Juno spacecraft gravity measurements provide evidence for normal modes of Jupiter

Abstract

The Juno spacecraft has been collecting data to shed light on the planet's origin and characterize its interior structure. The onboard gravity science experiment based on X-band and Ka-band dual-frequency Doppler tracking precisely measured Jupiter's zonal gravitational field. Here, we analyze 22 Juno's gravity passes to investigate the gravity field. Our analysis provides evidence of new gravity field features, which perturb its otherwise axially symmetric structure with a time-variable component. We show that normal modes of the planet could explain the anomalous signatures present in the Doppler data better than other alternative explanations, such as localized density anomalies and non-axisymmetric components of the static gravity field. We explain Juno data by p-modes having an amplitude spectrum with a peak radial velocity of 10-50 cm/s at 900-1200 μHz (compatible with ground-based observations) and provide upper bounds on lower frequency f-modes (radial velocity smaller than 1 cm/s). The new Juno results could open the possibility of exploring the interior structure of the gas giants through measurements of the time-variable gravity or with onboard instrumentation devoted to the observation of normal modes, which could drive spacecraft operations of future missions.

Fig. 1. Juno two-way range-rate (Doppler) residuals in mm/s for selected passes.

Different dynamical models are compared: static zonal gravity field (first row) or including normal modes (second row). The light black line is a moving average of the residuals, to highlight signatures near perijove if a zonal field is assumed. See Supplementary Figure 1 for Doppler residuals in all Juno perijove passes and different dynamical models.

Fig. 2. Normal modes model.

Radial velocity profile (a) and corresponding amplitude of normalized spherical harmonic coefficients (b). The model is depicted for ${\mathcal{v}}_{\max }$ = 50 cm/s, zero ${\mathcal{v}}_{\min }$, $f_{\mathrm{peak}}$ = 1210 μHz, and σ_f = 300 μHz. The letters f and the numbers indicate, respectively, f-modes and the radial order of p-modes, up to n = 7.

Fig. 3. Results for two slices of the full parameter space.

a ΔAIC value as a function of ${\mathcal{v}}_{\max }$ and f_peak, with ${\mathcal{v}}_{\min }$ = 0. b ΔAIC value as a function of ${\mathcal{v}}_{\max }$ and ${\mathcal{v}}_{\min }$, with f_peak = 1200 μHz. Both slices have σ_f = 300 μHz. Each circle is a different solution.

Fig. 4. Recovered radial velocity profile as a function of frequency.

Each line corresponds to a different model in the search grid. The opacity is proportional to Akaike weights: darker lines are for more likely models (decreasing ΔAIC values), conversely, lighter lines are for gradually less probable models (increasing ΔAIC values). The spread of the lines provides an indication of the uncertainty of the recovered amplitude spectrum.

Fig. 5. ΔAIC values as a function of vuniform, for a flat velocity profile with v(f)=vuniform.

The minimum ΔAIC corresponds to a solution that does not fit Juno Doppler data: such a model does not represent adequately Juno’s data.