Sean N. Raymond¹, Philip J. Armitage^2,3, and Dimitri Veras^4,5
Astrophysical Journal Letters 856, L7 Link to Article [DOI: 10.3847/2041-8213/aab4f6]
¹Laboratoire d’astrophysique de Bordeaux, Univ. Bordeaux, CNRS, B18N, allée Geoffroy Saint-Hilaire, 33615 Pessac, France
²JILA, University of Colorado and NIST, 440 UCB, Boulder, CO 80309-0440, USA
³Department of Astrophysical & Planetary Sciences, University of Colorado, Boulder, CO 80309-0391, USA
⁴Department of Physics, University of Warwick, Coventry CV4 7AL, UK
⁵Centre for Exoplanets and Habitability, University of Warwick, Coventry CV4 7AL, UK

‘Oumuamua was discovered passing through our solar system on a hyperbolic orbit. It presents an apparent contradiction, with colors similar to those of volatile-rich solar system bodies but with no visible outgassing or activity during its close approach to the Sun. Here, we show that this contradiction can be explained by the dynamics of planetesimal ejection by giant planets. We propose that ‘Oumuamua is an extinct fragment of a comet-like planetesimal born a planet-forming disk that also formed Neptune- to Jupiter-mass giant planets. On its pathway to ejection ‘Oumuamua’s parent body underwent a close encounter with a giant planet and was tidally disrupted into small pieces, similar to comet Shoemaker–Levy 9’s disruption after passing close to Jupiter. We use dynamical simulations to show that 0.1%–1% of cometary planetesimals undergo disruptive encounters prior to ejection. Rocky asteroidal planetesimals are unlikely to disrupt due to their higher densities. After disruption, the bulk of fragments undergo enough close passages to their host stars to lose their surface volatiles and become extinct. Planetesimal fragments such as ‘Oumuamua contain little of the mass in the population of interstellar objects but dominate by number. Our model makes predictions that will be tested in the coming decade by the Large Synoptic Survey Telescope.

Bethany L. EHLMANN^1,2, Robert HODYSS², Thomas F. BRISTOW³, George R. ROSSMAN¹,Eleonora AMMANNITO^4,6, M. Cristina DESANCTIS⁵, and Carol A. RAYMOND²
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13103]
¹Division of Geological & Planetary Sciences, California Institute of Technology, Pasadena, California 91125, USA
²Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California 91125, USA
³Exobiology Branch, NASA Ames Research Center, Moffett Field, California 94035, USA
⁴Department of Earth Planetary and Space Sciences, University of California, Los Angeles, California 90095–1567, USA
⁵Istituto di Astrofisica e Planetologia Spaziali, Istituto Nazionale di Astrofisica, 00133 Rome, Italy
⁶Present address: Italian Space Agency (ASI), 00133 Rome, Italy
Published by arrangement with John Wiley & Sons

Mg‐phyllosilicate‐bearing, dark surface materials on the dwarf planet Ceres have NH₄‐bearing materials, indicated by a distinctive 3.06 μm absorption feature. To constrain the identity of the Ceres NH₄‐carrier phase(s), we ammoniated ground particulates of candidate materials to compare their spectral properties to infrared data acquired by Dawn’s Visible and Infrared (VIR) imaging spectrometer. We treated Mg‐, Fe‐, and Al‐smectite clay minerals; Mg‐serpentines; Mg‐chlorite; and a suite of carbonaceous meteorites with NH₄‐acetate to exchange ammonium. Serpentines and chlorites showed no evidence for ammoniation, as expected due to their lack of exchangeable interlayer sites. Most smectites showed evidence for ammoniation by incorporation of NH₄⁺ into their interlayers, resulting in the appearance of absorptions from 3.02 to 3.08 μm. Meteorite samples tested had weak absorptions between 3.0 and 3.1 μm but showed little clear evidence for enhancement upon ammoniation, likely due to the high proportion of serpentine and other minerals relative to expandable smectite phases or to NH₄⁺ complexing with organics or other constituents. The wavelength position of the smectite NH₄ absorption showed no variation between IR spectra acquired under dry‐air purge at 25 °C and under vacuum at 25 °C to −180 °C. Collectively, data from the smectite samples show that the precise center wavelength of the characteristic ~3.05 μm v₃absorption in NH₄ is variable and is likely related to the degree of hydrogen bonding of NH₄‐H₂O complexes. Comparison with Dawn VIR spectra indicates that the hypothesis of Mg‐saponite as the ammonium carrier phase is the simplest explanation for observed data, and that Ceres dark materials may be like Cold Bokkeveld or Tagish Lake but with proportionally more Mg‐smectite.

N. G. RUDRASWAMI¹, D. FERNANDES¹, Agnelo DE ARAUJO¹, M. SHYAM PRASAD¹,and S. TAYLOR²
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13105]
¹National Institute of Oceanography (Council of Scientific and Industrial Research), Dona Paula, Goa 403004, India
²Cold Regions Research and Engineering Laboratory, 72 Lyme Road, Hanover, New Hampshire 03755–1290, USA
Published by arrangement with John Wiley & Sons

We studied 149 pyroxenes from 69 pyroxene‐bearing micrometeorites collected from deep‐sea sediments of the Indian Ocean and South Pole Water Well at Antarctica, Amundsen‐Scott South Pole station. The minor elements in pyroxenes from micrometeorites are present in the ranges as follows: MnO ~0.0–0.4 wt%, Al₂O₃ ~0.0–1.5 wt%, CaO ~0.0–1.0 wt%, Cr₂O₃ ~0.3–0.9 wt%, and FeO ~0.5–4 wt%. Their chemical compositions suggest that pyroxene‐bearing micrometeorites are mostly related to precursors from carbonaceous chondrites rather than ordinary chondrites. The Fe/(Fe+Mg) ratio of the pyroxenes and olivines in micrometeorites shows similarities to carbonaceous chondrites with values lying between 0 and 0.2, and those with values beyond this range are dominated by ordinary chondrites. Atmospheric entry of the pyroxene‐bearing micrometeorites is expected to have a relatively low entry velocity of <16 km s⁻¹ and high zenith angle (70–90°) to preserve their chemical compositions. In addition, similarities in the pyroxene and olivine mineralogical compositions between carbonaceous chondrites and cometary particles suggest that dust in the solar system is populated by materials from different sources that are chemically similar to each other. Our results on pyroxene chemical compositions reveal significant differences with those from ordinary chondrites. The narrow range in olivine and pyroxene chemical compositions are similar to those from carbonaceous chondrites, and a small proportion to ordinary chondrites indicates that dust is largely sourced from carbonaceous chondrite‐type bodies.

Xiao Hu (胡晓)^1,2, Jonathan C. Tan^1,3, Zhaohuan Zhu (朱照寰)², Sourav Chatterjee⁴, Tilman Birnstiel⁵, Andrew N. Youdin⁶, and Subhanjoy Mohanty⁷

Astrophysical Journal 857, 20 Link to Article [DOI: 10.3847/1538-4357/aaad08]
¹Department of Astronomy, University of Florida, Gainesville, FL 32611, USA
²Department of Physics and Astronomy, University of Nevada, Las Vegas, 4505 South Maryland Parkway, Las Vegas, NV 89154, USA
³Department of Physics, University of Florida, Gainesville, FL 32611, USA
⁴Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA), Physics and Astronomy, Northwestern University, Evanston, IL 60208, USA
⁵University Observatory, Faculty of Physics, Ludwig-Maximilians-Universität München, Scheinerstr. 1, D-81679 Munich, Germany
⁶Department of Astronomy and Steward Observatory, University of Arizona, 933 North Cherry Avenue, Tucson, AZ 85721, USA
⁷Imperial College London, 1010 Blackett Lab, Prince Consort Rd., London SW7 2AZ, UK

Systems with tightly packed inner planets (STIPs) are very common. Chatterjee & Tan proposed Inside-out Planet Formation (IOPF), an in situ formation theory, to explain these planets. IOPF involves sequential planet formation from pebble-rich rings that are fed from the outer disk and trapped at the pressure maximum associated with the dead zone inner boundary (DZIB). Planet masses are set by their ability to open a gap and cause the DZIB to retreat outwards. We present models for the disk density and temperature structures that are relevant to the conditions of IOPF. For a wide range of DZIB conditions, we evaluate the gap-opening masses of planets in these disks that are expected to lead to the truncation of pebble accretion onto the forming planet. We then consider the evolution of dust and pebbles in the disk, estimating that pebbles typically grow to sizes of a few centimeters during their radial drift from several tens of astronomical units to the inner, 1 au scale disk. A large fraction of the accretion flux of solids is expected to be in such pebbles. This allows us to estimate the timescales for individual planet formation and the entire planetary system formation in the IOPF scenario. We find that to produce realistic STIPs within reasonable timescales similar to disk lifetimes requires disk accretion rates of ~10⁻⁹ M _⊙ yr⁻¹ and relatively low viscosity conditions in the DZIB region, i.e., a Shakura–Sunyaev parameter of α ~ 10⁻⁴.