Callisto

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

2. Basic Parametres for Callisto • 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?? The New Solar System 1999 and Anderson et al 2001.

3. The surface of Callisto • 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)

4. Magnetic Field & Ocean • Galileo came in 1996. Base for new models • No internal magnetic field. (tectonically dead) • 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. (Kivelson et al, 1999) • 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. • 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. Schubert et al, 1981 Anderson et al, 1997

6. Anderson et al 1998 and 2001 • No ocean included. • Assumes hydrostatic stability • Based on gravitational data from flybys. • The gravitational coefficients in the well known Legendre Expansion. Approximates that all other than the monopol and the quadropoles are zero: J2(-C20), C21, S21, C22 and S22 Anderson et al 2001 • 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 C22.

7. Two layer model • Two limits: • A relatively pure ice outer shell, 300 km thick overlying a mixed ice and rock-metal interior (~2300 kg/m3) • A thick (>1000 km) ice and rock-metal outer shell (~1600 kg/m3) overlying a rock-metal core. Anderson et al. 2001

8. Three layer model • Outer shell has ~1000 kg/m3 • 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. Anderson et al. 2001

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

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

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. • Convecting heat flux not big enough to maintain an ocean • Most likely way of maintaining an ocean is by increasing the viscosity. Possibilities: • Antifreeze e.g. NH3 lowers temperature of ocean (and convecting ice) (Spohn and Schubert Icarus 2003) • Silicate particles in ice increase its viscosity • Very large ice grains • Non-Newtonian convection less efficient. A more glaciological approach. (Ruiz, Nature 2001) Spohn and Schubert, 2003 (with inspiration from prof. Nimmos powerpoints)

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. 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) Rock volume fraction Possible ocean Ice melting temp temperature Nagel et al 2004

14. 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. Temp dependent viscosity Temp independent viscosity 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