The effect of Jupiter oscillations on Juno gravity measurements

Abstract

Seismology represents a unique method to probe the interiors of giant planets. Recently, Saturn's f-modes have been indirectly observed in its rings, and there is strong evidence for the detection of Jupiter global modes by means of ground-based, spatially-resolved, velocimetry measurements. We propose to exploit Juno's extremely accurate radio science data by looking at the gravity perturbations that Jupiter's acoustic modes would produce. We evaluate the perturbation to Jupiter's gravitational field using the oscillation spectrum of a polytrope with index 1 and the corresponding radial eigenfunctions. We show that Juno will be most sensitive to the fundamental mode (n = 0), unless its amplitude is smaller than 0.5 cm/s, i.e. 100 times weaker than the n ∼ 4−11 modes detected by spatially-resolved velocimetry. The oscillations yield contributions to Juno's measured gravitational coefficients similar to or larger than those expected from shallow zonal winds (extending to depths less than 300 km). In the case of a strong f-mode (radial velocity ∼ 30 cm/s), these contributions would become of the same order as those expected from deep zonal winds (extending to 3000 km), especially on the low degree zonal harmonics, therefore requiring a new approach to the analysis of Juno data.

Measurement and implications of Saturn’s gravity field and ring mass

“…Another potential solution is to assume a time-varying field, generated by acoustic oscillations in the planet (28). Fundamental mode oscillations (f-mode, with radial order equal to zero) have been detected by analysis of density waves in the C ring (29).…”

“…An alternative Saturn gravity model assumes a time-variable field resulting from internal oscillation modes (28). Saturn's fundamental modes have been identified via density waves in the C ring (29), as observed in Cassini stellar occultation data.…”

Comparison of the Deep Atmospheric Dynamics of Jupiter and Saturn in Light of the Juno and Cassini Gravity Measurements

“…1) and are the only considerable source of north-south asymmetries on the gravity field. Other sources of north-south asymmetries can be internal oscillations (Durante et al, 2017) and the known north-south asymmetry in the magnetic field . However, internal oscillations can be expected to give fluctuating contributions from orbit to orbit whereas the measured odd harmonics are steady.…”

Ice Giant Seismology: Prospecting for Normal Modes

“…Φ m is numerically computed from the normal mode's perturbation density field [43]. Note that the influence of a normal mode of azimuthal order m appears at frequencies of (m ± 1)Ω 0 − mΩ p .…”