A COMPARISON OF INTERNAL STRUCTURE OF GANYMEDE AND TITAN. Dunaeva A.N., Kronrod V.A., Kuskov O.L. Vernadsky Institute of Geochemistry and Analytical Chemistry, Russian Academy of Sciences, Moscow, Russia

Ganymede and Titan:  are the two largest satellites in the Solar System;  were formed in the outer zones of their central planets (Jupiter and Saturn);  are regular satellites (their orbits and rotation are the same as the rotation of associated central planets);  satellites rotation is synchronous with their orbits;  low density of the satellites suggests that they could contain remarkable amounts of H 2 O. Jupiter Saturn Ganymede Titan

The main differences between Ganymede and Titan "Galileo", "Voyager" and "Cassini-Huygens" spacecraft missions to Jupiter and Saturn showed that Ganymede and Titan are different both external and internally: GanymedeTitan Atmosphere  Trace oxygen atmosphere Dense nitrogen atmosphere (~400 km): –N %, CH 4 and Ar - 1.6%, –CO 2 and other trace organics. –Free oxygen is absent. Magnetic field  A relatively strong intrinsic magnetic field and magnetosphere. Intrinsic magnetic field is absent. Climate  Exogenous climatic processes (evaporation, condensation, precipitation, cycle of substances, seasons) are not available. The seasonal weather patterns are similar to Earth, but governed by methane cycle (including winds, rains, seasons change, etc.). Surface features Two types of terrain: –Very old, highly cratered, dark regions; –Younger (but still ancient), lighter regions marked with an extensive array of grooves and ridges; Criovolcanism insignificant but important in the formation of the bright terrain. The surface is "complex, fluid-processed, and geologically young" (c): –Ridges, valleys, riverbeds, dunes, stable lakes of liquid hydrocarbons; –Minor amounts of relatively young impact craters; –Clearly defined criovolcanism.

Models of Ganymede and Titan. Ganymede: Ganymede’s general image from NASA, JPL Titan: Sohl F. et al., 2003 Mitri et al., 2009 Sohl F., 2010 Titan’s general image from NASA Grasset et al., 2005

Phase diagram of water and the temperature distribution in the Ganymede’s and Titan’s icy crust. Straight thin lines - conductive temperature profiles through the external (ice-Ih) crust. Dashed lines – adiabatic convective heat transfer in the water subcrustal ocean and in high-pressure ices. H, H Ih, H L - the distance from the satellite's surface (depth), the thickness of the external ice-Ih crust and of the inner liquid ocean respectively.

Calculation of the Ganymede's and Titan’s heat flux The thickness of the icy crust and internal ocean of Ganymede (blue) and Titan (black) via the heat flow through the satellites ice-Ih crust. F(mW/m 2 ) = [  o (R Sat - Н Ih ) /(Н Ih R Sat )]ln[T2 /Т1]  o = 567 W/m - thermal conductivity of ice Ih, R Sat – satellite’s radius, Н Ih – thickness of the icy crust, Т1 – satellite's surface temperature, Т2 – the temperature at the ice-Ih - liquid phase boundary, H w - the thickness of internal ocean, F – heat flux. [1] Bland, M.T., et al., 2009 [2] Mitri G., Showman A., 2008

Physical characteristics of the satellites Ganymede Titan Pressure at the surface, P[bar] 1.0e Temperature at the surface, T [K] Gravity acceleration, [m/s 2 ]; Radius, R [km] Average density, g/cm Mass, M [kg] e e24 Normalized moment of inertia, I/MR Models of the satellites internal structure described by the system of following equations:  Equations of hydrostatic equilibrium:,  The equations of the satellites mass and moment of inertia:,  The equation for calculating ice component concentration in mantle:  High-pressure water ices equations of state. Initinal data for modeling, problem setting and methods of solution  density of the water-ice shell,  average density of ice in mantle,  density of the rock–iron component,  average density of mantle = g/cm 3 (LL-chondrites)

The internal structure of Ganymede and Titan. In general three-layer models of satellites including the outer water-ice shell, mantle (rock or rock-ice) and the inner core (Fe-Si or Fe-FeS) can be made. Moreover, two-layer models (without inner core) could be realized. In this case satellite has significant large outer water-icy shell, but its inner core not forms. On this model the maximum possible thickness of the water-ice shell is about 900 km and 500 km for Ganymede and Titan respectively.

Water content and density gradients in large icy satellites of Jupiter and Saturn. The total water content in Ganymede is 46-48%, in Titan %.

Conclusion.  Ganymede and Titan are the similar in size and chemical composition: the density of the satellites’ rock material is typical for the hydrated L/LL chondrites.  The satellites do not differ in terms of bulk water content which in average is about 50 wt.% (water/rock ratio is close to 1).  Ganymede and Titan both may have subsurface oceans. –Internal ocean in the satellites not forms when the heat fluxes less than 3.3 mW/m 2 and 2.9 mW/m 2 for Titan and Ganymede respectively.  Internal structure of the satellites can differ fundamentally: –Ganymede is a completely differentiated body, with the inner region formed by separating of the original L/LL-chondritic substance into the silicate mantle and metallic core. –Titan is differentiated only partially: its inner areas are represented by a mixture of rock and ice components.  Equal content of bulk H 2 O and the same density of the satellites’ rock material allow to have assumption that Ganymede and Titan could have been formed from the planetesimals with similar composition corresponding to the ordinary L/LL- chondrites. Different conditions of the satellites’ formation from accretion disks led to major differences in their internal structure.