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

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Callisto

Callisto

Is it really undifferentiated?

ESS 298 Presentation 23.Nov 2004

Mads Dam Ellehøj


Basic parametres for callisto
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.


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)


Magnetic field ocean
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


Before galileo
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


Anderson et al 1998 and 2001
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.


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


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


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.


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??


An ocean
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)


Nagel et al 2004
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.


  • 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


  • 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


  • 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


Is it really undifferentiated
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


References
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