Using the density of Kuiper Belt Objects to constrain their composition and formation history

1C.J.Bierson,1F.Nimmo
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2019.01.027]
1Department of Earth and Planetary Sciences, UC Santa Cruz, Santa Cruz, CA 95064, USA
Copyright Elsevier

Telescopic observations of Kuiper Belt objects have enabled bulk density determinations for 17 objects. These densities vary systematically with size, perhaps suggesting systematic variations in bulk composition. We find this trend can be explained instead by variations in porosity arising from the higher pressures and warmer temperatures in larger objects. We are able to match the density of 14 of 17 KBOs within their 2σ errors with a constant rock mass fraction of 70%, suggesting a compositionally homogeneous, rock-rich reservoir. Because early 26Al would have removed too much porosity in small (∼ 100 km) KBOs we find the minimum formation time to be 4 Myr after solar system formation. This suggests that coagulation, and not gravitational collapse, was the dominant mechanism for KBO formation, or the gas disk lingered in the outer solar system. We also use this model to make predictions for the density of Makemake, 2007 OR10, and MU69

Effects of the Isothermal Region in Protoplanetary Disks and the Protostar Irradiation on the Disk Instability Model for Giant Planet Formation

Pengfei Tang and Liping Jin
Astrophysical Journal 871, 222 Link to Article [DOI: 10.3847/1538-4357/aafb6f ]
College of Physics, Jilin University Changchun, Jilin 130012, People’s Republic of China

We construct an analytical model of gravitationally unstable protoplanetary disks consisting of three regions: the inner region where the internal dissipation dominates the heating, the intermediate region where the central protostar irradiation dominates, and the outer region where background irradiation dominates. We use this analytical model and an evolutionary numerical model of protoplanetary disks to calculate the cooling time and find out the location of the isothermal region. We investigate the effects of the isothermal region on the disk instability model for giant planet formation. We find that the fragmentation region found in previous studies is contained in the isothermal region of a disk. In this case, the cooling time criterion is not applicable for fragmentation. Therefore, the constraint on the disk instability model caused by the cooling time criterion should be relieved. The viability of the disk instability model is improved. When the isothermal region is considered, the inner boundary of the fragmentation region is extended inward to ~20 au. We also show that if the contribution of the protostar irradiation to the disk surface temperature can be included in the cooling rate, the fragmentation region defined by the cooling time criterion can be extended inward to ~26 au. We find that a disk tends to be isothermal in the region where the cooling time criterion is satisfied. We also find that at the later stage of disk instability, the inner boundary of the fragmentation region is determined by the inner boundary of the gravitationally unstable region.

Mapping the Conditions for Hydrodynamic Instability on Steady-State Accretion Models of Protoplanetary Disks

Thomas Pfeil and Hubert Klahr
Astrophysical Journal 871, 150 Link to Article [DOI: 10.3847/1538-4357/aaf962 ]
Max-Planck-Institut für Astronomie, Königstuhl 17, D-69117 Heidelberg, Germany

Hydrodynamic instabilities in disks around young stars depend on the thermodynamic stratification of the disk and on the local rate of thermal relaxation. Here, we map the spatial extent of unstable regions for the Vertical Shear Instability (VSI), the Convective Overstability (COS), and the amplification of vortices via the Subcritical Baroclinic Instability (SBI). We use steady-state accretion disk models, including stellar irradiation, accretion heating, and radiative transfer. We determine the local radial and vertical stratification and thermal relaxation rate in the disk, which depends on the stellar mass, disk mass, and mass accretion rate. We find that passive regions of disks—that is, the midplane temperature dominated by irradiation—are COS unstable about one pressure scale height above the midplane and VSI unstable at radii >10 au. Vortex amplification via SBI should operate in most parts of active and passive disks. For active parts of disks (midplane temperature determined by accretion power), COS can become active down to the midplane. The same is true for the VSI because of the vertically adiabatic stratification of an internally heated disk. If hydrodynamic instabilities or other nonideal MHD processes are able to create α-stresses (>10−5) and released accretion energy leads to internal heating of the disk, hydrodynamic instabilities are likely to operate in significant parts of the planet-forming zones in disks around young stars, driving gas accretion and flow structure formation. Thus, hydrodynamic instabilities are viable candidates to explain the rings and vortices observed with the Atacama Large Millimeter/submillimeter Array and Very Large Telescope.

Petrogenesis and In Situ U‐Pb Geochronology of a Strongly Shocked L‐Melt Rock Northwest Africa 11042

1,2Yunhua Wu,3,4Weibiao Hsu
Journal of Geophysical Research, Planets (in Press) Link to Article [https://doi.org/10.1029/2018JE005743]
1Purple Mountain Observatory, Chinese Academy of Sciences, Nanjing, China
2University of Chinese Academy of Sciences, Beijing, China
3The State Key Laboratory of Planetary Science, Macau University of Science and Technology, Taipa, China
4CAS Center for Excellence in Comparative Planetology, China
Published by arrangement with John Wiley & Sons

Northwest Africa (NWA) 11042, originally classified as a primitive achondrite with no chondritic relicts, is rather a unique L‐melt rock. It is a severely shocked, igneous‐textured ultramafic rock composed of euhedral to subhedral olivine (Fa25.1±0.5) and pyroxenes (Low‐Ca pyroxene Fs20.7±0.8Wo4.2±1.0, and Ca‐rich pyroxene Fs11.5±0.5Wo37.6±1.2) with interstitial albitic plagioclase (Ab80.7±1.7Or5.0±0.7) that has been completely converted to maskelynite. Mineral compositions are similar to those of equilibrated L chondrites. Melt pockets are scattered throughout the sample, containing high‐pressure minerals including ringwoodite, wadsleyite, jadeite, and lingunite. Merrillite and apatite in NWA 11042 contain significantly higher REE abundances than those of ordinary chondrites, indicative of igneous fractional crystallization. In situ U‐Pb dating of apatite in NWA 11042 reveals an upper intercept age of 4479±43 Myr and a lower intercept age of 465±47 Myr on the normal U‐Pb concordia diagram. The upper intercept age recorded the time when NWA 11042 initially crystallized. This age is much younger than when the decay of short‐lived nuclides (e.g., 26Al) would act as a major heat source, suggesting melting and crystallization of NWA 11042 could be otherwise triggered by an impact event. The lower intercept age represents a reset age due to a later impact event, that is in coincidence with the disruption event of L chondrite parent body at ~470 Myr. NWA 11042 is an excellent example to link igneous‐textured meteorites with a chondritic parent body through shock‐induced melting.

Particle Size Effects on Mid‐IR Spectra of Lunar Analog Minerals in a Simulated Lunar Environment

1K. A. Shirley,1T. D. Glotch
Journal of Geophysical Research, Planets (in Press) Link to Article [https://doi.org/10.1029/2018JE005533]
1Department of Geosciences, Stony Brook University, Stony Brook, New York, USA
Published by arrangement with John Wiley & Sons

Mid‐infrared spectroscopic analysis of the Moon and other airless bodies requires a full accounting of spectral variation due to the unique thermal environment in airless body regoliths and the substantial differences between spectra acquired under airless body conditions and those measured in an ambient environment on Earth. Because there exists a thermal gradient within the upper 100s of microns of lunar regolith, the data acquired by the Diviner Lunar Radiometer Experiment are not isothermal with wavelength. While this complication has been previously identified, its effect on other known variables that contribute to spectral variation, such as particle size and porosity, have yet to be well characterized in the laboratory. Here we examine the effect of particle size on mid‐infrared spectra of silicates common to the Moon measured within a simulated lunar environment chamber. Under simulated lunar conditions, decreasing particle size is shown to enhance the spectral contrast of the Reststrahlen bands and transparency features, as well as shift the location of the Christiansen feature to longer wavelengths. This study shows that these variations are detectable at Diviner spectral resolution, and emphasizes the need for simulated environment laboratory datasets, as well as hyperspectral mid‐infrared instruments on future missions to airless bodies.

The Deposition of Heavy Elements in Giant Protoplanetary Atmospheres: The Importance of Planetesimal–Envelope Interactions

Claudio Valletta and Ravit Helled
Astrophysical Journal 871, 127 Link to Article [DOI: 10.3847/1538-4357/aaf427 ]
Center for Theoretical Astrophysics and Cosmology Institute for Computational Science, University of Zürich, Winterthurerstrasse 190, 8057 Zürich, Switzerland

In the standard model for giant planet formation, the planetary growth begins with accretion of solids, followed by a buildup of a gaseous atmosphere as more solids are accreted and, finally, by rapid accretion of gas. The interaction of the solids with the gaseous envelope determines the subsequent planetary growth and the final internal structure. In this work we simulate the interaction of planetesimals with a growing giant planet (proto-Jupiter) and investigate how different treatments of the planetesimal–envelope interaction affect the heavy-element distribution and the inferred core mass. We consider various planetesimal sizes and compositions, as well as different ablation and radiation efficiencies and fragmentation models. We find that in most cases the core reaches a maximum mass of ~2 M . We show that the value of the core’s mass mainly depends on the assumed size and composition of the solids, while the heavy-element distribution is also affected by the fate of the accreted planetesimals (ablation/fragmentation). Fragmentation, which is found to be important for planetesimals >1 km, typically leads to enrichment of the inner part of the envelope, while ablation results in enrichment of the outer atmosphere. Finally, we present a semianalytical prescription for deriving the heavy-element distribution in giant protoplanets.

Shock-generating Planetesimals Perturbed by a Giant Planet in a Gas Disk

M. Nagasawa1, K. K. Tanaka2, H. Tanaka2, H. Nomura3, T. Nakamoto3, and H. Miura4
Astrophysical Journal 871, 110 Link to Article [DOI: 10.3847/1538-4357/aaf795 ]
1Department of Physics, School of Medicine, Kurume University, 67 Asahi-machi, Kurume-city, Fukuoka 830-0011, Japan
2Astronomical Institute, Tohoku University, 6-3 Aramaki, Aoba-ku Sendai, Miyagi 980-8578, Japan
3Department of Earth and Planetary Sciences, Tokyo Institute of Technology, 2-12-1, Ookayama, Meguro-ku, Tokyo 152-8551, Japan
4Graduate School of Natural Sciences, Nagoya City University, Yamanohata 1, Mizuho-cho, Mizuho-ku, Nagoya 467-8501, Japan

We examined the excitations of planetesimals caused by the resonances of a giant planet in a protoplanetary gas disk. The highly excited planetesimals generate bow shocks, the mechanism of which results in chondrule formation, crystallization of silicate dust, and evaporation of icy planetesimals. The planetesimals beyond 2:1 resonance migrate owing to the gas drag and obtain the maximum eccentricity around 3:1 resonance, which is located at approximately half the planetary distance. The eccentricity depends on the parameters of the planetesimals and the Jovian planet, such as size and location, and the gas density of the disk. The maximum relative velocity of a 100 km sized planetesimal with respect to the gas disk reaches up to ~12 km s−1 in the case of Jupiter owing to secular resonance, which occurs because of the disk’s gravity. We find that if a Jovian-mass planet is located within 10 au, the planetesimals larger than 100 km gain sufficient velocity to cause the melting of chondrule precursors and crystallization of the silicate. The maximum velocity is higher for large planetesimals and eccentric planets. Planetesimals are trapped temporarily in the resonances and continue to have high speed over gsim1 Myr after the formation of a Jovian planet. This duration fits into the timescale of chondrule formation suggested by the isotopic data. The evaporation of icy planetesimals occurs when a Jovian planet is located within 15 au. This mechanism can be a new indicator of planet formation in exosystems if some molecules ejected from icy planetesimals are detected.