On the origin and dynamical evolution of Jupiter's moon Amalthea

Abstract

Interior to the orbits of Jupiter's iconic Galilean moons are four small satellites with individual mean radii, $R\lesssim 84$ 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. Nonetheless, 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 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 at least $h/r\gtrsim0.08$, 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.