Enceladus and the Icy Moons of Saturn (2016) 3071.pdf

WHY IS ENCELADUS ACTIVE WHEREAS MIMAS IS NOT? M. Neveu1 and A. R. Rhoden1, 1School of Earth & Space Exploration, Arizona State University, Tempe, AZ 85287, USA. Email: [email protected].

Context: Thermal models struggle to sustain an Our simulations produce an extant ocean on Ence- ocean on Enceladus, even with tidal heating [1-5]. In ladus if (1) it retains an insulating crust (Fig. 1); or (2) contrast, Mimas, Saturn’s innermost mid-sized moon, with Andrade-like dissipation, which yields an ice is surprisingly geologically inactive, although likely thickness of 50 km, or 25 km if dissipation is ten times differentiated [6]. An ocean on Mimas would lie below higher as suggested by [17]. In scenario (1), Mimas has warm ice, subject to tidal dissipation at a rate 30 times no ocean, but remains undifferentiated. In scenario (2), higher than on Enceladus [7]. Strong tides would drive Mimas also has an ocean. surface activity and circularize Mimas’ orbit, regard- Ongoing work: Because differentiation requires less of dissipation inside Saturn [7-9]. Yet, Mimas’ ice softening, conducive to runaway dissipation, a sce- orbit is four times as eccentric as Enceladus’, and not nario in which Mimas accretes rock before ice could in an eccentricity-type resonance with other moons. explain its present state [21]. It would then never heat Previous studies suggested that Enceladus’ lower sur- up enough for tidal feedbacks to take place, whereas face area:volume ratio and higher density (i.e. more Enceladus would. We are now testing such scenarios. abundant radionuclides) may have warmed it enough References: [1] Roberts J. H. and Nimmo F. (2008) for tidal dissipation to produce heat and melt, whereas Icarus, 194, 675–689. [2] Shoji D. et al. (2014) Icarus, Mimas may never have been warm enough to be suffi- 235, 75–85. [3] Roberts J. H. (2015) Icarus, 258, 54– ciently tidally heated [10-12]. However, none of these 66. [4] Travis B. J. and Schubert G. (2015) Icarus, 250, studies explicitly modeled tidal dissipation. 32–42. [5] Malamud U. and Prialnik D. (2016) Icarus, Model: We model the thermal evolution of Mimas 268, 1–11. [6] Tajeddine R. et al. (2014) Science, 346, and Enceladus from formation to the present. We use 322–324. [7] Meyer J. and Wisdom J. (2008) Icarus the 1-D code IcyDwarf [13,14] to model partial or full 193, 213–223. [8] Goldreich P. and Soter S. (1966) ice-rock differentiation, ammonia antifreeze, parame- Icarus, 5, 375–389. [9] Lainey V. et al. (2012) ApJ, terized convective heat transfer both in the shell and 752, 14. [10] Schubert G. et al. (2007) Icarus, 188, due to hydrothermal flow, and radiogenic, gravitation- 345–355. [11] Malamud U. and Prialnik D. (2013) al, and chemical heating. We have added viscoelastic Icarus, 225, 763–774. [12] Czechowski L. and Witek tidal heating [15-17], as well as porosity (model sim- P. (2015) Acta Geophys, 63, 900–921. [13] Desch S. J. plified from [18]) and its effects on thermal conductivi- et al. (2009) Icarus, 202, 694–714. [14] Neveu M. et ty [19,20]. Between simulations, we vary the time of al. (2015) JGR, 120, 123–154. [15] Henning W. G. et accretion (short- and long-lived radionuclide content), al. (2009) ApJ, 707, 1000. [16] Shoji D. et al. (2013) initial temperatures, porosities, and structure (homoge- Icarus, 226, 10–19. [17] McCarthy C. and Cooper R. neous or rocky core + icy shell), as well as tidal dissi- F. (2016) EPSL, 443, 185–194. [18] Neumann W. et al. pation models (elastic, Maxwell, Burgers, or Andrade). (2014) A&A, 567, A120. [19] Shoshany Y. et al. (2002) Results and discussion: Successful evolution sce- Icarus, 157, 219–227. [20] Krause M. et al. (2011) 42nd narios must yield differentiated moons [6], an extant LPSC, 2696. [21] Charnoz S. et al. (2011) Icarus, 216, ocean on Enceladus, but no dissipative interior on Mi- 535–550. mas (which likely precludes ocean emplacement or persistence). We have yet to find a set of realistic con- ditions that satisfies all criteria. Melting is favored in insulating interiors (undifferentiated and/or porous). Ocean sustainability hinges on tidal dissipation, but is also favored by undifferentiated crust and hy- drothermal circulation, which efficiently transports heat from the radiogenic core into the hydrosphere. Unlike previous findings [10-12], most sets of initial Fig. 1: Thermal evolution of Enceladus for accre- conditions that yield an ocean on Enceladus also yield tion 7 Myr after Ca-Al-rich inclusions, with initial one on Mimas, for which the positive tidal feedback on temperature 100 K and 20% initial porosity. Tidal dis- ocean persistence is stronger. This highlights the need sipation follows the Andrade model. Maxwell dissipa- to explicitly model tidal effects on thermal evolution. tion yields a thinner ocean and thicker crust.