EPSC Abstracts Vol. 13, EPSC-DPS2019-1887-1, 2019 EPSC-DPS Joint Meeting 2019 c Author(s) 2019. CC Attribution 4.0 license. Early resonances of Tethys and Dione: Implications for Ithaca Chasma Adrián Rodríguez (1), Hauke Hussmann (2) , Nelson Callegari Jr. (3) and Daigo Shoji (4) (1) Universidade Federal do Rio de Janeiro, Brasil ([email protected]), (2) DLR Institute of Planetary Research, Berlin, Germany, (3) Universidade Estadual Paulista (UNESP), Rio Claro, Brasil, (4) Tokyo Institute of Technology, Tokyo, Japan Abstract served in the outer Solar System. With a length of over 1000 km, a width of 50 – 100 km, a depth of about 3 We investigate the tidal evolution of Tethys identify- km, and rims up to about 0.5 km in height [2], Ithaca ing several possible resonance scenarios that could be Chasma is a very prominent feature with global extent relevant for intense tidal heating and the formation (Tethys’ radius R = 531 0.6 km [4]). (3) Due to ± of Ithaca Chasma early in the satellite’s history. By tidal interaction with Saturn, Tethys could have under- studying the phase-space we identify initial conditions gone phases of mean motion resonance lockings with for which Tethys and Dione can be trapped in a 2:1 its neighboring satellites prior to the present-day reso- mean motion resonance. From evolution models we nance with Mimas ([1]; [5]). show that capture of Tethys in this resonance is indeed Two hypotheses have been suggested as a mecha- possible. This scenario would imply a small semi- nism of Ithaca Chasma’s formation. One is that Ithaca major axis of Tethys early in its history and small Q- Chasma’s origin is related to the large impact that has values (high dissipation rates) of Saturn. Various reso- formed the Odysseus basin (400 km in diameter), lo- nance scenarios are obtained including later evolution cated on the opposite side of Ithaca Chasma. As an al- out of resonance. As a result of the early 2:1 resonance ternative hypothesis, [1] suggested global expansion of tidal heating can be significantly enhanced in Tethys. the satellite due to formation of a subsurface ocean on Depending on the assumed dissipation factor of Saturn Tethys while locked in a 3:2 resonance with Dione as at the time of the resonance, the resulting global dis- origin of the rift system. Both mechanisms (Odysseus sipation rate ranges between 20 and several hundred impact and ocean formation) do not match the obser- GW and could provide the heat pulse associated with vations very well. the formation of Ithaca Chasma, a global rift system Here we investigate the possibility of Tethys being on Tethys. In addition, the orbital expansion due to trapped in a 2:1 mean motion resonance with Dione tides will lead to surface stresses that could trigger the early in its history and that Ithaca Chasma was formed formation of Ithaca Chasma. The stress pattern is con- in that early phase consistent with the cratering record sistent with the orientation of faults of Ithaca Chasma suggesting an age of about 4 Ga [2]. at high latitudes but the orientation does not match for the equatorial regions. Expansion due to freezing of an 2. Summary and Conclusions early subsurface ocean as suggested in previous stud- ies could be an additional factor required to explain Tidal friction of Tethys for eccentricities of about 0.01 the formation and orientation of the rift system at low and a semi-major axis of about 223,000 km would re- latitudes. sult in a global dissipation rate of 70 GW at the 2:1 resonance with Dione, for the rheologic adopted in this work. According to [1] 60 – 100 GW would be rep- 1. Introduction resentative for Tethys’ global heat flux during forma- Tethys, one of the icy moons of Saturn, is remarkable tion of Ithaca Chasma. Thus we conclude that a pos- in several ways: (1) the satellite has the smallest den- sible passage of Tethys through the 2:1 mean motion 3 sity (985 kg m− ) of all icy moons in the Solar System resonance with Dione could be responsible for a peak [4] close to the density of H2O-ice, which implies an in Tethys’ heating rate leading to formation of Ithaca extremely small silicate fraction in the satellite’s inte- Chasma. rior. (2) With Ithaca Chasma Tethys’ surface displays a large rift system [3] that is among the largest ob- References [1] Chen, E.M.A., Nimmo, F., 2008. Implications from Ithaca Chasma for the thermal and or- bital history of Tethys. Geophys. Res. Lett. 35, L19203. [2] Giese, B., Wagner, R., Neukum, G., Helfenstein, P., Thomas, P.C., 2007. Tethys: lithos- pheric thickness and heat flux from flexurally supported toography at Ithaca Chasma. Geophys. Res. Lett. 34, L21203. [3] Jaumann, R., Clark, R.N., Nimmo, F., Hendrix, A.R., Buratti, B.J., Denk, T., Moore, J.M., Schenk, P.M., Os- tro, S.J., Srama, R., 2017. Icy satellites: geological evo- lution and sur- face processes. In: Dougherty, M.K., Es- posito, L.W., Krimigis, S.M. (Eds.), Saturn from Cassini- Huygens. vol. 2009, Springer, pp. 637 – 681. [4] Thomas, P.C., 2010. Sizes, shapes, and derived proper- ties of the Saturnian satellites after the Cassini nominal mission. Icarus 208, 395 – 401. [5] Zhang, K., Nimmo, F., 2012. Late-stage impacts and the orbital and thermal evolution of Tethys. Icarus 218, 348 – 355..