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Abstract
Abstract The time variation of a planetary magnetic field can reveal important aspects of a planet's interior structure. Searching for time variation in planetary magnetic fields other than Earth has proved challenging owing to the small number of spacecraft missions flown to date, but such a detection may be possible given a sufficiently long baseline for comparison. Here we leverage 38 years of spacecraft magnetometer measurements to search for time variation in Saturn’s internal magnetic field. To isolate the possible signal of time variation, we remove a contemporary high-resolution internal field model, derived from Cassini data, as well as a best-fitting external magnetodisk field model from each of four past mission data sets: Pioneer 11 (1979), Voyager 1 (1980), Voyager 2 (1981), and Cassini Saturn Orbit Insertion (2004). We then attempt to fit the resulting signal with an axisymmetric internal field model. Overall, we find no evidence of time variation on a multidecadal timescale. Our results lend support to the existence of a stably stratified layer in Saturn and have comparative planetology implications for Jupiter’s interior structure and dynamics.
Highlights
After removing a best-fitting external field model (Section 2.3) from the processed magnetometer data (Section 2.1), we explore whether the inferred time variation signal can be explained by a purely axisymmetric internal field model
If the differences here were entirely due to time variation of the planet’s internal magnetic field, the rate of variation would appear to be typically ∼0.01% of the total field per year at close approach (see Figures 1(a)–(d) for the total field magnitude measured by each spacecraft, for comparison)
This rate is similar to previous bounds placed on Saturn’s time variation based on earlier Cassini results (Cao et al 2011), and at least two orders of magnitude smaller than the change in Jupiter’s magnetic field (∼1% yr−1; Moore et al 2019)
Summary
Spacecraft observations of Saturn over the past few decades have revealed surprising features such as the planet’s excess luminosity (Pollack et al 1977; Fortney 2011; Leconte & Chabrier 2013), deep zonal jets (Chachan & Stevenson 2019; Iess et al 2019; Militzer et al 2019), and its highly axisymmetric magnetic field (Acuña & Ness 1980; Connerney et al 1982; Burton et al 2009, 2010; Dougherty et al 2018; Cao et al 2020). We note that subsequent work has relaxed the condition that the field be steady (see, e.g., Kaiser & Tilgner 2014) Such a layer could arise owing to the immiscibility of hydrogen and helium in a narrow range of interior conditions possibly found in Jupiter and Saturn (Lorenzen et al 2009; Militzer & Hubbard 2013). This immiscibility would lead to a dissolution zone near the top of the dynamo region, with helium raining down and leaving a less dense, helium-depleted, stably stratified layer (Stevenson 1979)
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