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

Abstract

The interior structure of Saturn, the depth of its winds, and the mass and age of its rings constrain its formation and evolution. In the final phase of the Cassini mission, the spacecraft dived between the planet and its innermost ring, at altitudes of 2600 to 3900 kilometers above the cloud tops. During six of these crossings, a radio link with Earth was monitored to determine the gravitational field of the planet and the mass of its rings. We find that Saturn’s gravity deviates from theoretical expectations and requires differential rotation of the atmosphere extending to a depth of at least 9000 kilometers. The total mass of the rings is (1.54 ± 0.49) × 1019 kilograms (0.41 ± 0.13 times that of the moon Mimas), indicating that the rings may have formed 107 to 108 years ago.

Saturn's Deep Atmospheric Flows Revealed by the Cassini Grand Finale Gravity Measurements

“…With a conservatively modified cloud‐level wind, extended to a depth of around 8,800 km, all the relevant gravity harmonics can be explained, taking into account the associated uncertainties in both the measurements and the model solutions. In most latitudes the optimal top level wind is similar to the observed wind, with the largest deviations found around latitudes 25–35 ∘ north and south, similar to the deviation found by Iess et al () but twice as small. This is a result of the different SB background density and gravity harmonics solutions used here, as discussed above.…”

“…In most latitudes the wind solution is very similar to the observed cloud‐level wind and is well within the expected uncertainties discussed in section 1). The largest deviations are around latitudes 25–35 ∘ north and south, similar in location to the findings of Iess et al (), but about half the size. The wind decay profile and the resulting flow structure (Figures d and e) reveal that the wind behaves nearly barotropicly in the equatorial region (extending all the way to the equatorial plain in the direction of the spin axis) but baroclinicly outside latitudes 20 ∘ N and 20 ∘ S, that is, decaying before reaching the equatorial plain.…”

“…However, aside from some observed variations in the wind strength between the upper and middle troposphere, there has been very little knowledge on the nature of the flow below the cloud level. A possible answer was enabled by the gravity experiment conducted during the Grand Finale phase of NASA's Cassini spacecraft (Iess et al, ). The measured gravity field (Figure c, red and green dots) was found to be considerably different from that predicted by typical internal rigid body (RB) models (gray dots), especially for gravity harmonics higher than J 6 .…”

“…The measured gravity field (Figure c, red and green dots) was found to be considerably different from that predicted by typical internal rigid body (RB) models (gray dots), especially for gravity harmonics higher than J 6 . Initial estimates for the depth of the winds indicated very deep winds penetrating to a depth of more than 9,000 km (Iess et al, ). This estimate, however, required a substantial modification of the meridional profile of the cloud‐level winds and was determined by matching only the even gravity harmonics J 8 , and J 10 .…”

Lower-order zonal gravitational coefficients caused by zonal circulations inside gaseous planets: Convective flows and numerical comparison between modeling approaches

“…In modeling the zonal‐circulation‐related distortion, several authors have employed the so‐called Thermal Wind Equation (TWE) for density ρ , gravity and pressure p (Kaspi et al, 2018; Iess et al, 2019; Debras and Chabrier, 2019; Galanti et al, 2019),…”