Francesco C. Pignatale¹, Sébastien Charnoz^1,2, Pascal Rosenblatt^3,4, Ryuki Hyodo⁵, Tomoki Nakamura⁶, and Hidenori Genda⁵

Astrophysical Journal 853, 118 Link to Article [DOI: 10.3847/1538-4357/aaa23e]
¹Institut de Physique du Globe de Paris (IPGP), 1 rue Jussieu, F-75005, Paris, France
²Institut de Physique du Globe/Universite Paris Diderot/CEA/CNRS, F-75005 Paris, France
³Royal Observatory of Belgium, Avenue circulaire 3, B-1180 Uccle, Belgium
⁴Now at ACRI-ST, 260 route du pin-montard-BP 234, F-06904 Sophia-Antipolis Cedex, France
⁵Earth-Life Science Institute/Tokyo Institute of Technology, 152-8550 Tokyo, Japan
⁶Tohoku University, 980-8578 Miyagi, Japan

The origin of Phobos and Deimos in a giant impact-generated disk is gaining larger attention. Although this scenario has been the subject of many studies, an evaluation of the chemical composition of the Mars’s moons in this framework is missing. The chemical composition of Phobos and Deimos is unconstrained. The large uncertainties about the origin of the mid-infrared features; the lack of absorption bands in the visible and near-infrared spectra; and the effects of secondary processes on the moons’ surfaces make the determination of their composition very difficult using remote sensing data. Simulations suggest a formation of a disk made of gas and melt with their composition linked to the nature of the impactor and Mars. Using thermodynamic equilibrium, we investigate the composition of dust (condensates from gas) and solids (from a cooling melt) that result from different types of Mars impactors (Mars-, CI-, CV-, EH-, and comet-like). Our calculations show a wide range of possible chemical compositions and noticeable differences between dust and solids, depending on the considered impactors. Assuming that Phobos and Deimos resulted from the accretion and mixing of dust and solids, we find that the derived assemblage (dust-rich in metallic iron, sulfides and/or carbon, and quenched solids rich in silicates) can be compatible with the observations. The JAXA’s Martian Moons eXploration (MMX) mission will investigate the physical and chemical properties of Phobos and Deimos, especially sampling from Phobos, before returning to Earth. Our results could be then used to disentangle the origin and chemical composition of the pristine body that hit Mars and suggest guidelines for helping in the analysis of the returned samples.

Cui Wenyuan^1,2, Jiang Xiaohua¹, Shi Jianrong^3,4, Zhao Gang^3,4, and Zhang Bo¹

Astrophysical Journal 854, 131 Link to Article [DOI: 10.3847/1538-4357/aaa75f]
¹Department of Physics, Hebei Normal University, Shijiazhuang 050024, People’s Republic of China
²School of Space Science and Physics, Shandong University at Weihai, Weihai 264209, People’s Republic of China
³Key Lab of Astronomy, National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100012, People’s Republic of China
⁴School of Astronomy and Space Science, University of Chinese Academy of Sciences, Beijing 100049, People’s Republic of China

The origin of Phobos and Deimos in a giant impact-generated disk is gaining larger attention. Although this scenario has been the subject of many studies, an evaluation of the chemical composition of the Mars’s moons in this framework is missing. The chemical composition of Phobos and Deimos is unconstrained. The large uncertainties about the origin of the mid-infrared features; the lack of absorption bands in the visible and near-infrared spectra; and the effects of secondary processes on the moons’ surfaces make the determination of their composition very difficult using remote sensing data. Simulations suggest a formation of a disk made of gas and melt with their composition linked to the nature of the impactor and Mars. Using thermodynamic equilibrium, we investigate the composition of dust (condensates from gas) and solids (from a cooling melt) that result from different types of Mars impactors (Mars-, CI-, CV-, EH-, and comet-like). Our calculations show a wide range of possible chemical compositions and noticeable differences between dust and solids, depending on the considered impactors. Assuming that Phobos and Deimos resulted from the accretion and mixing of dust and solids, we find that the derived assemblage (dust-rich in metallic iron, sulfides and/or carbon, and quenched solids rich in silicates) can be compatible with the observations. The JAXA’s Martian Moons eXploration (MMX) mission will investigate the physical and chemical properties of Phobos and Deimos, especially sampling from Phobos, before returning to Earth. Our results could be then used to disentangle the origin and chemical composition of the pristine body that hit Mars and suggest guidelines for helping in the analysis of the returned samples.

Takashi Ishihara¹, Naoki Kobayashi², Kei Enohata², Masayuki Umemura³, and Kenji Shiraishi⁴

Astrophysical Journal 854, 81 Link to Article [DOI: 10.3847/1538-4357/aaa976]
¹Graduate School of Environmental and Life Science, Okayama University, Okayama 700-8530, Japan
²Department of Computational Science and Engineering, Graduate School of Engineering, Nagoya University, Nagoya 464-8603, Japan
³Center for Computational Sciences, University of Tsukuba, Tsukuba 305-8577, Japan
⁴Institute of Materials and Systems for Sustainability, Nagoya University, Nagoya 464-8601, Japan

The coagulation of dust particles is a key process in planetesimal formation. However, the radial drift and bouncing barriers are not completely resolved, especially for silicate dust. Since the collision velocities of dust particles are regulated by turbulence in a protoplanetary disk, turbulent clustering should be properly treated. To that end, direct numerical simulations (DNSs) of the Navier–Stokes equations are requisite. In a series of papers, Pan & Padoan used a DNS with Reynolds number Re ~ 1000. Here, we perform DNSs with up to Re = 16,100, which allow us to track the motion of particles with Stokes numbers of 0.01  St  0.2 in the inertial range. Through the DNSs, we confirm that the rms relative velocity of particle pairs is smaller by more than a factor of two, compared to that by Ormel & Cuzzi. The distributions of the radial relative velocities are highly non-Gaussian. The results are almost consistent with those by Pan & Padoan or Pan et al. at low Re. Also, we find that the sticking rates for equal-sized particles are much higher than those for different-sized particles. Even in the strong-turbulence case with α-viscosity of 10⁻², the sticking rates are as high as 50% and the bouncing probabilities are as low as ~10% for equal-sized particles of St  0.01. Thus, turbulent clustering plays a significant role in the growth of centimeter-sized compact aggregates (pebbles) and also enhances the solid abundance, which may lead to the streaming instability in a disk.

Lev Arzamasskiy¹ and Roman R. Rafikov^2,3

Astrophysical Journal 854, 84 Link to Article [DOI: 10.3847/1538-4357/aaa8e8]
¹Department of Astrophysical Sciences, Princeton University, Ivy Lane, Princeton, NJ 08540, USA
²Centre for Mathematical Sciences, Department of Applied Mathematics and Theoretical Physics, University of Cambridge, Wilberforce Road, Cambridge CB3 0WA, UK
³Institute for Advanced Study, Einstein Drive, Princeton, NJ 08540, USA

Spiral density waves are known to exist in many astrophysical disks, potentially affecting disk structure and evolution. We conduct a numerical study of the effects produced by a density wave, evolving into a shock, on the characteristics of the underlying disk. We measure the deposition of angular momentum in the disk by spiral shocks of different strengths and verify the analytical prediction of Rafikov for the behavior of this quantity, using shock amplitude (which is potentially observable) as the input variable. Good agreement between theory and numerics is found as we vary the shock amplitude (including highly nonlinear shocks), disk aspect ratio, equation of state, radial profiles of the background density and temperature, and pattern speed of the wave. We show that high numerical resolution is required to properly capture shock-driven transport, especially at small wave amplitudes. We also demonstrate that relating the local mass-accretion rate to shock dissipation in rapidly evolving disks requires accounting for the time-dependent contribution to the angular momentum budget caused by the time dependence of the radial pressure support. We provide a simple analytical prescription for the behavior of this contribution and demonstrate its excellent agreement with the simulation results. Using these findings, we formulate a theoretical framework for studying the one-dimensional (in radius) evolution of shock-mediated accretion disks, which can be applied to a variety of astrophysical systems.

Kunihiro Myojo¹, Tetsuya Yokoyama¹, Satoki Okabayashi¹, Shigeyuki Wakaki², Naoji Sugiura³, and Hikaru Iwamori^1,4
Astrophysical Journal 853, 48 Link to Article [DOI: 10.3847/1538-4357/aa9f2e]
¹Department of Earth and Planetary Sciences, Tokyo Institute of Technology, Meguro, Tokyo 152-8551, Japan
²Kochi Institute for Core Sample Research, Japan Agency for Marine-Earth Science and Technology, 200 Monobe Otsu, Nankoku City, Kochi, Japan
³Emeritus professor, Department of Earth and Planetary Science, University of Tokyo, Japan
⁴Japan Agency for Marine-Earth Science and Technology, Japan

Nucleosynthetic isotope anomalies in meteorites are useful for investigating the origin of materials in the protoplanetary disk and dynamical processes of planetary formation. In particular, calcium and aluminum-rich inclusions (CAIs) found in chondrites are key minerals for decoding the initial conditions of the solar system before the accretion of small planetary bodies. In this study, we report isotopic analyses for three Allende CAIs, fluffy type A (FTA), type B, and fine-grained spinel rich (FS) inclusions, with a specific emphasis on the measurements of ⁸⁴Sr/⁸⁶Sr ratios. It was found that the average μ ⁸⁴Sr values (10⁶ relative deviations from a standard material) were 175, 129, and 56 ppm for the samples of FTA, type B, and FS inclusions, respectively. Additionally, the FTA samples exhibited heterogeneous μ ⁸⁴Sr values, while those for the type B and FS inclusions were homogeneous within individual inclusions. The elevated μ ⁸⁴Sr values were most likely explained by the relative enrichment of r-process nuclides in the CAI formation region. The variation of μ ⁸⁴Sr values between the FTA and type B inclusions, as well as within the FTA inclusion, suggests the presence of multiple CAI source reservoirs with distinct isotopic compositions, which is either inherited from isotopic heterogeneity in the molecular cloud or caused by the selective destruction of r-process-enriched supernova grains via nebular thermal processing. On the other hand, the reaction between a refractory precursor of the FS inclusion and a gaseous reservoir enriched in Mg, Si, and ¹⁶O resulted in the lowest μ ⁸⁴Sr values for the FS inclusion.

J. Escobar-Cerezo¹, A. Penttilä², T. Kohout^2,3, O. Muñoz¹, F. Moreno¹, and K. Muinonen^2,4
Astrophysical Journal 853, 71 Link to Article [DOI: 10.3847/1538-4357/aaa24d]
¹Instituto de Astrofísica de Andalucía, CSIC, Glorieta de la Astronomía s/n, E-18008 Granada, Spain
²Department of Physics, P.O. Box 64, FI-00014 University of Helsinki, Finland
³Institute of Geology, The Czech Academy of Sciences, Prague, Czech Republic
⁴National Land Survey of Finland, Finnish Geospatial Research Institute, P.O. Box 84, FI-00521 Helsinki, Finland

Lunar soil spectra differ from pulverized lunar rocks spectra by reddening and darkening effects, and shallower absorption bands. These effects have been described in the past as a consequence of space weathering. In this work, we focus on the effects of nanophase iron (npFe⁰) inclusions on the experimental reflectance spectra of lunar regolith particles. The reflectance spectra are computed using SIRIS3, a code that combines ray optics with radiative-transfer modeling to simulate light scattering by different types of scatterers. The imaginary part of the refractive index as a function of wavelength of immature lunar soil is derived by comparison with the measured spectra of the corresponding material. Furthermore, the effect of adding nanophase iron inclusions on the reflectance spectra is studied. The computed spectra qualitatively reproduce the observed effects of space weathered lunar regolith.

Alan M. Gersch, Lori M. Feaga, and Michael F. A’Hearn¹
Astrophysical Journal 854, 149 Link to Article [DOI: 10.3847/1538-4357/aa9795]
Department of Astronomy, University of Maryland, College Park, MD 20742-2421, USA
¹Deceased.

We have adapted Coupled Escape Probability, a new exact method of solving radiative transfer problems, for use in asymmetrical spherical situations for use in modeling optically thick cometary comae. Here we present the extension of our model and corresponding results for two additional primary volatile species of interest, H₂O and CO₂, in purely theoretical comets. We also present detailed modeling and results for the specific examples of CO, H₂O, and CO₂ observations of C/2009 P1 Garradd by the Deep Impact flyby spacecraft.

Rainer Schräpler¹, Jürgen Blum¹, Sebastiaan Krijt^2,3, and Jan-Hendrik Raabe¹
Astrophysical Journal 853, 74 Link to Article [DOI: 10.3847/1538-4357/aaa0d2]
¹Institut für Geophysik und extraterrestrische Physik, Technische Universität Braunschweig, Mendelssohnstr. 3, D-38106 Braunschweig, Germany
²Department of the Geophysical Sciences, The University of Chicago, 5734 South Ellis Avenue, Chicago, IL 60637, USA
³Hubble Fellow.

In a protoplanetary disk, dust aggregates in the μm to mm size range possess mean collision velocities of 10–60 m s⁻¹ with respect to dm- to m-sized bodies. We performed laboratory collision experiments to explore this parameter regime and found a size- and velocity-dependent threshold between erosion and growth. By using a local Monte Carlo coagulation calculation and along with a simple semi-analytical timescale approach, we show that erosion considerably limits particle growth in protoplanetary disks and leads to a steady-state dust-size distribution from μm- to dm-sized particles.

Erik Weaver, Andrea Isella, and Yann Boehler
Astrophysical Journal 853, 113 Link to Article [DOI: 10.3847/1538-4357/aaa481]
Department of Physics and Astronomy, Rice University, 6100 Main Street, MS-108, Houston, TX 77005 USA

The accurate measurement of temperature in protoplanetary disks is critical to understanding many key features of disk evolution and planet formation, from disk chemistry and dynamics, to planetesimal formation. This paper explores the techniques available to determine temperatures from observations of single, optically thick molecular emission lines. Specific attention is given to issues such as the inclusion of optically thin emission, problems resulting from continuum subtraction, and complications of real observations. Effort is also made to detail the exact nature and morphology of the region emitting a given line. To properly study and quantify these effects, this paper considers a range of disk models, from simple pedagogical models to very detailed models including full radiative transfer. Finally, we show how the use of the wrong methods can lead to potentially severe misinterpretations of data, leading to incorrect measurements of disk temperature profiles. We show that the best way to estimate the temperature of emitting gas is to analyze the line peak emission map without subtracting continuum emission. Continuum subtraction, which is commonly applied to observations of line emission, systematically leads to underestimation of the gas temperature. We further show that once observational effects such as beam dilution and noise are accounted for, the line brightness temperature derived from the peak emission is reliably within 10%–15% of the physical temperature of the emitting region, assuming optically thick emission. The methodology described in this paper will be applied in future works to constrain the temperature, and related physical quantities, in protoplanetary disks observed with ALMA.

N. G. Rudraswami¹, D. Fernandes¹, A. K. Naik¹, M. Shyam Prasad¹, J. D. Carrillo-Sánchez², J. M. C. Plane², W. Feng^2,3, and S. Taylor⁴
Astrophysical Journal 853, 38 Link to Article [DOI: 10.3847/1538-4357/aaa5f7]
¹National Institute of Oceanography (Council of Scientific and Industrial Research), Dona Paula, Goa 403004, India
²School of Chemistry, University of Leeds, Leeds LS2 9JT, UK
³NCAS, School of Earth and Environment, University of Leeds, Leeds, LS2 9JT, UK
⁴Cold Regions Research and Engineering Laboratory, 72 Lyme Road, Hanover, NH 03755–1290, USA

All known extraterrestrial dust (micrometeoroids) entering the Earth’s atmosphere is anticipated to have a significant contribution from ordinary chondritic precursors, as seen in meteorites, but this is an apparent contradiction that needs to be addressed. Ordinary chondrites represent a minor contribution to the overall meteor influx compared to carbonaceous chondrites, which are largely dominated by CI and/or CM chondrites. However, the near-Earth asteroid population presents a scenario with sufficient scope for generation of dust-sized debris from ordinary chondritic sources. The bulk chemical composition of 3255 micrometeorites (MMs) collected from Antarctica and deep-sea sediments has shown Mg/Si largely dominated by carbonaceous chondrites, and less than 10% having ordinary chondritic precursors. The chemical ablation model is combined with different initial chondritic compositions (CI, CV, L, LL, H), and the results clearly indicate that high-density (≥2.8 g cm⁻³) precursors, such as CV and ordinary chondrites in the size range 100–700 μm and zenith angle 0°–70°, ablate at much faster rates and lose their identity even before reaching the Earth’s surface and hence are under-represented in our collections. Moreover, their ability to survive as MMs remains grim for high-velocity micrometeoroids (>16 km s⁻¹). The elemental ratio for CV and ordinary chondrites are also similar to each other irrespective of the difference in the initial chemical composition. In conclusion, MMs belonging to ordinary chondritic precursors’ concentrations may not be insignificant in thermosphere, as they are found on Earth’s surface.