Resolving the Latitudinal Short‐Scale Gravity Field of Jupiter Using Slepian Functions

Abstract

Early gravity measurements performed by the Juno spacecraft determined Jupiter's low‐degree gravity harmonics, including the first estimate of the planet's north‐south asymmetric field. The retrieved information was used to infer that the strong zonal winds visible at the cloud tops must extend down a few thousand kilometers, where they are suppressed in the deep interior. The next frontier for the Juno gravity experiment includes, among other goals, the determination of Jupiter's small‐scale gravity field with high accuracy, and its relation to atmospheric circulation at shorter length scales. The geometry of the Juno closest approaches to the planet poses a challenge to this task, as they span latitudes between 4°N and 29°N over the course of the nominal mission. Since Doppler measurements are the most sensitive to gravity anomalies when the spacecraft is close to the body, observations of Jupiter's gravity field are mostly concentrated in the northern hemisphere, while the traditional spherical harmonic functions are not orthonormal over a latitudinal subdomain. Here we define customized Slepian functions, which are orthogonal in a specific latitude range and are optimized to represent Jupiter's local surface gravity at north latitudes. We show that with the new functions, the short‐scale latitudinal variability of the gravity field is resolved with high accuracy between 15°S and 45°N latitude. Furthermore, preliminary results show that the estimated values for the Slepian coefficients from the Juno data match the predictions obtained using thermal wind balance to relate the dynamical density anomalies and the winds with an optimized scale height.

Orbit determination methods for interplanetary missions: development and use of the Orbit14 software

“…We found the same features for the gravitational and tidal field of Jupiter, showing further details of the Juno gravity experiment. We demonstrated that Orbit14 is able to provide an independent estimation of the dynamical parameters of Jupiter, working in parallel with other research groups which employ the NASA OD software MONTE (Iess et al, 2018;Parisi et al, 2020).…”

“…For the future experiments with a larger amount of data, some aspects need to be further investigated. First of all, it will be important to see if the remaining signals at the closest approaches will increase in magnitude and if random accelerations will be able to still absorb them or other models related to physical processes will be required (see Durante et al 2017;Parisi et al 2020). Moreover, the use of a satellite-dependent formulation of the tides on Jupiter will be essential to confirm possible deviations of the tidal parameters of Jupiter from their static values (see Notaro et al 2019).…”

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“…The high correlations between zonal harmonics beyond degree 10 due to the uneven spatial resolution prevent the estimation of the gravity moments with the required accuracy. A similar effect was investigated with the Juno gravity experiment, which sought to circumvent this limitation by either adopting a different mathematical base than spherical harmonics or setting constraints on the surface gravity from realistic physical considerations (Galanti et al 2017;Parisi et al 2020).…”

“…The gap between the Canberra and Madrid data is due to the rising and setting times of the stations (handover), as well as the limit set on the elevation angle. All simulated Doppler data from all arcs are combined into a multiarc solution for the solved-for parameters (Parisi et al 2020(Parisi et al , 2021.…”

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