Origins, Worlds, and Life A Decadal Strategy for Planetary Science and Astrobiology 2023-2032 (2022) / Chapter Skim
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5 Question 2: Accretion in the Outer Solar System
5 Question 2: Accretion in the Outer Solar System
Pages 130-163
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From page 130... ...
The outer solar system, stretching from Jupiter to the Oort cloud, contains the keys to understanding the formation and early evolution of our planetary system. 1 Gas giant planet formation is a primary open question in theoretical astrophysics, with Jupiter and Saturn used to calibrate and test new models.
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From page 131... ...
Numerical models suggest that pebble accretion in combination with protoplanet mergers can form the giant planet cores before the solar nebula dissipates (Levison et al. 2015 and references therein; Johansen and Lambrechts 2017)
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From page 132... ...
In general, core accretion requires a heavy-element core, but the presence of a core in the disk instability model cannot be ruled out. Overall, better understanding of giant planet origin requires improved information on their PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION 5-3
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Further constraints on the origin of Uranus and Neptune would be greatly improved by improved measurements of their gravitational and magnetic fields and atmospheric compositions. Q2.1c What Were the Primordial Internal Structures of Giant Planets?
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Details on the evolution of the internal structures of giant planets are given in chapter 10 and references therein. Q2.1d What Were the Roles of Early Giant Impacts and Magnetic Fields in Shaping the Properties of the Outer Planets?
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● Better determine the formation and early evolution of the outer planets through improved numerical simulations and theoretical models. 2 Trapped normal modes in the interiors of giant planets can create a velocity field that can be sensed remotely.
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Understanding the origin of giant planet heavy element fractions hinges on our incomplete knowledge of the bulk heavy element fractions themselves. For Jupiter, abundances of most heavy elements detected by the Galileo probe and remote sensing (Figure 5.3)
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, a fundamental challenge to understanding the types of solids accreted during planetary formation. Equilibrium condensation of a protosolar-abundance mixture would produce more ice than rock, if the condensation temperature is low PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION 5-8
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From page 138... ...
New compositional measurements, particularly in Uranus and Neptune as a contrast to Jupiter, are needed in the coming decade to constrain increasingly complex models of giant planet formation. The potential PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION 5-9
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. The main processes thought to have produced circumplanetary disks -- gas accretion and giant impacts -- would have occurred during or near the end stages of giant planet accretion.
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. It has been argued to be less likely because they PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION 5-12
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, dynamical transport of heliocentric planetesimals and dwarf planets throughout the giant planet region would have occurred but would have been especially important for Neptune as it migrated outward through the primordial Kuiper belt population. Capture into the satellite region of a giant planet could have occurred by direct collision with a pre-existing regular satellite or by tidal stripping of a binary (e.g., Agnor and Hamilton 2006; Nogueira et al.
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. It is possible that such cavities could be detectable via polarimetric measurements of accreting extra-solar giant planets (sensitive to magnetic fields)
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Q2.4 HOW DID THE GIANT PLANETS GRAVITATIONALLY INTERACT WITH EACH OTHER, THE PROTOSOLAR DISK, AND SMALLER BODIES IN THE OUTER SOLAR SYSTEM? It is now thought that the giant planets did not form where they currently reside, but instead migrated inward and/or outward because of disk torques during the protosolar nebular phase or by later gravitational interactions with remnant planetesimals.
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of giant planet migration, as well as, potentially, the formation of the Moon and the asteroids. Presently there is no fully accepted, self-consistent model for the simultaneous formation of the giant planets during PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION 5-16
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From page 146... ...
For example, if Neptune entered the primordial Kuiper belt at ~20 AU, it probably had to form near that location. In turn, that means the giant planets once had a different configuration and a migration mechanism had to move them to where we see them today (Nesvorný 2018 and references therein)
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. This migration dynamically affects the terrestrial planets, asteroid belt, and primordial Kuiper belt (e.g., Nesvorný 2018)
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. Strategic Research for Q2.4 ● Determine the timing, extent and effects of giant planet migration by measurement of impact basin ages on the terrestrial planets, compositional and isotopic constraints on early terrestrial planet evolution, including the origin of the Moon, and studies of impact crater populations on diverse outer solar system bodies ● Further constrain the dynamical structure of the distant trans-Neptunian population, including classical and resonant objects, so-called "detached" objects (Q2.6)
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From page 150... ...
that quantity allows the depletion of the primordial Kuiper belt by Neptune's migration to explain various small body populations captured during the giant planet instability (Nesvorný and Vokrouhlický 2016; Vokrouhlický et al. 2016; 2019; Morbidelli and Nesvorný 2020 and references therein)
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This suggests that among accretional, PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION 5-22
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From page 153... ...
via remote sensing by spacecraft and ground /space-based telescopic observations ● Improve understanding of giant impacts between ice-rock protoplanets through state-of-the art numerical simulations of giant impacts ● Improve the understanding of the accretion and internal differentiation, of icy moons and TNOs, including ocean formation, through interpretation of spacecraft data in terms of consistent physico-chemical models and laboratory experiments on ices and carbonaceous materials. ● Improve the understanding of binary system, TNO family, and ring formation through theory, observations, and numerical modeling Q2.6 HOW DID THE ORBITAL STRUCTURE OF THE TRANS-NEPTUNIAN BELT, THE OORT CLOUD, AND THE SCATTERED DISK ORIGINATE, AND HOW DID GRAVITATIONAL INTERACTIONS IN THE EARLY OUTER SOLAR SYSTEM LEAD TO SCATTERING AND EJECTION?
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. Here small bodies scattered into the giant planet zone from the primordial Kuiper belt experienced giant planet encounters, with many placed onto orbits with very large semimajor axes.
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these encounters are happening concurrently with Neptune's migration through the primordial Kuiper belt, which sends approximately 20 Earth masses of TNOs into the giant planet zone. Some TNOs could have been located at the right place and time to be captured within stable reservoirs across the solar system by PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION 5-26
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. However, objects ejected from the primordial Kuiper belt have high inclinations, and giant planet encounters can capture a small fraction of them within Jupiter and Neptune's L4 and L5 locations (Vokrouhlický et al.
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1997. Giant planet formation by gravitational instability.
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2017. Ring formation around giant planets by tidal disruption of a single passing large Kuiper belt object.
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