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Measurement and Implications of Saturns Gravity Field and Ring Mass Presentation prepared by Burkhard Militzer UC Berkeley Authors of the article in Science: L. Iess, B. Militzer, Y. Kaspi, P. Nicholson, D. Durante, P. Racioppa, A. Anabtawi, E. Galanti, W. Hubbard, M. J. Mariani, P. Tortora, S. Wahl, M. Zannoni 1. Saturns Gravity Field The Cassini Spacecraft Measured
Saturns Gravity Field when it dove inside its rings before eventually burning up in its atmosphere Saturns Gravity is highly unusual, cannot be matched with Uniform Rotation Models Puzzling Cassini Gravity Data with unusually large J6, J8 and J10 Models match mass, radius, and J2 from Jacobson solution. All J2n multiplied by 10 Range of Model Predictions assuming
Uniform Rotation Cassini Data Rev 273+274 solution May 1, 2017 May 19, 2017 J4 938.619 -933.187
J6 80.532 81.737 J8 -8.950 -8.680 J10 1.076
1.129 J12 -0.157 -0.147 J14 0.0215 0.0234 Unusual Model 1
(BM) Unusual Model 2 (BM) May 29, 2017 -934.5792 -934.5792 -943.6830 86.5215
80.6008 82.2691 -14.3704 4.9910 -0.6670 0.5332 No model that -8.6693 -8.9477 assumed uniform 1.0721 1.1190 rotation (no winds)
-0.1459 could match-0.1540 the observations. 0.0213 0.0227 Effects of Differential Rotation Effects of Differential Rotation Rotational flattening from uniform, solid-body rotation Flattening from differential rotation
(exaggerated). Parameter Ranges in our Models that are consistent with the Spacecraft measurements Saturn is predicted to have a rock-ice core worth 15-18 Earth masses. Typical for Giant Planet Interiors Take Jupiter as an Example Jupiter model
Militzer et al. (2016) JGR, 121. Comparison of Saturn and Jupiter Saturns interior Jupiters interior 2. Mass of Saturns Rings How Do we Relate our Ring Mass Measurement to the Age of the Rings? 1. Assume rings started as pure water ice and have
been steadily darkened by the infall of meteoroid material (Cuzzi, Estrada, 1998). The meteoroid flux (mass/year) is known. The color of the rings tell us about todays rock-ice ratio. Our new mass measurements thus tell us when the rings formed and for how long meteoroids have polluted them. 2. Rate of recession of the small satellites from the rings due to gravitational torques (Borderies et al, 1984) 3. Evolution of unconfined edges such as the inner edges of the A and B rings (Estrada et al. 2015) All these estimates depend on the total ring mass. Pre-Cassini Ring Mass Estimate The estimated total ring mass was
2.8 1019 kg or 0.75 Mimas masses and became the standard pre-Cassini value. However, it was argued that substantially -6 more mass might be hidden opaque Mimas:
moon of Saturn, massin = the 6.3 parts of. the B ring. Mearth 2000 Mimas masses ~ 1 lunar mass. 16000 Mimas masses ~ 1 Earth mass. Cassini observations of density waves Cassini observations of density waves have led to local surface density estimates in the A, B and C rings.
The B ring density is surprisingly low, with a mean value of ~600 kg/m2. Likely total ring mass of 0.40 Mimas masses. However, it is conceivable that some mass in the rings does not contribute to the density waves. First Determination of Saturns Ring Mass from Gravity Directly from the gravity signal, we determined a total mass of the main rings A, B and C = 0.41+-0.13 Mimas masses. Mass of diffuse D, F, G, and E rings
assumed to be negligible. An indication that the rings are young, Possible Origin of Saturns Rings The rings weigh 0.41 Mimas masses today but were much more massive earlier. Rings possibly came about from the capture and following gravitational disruption of a comet or Centaur (Dones, 1991; Dones et al. 2007)
Alternatively they came from an earlier population of mid-sized icy satellites (uk, ApJ, 2016). Their orbits became unstable and there was at least one giant collision that products the guzzilion of ring particles that we see today Conclusions Saturn is predicted to rotate differentially based on even gravity coefficient. The equatorial regions rotate about 4% faster than the deep interior. Winds are at least 9000 km deep. Saturn rings have a low mass and are young (10-100
million years). This points to a dramatic collision in our recent solar system history.
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