¹Munan Gong (龚 慕南),²Xiaochen Zheng (郑 晓晨),^2,3,4Douglas N. C. Lin (林潮),¹Kedron Silsbee,⁵Clement Baruteau,¹Shude Mao (毛 淑德)
The Astrophysical Journal 883, 164 Link to Article [DOI
https://doi.org/10.3847/1538-4357/ab3e70]
¹Max-Planck Institute for Extraterrestrial Physics, Garching by Munich, D-85748, Germany
²Department of Astronomy and Center for Astrophysics, Tsinghua University, Beijing 10086, People’s Republic of China
³Institute for Advanced Studies, Tsinghua University, Beijing 10086, People’s Republic of China
⁴Department of Astronomy and Astrophysics, University of California Santa Cruz, Santa Cruz, CA 95064, USA
⁵Institut de Recherche en Astrophysique et Planétologie (IRAP), 14 avenue Edouard Belin, F-31400 Toulouse, France

Chondrules are silicate spheroids found in meteorites, and they serve as important fossil records of the early solar system. In order to form chondrules, chondrule precursors must be heated to temperatures much higher than the typical conditions in the current asteroid belt. One proposed mechanism for chondrule heating is the passage through bow shocks of highly eccentric planetesimals in the protoplanetary disk in the early solar system. However, it is difficult for planetesimals to gain and maintain such high eccentricities. In this paper, we present a new scenario in which planetesimals in the asteroid belt region are excited to high eccentricities by the Jovian sweeping secular resonance in a depleting disk, leading to efficient formation of chondrules. We study the orbital evolution of planetesimals in the disk using semi-analytic models and numerical simulations. We investigate the dependence of eccentricity excitation on the planetesimal’s size, as well as the physical environment and the probability for chondrule formation. We find that 50–2000 km planetesimals can obtain eccentricities larger than 0.6 and cause effective chondrule heating. Most chondrules form in high-velocity shocks, in low-density gas, and in the inner disk. The fraction of chondrule precursors that become chondrules is about 4%–9% between 1.5 and 3 au. Our model implies that the disk depletion timescale is τ _dep ≈ 1 Myr, comparable to the age spread of chondrules, and that Jupiter formed before chondrules, no more than 0.7 Myr after the formation of calcium aluminum inclusions.

^1,2Fei Dai,^2,4Kento Masuda,²Joshua N. Winn,³Li Zeng
The Astrophysical Journal 883, 79 Link to Article [DOI
https://doi.org/10.3847/1538-4357/ab3a3b]
¹Department of Physics and Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
²Department of Astrophysical Sciences, Princeton University, 4 Ivy Lane, Princeton, NJ 08544, USA
³Department of Earth & Planetary Sciences, Harvard University, 20 Oxford Street, Cambridge, MA 02138, USA
⁴NASA Sagan Fellow.

Terrestrial planets have been found orbiting Sun-like stars with extremely short periods—some as short as 4 hr. These “ultra-short-period planets” or “hot Earths” are so strongly irradiated that any initial H/He atmosphere has probably been lost to photoevaporation. As such, the sample of hot Earths may give us a glimpse at the rocky cores that are often enshrouded by thick H/He envelopes on wider-orbiting planets. However, the mass and radius measurements of hot Earths have been derived from a hodgepodge of different modeling approaches, and include several cases of contradictory results. Here, we perform a homogeneous analysis of the complete sample of 11 known hot Earths with an insolation exceeding 650 times that of the Earth. We combine all available data for each planet, incorporate parallax information from Gaia to improve the stellar and planetary parameters, and use Gaussian process regression to account for correlated noise in the radial-velocity data. The homogeneous analysis leads to a smaller dispersion in the apparent composition of hot Earths, although there does still appear to be some intrinsic dispersion. Most of the planets are consistent with an Earth-like composition (35% iron and 65% rock), but two planets (K2-141b and K2-229b) show evidence for a higher iron fraction, and one planet (55 Cnc e) has either a very low iron fraction or an envelope of low-density volatiles. All of the planets are less massive than 8 M _⊕, despite the selection bias toward more massive planets, suggesting that 8 M _⊕ is the critical mass for runaway accretion.

¹Tetsuya Yokoyama,^1,2Yuichiro Nagai,^1,2Ryota Fukai,²Takafumi Hirata
The Astrophysical Journal 883, 62 Link to Article [DOI
https://doi.org/10.3847/1538-4357/ab39e7]
¹Department of Earth and Planetary Sciences, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8551, Japan
²Geochemical Research Center, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan

New high-precision Mo isotopic data were obtained for 10 iron meteorites and two carbonaceous, five ordinary, and two rumuruti chondrites. A clear isotopic dichotomy is observed in μ ⁱ Mo−μ ⁹⁴Mo diagrams between the CC meteorites (carbonaceous chondrites and IVB irons) and other noncarbonaceous (NC) meteorites. The Mo isotope variabilities within the CC meteorites can indicate either s-process matter distributed heterogeneously throughout various chondritic components in the different outer solar system materials or that generated by a local parent-body processing. In contrast, the presence of two end-member components for the Mo isotope composition, that is, NC-A and NC-B, was suggested in the NC reservoir. The NC-B component represents the remaining counterpart of the gaseous source reservoir for type B calcium-aluminum-rich inclusions, which was presumably formed via thermal processing that destroyed r-process-rich carriers. Two models were proposed to consider the observed Mo isotope variability among the NCs. In model 1, the NC-A reservoir was formed closer to the Sun than the NC-B reservoir by another thermal processing that destroyed s-process-depleted phases. The Mo isotopic composition of the NC region changed via outward motion of particles from the two reservoirs, resulting in a gradual change from NC-A- to NC-B-like components as a function of the heliocentric distance. In model 2, the Mo isotopic composition in individual NCs is controlled by the amount of metal and matrix-like material that is removed from and added to the NC-B reservoir. Such a fractionation process most likely occurred locally in time and/or space in the inner solar system.

¹Honglei Lin,²Rui Xu,¹Wei Yang,¹Yangting Lin,¹Yong Wei,¹Sen Hu,²Zhiping He,³Le Qiao,¹Weixing Wan
Journal of Geophysical Research, Planets (in Press) Link to Article [https://doi.org/10.1029/2019JE006076]
¹Key Laboratory of Earth and Planetary Physics, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, China
²Key Laboratory of Space Active Opto‐Electronics Technology, Shanghai
Institute of Technical Physics, Chinese Academy of Sciences, Shanghai, China
³Shandong Provincial Key Laboratory of Optical Astronomy and Solar‐Terrestrial Environment, Institute of Space Sciences, Shandong University, Weihai, China
Published by arrangement with John Wiley & Sons

China’s Chang’E‐4 (CE‐4) mission successfully landed in Von Kármán crater within South Pole‐Aitken basin of the Moon. The Visible and Near‐Infrared Imaging Spectrometer (VNIS) on board Yutu‐2 rover investigated the photometric properties of lunar regolith. Seven VNIS measurements were conducted on a small lunar surface with a diameter <5 m by the rover rotating at the center, with the phase angles from 39.6 to 97.1° obtained in the similar observational geometry of solar altitude and observation angle. The phase function, which varies in different wavelength, is derived using a third‐order polynomial fitting, in combination with the calibration and comparison of orbital/in situ VNIS data at the Chang’E‐4 landing site and the same regions. After the photometrical correction of the spectra with the phase function, the derived FeO contents and optical maturity parameters of the regolith reduce much of their deviations, which is consistent with the homogeneity of the regolith and hence demonstrates the significance of the photometric correction on the VNIS spectra.

¹S. A. Singerling,¹A. J. Brearley
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13450]
¹Department of Earth & Planetary Sciences, University of New Mexico, MSC‐03 2040, Albuquerque, New Mexico, 87131 USA
Published by arrangement with John Wiley & Sons

The presence of primary iron sulfides that appear to be aqueously altered in CM and CR carbonaceous chondrites provides the potential to study the effects and, by extension, the conditions of aqueous alteration. In this work, we have used SEM, TEM, and EPMA techniques to characterize primary sulfides that show evidence of secondary alteration. The alteration styles consist of primary pyrrhotite altering to secondary pentlandite (CMs only), magnetite (CMs and CRs), and phyllosilicates (CMs only) in grains that initially formed by crystallization from immiscible sulfide melts in chondrules (pyrrhotite‐pentlandite intergrowth [PPI] grains). Textural, microstructural, and compositional data from altered sulfides in a suite of CM and CR chondrites have been used to constrain the conditions of alteration of these grains and determine their alteration mechanisms. This work shows that the PPI grains exhibit two styles of alteration—one to form porous pyrrhotite‐pentlandite (3P) grains by dissolution of precursor PPI grain pyrrhotite and subsequent secondary pentlandite precipitation (CMs only), and the other to form the altered PPI grains by pseudomorphic replacement of primary pyrrhotite by magnetite (CMs and CRs) or phyllosilicates (CMs only). The range of alteration textures and products is the result of differences in conditions of alteration due to the role of microchemical environments and/or brecciation. Our observations show that primary sulfides are sensitive indicators of aqueous alteration processes in CM and CR chondrites.

¹T.J.Bowling,² B.C.Johnson,³ S.E.Wiggins,⁴E.L.Walton,²H.J.Melosh,⁵T.G.Sharp
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2020.113689]
¹Department of Geophysical Sciences, University of Chicago, United States of America
²Department of Earth, Atmospheric, and Planetary Sciences, Purdue University, United States of America
³Department of Earth, Environmental, and Planetary Sciences, Brown University, United States of America
⁴Department of Physical Sciences, MacEwan University, Canada
⁵School of Earth and Space Exploration, Arizona State University, United States of America
Copyright Elsevier

Martian meteorites are currently the only rock samples from Mars available for direct study in terrestrial laboratories. Linking individual specimens back to their source terrains is a major scientific priority, and constraining the size of the impact craters from which each sample was ejected is a critical step in achieving this goal. During ejection from the surface of Mars by hypervelocity impacts, these meteorites were briefly compressed to high temperatures and pressures. The period of time that these meteorites spent at high pressure during ejection, or the ‘dwell time’, has been used to infer the size of the crater from which they were ejected. This inference requires assumptions that relate shock duration to impactor size, and the relation used by many authors is neither physically motivated nor accurate. Using the iSALE2D shock physics code we simulate vertical impacts at high resolution to investigate the dwell time that basaltic rocks from Mars (shergottites) spend at high pressure and temperature during ejection. Future simulation of oblique impacts will lead to more accurate dwell time estimates. Ultimately, we find that dwell time is insensitive to changes in impact velocity but for a given impact, dwell times are longer for material originating from greater depth and material that experiences higher shock pressures. Using our results, we provide scaling laws for estimating impactor size. During the formation of craters 1.9, 14, and 104 km in diameter, material capable of escaping Mars will have mean dwell times of 1, 10, and 100 ms, respectively.

^1,2Hendrix, A.R.,^1,2Vilas, F.
Geophysical Research Letters 46, 14307-14317 Link to Article [DOI: 10.1029/2019GL085883]
¹Planetary Science Institute, Tucson, AZ, United States
²SSERVI/TREX, Planetary Science Institute, Tucson, AZ, United States

We currently do not have a copyright agreement with this publisher and cannot display the abstract here

^1,2,3Wakita, S.,¹Genda, H.,⁴Kurosawa, K.,⁵Davison, T.M.
Geophysical Research Letters 46, 13678-13686 Link to Article [DOI: 10.1029/2019GL085174]
¹Earth-Life Science Institute, Tokyo Institute of Technology, Meguro, Japan
²Department of Earth, Environmental and Planetary Sciences, Brown University, Providence, RI, United States
³Now at Department of Earth, Atmospheric, and Planetary Sciences, Purdue University, West Lafayette, IN, United States
⁴Planetary Exploration Research Center, Chiba Institute of Technology, Narashino, Japan
⁵Department of Earth Science and Engineering, Imperial College London, London, United Kingdom

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^1,2Stefanik, M.,¹Cesnek, M.,¹Sklenka, L.,³ Kmjec, T.,^1,4Miglierini, M.
Radiation Physics and Chemistry 171, 108675 Link to Article [DOI: 10.1016/j.radphyschem.2019.108675]
¹Czech Technical University in Prague, Faculty of Nuclear Sciences and Physical Engineering, Brehova 7, Prague, 115 19, Czech Republic
²Nuclear Physics Institute of The Czech Academy of Sciences, P.r.i., Rez 130, Rez, 250 68, Czech Republic
³Charles University, Faculty of Mathematics and Physics, Ke Karlovu 3, Prague, 120 00, Czech Republic
⁴Slovak University of Technology in Bratislava, Faculty of Nuclear Engineering and Information Technology, Ilkovicova 3, Bratislava, 812 19, Slovakia

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¹Richard C.Greenwood,²Thomas H.Burbine,¹Ian A.Franchi
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2020.02.004]
¹Planetary and Space Sciences, School of Physical Sciences, The Open University, Walton Hall, Milton Keynes MK7 6AA, United Kingdom
²Department of Astronomy, Mount Holyoke College, South Hadley, MA 01075, USA
Copyright Elsevier

Meteorites provide a unique insight into early Solar System processes. However, to fully interpret this record requires that these meteorites are related back to their source asteroids and ultimately to the original planetesimal population that formed early in Solar System history. As a first step in this process an assessment has been undertaken of the likely number of distinct source asteroids sampled by meteorites and related extraterrestrial materials. The results of this survey indicate that there are between 95 and 148 parent bodies represented in our sample collections. This number has been steadily increasing as new “anomalous” meteorites are characterized. Attempts to link these parent bodies to identified asteroidal sources has so far been of limited success, due to the non-unique reflectance spectra of almost all known asteroids. Asteroid (4) Vesta and the HEDs (howardites, eucrite, diogenite) meteorites is the best example of a relatively non-disputed asteroid-meteorite linkage.

As part of this study the “parent body” concept has been examined and it is found to be a widely, but loosely, used term in the literature to designate “a body that supplies meteorites to Earth.” This concept could be rendered more meaningful by discriminating between primary and secondary parent bodies. A primary parent body is the source asteroid from which the meteorite is ultimately derived, and a secondary parent body is an asteroid derived through impact or break-up of the primary body. A clear example of this usage is provided by (4) Vesta, with the main asteroid being the primary parent body and the Vestoids representing secondary parent bodies. The concept of primary vs. secondary parent bodies may have important implications for early Solar System evolution. Chondritic parent bodies are known to have accreted between 1 and 4 Myr after CAIs. This timing difference may reflect the fact that their source asteroids, particularly those of the carbonaceous chondrites, are secondary bodies, with the original CAI-bearing primary bodies destroyed during early collisional processing.

The number of primary parent bodies represented by meteorites (95 to 148) appears low when compared to the estimated number of asteroids in the main belt (> 100,000 with diameters exceeding ∼2 km). A range of potential reasons may explain this apparent mismatch: i) meteorites provide an unrepresentative sampling of the main belt, ii) the belt may only contain a limited number of primary parent bodies, iii) meteorites may be preferentially derived from the ∼120 identified asteroid families, iv) loosely consolidated types are filtered by Earth’s atmosphere, v) multiple, near-identical, “clone” parent bodies may be present in the belt. At present, it is not possible to determine which of these potential mechanisms are dominant and all may be operating to a greater or lesser extent.

Based on classical accretion models the meteorite record appears to be highly unrepresentative of the primordial asteroid population. In contrast, pebble accretion models suggest that these first-generation bodies may have been relatively large, in which case meteorites may provide a more unbiased record of early Solar System processes.