¹Ye Li,^1,2Weibiao Hsu
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13061]
¹Purple Mountain Observatory, Chinese Academy of Sciences, Nanjing, China
²Space Science Institute, Macau University of Science and Technology, Macau, China
Published by arrangement with John Wiley & Sons

Here we report in situ secondary ionization mass spectrometry Ca‐phosphate U‐Pb ages for an L‐impact melt breccia (NWA 7251), which are integrated with petrological and mineral chemical studies of this meteorite. NWA 7251 is a heavily shocked rock that is composed mainly of the chondrite host, impact melt portion, and melt veins (crosscutting and pervasive type). The host is an L4 chondrite that has been shocked to S4. The impact melt portion has a fine‐grained igneous texture, and is composed mainly of olivine, low‐Ca pyroxene, high‐Ca pyroxene, and albitic glass. The impact melt was generated at pressure of >30–35 GPa and temperature of >1300–1500 °C during an impact event. The Ca‐phosphate grains in the host were affected by a shock heating event. Most of the Ca‐phosphate grains in the melt were neocrystallized, but relatively large grains enclosed by or adjacent to metal veins or melt globules are likely inherited. The U‐Pb isotopic systematics of Ca‐phosphates in NWA 7251 yield an upper intercept age of 4457 ± 56 Ma and a lower intercept age of 574 ± 82 Ma on the normal U‐Pb concordia diagram. The age of 4457 ± 56 Ma is interpreted to be related to an early shocking event rather than the thermal metamorphism of the parent body. The impact melt and veins in NWA 7251 were generated at 574 ± 82 Ma, resulting from disruption of the L chondrite parent body.

¹Jennifer K. Harris, ¹Peter M. Grindrod
¹Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13065]
¹Earth Sciences, Natural History Museum, , London, UK
Published by arrangement with John Wiley & Sons

The mineralogy of Mars is well understood on a qualitative level at a global scale due to satellite data. Quantitative analysis of visible and near‐infrared (VNIR) satellite data is a desirable but nontrivial task, due partly to the nonlinearity of VNIR reflectance spectra from the mineral mixtures of the Martian surface. In this study, we investigated the use of the Hapke radiative transfer model to generate linearly mixed single scattering albedo data from nonlinearly mixed VNIR reflectance data and then quantitatively analyzed them using the linear spectral mixture model. Simplifications to the Hapke equation were tested accounting for variables that would be unknown when using satellite data. Mineral mixture spectra from the RELAB spectral library were degraded to test the robustness of the unmixing technique in the face of data that mimic some of the complexities of satellite spectral data collected at Mars. A final test was performed on spectra from shergottite meteorites to assess the technique against real Martian mineral mixtures. The simplified Hapke routine produced robust abundance estimates within 5–10% accuracy when applied to laboratory standard spectra from the synthetic mixtures of igneous minerals in agreement with previous studies. The results of tests involving degraded data to mimic the low spectral contrast of the Martian surface and the lack of a priori knowledge of the constituent mineral spectral endmembers, however, were less encouraging, with errors in abundance estimation greater than 25%. These results cast doubt on the utility of Hapke unmixing for the quantitative analysis of VNIR data of the surface of Mars.

¹C. Fry, ¹C. Samson, ^2,3P. J. A. McCausland,¹M. Ralchenko,¹T. K. McLeod
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13067]
¹Department of Earth Sciences, Carleton University, Ottawa, Ontario, Canada
²Department of Earth Sciences, Western University, London, Ontario, Canada
³Centre for Planetary Science and Exploration, Western University, London, Ontario, Canada
Published by arrangement with John Wiley & Sons

This study tested the feasibility of using 3‐D laser imaging to measure the bulk density of iron meteorites. 3‐D laser imaging is a technique in which a 3‐D model of an object is built after aligning and merging individual detailed images of its surface. Assuming that the mass of the object is known, the volume of the model is calculated by software and an estimate of bulk density can be obtained by dividing mass by volume. The 3‐D laser imaging technique was used to determine the density of 46 fragments from 11 different iron meteorites. The technique proved to be robust and was applied successfully to study samples ranging close to four orders of magnitude in mass (8 g to 156 kg) and exhibiting a variety of surface textures (e.g., cracks, regmaglypts), reflectivities (e.g., polished surfaces, fusion crust, rust), and morphologies (e.g., sharp angular edges, shrapnel tendrils). Three metrics were considered to estimate the error associated with density measurements: the range accuracy of the laser camera, image alignment error, and inter‐operator variability during model building. Inter‐operator variability was the largest source of error and was highest when assembling models of samples which either lacked distinctive features or were very complex in shape. Excluding two anomalous Zagora samples where silicate inclusions might have lowered density, the densities measured using 3‐D laser imaging ranged from 6.98 to 7.93 g cm−3, consistent with previous studies. There is overlap between bulk density and iron meteorite class, and therefore bulk density cannot be used in isolation as a classification criterion. It is a good indicator, however, of weathering effects and of the potential presence of low‐density inclusions.

¹Gregory A.Brennecka, ²Yuri Amelin, ¹Thorsten Kleine
Earth and Planetary Science Letters 490, 1-10 Link to Article [https://doi.org/10.1016/j.epsl.2018.03.010]
¹Institut für Planetologie, University of Münster, Wilhelm-Klemm-Str. 10, 48149 Münster, Germany
²Research School of Earth Sciences, The Australian National University, Canberra ACT 0200, Australia
Copyright Elsevier

The crystallization ages of planetary crustal material (given by basaltic meteorites) and planetary cores (given by iron meteorites) provide fiducial marks for the progress of planetary formation, and thus, the absolute ages of these objects fundamentally direct our knowledge and understanding of planet formation and evolution. The lone precise absolute age of planetary core material was previously obtained on troilite inclusions from the IVA iron meteorite Muonionalusta. This previously reported Pb–Pb age of 4565.3 ± 0.1 Ma—assuming a 238U/235U =137.88—only post-dated the start of the Solar System by approximately 2–3 million years, and mandated fast cooling of planetary core material. Since an accurate Pb–Pb age requires a known 238U/235U of the sample, we have measured both 238U/235U and Pb isotopic compositions of troilite inclusions from Muonionalusta. The measured 238U/235U of the samples range from ∼137.84 to as low as ∼137.22, however based on Pb and U systematics, terrestrial contamination appears pervasive and has affected samples to various extents for Pb and U. The cause of the relative 235U excess in one sample does not appear to be from terrestrial contamination or the decay of short-lived 247Cm, but is more likely from fractionation of U isotopes during metal–silicate separation during core formation, exacerbated by the extreme U depletion in the planetary core. Due to limited Pb isotopic variation and terrestrial disturbance, no samples of this study produced useful age information; however the clear divergence from the previously assumed 238U/235U of any troilite in Muonionalusta introduces substantial uncertainty to the previously reported absolute age of the sample without knowledge of the 238U/235U of the sample.

Uncertainties associated with U isotope heterogeneity do not allow for definition of a robust age of solidification and cooling for the IVA core. However, one sample of this work—paired with previous work using short-lived radionuclides—suggests that the cooling age of the IVA core may be significantly younger than previously thought. This work indicates the metallic cores of protoplanetary bodies solidified no earlier than the first ∼5–10 million years of the Solar System.

R. H. Hewins^1,2, C. Condie^1,3, M. Morris⁴, M. L. A. Richardson⁵, N. Ouellette⁴, and M. Metcalf⁴
Astrophysical Journal Letters 855, L17 Link to Article [DOI: 10.3847/2041-8213/aab15b]
¹EPS, Rutgers University, Piscataway NJ 08816, USA
²IMPMC, MNHN, UPMC, Sorbonne Universités, Paris F-75005, France
³Natural Science, Middlesex Community College, Edison, NJ 08818, USA
⁴Physics, SUNY at Cortland, NY 13045, USA
⁵Sub-department of Astrophysics, University of Oxford, Keble Road, Oxford OX1 3RH, UK

It has been proposed that some meteorites, CB and CH chondrites, contain material formed as a result of a protoplanetary collision during accretion. Their melt droplets (chondrules) and FeNi metal are proposed to have formed by evaporation and condensation in the resulting impact plume. We observe that the skeletal olivine (SO) chondrules in CB_b chondrites have a blebby texture and an enrichment in refractory elements not found in normal chondrules. Because the texture requires complete melting, their maximum liquidus temperature of 1928 K represents a minimum temperature for the putative plume. Dynamic crystallization experiments show that the SO texture can be created only by brief reheating episodes during crystallization, giving a partial dissolution of olivine. The ejecta plume formed in a smoothed particle hydrodynamics simulation served as the basis for 3D modeling with the adaptive mesh refinement code FLASH4.3. Tracer particles that move with the fluid cells are used to measure the in situ cooling rates. Their cooling rates are ~10,000 K hr⁻¹ briefly at peak temperature and, in the densest regions of the plume, ~100 K hr⁻¹ for 1400–1600 K. A small fraction of cells is seen to be heating at any one time, with heating spikes explained by the compression of parcels of gas in a heterogeneous patchy plume. These temperature fluctuations are comparable to those required in crystallization experiments. For the first time, we find an agreement between experiments and models that supports the plume model specifically for the formation of CB_b chondrules.

Charli M. Sakari¹ (>10)
Astrophysical Journal Letters 854, L20 Link to Article [DOI: 10.3847/2041-8213/aaa9b4]
¹Department of Astronomy, University of Washington, Seattle, WA 98195-1580, USA

A high-resolution spectroscopic analysis is presented for a new highly r-process-enhanced ([Eu/Fe] = 1.27, [Ba/Eu] = −0.65), very metal-poor ([Fe/H] = −2.09), retrograde halo star, RAVE J153830.9–180424, discovered as part of the R-Process Alliance survey. At V = 10.86, this is the brightest and most metal-rich r-II star known in the Milky Way halo. Its brightness enables high-S/N detections of a wide variety of chemical species that are mostly created by the r-process, including some infrequently detected lines from elements like Ru, Pd, Ag, Tm, Yb, Lu, Hf, and Th, with upper limits on Pb and U. This is the most complete r-process census in a very metal-poor r-II star. J1538–1804 shows no signs of s-process contamination, based on its low [Ba/Eu] and [Pb/Fe]. As with many other r-process-enhanced stars, J1538–1804’s r-process pattern matches that of the Sun for elements between the first, second, and third peaks, and does not exhibit an actinide boost. Cosmo-chronometric age-dating reveals the r-process material to be quite old. This robust main r-process pattern is a necessary constraint for r-process formation scenarios (of particular interest in light of the recent neutron star merger, GW170817), and has important consequences for the origins of r-II stars. Additional r-I and r-II stars will be reported by the R-Process Alliance in the near future.

Siteng Fan and Konstantin Batygin
Astrophysical Journal Letters 851, L37 Link to Article [DOI: 10.3847/2041-8213/aa9f0b]
Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125, USA

Over the course of the last decade, the Nice model has dramatically changed our view of the solar system’s formation and early evolution. Within the context of this model, a transient period of planet–planet scattering is triggered by gravitational interactions between the giant planets and a massive primordial planetesimal disk, leading to a successful reproduction of the solar system’s present-day architecture. In typical realizations of the Nice model, self-gravity of the planetesimal disk is routinely neglected, as it poses a computational bottleneck to the calculations. Recent analyses have shown, however, that a self-gravitating disk can exhibit behavior that is dynamically distinct, and this disparity may have significant implications for the solar system’s evolutionary path. In this work, we explore this discrepancy utilizing a large suite of Nice model simulations with and without a self-gravitating planetesimal disk, taking advantage of the inherently parallel nature of graphic processing units. Our simulations demonstrate that self-consistent modeling of particle interactions does not lead to significantly different final planetary orbits from those obtained within conventional simulations. Moreover, self-gravitating calculations show similar planetesimal evolution to non-self-gravitating numerical experiments after dynamical instability is triggered, suggesting that the orbital clustering observed in the distant Kuiper Belt is unlikely to have a self-gravitational origin.

Pablo Benítez-Llambay and Martin E. Pessah
Astrophysical Journal Letters 855, L28 Link to Article [DOI: 10.3847/2041-8213/aab2ae]
Niels Bohr International Academy, Niels Bohr Institute, Blegdamsvej 17, DK-2100 Copenhagen Ø, Denmark

Fast inward migration of planetary cores is a common problem in the current planet formation paradigm. Even though dust is ubiquitous in protoplanetary disks, its dynamical role in the migration history of planetary embryos has not been assessed. In this Letter, we show that the scattered pebble-flow induced by a low-mass planetary embryo leads to an asymmetric dust-density distribution that is able to exert a net torque. By analyzing a large suite of multifluid hydrodynamical simulations addressing the interaction between the disk and a low-mass planet on a fixed circular orbit, and neglecting dust feedback onto the gas, we identify two different regimes, gas- and gravity-dominated, where the scattered pebble-flow results in almost all cases in positive torques. We collect our measurements in a first torque map for dusty disks, which will enable the incorporation of the effect of dust dynamics on migration into population synthesis models. Depending on the dust drift speed, the dust-to-gas mass ratio/distribution, and the embryo mass, the dust-induced torque has the potential to halt inward migration or even induce fast outward migration of planetary cores. We thus anticipate that dust-driven migration could play a dominant role during the formation history of planets. Because dust torques scale with disk metallicity, we propose that dust-driven outward migration may enhance the occurrence of distant giant planets in higher-metallicity systems.

Tom Seccull¹, Wesley C. Fraser¹, Thomas H. Puzia², Michael E. Brown³, and Frederik Schönebeck⁴
Astrophysical Journal Letters 851, L12 Link to Article [DOI: 10.3847/2041-8213/aab3dc]
¹Astrophysics Research Centre, Queen’s University Belfast, Belfast BT7 1NN, UK
²Institute of Astrophysics, Pontificia Universidad Católica de Chile, Av. Vincuña Mackenna 4860, 7820436, Santiago, Chile
³Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125, USA
⁴Astronomisches Rechen-Institut, Zentrum für Astronomie der Universität Heidelberg, Mönchhofstraße 12-14, D-69120 Heidelberg, Germany

Models of the Solar System’s dynamical evolution predict the dispersal of primitive planetesimals from their formative regions among the gas-giant planets due to the early phases of planetary migration. Consequently, carbonaceous objects were scattered both into the outer asteroid belt and out to the Kuiper Belt. These models predict that the Kuiper Belt should contain a small fraction of objects with carbonaceous surfaces, though to date, all reported visible reflectance spectra of small Kuiper Belt Objects (KBOs) are linear and featureless. We report the unusual reflectance spectrum of a small KBO, (120216) 2004 EW₉₅, exhibiting a large drop in its near-UV reflectance and a broad shallow optical absorption feature centered at ~700 nm, which is detected at greater than 4σsignificance. These features, confirmed through multiple epochs of spectral photometry and spectroscopy, have respectively been associated with ferric oxides and phyllosilicates. The spectrum bears striking resemblance to those of some C-type asteroids, suggesting that 2004 EW₉₅ may share a common origin with those objects. 2004 EW₉₅ orbits the Sun in a stable mean motion resonance with Neptune, at relatively high eccentricity and inclination, suggesting it may have been emplaced there by some past dynamical instability. These results appear consistent with the aforementioned model predictions and are the first to show a reliably confirmed detection of silicate material on a small KBO.

J. Martin Laming¹ et al. (>10)
Astrophysical Journal Letters 851, L12 Link to Article [DOI: 10.3847/2041-8213/aa9bf0]
¹Space Science Division, Naval Research Laboratory, Code 7684, Washington, DC 20375, USA

We compare element and isotopic fractionations measured in bulk solar wind samples collected by NASA’s Genesis mission with those predicted from models incorporating both the ponderomotive force in the chromosphere and conservation of the first adiabatic invariant in the low corona. Generally good agreement is found, suggesting that these factors are consistent with the process of solar wind fractionation. Based on bulk wind measurements, we also consider in more detail the isotopic and elemental abundances of O. We find mild support for an O abundance in the range 8.75–8.83, with a value as low as 8.69 disfavored. A stronger conclusion must await solar wind regime-specific measurements from the Genesis samples.