1. Callisto Is it really undifferentiated? ESS 298 Presentation 23.Nov 2004 Mads Dam Ellehøj

2. Basic Parametres for Callisto Mean distance from Jupiter (km) 1,8E6 Period (days) 16.7 Eccentricity0.007 Radius (km) 2400 Mean density (kg/m 3 ) 1850 g (m/s 2 ) 1.24 C/MR 2 (Moment of Intertia) 0.355 The New Solar System 1999 and Anderson et al 2001. In a big gap between Ganymede (1,070,000 km) and Leda (11,094,000 km) Plus no Laplace Resonance. No big tidal heating Density of rock is much higher and density of ice is much lower. Low MoI indicates more homogenous body than for example Io (0.378) 0.38 is an expected value (based on Callisto’s size and mass) of a homogenous body of a mixture of ice and rock. (Anderson et al 1998) Not homogenous??

3. Heavily cratered. Saturated. Seems to be no tectonic activity Not many small craters. Seems to have eroded away by sublimation of the ice. (remember in class) IR spectra and radiative transfer models show that the top layer seems to consist of a mixture between rock and ice. (J.R Spencer, 1987 and Calvin et al, 1995) The surface of Callisto

4. Magnetic Field & Ocean Galileo came in 1996. Base for new models Galileo came in 1996. Base for new models No internal magnetic field. No internal magnetic field. (tectonically dead) (tectonically dead) Induced magnetic field indicates ocean. (Khurana et al, 1998) Induced magnetic field indicates ocean. (Khurana et al, 1998) Ocean proposed to be tens of kilometres thick, but also tens of kilometres under surface for magnetoconvective field to have right magnitude. Ocean proposed to be tens of kilometres thick, but also tens of kilometres under surface for magnetoconvective field to have right magnitude. (Kivelson et al, 1999) (Kivelson et al, 1999) Could have absorbed seismic waves from the Valhalla impact. No opposite focusing. Could have absorbed seismic waves from the Valhalla impact. No opposite focusing. http://science.nasa.gov/newhome/headlines/images/galileo/flyby_big.gif http://cc.oulu.fi/tati/JR/TerrPlanets/Pl1_2001/T_Suokas/valhalla.gif

5. Before Galileo Previous models of Callisto have solid cores surrounded by water or ice mantles. Schubert et al, 1981 showed (Based on accretion temperatures) that a separation of rock and ice did not happen. Callisto seemed to be undifferentiated. Previous models of Callisto have solid cores surrounded by water or ice mantles. Schubert et al, 1981 showed (Based on accretion temperatures) that a separation of rock and ice did not happen. Callisto seemed to be undifferentiated. On the edge: Anderson et al 1997 stated that (based on a two layer model and gravitational data from the C3 flyby) it was likely undifferentiated. On the edge: Anderson et al 1997 stated that (based on a two layer model and gravitational data from the C3 flyby) it was likely undifferentiated. Models did not include an ocean. Models both for and against differentiation. Models did not include an ocean. Models both for and against differentiation. Schubert et al, 1981 Anderson et al, 1997

6. Anderson et al 1998 and 2001 No ocean included. No ocean included. Assumes hydrostatic stability Assumes hydrostatic stability Based on gravitational data from flybys. Based on gravitational data from flybys. The gravitational coefficients in the well known Legendre Expansion. The gravitational coefficients in the well known Legendre Expansion. Approximates that all other than the monopol and the quadropoles are zero: J 2 (-C 20 ), C 21, S 21, C 22 and S 22 Assumes that Callistos spherical harmonical degree 2 is due to the tidal and rotational distortion because of synchronous rotation. The model creates possible hydrostatic structures consistent with the observed values of mean density and C 22. Anderson et al 2001

7. Two limits: A relatively pure ice outer shell, 300 km thick overlying a mixed ice and rock-metal interior (~2300 kg/m 3 ) A thick (>1000 km) ice and rock-metal outer shell (~1600 kg/m 3 ) overlying a rock-metal core. Two layer model Anderson et al. 2001

8. Three layer model Anderson et al. 2001 Outer shell has ~1000 kg/m 3 In every case, a significant portion of Callisto has big density. Which means a mixture of ice and rock or rock-metal. Core of rock or rock-metal appears. Whatever the distribution, it seems like a certain amount of ice and rock are mixed to depths at at least 1000 km, and perhaps to the center.

9. Concludes that Callisto is not completely differentiated,Concludes that Callisto is not completely differentiated, but not undifferentiated aswell. but not undifferentiated aswell. Because ice convection is needed to remove radioactive heatingBecause ice convection is needed to remove radioactive heating (and therefore creates higher density of rocks with depth) (and therefore creates higher density of rocks with depth) the authors prefer: 1. A twolayer model with a large homogenous ice-rock-metal core (but still no more than 25% of radius) surrounded by a (but still no more than 25% of radius) surrounded by a pure iceshell. Or 2. A similar threelayer model also with a core. Concludes that:

10. Iron cores are a problem. Temperatures too high in seperation.Iron cores are a problem. Temperatures too high in seperation. No magnetic field. No magnetic field. Ice-rock differentiation must be a slowIce-rock differentiation must be a slow process, but ongoing. process, but ongoing. Maybe created by a slow accretion.Maybe created by a slow accretion. Partially differentiated, but what about the ocean??Partially differentiated, but what about the ocean?? SO:

11. An ocean As seen in the class: Thermal evolution of an ocean will be controlled by balance between heat added (from below) and heat transported to the surface. Thermal evolution of an ocean will be controlled by balance between heat added (from below) and heat transported to the surface. Convecting heat flux not big enough to maintain an ocean Convecting heat flux not big enough to maintain an ocean Most likely way of maintaining an ocean is by increasing the viscosity. Possibilities: Most likely way of maintaining an ocean is by increasing the viscosity. Possibilities: Antifreeze e.g. NH 3 lowers temperature of ocean (and convecting ice)Antifreeze e.g. NH 3 lowers temperature of ocean (and convecting ice) (Spohn and Schubert Icarus 2003) (Spohn and Schubert Icarus 2003) Silicate particles in ice increase its viscositySilicate particles in ice increase its viscosity Very large ice grainsVery large ice grains Non-Newtonian convection less efficient.Non-Newtonian convection less efficient. A more glaciological approach. A more glaciological approach. (Ruiz, Nature 2001) (Ruiz, Nature 2001) (with inspiration from prof. Nimmos powerpoints) Spohn and Schubert, 2003

12. Nagel et al 2004 Recent work. A model for incomplete differentiation of a solid Callisto Introduces ”close packing limit” – a measure of the volume fraction of rock/ice A complete model. Takes lot into account, e.g.: Ice phase transitions (with limits, though) Creep of ice Temperature dependent viscosity Only longlived radiogenic isotopes.(good or not good depends of accretion time scale) Does not take ammonia presence into account in the modeling. To hard.

13. Nagel et al 2004 The rock will warm surrounding ice. Heat is transferred by convection. Creates separation of ice and rock. Results show a undifferentiated top layer (caused by high viscosity and low surface temp). Consistent with observations. Works as an isolator for the underneath. Might have an ocean. Ice melting temp meets temperature. Radially increasing Temperatures. No deep melting because ice melting temp Increases with depth (pressure) Possible ocean temperature Ice melting temp Rock volume fraction

14. Temp dependent viscosityTemp independent viscosity The same is seen: Cold downwelling plume erodes top layer from below. Driven by negative buoyancy of rock. The upwelling plume is seen under the poles. Temperature here reaches melting temp. For independent viscosity, clearly convection driven by thermal buoyancy. Rock concentration temperature Nagel et al 2004

15. SO: Callisto is partially differentiated. Slow separation of rock and ice is ongoing. No simple explanation for ocean. Upwelling plumes are relatively local. But, if ammonia, things would be very different. Near surface ocean could be realistic

16. Is it really undifferentiated? No metallic core. Would need higher temperatures than the ice allows. Nonhydrostatic? Models don’t account for this. (McKinnon, 1997) But likely partially differentiated: For example (from figure in Nagel et al 2004) Upper layer of mixture of rock and ice ~300 km Middle layer with lots of ice (ocean??) ~400 km ”Core” with big rock fraction ~1700 km Maybe still ongoing separation of rock and ice. Slowly removing the heat. Slow accretion models (Canup and Ward, 2002) show that is it possible to create a partially undifferentiated Callisto. Formed cold. Ocean is still not incorperated in the models. Future will show. http://www.jpl.nasa.gov/releases/98/glcallistoocean.html

17. References Anderson et al, 2001. Shape, mean radius, gravity field and interior structure of Callisto. Icarus 153, 157-161. Anderson et al, 1998. Distribution of Rock, Metals and Ices in Callisto, Science 280, 1573-1576. Anderson et al, 1997. Gravitational evidence for an undifferentiated Callisto, Nature 387, 264-266. Calvin et al, 1995. J.Geophys Res. 100, 19041 Canup and Ward, 2002. Formation of the Galilean Sattelites: Conditions of accretion. The Astronomical Journal 124, 3404-3423. J.R Spencer, 1987. Ibid. 70, 99 Khurana et al, 1998. Induced magnetic fields as evidence for subsurface oceans in Europa and Callisto. Nature 395, 777-780. Kivelson et al, 1999. Europa and Callisto: Induced or itrinsic in a periodically varying plasma environment. J Geophys. Res. 104, 4609-4625. McKinnon, 1997. Mystery of Callisto: Is it undifferentiated? Icarus 130, 540-543. Nagel et al, 2004. A model for the interior structure, evolution, and differentiation of Callisto, Icarus 169, 402-412. Ruiz, 2001, The Stability against freezing of an internal liquid-water ocean in Callisto. Nature 412, 409-411. Spohn and Schubert, 2003. Oceans in the icy Galilean satellites of Jupiter? Icarus 161, 456-467. The New Solar System, 1999. Beatty, Petersen and Chaikin, 4th Ed., Cambridge Uni. Press. 100% Jenna, 2001