¹S.J.Weidenschilling
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13270]
Planetary Science Institute, Tucson, Arizona, 85719 USA
Published by arrangement with John Wiley & Sons

Thermal models of asteroids generally assume that they accreted either instantaneously or over an extended interval with a prescribed growth rate. It is conventionally assumed that the onset of accretion of chondrite parent bodies was delayed until a substantial fraction of the initial 26Al had decayed. However, this interval is not consistent with the early melting, and differentiation of parent bodies of iron meteorites. Formation time scales are tested by dynamical simulations of accretion from small primary planetesimals. Gravitational accretion yields rapid runaway growth of large planetary embryos until most smaller bodies are depleted. In a given simulation, all asteroid‐sized bodies have comparable growth times, regardless of size. For plausible parameters, growth times are shorter than the lifetime of 26Al, consistent with thermal models that assume instantaneous accretion. Rapid growth after planetesimal formation is consistent with differentiation of parent bodies of iron meteorites, but not with the assumed delay in formation of chondritic bodies. After the initial growth stage, there is an interval of slower evolution until the belt is stirred and the embryos are dynamically removed. During this interval, a fraction of asteroid‐sized bodies experience large accretional impacts, allowing bodies of the same final size to have very different histories of radius versus time. Accretion from small primary planetesimals leaves some fraction of material in bodies small enough to preserve CAIs while avoiding heating by 26Al. Unheated material can be a significant fraction of the mass that remains after large embryos are removed from the Main Belt.

¹Hannah H. Kaplan,²Ralph E. Milliken,³Conel M. O’D. Alexander,⁴Christopher D. K. Herd
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13264]
¹Department of Space Studies, Southwest Research Institute, Boulder, Colorado, 80302 USA
²Department of Earth, Environmental, and Planetary Sciences, Brown University, , Providence, Rhode Island, 02912 USA
³Department of Terrestrial Magnetism, Carnegie Institution of Washington, , Washington, DC, 20015 USA
⁴Department of Earth and Atmospheric Sciences, University of Alberta, , Edmonton, AB T6G 2E3 Canada
Published by arrangement with John Wiley & Sons

Insoluble organic matter (IOM) is the major organic component of chondritic meteorites and may be akin to organic materials from comets and interplanetary dust particles (IDPs). Reflectance spectra of IOM in the range 0.35–25 μm are presented as a tool for interpreting organic chemistry from remote measurements of asteroids, comets, IDPs, and other planetary bodies. Absorptions in the IOM spectra were strongly related to elemental H/C (atom) ratio. The aliphatic 3.4 μm absorption in IOM spectra increased linearly in strength with increasing H/C for H/C > 0.4, but was absent at lower H/C values. When meteorite spectra from the Reflectance Experiment Laboratory (RELAB) spectral catalog (n = 85) were reanalyzed at 3.4 μm, this detection limit (H/C > 0.4) persisted. Aromatic absorption features seen in IOM spectra were not observed in the meteorite spectra due to overlapping absorptions. However, the 3.4 μm aliphatic absorption strength for the bulk meteorites was correlated with both H/C of the meteorite’s IOM and bulk C (wt%). Gaussian modeling of the 3 μm region provided an additional estimate of bulk C for the meteorites, along with bulk H (wt%), which is related to phyllosilicate abundance. These relationships lay the foundation for determining organic and phyllosilicate abundances from reflectance spectra. Both the full IOM spectra and the spectral parameters discussed here will aid in the interpretation of data from asteroid missions (e.g., OSIRIS‐REx, Hayabusa2), and may be able to place unknown spectral samples within the context of the meteorite collection.

Kelly E. Miller, Christopher R. Glein, and J. Hunter Waite Jr.
Astrophysical Journal 871, 59 Link to Article [DOI: 10.3847/1538-4357/aaf561 ]
Southwest Research Institute, 6220 Culebra Rd., San Antonio, TX 78238, USA

Since its discovery in the first half of the 20th century, scientists have puzzled over the origins of Titan’s atmosphere. Current models suggest that atmospheric N₂ on Titan may have originated from NH₃-bearing ice with N-isotopic ratios similar to those observed in NH₂ in cometary comae (¹⁴N/¹⁵N ~ 136). In contrast, N₂ ice appears to be too ¹⁵N poor to explain Titan’s atmosphere (¹⁴N/¹⁵N ~ 168). Additionally, data from the Rosetta mission to comet 67P/Churyumov–Gerasimenko suggest that the Ar/N₂ ratio of outer solar system planetesimals may be too high for a comet-like N₂ source on Titan. The Rosetta mission also revealed an astonishing abundance of N-bearing complex organic material. While thermal fractionation of cometary sources during Titan accretion may explain the loss of N₂– and Ar-rich ices, more refractory materials such as complex organics would be retained. Later heating in the interior may lead to volatilization of accreted organics, consistent with Cassini–Huygens measurements of ⁴⁰Ar that suggest outgassing from the interior may have played a role in atmosphere formation. Here, we develop a three endmember mixing model for N isotopes and the ³⁶Ar/¹⁴N ratio of Titan’s atmosphere, and consider the implications for the source of atmospheric methane. Our model suggests that Titan’s interior is likely warm, and that N from accreted organics may contribute on the order of 50% of Titan’s present-day nitrogen atmosphere.

Chen-En Wei¹, Hideko Nomura¹, Jeong-Eun Lee², Wing-Huen Ip³, Catherine Walsh⁴, and T. J. Millar^5,6
Astrophysical Journal 870, 129 Link to Article [DOI: 10.3847/1538-4357/aaf390 ]
¹Department of Earth and Planetary Sciences, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro, Tokyo, 152-8551, Japan
²School of Space Research, Kyung Hee University, Seocheon-Dong, Giheung-Gu, Yongin-Si, Gyeonggi-Do, 446-701, Republic of Korea
³Graduate Institute of Astronomy, National Central University, No. 300, Zhongda Road, Zhongli Dist., Taoyuan City 32001, Taiwan
⁴School of Physics and Astronomy, University of Leeds, Leeds, LS2 9JT, UK
⁵Astrophysics Research Centre, School of Mathematics and Physics, Queen’s University Belfast, University Road, Belfast, BT7 1NN, UK
⁶Leiden Observatory, Leiden University, P.O. Box 9513, 2300 RA Leiden, The Netherlands

The bulk composition of Earth is dramatically carbon-poor compared to that of the interstellar medium, and this phenomenon extends to the asteroid belt. To interpret this carbon deficit problem, the carbonaceous component in grains must have been converted into the gas phase in the inner regions of protoplanetary disks (PPDs) prior to planetary formation. We examine the effect of carbon grain destruction on the chemical structure of disks by calculating the molecular abundances and distributions using a comprehensive chemical reaction network. When carbon grains are destroyed and the elemental abundance of the gas becomes carbon-rich, the abundances of carbon-bearing molecules, such as HCN and carbon-chain molecules, increase dramatically near the midplane, while oxygen-bearing molecules, such as and , are depleted. We compare the results of these model calculations with the solid carbon-to-silicon fraction in the solar system. Although we find a carbon depletion gradient, there are some quantitative discrepancies: the model shows a higher value at the position of the asteroid belt and a lower value at the location of Earth. In addition, using the obtained molecular abundance distributions, coupled with line radiative transfer calculations, we make predictions for ALMA to potentially observe the effect of carbon grain destruction in nearby PPDs. The results indicate that HCN, , and c- may be good tracers.