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[39.06] Callisto Thermal Model

E. Jensen (UCLA Dept of ESS)

When Galileo flew by Callisto, data from the magnetometer showed strong evidence for the presence of an ocean. Here, we look at constraining the gravitationally determined range of internal structures for Callisto using this information. A thermal evolution model of Callisto shows that Callisto has a large rocky core surrounded by a roughly equal distribution of ice and rock mix layer. The mix layer is surrounded by an ocean and ice shell.

A sizable rocky interior of Callisto would provide the radioactive energy required to allow the existence of a global sized ocean. A three layer model was used for Callisto where the core consisted of silica rock, the middle layer of silica rock and ice, and the outer layer of (initially) liquid water and ice I. On Callisto, the surface heat flux releases more heat into space than radioactive decay in the silica rocks provides. This leads to cooling as seen through a gradually decreasing temperature in the ocean. With the growth of the ice I layer, the ocean also experiences a gradual increase in pressure. Blue circles in the Figure demonstrate the interior structures possible for Callisto to maintain an ocean for 4.5 billion years.

The white triangles indicate possible interior structures for Callisto, while the blue circle indicate the interior structures which allow an ocean to exist for 4.5 billion years. The radius of the silica core with respect to the radius of Callisto is plotted on the y-axis. The density of the middle layer of ice and rock mix (which in turn indicates the thickness of this layer) is plotted on the x-axis in X 10³ kg/m^{3}. Finally, the density of the outer ice I and liquid water (or in the case of the white triangles, ice III) is selected to the allowable density of 1.050 X 10³ kg/m^{3}.

The author(s) of this abstract have provided an email address for comments about the abstract: firerock@alum.mit.edu