The gravitational imprint of an interior-orbital resonance in Jupiter-Io

Abstract

At mid-mission perijove 17, NASA's Juno mission has revealed a $7\sigma$ discrepancy between Jupiter's observed high-degree tidal response and the theoretical equilibrium tidal response, namely the Love number $k_{42}$. 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 $\Delta k_{42}$ 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 ($r\gtrsim0.7R_J$) produces an interior-orbital resonance with Io that modifies Jupiter's tidal response in $\Delta k_{42}\sim-11\%$, allowing us to fit Juno's $k_{42}$. 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 $_4^2g_1$ mode ($\ell,m,n=4,2,1$) with resonant tidal dissipation reaching $Q_4\sim1000$. Jupiter's dilute core evolution path and the dissipation mechanism for the resonant $_4^2g_1$ mode are uncertain and motivate future analysis. No other alternative exists so far to explain the $7\sigma$ discrepancy in Juno $k_{42}$. Our proposed interior-orbital resonance can be tested by Juno observations of $k_{42}$ 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 $_4^2g_1$ mode oscillation frequency.