¹Reddy, Vishnu et al. (>10)
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2019.02.018]
¹Lunar and Planetary Laboratory, University of Arizona, 1629 E University Blvd, Tucson, AZ 85721, USA
Coypright Elsevier

Impacts due to near-Earth objects (NEOs) are responsible for causing some of the great mass extinctions on Earth. While nearly all NEOs of diameter > 1 km, capable of causing a global climatic disaster, have been discovered and have negligible chance of impacting in the near future, we are far from completion in our effort to detect and characterize smaller objects. In an effort to test our preparedness to respond to a potential NEO impact threat, we conducted a community-led global planetary defense exercise with support from the NASA Planetary Defense Coordination Office. The target of our exercise was 2012 TC4, the ~10 m diameter asteroid that made a close pass by the Earth on 2017 October 12 at a distance of about 50,000 km. The goal of the TC4 observing campaign was to recover, track, and characterize 2012 TC4 as a hypothetical impactor in order to exercise the global planetary defense system involving observations, modeling, prediction, and communication. We made three attempts with the Very Large Telescope (VLT) on 2017 July 27, 31 and on 2017 August 5 and recovered 2012 TC4 within its ephemeris uncertainty at 2.2 arcmin from the nominal prediction. At visual magnitude V = 27, the recovery of 2012 TC4 is the faintest NEA detection thus far. If an impact during the 2017 close approach had been possible based on the 2012 astrometric data, these recovery observations would have been sufficient to confirm or rule out the impact. The first automatic detection by a survey (Pan-STARRS1) was on September 25, which is the earliest that 2012 TC4 would have been discovered in survey mode, if it had not been discovered in 2012. We characterized 2012 TC4 using photometry, spectroscopy and radar techniques. Based on photometric observations, we determined a rotation period of 12.2 min with an amplitude of 0.9 magnitudes. An additional lower amplitude period was detected, indicating that 2012 TC4 was in a state of non-principle axis rotation. The combined visible and near-infrared spectrum puts it in the taxonomic X-class. Radar observations at 3.75 m resolution placed only one to two range pixels on the asteroid and barely resolved it; this suggests that 2012 TC4 is less than about 20 m on its long axis. We also estimated the average circular polarization, SC/OC, of 2012 TC4 to be 0.57, which is relatively high among NEOs observed by radar to date (averaging 0.34 ± 0.28). High circular polarization ratios can be caused by a variety of mechanisms such as near-surface roughness, curvature at scales comparable to that of the radar wavelength, and a high refractive index. We also performed a probabilistic impact risk assessment exercise for hypothetical impactors based on the 2012 TC4 observing campaign. This exercise was performed as part of ongoing efforts to advance effective impact risk models and assessment processes for planetary defense. The 2012 TC4 close approach provided a valuable opportunity to test the application of these methods using realistically evolving observational data to define the modeling inputs. To this end, risk assessments were calculated at several epochs before and during the close approach, incorporating new information about 2012 TC4 as it became available. Two size ranges were assessed—one smaller size range (H = 26.7) similar to the actual 2012 TC4, and one larger size range (H = 21.9) to produce a greater-damage scenario for risk assessment. Across the epochs, we found that only irons caused significant damage for smaller size. For the larger size case, however, hydrous stones caused the greatest damage, anhydrous stones caused the least damage, and irons caused moderate damage. We note that the extent of damage depends on composition in different size regimes and, after astrometry, size is the most important physical property to determine for an incoming object.

^1,2S.Schröder,¹K.Rammelkamp,¹D.S.Vogt,²O.Gasnault,^1,3H.-W.Hübers
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2019.02.017]
¹Deutsches Zentrum für Luft- und Raumfahrt (DLR), Institut für Optische Sensorsysteme, Berlin, Germany
²Institut de Recherche en Astrophysique et Planétologie (IRAP, Université de Toulouse, CNRS, CNES, UPS), Toulouse, France
³Humboldt-Universität zu Berlin, Institut für Physik, Be
Copyright Elsevier

For in-situ geochemical analysis of the surface of Mars, laser-induced breakdown spectroscopy (LIBS) is a very useful technique and the first extraterrestrial LIBS instrument ChemCam will soon be followed by others. Appropriate normalization of real mission data that is taken under varying experimental and environmental conditions from diverse geologic samples is an ongoing topic. One approach is the scaling to emission line intensities of carbon and oxygen from the CO2-dominated low-pressure martian atmosphere as an internal standard. Here, we performed several experiments to examine the emission of carbon and oxygen from a simulated martian atmosphere on simple, mostly mono-elemental samples, and to compare the emission characteristics of the elements of both origins. Differences in laser irradiance were found to have the biggest impact on sample emission lines scaled to C and O emission of the atmosphere. The temporal behavior shown in time-resolved LIBS measurements is dominated by the degree of ionization, but the ratio of the neutral emission from the sample to neutral emission from the atmosphere also varies over time. The effect of different grain sizes was minor in comparison to the high intrinsic variation in the LIBS data. Different samples were found to affect the absolute intensity of atmospheric C(I) more than O(I). Furthermore, the C(I) emission was found to be inseparably superimposed by iron if the latter was present in the target. The results indicate limitations of the general suitability of atmospheric carbon and oxygen emission for normalization purposes of martian LIBS data.

Peter Hoppe, Jan Leitner, and János Kodolányi
Astrophysical Journal 869, 47 Link to Article [DOI: 10.3847/1538-4357/aaec0a]
Max Planck Institute for Chemistry, Hahn-Meitner-Weg 1, D-55128 Mainz, Germany

We report high-resolution (<100 nm) Mg and Si isotope data of 12 presolar silicate grains (230–440 nm) from red giant and/or asymptotic giant branch stars that were previously identified based on their anomalous O-isotopic compositions (11 Group 1 grains and one Group 2 grain) in five primitive meteorites. The data were acquired by NanoSIMS ion imaging with the new Hyperion ion source that permits Mg and Si isotope measurements of presolar silicates with higher precision than was possible before. For a subset of five Group 1 (“category A”) grains, ²⁵Mg/²⁴Mg and ²⁹Si/²⁸Si ratios correlate with the inferred initial ¹⁸O/¹⁶O ratios of their parent stars, a measure of stellar metallicity. The Mg and Si isotope data of category A grains show positive correlations in the δ²⁵Mg–δ ²⁶Mg, δ ²⁹Si–δ ³⁰Si, and δ ²⁵Mg–δ ²⁹Si spaces. The correlations between O-, Mg, and Si-isotopic compositions are best explained by Galactic chemical evolution (GCE), with only minor imprints of nucleosynthetic and mixing processes in the grains’ parent stars. Six Group 1 silicate (“category B”) grains have close-to-normal Mg and Si isotopic compositions, possibly the result of isotope exchange in interstellar space or the meteorite parent bodies. For Si in category A grains, we find, with ~2σ significance, a slightly shallower slope in the δ ²⁹Si–δ ³⁰Si space for the GCE than inferred from presolar SiC mainstream grains. The 2σ upper limit on the slope for the linear trend in the δ ²⁵Mg–δ ²⁶Mg space of category A grains is slightly lower than the slope-1 predicted by GCE models around solar metallicity.

Feng Long (龙凤)^1,2 et al. (>10)
Astrophysical Journal 869, 17 Link to Article [DOI: 10.3847/1538-4357/aae8e1]
¹Kavli Institute for Astronomy and Astrophysics, Peking University, Beijing 100871, People’s Republic of China

Rings are the most frequently revealed substructure in Atacama Large Millimeter/submillimeter Array (ALMA) dust observations of protoplanetary disks, but their origin is still hotly debated. In this paper, we identify dust substructures in 12 disks and measure their properties to investigate how they form. This subsample of disks is selected from a high-resolution (~012) ALMA 1.33 mm survey of 32 disks in the Taurus star-forming region, which was designed to cover a wide range of brightness and to be unbiased to previously known substructures. While axisymmetric rings and gaps are common within our sample, spiral patterns and high-contrast azimuthal asymmetries are not detected. Fits of disk models to the visibilities lead to estimates of the location and shape of gaps and rings, the flux in each disk component, and the size of the disk. The dust substructures occur across a wide range of stellar mass and disk brightness. Disks with multiple rings tend to be more massive and more extended. The correlation between gap locations and widths, the intensity contrast between rings and gaps, and the separations of rings and gaps could all be explained if most gaps are opened by low-mass planets (super-Earths and Neptunes) in the condition of low disk turbulence (α = 10⁻⁴). The gap locations are not well correlated with the expected locations of CO and N₂ ice lines, so condensation fronts are unlikely to be a universal mechanism to create gaps and rings, though they may play a role in some cases.

^1,2 Courtney J. Sprain,^1,3Paul R. Renne⁴Loÿc Vanderkluysen,⁵Kanchan Pande,¹Stephen Self,¹Tushar Mittal
Science 363, 866-870 Link to Article [DOI: 10.1126/science.aav1446]
¹Department of Earth and Planetary Science, University of California, Berkeley, 307 McCone Hall, Berkeley, CA 94720-4767, USA.
²Geomagnetism Laboratory, Department of Earth, Ocean and Ecological Sciences, University of Liverpool, Liverpool L69 7ZE, UK.
³Berkeley Geochronology Center, 2455 Ridge Road, Berkeley, CA 94709, USA.
⁴Department of Biodiversity, Earth and Environmental Science, Drexel University, 3245 Chestnut Street, PISB 123, Philadelphia, PA 19104, USA.
⁵Department of Earth Sciences, Indian Institute of Technology Bombay, Powai, Mumbai 400 076, India.
Reprinted with permission of AAAS

Late Cretaceous records of environmental change suggest that Deccan Traps (DT) volcanism contributed to the Cretaceous-Paleogene boundary (KPB) ecosystem crisis. However, testing this hypothesis requires identification of the KPB in the DT. We constrain the location of the KPB with high-precision argon-40/argon-39 data to be coincident with changes in the magmatic plumbing system. We also found that the DT did not erupt in three discrete large pulses and that >90% of DT volume erupted in <1 million years, with ~75% emplaced post-KPB. Late Cretaceous records of climate change coincide temporally with the eruption of the smallest DT phases, suggesting that either the release of climate-modifying gases is not directly related to eruptive volume or DT volcanism was not the source of Late Cretaceous climate change.

¹Blair Schoene, ¹Michael P. Eddy, ²Kyle M. Samperton, ³C. Brenhin Keller, ¹Gerta Keller, ⁴Thierry Adatte, ⁵Syed F. R. Khadri
Science 363, 862-866 Link to Article [DOI: 10.1126/science.aau2422]
¹Department of Geosciences, Princeton University, Princeton, NJ, USA.
²Nuclear and Chemical Sciences Division, Lawrence Livermore National Laboratory, Livermore, CA, USA.
³Berkeley Geochronology Center, Berkeley, CA, USA.
⁴ISTE, Institut des Sciences de la Terre, Université de Lausanne, GEOPOLIS, Lausanne, Switzerland.
⁵Department of Geology, Amravati University, Amravati, India.
Reprinted with permission from AAAS

Temporal correlation between some continental flood basalt eruptions and mass extinctions has been proposed to indicate causality, with eruptive volatile release driving environmental degradation and extinction. We tested this model for the Deccan Traps flood basalt province, which, along with the Chicxulub bolide impact, is implicated in the Cretaceous-Paleogene (K-Pg) extinction approximately 66 million years ago. We estimated Deccan eruption rates with uranium-lead (U-Pb) zircon geochronology and resolved four high-volume eruptive periods. According to this model, maximum eruption rates occurred before and after the K-Pg extinction, with one such pulse initiating tens of thousands of years prior to both the bolide impact and extinction. These findings support extinction models that incorporate both catastrophic events as drivers of environmental deterioration associated with the K-Pg extinction and its aftermath.

Udry¹ et al. (>10)
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13252]
¹Department of Geoscience, University of Nevada Las Vegas, Las Vegas, Nevada, 89154 USA
Published by arrangement with John Wiley & Sons

We present petrologic and isotopic data on Northwest Africa (NWA) 4799, NWA 7809, NWA 7214, and NWA 11071 meteorites, which were previously classified as aubrites. These four meteorites contain between 31 and 56 vol% of equigranular, nearly endmember enstatite, Fe,Ni metal, plagioclase, terrestrial alteration products, and sulfides, such as troilite, niningerite, daubréelite, oldhamite, and caswellsilverite. The equigranular texture of the enstatite and the presence of the metal surrounding enstatite indicate that these rocks were not formed through igneous processes like the aubrites, but rather by impact processes. In addition, the presence of pre‐terrestrially weathered metal (7.1–14 vol%), undifferentiated modal abundances compared to enstatite chondrites, presence of graphite, absence of diopside and forsterite, low Ti in troilite, and high Si in Fe,Ni metals suggest that these rocks formed through impact melting on chondritic and not aubritic parent bodies. Formation of these meteorites on a parent body with similar properties to the EHa enstatite chondrite parent body is suggested by their mineralogy. These parent bodies have undergone impact events from at least 4.5 Ga (NWA 11071) until at least 4.2 Ga (NWA 4799) according to ³⁹Ar‐⁴⁰Ar ages, indicating that this region of the solar system was heavily bombarded early in its history. By comparing NWA enstatite chondrite impact melts to Mercury, we infer that they represent imperfect petrological analogs to this planet given their high metal abundances, but they could represent important geochemical analogs for the behavior and geochemical affinities of elements on Mercury. Furthermore, the enstatite chondrite impact melts represent an important petrological analog for understanding high‐temperature processes and impact processes on Mercury, due to their similar mineralogies, Fe‐metal‐rich and FeO‐poor silicate abundances, and low oxygen fugacity.

Karel ZAK¹, Roman SKALA¹, Andreas PACK², Lukas ACKERMAN¹, and Sarka KRIZOVA^1,3
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13251]
¹Institute of Geology of the Czech Academy of Sciences, Rozvojova 269, CZ-165 00 Praha 6, Czech Republic
²Geowissenschaftliches Zentrum, Abteilung Isotopengeologie, Universität Göttingen,Goldschmidtstraße 1, D-37077 Göttingen, Germany
³Institute of Geochemistry, Mineralogy and Mineral Resources, Faculty of Science,Charles University, Albertov 6, CZ-128 43 Praha 2, Czech Republic
Published by arrangement with John Wiley & Sons

Major and trace element analyses and triple oxygen isotope measurements were performed on 11 individual specimens of Australasian tektites (AAT) with exactly known field positions from Laos. The sample set was dominated by Muong Nong‐type tektites (MNAAT), including separated layers of glass of different appearance and chemistry from four samples. This first larger set of oxygen isotope data of MNAAT revealed the δ¹⁸O range 8.7 ≤ δ¹⁸O ≤ 11.6‰ on VSMOW2 scale (12 analyses), only slightly wider than the previously reported range for splash‐form AAT. The Δ’¹⁷O values of MNAAT (−0.098 ≤ Δ’¹⁷O ≤ −0.069‰; 12 analyses) and splash‐form AAT (−0.080 ≤ Δ’¹⁷O ≤ −0.068‰; three analyses) are all in the range of data typical for terrestrial crustal rocks, with no mass‐independent oxygen isotope fractionation (from impactor or from exchange with atmospheric O₂) being observed.

M. R. Mumpower¹, T. Kawano¹, T. M. Sprouse², N. Vassh², E. M. Holmbeck², R. Surman², and P. Möller¹
Astrophysical Journal 869, 14 Link to Article [DOI: 10.3847/1538-4357/aaeaca]
¹Theoretical Division, Los Alamos National Laboratory Los Alamos, NM 87545, USA
²Department of Physics, University of Notre Dame Notre Dame, IN 46556, USA

We present β-delayed neutron emission and β-delayed fission (βdf) calculations for heavy, neutron-rich nuclei using the coupled Quasi-Particle Random Phase Approximation plus Hauser-Feshbach (QRPA+HF) approach. From the initial population of a compound nucleus after β-decay, we follow the statistical decay, taking into account competition between neutrons, γ-rays, and fission. We find a region of the chart of nuclides where the probability of βdf is ~100%, which likely prevents the production of superheavy elements in nature. For a subset of nuclei near the neutron dripline, neutron multiplicity and the probability of fission are both large, leading to the intriguing possibility of multi-chance βdf, a decay mode for extremely neutron-rich heavy nuclei. In this decay mode, β-decay can be followed by multiple neutron emission, leading to subsequent daughter generations that each have a probability to fission. We explore the impact of βdf in rapid neutron-capture process (r-process) nucleosynthesis in the tidal ejecta of a neutron star–neutron star merger and show that it is a key fission channel that shapes the final abundances near the second r-process peak.

Sakari¹ et al. (>10)
Astrophysical Journal 868, 110 Link to Article [DOI: 10.3847/1538-4357/aae9df]
Space Science Division, U.S. Naval Research Laboratory, Washington, DC 20375, USA
¹ Department of Astronomy, University of Washington, Seattle, WA 98195-1580, USA

This paper presents the detailed abundances and r-process classifications of 126 newly identified metal-poor stars as part of an ongoing collaboration, the R-Process Alliance. The stars were identified as metal-poor candidates from the RAdial Velocity Experiment (RAVE) and were followed up at high spectral resolution (R ~ 31,500) with the 3.5 m telescope at Apache Point Observatory. The atmospheric parameters were determined spectroscopically from Fe i lines, taking into account non-LTE corrections and using differential abundances with respect to a set of standards. Of the 126 new stars, 124 have [Fe/H] < −1.5, 105 have [Fe/H] < −2.0, and 4 have [Fe/H] < −3.0. Nine new carbon-enhanced metal-poor stars have been discovered, three of which are enhanced in r-process elements. Abundances of neutron-capture elements reveal 60 new r-I stars (with +0.3 ≤ [Eu/Fe] ≤ +1.0 and [Ba/Eu] < 0) and 4 new r-II stars (with [Eu/Fe] > +1.0). Nineteen stars are found to exhibit a “limited-r” signature ([Sr/Ba] > +0.5, [Ba/Eu] < 0). For the r-II stars, the second- and third-peak main r-process patterns are consistent with the r-process signature in other metal-poor stars and the Sun. The abundances of the light, α, and Fe-peak elements match those of typical Milky Way (MW) halo stars, except for one r-I star that has high Na and low Mg, characteristic of globular cluster stars. Parallaxes and proper motions from the second Gaia data release yield UVW space velocities for these stars that are consistent with membership in the MW halo. Intriguingly, all r-II and the majority of r-I stars have retrograde orbits, which may indicate an accretion origin.