The Gravitational Imprint of an Interior–Orbital Resonance in Jupiter–Io

Abstract

At mid-mission perijove 17, NASA’s Juno mission has revealed a 7σ discrepancy between Jupiter’s observed high-degree tidal response and the theoretical equilibrium tidal response, namely, the Love number k42. Here we propose an interpretation for this puzzling disagreement based on an interior–orbital resonance between internal gravity waves trapped in Jupiter’s dilute core and the orbital motion of Io. We use simple Jupiter models to calculate a fractional correction Δk42 to the equilibrium tidal response that comes from the dynamical tidal response of a g–mode trapped in Jupiter’s dilute core. Our results suggest that an extended dilute core (r ≳ 0.7 RJ) produces an interior–orbital resonance with Io that modifies Jupiter’s tidal response in Δk42 ∼ −11%, allowing us to fit Juno’s k42. In our proposed self-consistent scenario, Jupiter’s dilute core evolves in resonant locking with Io’s orbital migration, which allows the interior–orbital resonance to persist over geological timescales. This scenario requires a dilute core that becomes smoother or shrinks over time, together with a 24g1 mode (ℓ, m, n = 4, 2, 1) with resonant tidal dissipation reaching Q4 ∼ 1000. Jupiter’s dilute core evolution path and the dissipation mechanism for the resonant 24g1 mode are uncertain and motivate future analysis. No other alternative exists so far to explain the 7σ discrepancy in Juno k42. Our proposed interior–orbital resonance can be tested by Juno observations of k42 tides raised on Jupiter by Europa, as obtained at the end of the extended mission (mid-2025), and by future seismological observations of Jupiter’s 24g1 mode oscillation frequency.