Published September 20, 2025 | Published
Journal Article Open

On the Origin and Dynamical Evolution of Jupiter's Moon Amalthea

  • 1. ROR icon California Institute of Technology

Abstract

Interior to the orbits of Jupiter’s iconic Galilean moons are four small satellites with individual mean radii R ≲ 84 km. Multiple lines of evidence suggest that these bodies formed at a more distant location in Jupiter’s circumplanetary disk before coming to reside at their current short-period orbits. However, how these moons dynamically evolved to such a location has yet to be explained in the emerging paradigm of Jovian satellite formation. Here, we present a quantitative model for the origin of the largest of these inner moons, Amalthea, that can be extended to its neighbor, Thebe, and to other small bodies in astrophysical disks. We propose that Amalthea’s anomalous features are due to it having formed alongside the Galileans in a reservoir of satellitesimals located at a large Jovian-centric distance. As the innermost Galilean, Io, migrated inward from this reservoir, it captured the satellitesimal Amalthea into resonance and shepherded the small body to its modern neighborhood. During this migration through the disk, dissipative forcing from aerodynamic drag induces overstable librations in the Io–Amalthea resonance, such that only a narrow range of nebular parameters can accommodate the requisite long-range transport. In particular, the disk aspect ratio, h/r, emerges as the key variable. Our calculations indicate that the circumjovian disk had a scale height of h/r ≳ 0.08, implying a relatively hot, actively accreting disk during the epoch of satellite formation. These results thus shed light on the evolution of the Jovian system, along with the more general phenomenon of satellite–disk interactions.

Copyright and License

© 2025. The Author(s). Published by the American Astronomical Society. Original content from this work may be used under the terms of the Creative Commons Attribution 4.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.

Acknowledgement

I.R.B. is grateful to the Ahmanson Foundation for its financial support, to Hanno Rein and Dan Tamayo for their help with the Rebound implementation, and to Fred Adams, Matthew Belyakov, and Gabriele Pichierri for useful discussions. K.B. is thankful for the support of the David and Lucile Packard Foundation, and the National Science Foundation (grant No. AST 2408867). Both authors thank Caltech and 3CPE.

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Additional details

Created:
September 19, 2025
Modified:
September 19, 2025