the depth at which the detected olivine originated was a matter of debate. Later works raised instead the possibility that the olivine had an exogenous origin, based on the geologic and spectral features of the deposits. In this work, we quantitatively explore the proposed scenario of a exogenous origin for the detected vestan olivine to investigate whether its presence on Vesta can be explained as a natural outcome of the collisional history of the asteroid over the last one or more billion years. To perform this study we took advantage of the impact contamination model previously developed to study the origin and amount of dark and hydrated materials observed by Dawn on Vesta, a model we updated by performing dedicated hydrocode impact simulations. We show that the exogenous delivery of olivine by the same impacts that shaped the vestan surface can offer a viable explanation for the currently identified olivine-rich sites without violating the constraint posed by the lack of global olivine signatures on Vesta. Our results indicate that no mantle excavation is in principle required to explain the observations of the Dawn mission and support the idea that the vestan crust could be thicker than indicated by simple geochemical models based on the Howardite–Eucrite–Diogenite family of meteorites. © 2016 Elsevier Inc.

Macke S.J. R.J.,University of Central Florida | Britt D.T.,University of Central Florida | Consolmagno S.J. G.J.,Specola Vaticana
Planetary and Space Science | Year: 2010

The Archimedean glass bead method for determining meteorite bulk density has become widely applied. We used well characterized, zero-porosity quartz and topaz samples to determine the systematic error in the glass bead method to support bulk density measurements of meteorites for our ongoing meteorite survey. Systematic error varies according to bead size, container size and settling method, but in all cases is less than 3%, and generally less than 2%. While measurements using larger containers (above 150 cm3) exhibit no discernible systematic error but much reduced precision, higher precision measurements with smaller containers do exhibit systematic error. For a 77 cm3 container using 40-80 μm diameter beads, the systematic error is effectively eliminated within measurement uncertainties when a "secured shake" settling method is employed in which the container is held securely to the shake platform during a 5 s period of vigorous shaking. For larger 700-800 μm diameter beads using the same method, bulk volumes are uniformly overestimated by 2%. Other settling methods exhibit sample-volume-dependent biases. For all methods, reliability of measurement is severely reduced for samples below ∼5 cm3 (10-15 g for typical meteorites), providing a lower-limit selection criterion for measurement of meteoritical samples. © 2009 Elsevier Ltd. All rights reserved.

Opeil C.P.,Boston College | Consolmagno G.J.,Specola Vaticana | Safarik D.J.,Los Alamos National Laboratory | Britt D.T.,University of Central Florida
Meteoritics and Planetary Science | Year: 2012

In our ongoing survey of meteorite physical properties, we have to date measured the thermal conductivity for seventeen stony meteorites at temperatures ranging from 5K to 300K. Here, we report new results for nine ordinary chondrites, one enstatite chondrite, and the basaltic achondrites Frankfort (howardite) and Los Angeles (shergottite). We find that thermal conductivity is significantly lower than would be expected from averaging the laboratory conductivities of their constituent minerals, with a dependence on temperature different from the expected conductivity of pure minerals. In addition, we find a linear relationship between the inverse of the porosity of the samples measured and their thermal conductivity, regardless of meteorite composition or type. We conclude that thermal conductivity is controlled by the presence of shock-induced microcracks within the meteorites, which provide a barrier to the transmission of thermal energy via phonons. In contrast to conductivity, our first measurement of heat capacity as a function of temperature (on Los Angeles) suggests that heat capacity is primarily a function of oxide composition and is not strongly affected by the physical state of the sample. © 2012 The Meteoritical Society.

Macke R.J.,University of Central Florida | Consolmagno G.J.,Specola Vaticana | Britt D.T.,University of Central Florida
Meteoritics and Planetary Science | Year: 2011

We report physical properties (bulk and grain density, magnetic susceptibility, and porosity) measured using nondestructive and noncontaminating methods for 195 stones from 63 carbonaceous chondrites. Grain densities over the whole population average 3.44gcm -3, ranging from 2.42gcm -3 (CI1 Orgueil) to 5.66gcm -3 (CB Bencubbin). Magnetic susceptibilities (in log units of 10 -9m 3kg -1) averaged logχ=4.22, ranging from 3.23 (CV3 Axtell) to 5.79 (CB Bencubbin). Porosities averaged 17%, ranging from 0 (for a number of meteorites) to 41% (for one stone of the CO Ornans). Notably, we found significant differences in porosity between the oxidized and reduced CV subgroups, with the porosities of CV o averaging approximately 20% and CV r porosities approximately 4%. Overall, porosities of carbonaceous chondrite falls trend with petrographic type, from type 1 (CI) near 35%, type 2 (CM, CR) averaging 23%, type 3 (CV, CO) 21%, to type 4 (CK and some CO) averaging 15%. There is also a significant decrease in porosity between meteorites of shock stage S1 and those of S2, indicative of shock compression. © The Meteoritical Society, 2011.

Macke R.J.,University of Central Florida | Britt D.T.,University of Central Florida | Consolmagno G.J.,Specola Vaticana
Meteoritics and Planetary Science | Year: 2011

+Abstract-: As part of a large-scale survey of meteorite bulk and grain densities, porosities, and magnetic susceptibilities, we measured these properties for 174 stones from 106 achondritic meteorites. These include four lunar meteorites, 15 stones from 10 shergottites, nakhlites, and chassignites (SNCs), 96 stones from 56 howardites, eucrites, and diogenites (HEDs), 17 stones from nine aubrites, two angrites, and 16 stones from 10 ureilites, four stones of three acapulcoites, as well as four stones of three lodranites, and 15 stones from eight primitive achondrites. Those meteorites derived from basalts and crustal material of differentiated parent bodies have lower densities and magnetic susceptibilities, on an average, than the more primitive achondrites, which have a higher percentage metal. A notable exception is the one chassignite in the study (Chassigny), which has a high grain density of 3.73±0.04gcm-3. Ureilites have magnetic susceptibilities consistent with primitive achondrites, but lower grain densities. Porosities do not vary considerably between most of the groups, with most stones 5-14% porous, although on an average, ureilites and brachinites have lower porosities, with most stones less than 7% porous. For primitive achondrites, the higher metal content causes finds to exhibit weathering effects similar to what is observed in ordinary chondrites, with a reduction in grain density, magnetic susceptibility, and porosity as compared with unweathered falls. For lunites, SNCs, and HEDs, no such effect is observed. We also observe that grain density and magnetic susceptibility used in conjunction distinguish shergottites, nakhlites, and chassignites from each other. Shergottites and nakhlites have low grain densities (averaging 3.31 and 3.41gcm-3, respectively) whereas Chassigny is 3.7gcm-3. In magnetic susceptibility, shergottities and chassignites are similar (averaging 2.85 and 2.98 in log units of 10-9m3kg-1, respectively) with nakhlites averaging higher at 3.42. © The Meteoritical Society, 2011.

Brown D.,Specola Vaticana
Astrophysics and Space Science | Year: 2010

One of the ways by which subdwarf B stars are thought to form is through binary star interactions. The metallicity of the sdB progenitor stars in such binary systems should not seem to be a major factor in the formation of sdB stars. However, given the different environments in which sdB stars are found, binary population synthesis simulations have been conducted in order to examine how metallicity might be a subtle factor in the formation of sdB stars in such environments. This is then applied to clusters of stars and to the UV Upturn phenomenon. © 2010 Springer Science+Business Media B.V.

Gabriele Gionti S.J.,Specola Vaticana | Gabriele Gionti S.J.,University of Arizona
International Journal of Geometric Methods in Modern Physics | Year: 2012

Recent results in Local Regge Calculus are confronted with Spin-Foam Formalism. Introducing BarrettCrane Quantization in Local Regge Calculus makes it possible to associate a unique Spin j h with an hinge h, fulfilling one of the requirements of Spin-Foam definition. It is shown that inter-twiner terms of Spin-Foam can follow from the closure constraint in Local Regge Calculus. © 2012 World Scientific Publishing Company.

Loading Specola Vaticana collaborators
Loading Specola Vaticana collaborators