Enceladus and the Icy Moons of Saturn (2016) 3009.pdf

Orbits and rotation states of Enceladus and other moons of Saturn Matthew S. Tiscareno, SETI Institute, Mountain View CA 94086, USA ([email protected]) While Enceladus rotates once per orbit, so that it References always keeps the same face towards Saturn, its non- [1] P. C. Thomas, R. Tajeddine, M. S. Tiscareno, J. A. Burns, spherical shape and non-circular orbit together give rise J. Joseph, T. J. Loredo, P. Helfenstein, and C. Porco. Enceladus’s to a wobble (a “libration”) upon that base rotation state. measured physical libration requires a global subsurface ocean. Icarus, 264:37–47, January 2016. doi: 10.1016/j.icarus.2015.08. The rotational libration has been measured with an am- 037. ◦ ◦ plitude of 0.120 ± 0.014 , which is large enough to [2] H.-W. Hsu, F. Postberg, Y. Sekine, T. Shibuya, S. Kempf, demonstrate that the core of Enceladus cannot be rigidly M. Horanyi,´ A. Juhasz,´ N. Altobelli, K. Suzuki, Y. Masaki, connected to the surface [1]. This proves that its subsur- T. Kuwatani, S. Tachibana, S.-I. Sirono, G. Moragas- Klostermeyer, and R. Srama. Ongoing hydrothermal activities face water, already inferred to exist via geyser activity [2] within Enceladus. Nature, 519:207–210, March 2015. doi: and to be at least regional in scope via gravity measure- 10.1038/nature14262. ments [3], in fact extends globally [1]. [3] L. Iess, D. J. Stevenson, M. Parisi, D. Hemingway, R. A. Ja- We will describe the method we used to carry out cobson, J. I. Lunine, F. Nimmo, J. W. Armstrong, S. W. As- mar, M. Ducci, and P. Tortora. The gravity field and interior this measurement and will review the method’s previous structure of Enceladus. Science, 344:78–80, April 2014. doi: applications to Janus and Epimetheus [4] and Mimas [5], 10.1126/science.1250551. as well as the prospects for applying it to other moons of [4] M. S. Tiscareno, P. C. Thomas, and J. A. Burns. The rotation of Saturn. Janus and Epimetheus. Icarus, 204:254–261, November 2009. doi: 10.1016/j.icarus.2009.06.023. We will also review the orbit of Enceladus and the [5] R. Tajeddine, N. Rambaux, V. Lainey, S. Charnoz, A. Richard, other moons of Saturn, identifying resonances and other A. Rivoldini, and B. Noyelles. Constraints on Mimas’ interior effects that shape their current behavior. For exam- from Cassini ISS libration measurements. Science, 346:322–324, October 2014. doi: 10.1126/science.1255299. ple, not only is Enceladus in a 2:1 inner Lindblad res- [6] M. M. Hedman and P. D. Nicholson. The B-ring’s surface mass onance with Dione, the two nearby 2:1 corotation res- density from hidden density waves: Less than meets the eye? onances (eccentricity-type and inclination-type) are in a Icarus, 2016. in press (arXiv:1601.07955). near-resonant state and also significantly affect the or- [7] S. Kempf, M. Horanyi, R. Srama, and N. Altobelli. Exogenous dust delivery into the Saturnian system and the age of Saturn’s bital behavior of Enceladus. rings. European Planetary Science Congress Abstracts, 10:411, Consensus on the dynamical history of the small and October 2015. mid-size moons of Saturn is currently in flux. Although [8] V. Lainey, R. A. Jacobson, R. Tajeddine, N. J. Cooper, C. Murray, origins scenarios are easier to construct when set near the V. Robert, G. Tobie, T. Guillot, S. Mathis, F. Remus, J. Desmars, J.-E. Arlot, J.-P. De Cuyper, V. Dehant, D. Pascu, W. Thuillot, beginning of solar system history, longtime suspicions C. Le Poncin-Lafitte, and J.-P. Zahn. New constraints on Saturn’s that Saturn’s rings are only ∼100 Myr old have recently interior from Cassini astrometric data. 2015. arXiv:1510.05870. intensified due to realizations that the rings are less mas- [9]M. Cuk,´ L. Dones, and D. Nesvorny.´ Dynamical Evidence for a sive than previously thought [6] and that the pollution Late Formation of Saturn’s Moons. Astrophys. J., 820:97, April 2016. doi: 10.3847/0004-637X/820/2/97. of the rings’ pristine water ice by interplanetary dust is [10] S. Charnoz, J. Salmon, and A. Crida. The recent formation of Sat- faster than previously thought [7]. The simultaneous re- urn’s moonlets from viscous spreading of the main rings. Nature, alization that orbital evolution of moon orbits due to Sat- 465:752–754, June 2010. doi: 10.1038/nature09096. urn’s internal friction is faster than previously thought [8] [11] A. Crida and S. Charnoz. Formation of Regular Satellites from Ancient Massive Rings in the Solar System. Science, 338:1196, has led some to conclude that even the mid-sized moons November 2012. doi: 10.1126/science.1226477. (e.g., Enceladus) cannot be more than 100 Myr old [9]. Genesis from a young massive ring, rather than from the protosatellite disk, was previously discussed for the small [10] and mid-sized [11] moons and should be re- visited in light of these new results.