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The Origin of Neptune's Unusual Satellites from a Planetary Encounter

Li, Daohai LU orcid and Christou, Apostolos A. (2020) In The Astronomical Journal 159(4).
Abstract

The Neptunian satellite system is unusual, comprising Triton, a large (∼2700 km) moon on a close-in, circular, yet retrograde orbit, flanked by Nereid, the largest irregular satellite (∼300 km) on a highly eccentric orbit. Capture origins have been previously suggested for both moons. Here we explore an alternative in situ formation model where the two satellites accreted in the circum-Neptunian disk and are imparted irregular and eccentric orbits by a deep planetary encounter with an ice giant (IG), like that predicted in the Nice scenario of early solar system development. We use N-body simulations of an IG approaching Neptune to 20 Neptunian radii (R Nep), through a belt of circular prograde regular satellites at 10-30 R... (More)

The Neptunian satellite system is unusual, comprising Triton, a large (∼2700 km) moon on a close-in, circular, yet retrograde orbit, flanked by Nereid, the largest irregular satellite (∼300 km) on a highly eccentric orbit. Capture origins have been previously suggested for both moons. Here we explore an alternative in situ formation model where the two satellites accreted in the circum-Neptunian disk and are imparted irregular and eccentric orbits by a deep planetary encounter with an ice giant (IG), like that predicted in the Nice scenario of early solar system development. We use N-body simulations of an IG approaching Neptune to 20 Neptunian radii (R Nep), through a belt of circular prograde regular satellites at 10-30 R Nep. We find that half of these primordial satellites remain bound to Neptune and that 0.4%-3% are scattered directly onto wide and eccentric orbits resembling that of Nereid. With better matches to the observed orbit, our model has a success rate comparable to or higher than capture of large Nereid-sized irregular satellites from heliocentric orbit. At the same time, the IG encounter injects a large primordial moon onto a retrograde orbit with specific angular momentum similar to Triton's in 0.3%-3% of our runs. While less efficient than capture scenarios, our model does indicate that an in situ origin for Triton is dynamically possible. We also simulate the post-encounter collisional and tidal orbital evolution of Triton analog satellites and find they are decoupled from Nereid on timescales of ∼104 yr, in agreement with Cuk & Gladman.

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Contribution to journal
publication status
published
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in
The Astronomical Journal
volume
159
issue
4
article number
184
publisher
IOP Publishing
external identifiers
  • scopus:85086599384
ISSN
0004-6256
DOI
10.3847/1538-3881/ab7cd5
language
English
LU publication?
yes
id
09548e19-c8ea-4e37-a547-9d47651875fa
date added to LUP
2021-01-11 15:34:33
date last changed
2024-04-03 20:56:20
@article{09548e19-c8ea-4e37-a547-9d47651875fa,
  abstract     = {{<p>The Neptunian satellite system is unusual, comprising Triton, a large (∼2700 km) moon on a close-in, circular, yet retrograde orbit, flanked by Nereid, the largest irregular satellite (∼300 km) on a highly eccentric orbit. Capture origins have been previously suggested for both moons. Here we explore an alternative in situ formation model where the two satellites accreted in the circum-Neptunian disk and are imparted irregular and eccentric orbits by a deep planetary encounter with an ice giant (IG), like that predicted in the Nice scenario of early solar system development. We use N-body simulations of an IG approaching Neptune to 20 Neptunian radii (R <sub>Nep</sub>), through a belt of circular prograde regular satellites at 10-30 R <sub>Nep</sub>. We find that half of these primordial satellites remain bound to Neptune and that 0.4%-3% are scattered directly onto wide and eccentric orbits resembling that of Nereid. With better matches to the observed orbit, our model has a success rate comparable to or higher than capture of large Nereid-sized irregular satellites from heliocentric orbit. At the same time, the IG encounter injects a large primordial moon onto a retrograde orbit with specific angular momentum similar to Triton's in 0.3%-3% of our runs. While less efficient than capture scenarios, our model does indicate that an in situ origin for Triton is dynamically possible. We also simulate the post-encounter collisional and tidal orbital evolution of Triton analog satellites and find they are decoupled from Nereid on timescales of ∼10<sup>4</sup> yr, in agreement with Cuk &amp; Gladman.</p>}},
  author       = {{Li, Daohai and Christou, Apostolos A.}},
  issn         = {{0004-6256}},
  language     = {{eng}},
  month        = {{04}},
  number       = {{4}},
  publisher    = {{IOP Publishing}},
  series       = {{The Astronomical Journal}},
  title        = {{The Origin of Neptune's Unusual Satellites from a Planetary Encounter}},
  url          = {{http://dx.doi.org/10.3847/1538-3881/ab7cd5}},
  doi          = {{10.3847/1538-3881/ab7cd5}},
  volume       = {{159}},
  year         = {{2020}},
}