Marc Neveu^1,2 and Pierre Vernazza³
Astrophysical Journal 875, 30 Link to Article [DOI: 10.3847/1538-4357/ab0d87 ]
¹University of Maryland, 4296 Stadium Dr., College Park, MD 20742, USA
²NASA Goddard Space Flight Center, 8800 Greenbelt Rd., Greenbelt, MD 20770, USA
³Aix-Marseille Université, CNRS, Laboratoire d’Astrophysique de Marseille, 38 Rue Frederic Joliot Curie, F-13013 Marseille, France

The parent bodies of ordinary chondrites, carbonaceous CM chondrites, and interplanetary dust particles (IDPs) represent most of the mass of the solar system’s small (D ≤ 250 km) bodies. The times of formation of the ordinary and carbonaceous CM chondrite parent bodies have previously been pinpointed, respectively, to ≈2 and 3–4 million years after calcium–aluminum-rich inclusions (CAIs). However, the timing of the formation of IDP parent bodies such as P- and D-type main-belt asteroids and Jupiter Trojans has not been tightly constrained. Here, we show that they formed later than 5–6 million years after CAIs. We use models of their thermal and structural evolution to show that their anhydrous surface composition would otherwise have been lost due to melting and ice-rock differentiation driven by heating from the short-lived radionuclide ²⁶Al. This suggests that IDP-like volatile-rich small bodies may have formed after the gas of the protoplanetary disk dissipated and thus later than the massive cores of the giant planets. It also confirms an intuitive increase in formation times with increased heliocentric distance, and suggests that there may have been a gap in time between the formation of carbonaceous chondrite (chondrule-rich) and IDP (chondrule-poor) parent bodies.

John B. Biersteker¹, Benjamin P. Weiss¹, Philip Heinisch², David Herčik², Karl-Heinz Glassmeier², and Hans-Ulrich Auster²
Astrophysical Journal 875, 39 Link to Article [DOI: 10.3847/1538-4357/ab0f2a ]
¹Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139-4307, USA
²Technische Universität Braunschweig, Mendelssohnstrasse 3, D-38106 Braunschweig, Germany

The remanent magnetization of solar system bodies reflects their accretion mechanism, the space environment in which they formed, and their subsequent geological evolution. In particular, it has been suggested that some primitive bodies may have formed large regions of coherent remanent magnetization as a consequence of their accretion in a background magnetic field. Measurements acquired by the Rosetta Magnetometer and Plasma Monitor have shown that comet 67P/Churyumov–Gerasimenko (67P) has a surface magnetic field of less than 0.9 nT. To constrain the spatial scale and intensity of remanent magnetization in 67P, we modeled its magnetic field assuming various characteristic spatial scales of uniform magnetization. We find that for regions of coherent magnetization with ≥10 cm radius, the specific magnetic moment is  5 × 10⁻⁶ . If 67P formed during the lifetime of the solar nebula and has not undergone significant subsequent collisional or aqueous alteration, this very low specific magnetization is inconsistent with its formation from the gentle gravitational collapse of a cloud of millimeter-sized pebbles in a background magnetic field 3 μT. Given the evidence from other Rosetta instruments that 67P formed by pebble-pile processes, this would indicate that the nebular magnetic field was 3 μT at 15–45 au from the young Sun. This constraint is consistent with theories of magnetically driven evolution of protoplanetary disks.

Ryan Miranda¹ and Roman R. Rafikov^1,2
Astrophysical Journal 875, 37 Link to Article [DOI: 10.3847/1538-4357/ab0f9e ]
¹Institute for Advanced Study, Einstein Drive, Princeton, NJ 08540, USA
²Centre for Mathematical Sciences, Department of Applied Mathematics and Theoretical Physics, University of Cambridge, Wilberforce Road, Cambridge CB3 0WA, UK

Recent observations of protoplanetary disks, as well as simulations of planet–disk interaction, have suggested that a single planet may excite multiple spiral arms in the disk, in contrast to the previous expectations based on linear theory (predicting a one-armed density wave). We reassess the origin of multiple arms in the framework of linear theory by solving for the global two-dimensional response of a nonbarotropic disk to an orbiting planet. We show that the formation of a secondary arm in the inner disk, at about half of the orbital radius of the planet, is a robust prediction of linear theory. This arm becomes stronger than the primary spiral at several tenths of the orbital radius of the planet. Several additional, weaker spiral arms may also form in the inner disk. On the contrary, a secondary spiral arm is unlikely to form in the outer disk. Our linear calculations, fully accounting for the global behavior of both the phases and amplitudes of perturbations, generally support the recently proposed WKB phase argument for the secondary arm origin (as caused by the intricacy of constructive interference of the azimuthal harmonics of the perturbation at different radii). We provide analytical arguments showing that the process of a single spiral wake splitting up into multiple arms is a generic linear outcome of wave propagation in differentially rotating disks. It is not unique to planet-driven waves and also occurs in linear calculations of spiral wakes freely propagating with no external torques. These results are relevant for understanding formation of multiple rings and gaps in protoplanetary disks.

Olivier Mousis¹, Thomas Ronnet^1,2, and Jonathan I. Lunine³
Astrophysical Journal 875, 9 Link to Article [DOI: 10.3847/1538-4357/ab0a72 ]
¹Aix Marseille Université, CNRS, CNES, LAM, Marseille, France
²Lund Observatory, Department of Astronomy and Theoretical Physics, Lund University, Box 43, SE-221 00 Lund, Sweden
³Department of Astronomy, Cornell University, Ithaca, NY 14853, USA

Argon, krypton, xenon, carbon, nitrogen, sulfur, and phosphorus have all been measured and found to be enriched by a quasi uniform factor in the 2–4 range, compared to their protosolar values, in the atmosphere of Jupiter. To elucidate the origin of these volatile enrichments, we investigate the possibility of an inward drift of particles made of amorphous ice and adsorbed volatiles, and their ability to enrich in heavy elements the gas phase of the protosolar nebula, once they cross the amorphous-to-crystalline ice transition zone, following the original idea formulated by Monga & Desch. To do so, we use a simple accretion disk model coupled to modules depicting the radial evolution of icy particles and vapors, assuming growth, fragmentation, and crystallization of amorphous grains. We show that it is possible to accrete supersolar gas from the nebula onto proto-Jupiter’s core to form its envelope, and allowing it to match the observed volatile enrichments. Our calculations suggest that nebular gas, with a metallicity similar to that measured in Jupiter, can be accreted by its envelope if the planet is formed in the ~0.5–2 Myr time range and in the 0.5–20 au distance range from the Sun, depending on the adopted viscosity parameter of the disk. These values match a wide range of Jupiter’s formation scenarios, including in situ formation and migration/formation models.