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Three-Body and Spin-Orbit Resonances in the Pluto System

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Three-Body and Spin-Orbit Resonances in the Pluto System Pluto System After New Horizons 2019 (LPI Contrib. No. 2133) 7072.pdf THREE-BODY AND SPIN-ORBIT RESONANCES IN THE PLUTO SYSTEM. D. P. Ham- ilton1 and T. de Santana2, Astronomy Department, University of Maryland, (College Park, MD) dpham- [email protected], 2UNESP, (Guaratingueta, SP, Brazil) [email protected]. Introduction: The four small satellites of Pluto, satellites, as it is insensitive to satellite spin rates and Styx, Nix, Kerberos, and Hydra, orbit near simple can drive initially small obliquities toward 90 degrees. two-body mean motion resonances with Pluto’s large More exotic families of spin-orbit resonances also moon Charon: the 3:1, 4:1, 5:1, and 6:1, respectively. exist, including those that involve multiple satellite Somewhat paradoxically, however, while near the orbital frequencies, satellite spin rates, or both. These resonances, the satellites are decidedly not in the res- resonances, however, require more specialized cir- onances. How can these resonances be simultaneously cumstances to activate [6]. Nevertheless, given the important (suggested by the satellites’ proximity to unknown, but likely complicated, past history of the them) and unimportant (all resonances are inactive)? satellites we remain open to all of these possibilities. Further mysteries involve the rotation states of the Numerical Approach: We use the N-body code satellites which all feature non-synchronous rotation hnbody for both the three-body resonances and secu- and large obliquities. lar spin-orbit resonances. We are currently evaluating Three-Body Resonance: Showalter and Hamil- several promising scenarios and will report on our ton [1] showed Hydra, Nix and Styx do participate in most recent results. On a longer timescale, we are a 2:5:3 three-body resonance that is analogous to the extending hnbody to handle the spin-orbit resonances 2:3:1 three-body resonance that affects Jupiter’s Gan- that depend explicitly on the spin rates of the satel- ymede, Europa, and Io. At Jupiter, additional 2:1 res- lites [5]. onances between Io and Europa and between Europa References: [1] Showalter M. R. and Hamilton D. and Ganymede are also active; at Pluto, conversely, DP. (2015) Nature, 522, 45–49. [2] De Santana T., no two-body resonances are active. We suggest a his- Hamilton D. P. and Winter O. C. (2018) DPS, Ab- tory for Pluto’s satellites, driven by the tidal migra- stract #50. [3] Ward, W. R. and Hamilton D. P. tion of Charon, that first temporarily put satellites in (2004) AJ, 128, 2501–2509. [4] Hamilton D. P. and two-body resonances, then activated the 3:5:2 three- Ward W. R. (2004) AJ, 128, 2510–2517. body resonance, and finally moved the satellites out [5] Rogozinski, Z. and Hamilton D. P. (2018) DDA of the original resonances [2]. The two body reso- #49, Abstract #10. [6] Quillen A. C., Nichols- nances that would most naturally activate the ob- Fleming, F., Chen Y.-Y. and Noyelles, B. (2017) Ica- served three-body resonance are the 5:4 Styx-Nix and rus, 293, 94–113. 2:1 Styx-Hydra two-body resonances. When the two body resonances both involve the pericenter of Styx, the three-body resonance activates. We also show that the three-body resonance, once activated, can drive the system away from the two-body resonances, thus leaving a system near, but not in, these resonances. A similar situation may have also occurred for Kerberos, which today is found close to, but not precisely in, the 43:85:42 Styx-Nix-Kerberos three-body resonance. Spin-Orbit Resonance: Hamilton and Ward [3][4] invoked a spin-orbit resonance to explain the 27 degree tilt of Saturn and there is some hope that a similar mechanism can be found to explain the more extreme tilt of Uranus [5]. These spin-orbit resonanc- es may also be important in the Pluto system. There, tidal migration of Charon early in Pluto’s history causes the relevant frequencies to slowly change, bringing satellites into spin-orbit resonances that can potentially drive obliquities up. The main secular spin-orbit resonance studied by [3][4] is an attractive possibility for producing the obliquities of the small .
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