P032-0002
A Kuiper Belt Carol (in prose), Being an Origin Story of Pluto and Charon
Thursday, 10 December 2020
Poster
William B McKinnon1, Christopher R Glein2, Tanguy Bertrand3, Alyssa Rhoden4, Alan Stern5, Harold A Weaver Jr6, Catherine Olkin7, John R Spencer8, Leslie Ann Young7 and New Horizons Science Team, (1)Washington University in St Louis, St. Louis, MO, United States, (2)Southwest Research Institute, San Antonio, TX, United States, (3)NASA Ames Research Center, Moffett Field, CA, United States, (4)Southwest Research Institute Boulder, Boulder, United States, (5)Southwest Research Institute San Antonio, San Antonio, TX, United States, (6)The Johns Hopkins University, Laurel, MD, United States, (7)Southwest Research Institute Boulder, Boulder, CO, United States, (8)Southwest Research Institute, Boulder, CO, United States
Abstract:
From the
New Horizons encounters with Pluto, Charon and Arrokoth, we present the following tale ... in the gaseous and dusty protosolar disk, collective instabilities caused local concentrations of small particles (pebbles) to exceed the threshold for gravitational instability, creating the initial planetesimals in the original trans-Neptunian region (~20-30+ AU), with characteristic sizes near 100 km (a model robustly supported by Arrokoth data). These planetesimals grew by mutual collisions followed by relatively efficient, gas-drag mediated pebble accretion. Once the gas dissipated, however, hierarchical coagulation took over once more. This gas-free planetesimal disk was an ideal dynamical environment — in terms of sufficient number density and low encounter speeds — for the accretion of dwarf-planet mass bodies, and plausibly saw thousands of relatively “giant” impacts among them, including that which birthed Charon.
Neptune began migrating into the planetesimal disk within a few 10s of Myr, ultimately leading to the giant planet instability. The trans-Neptunian bodies were scattered, encounter velocities increased so that giant impacts could no longer yield Charon-like outcomes (except rarely), and the modern Kuiper belt was installed (modern in structure; the populations were larger than today, and have been decreasing ever since). Pluto-Charon was ultimately captured into orbital resonance with Neptune, and its outward migration did not end until Neptune’s did, perhaps ~100 Myr after the beginning of the Solar System.
Numerical models imply a relatively slow Charon-forming collision, between partially differentiated precursors, consistent with late growth of the progenitors. Only by forming in the latter, waning half of protoplanetary gas nebula’s nominal lifetime of ~5 Myr can the combination of pebble and planetesimal accretion (and negligible 26Al heating) yield partially differentiated bodies of proto-Pluto scale. Giant impact energetics favor a hot, ocean-bearing start for Pluto and possibly for Charon as well, depending on subsequent tidal evolution details (described herein), and subject to the still unmodeled effects of material strength during the collision. Not so much the spirit of Kuiper belt past, the Pluto system embodies the spirit of dwarf planet exploration yet to come.
