Callisto

Callisto’s Nonresonant Orbit As An Outcome Of Circum-Jovian Disk Substructure

By Keith Cowing

Status Report

astro-ph.EP

January 9, 2026

Filed under astro-ph.EP, Callisto, Europa, Galilean moons, Ganymede, icy world, Io, Jupiter

Callisto’s Nonresonant Orbit As An Outcome Of Circum-Jovian Disk Substructure — Schematic of the circum-Jovian decretion disk. Gas and dust from the circumsolar accretion disk are subsumed into the Jovian disk from approximately one hydrostatic scale height via meridional flows, and move outward beyond the magnetospheric truncation radius. Decretion M˙ is driven by turbulence manifesting as a macroscopic viscosity, and parametrized by the Shakura-Sunyaev α parameter. The four Galilean moons are envisioned to form beyond the ice-line and pressure bump, and undergo Type-I migration inwards. The bump serves as a migration trap, preventing Callisto from convergent migration into resonance with Io, Europa, and Ganymede. — astro-ph.EP

The Galilean moons of Io, Europa, and Ganymede exhibit a 4:2:1 commensurability in their mean motions, a configuration known as the Laplace resonance.

The prevailing view for the origin of this three-body resonance involves the convergent migration of the moons, resulting from gas-driven torques in the circum-Jovian disk wherein they accreted. To account for Callisto’s exclusion from the resonant chain, a late and/or slow accretion of the fourth and outermost Galilean moon is typically invoked, stalling its migration.

Here, we consider an alternative scenario in which Callisto’s nonresonant orbit is a consequence of disk substructure. Using a suite of N-body simulations that self-consistently account for satellite-disk interactions, we show that a pressure bump can function as a migration trap, isolating Callisto and alleviating constraints on its timing of accretion. Our simulations position the bump interior to the birthplaces of all four moons.

In exploring the impact of bump structure on simulation outcomes, we find that it cannot be too sharp nor flat to yield the observed orbital architecture. In particular, a “Goldilocks” zone is mapped in parameter space, corresponding to a well-defined range in bump aspect ratio. Within this range, Io, Europa, and Ganymede are sequentially trapped at the bump, and ushered across it through resonant lockstep migration with their neighboring, exterior moon.

The implications of our work are discussed in the context of uncertainties regarding Callisto’s interior structure, arising from the possibility of non-hydrostatic contributions to its shape and gravity field, unresolved by the Galileo spacecraft.

Teng Ee Yap, Konstantin Batygin

Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2601.00786 [astro-ph.EP] (or arXiv:2601.00786v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2601.00786
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Journal reference: The Astrophysical Journal, 995(2), 218 (2025)
Related DOI:
https://doi.org/10.3847/1538-4357/ae171c
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Submission history
From: Teng Ee Yap
[v1] Fri, 2 Jan 2026 18:41:18 UTC (5,528 KB)
https://arxiv.org/abs/2601.00786

Astrobiology,

Keith Cowing

Explorers Club Fellow, ex-NASA Space Station Payload manager/space biologist, Away Teams, Journalist, Lapsed climber, Synaesthete, Na’Vi-Jedi-Freman-Buddhist-mix, ASL, Devon Island and Everest Base Camp veteran, (he/him) 🖖🏻

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