The landscape of Saturn’s internal magnetic field from the Cassini Grand Finale

Abstract

The Cassini mission entered the Grand Finale phase in April 2017 and executed 22.5 highly inclined, close-in orbits around Saturn before diving into the planet on September 15th 2017. Here we present our analysis of the Cassini Grand Finale magnetometer (MAG) dataset, focusing on Saturn’s internal magnetic field. These measurements demonstrate that Saturn’s internal magnetic field is exceptionally axisymmetric, with a dipole tilt less than 0.007 degrees (25.2 arcsecs). Saturn’s magnetic equator was directly measured to be shifted northward by ∼ 0.0468 ± 0.00043 (1σ) RS, 2820±26 km, at cylindrical radial distances between 1.034 and 1.069 RS from the spin-axis. Although almost perfectly axisymmetric, Saturn’s internal magnetic field exhibits features on many characteristic length scales in the latitudinal direction. Examining Br at the a=0.75RS, c=0.6993RS isobaric surface, the degree 4 to 11 contributions correspond to latitudinally banded magnetic perturbations with characteristic width ∼15∘, similar to that of the off-equatorial zonal jets observed in the atmosphere of Saturn. Saturn’s internal magnetic field beyond 60∘, in particular the small-scale features, are less well constrained by the available measurements, mainly due to incomplete spatial coverage in the polar region. Magnetic fields associated with the ionospheric Hall currents were estimated and found to contribute less than 2.5 nT to Gauss coefficients beyond degree 3. The magneto-disk field features orbit-to-orbit variations between 12 nT and 15.4 nT along the close-in part of Grand Finale orbits, offering an opportunity to measure the electromagnetic induction response from the interior of Saturn. A stably stratified layer thicker than 2500 km likely exists above Saturn’s deep dynamo to filter out the non-axisymmetric internal magnetic field. A heat transport mechanism other than pure conduction, e.g. double diffusive convection, must be operating within this layer to be compatible with Saturn’s observed luminosity. The latitudinally banded magnetic perturbations likely arise from a shallow secondary dynamo action with latitudinally banded differential rotation in the semi-conducting layer.