On the Internal Stucure of the Planet Mars

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Abstract
The hypothesis that Mars is of similar constitution to the Earth is investigated. On this basis, the planet can have no liquid core and will consist of two solid zones: an inner one with properties similar to the mantle of the Earth and an outer one similar to the outer shell of the Earth. Seismic data for the Earth have shown that the bulk-modulus is a (different) linear function of the pressure in each zone, and these laws enable a series of two-zone models to be calculated with the mass of the inner zone as parameter. Numerical values have been found for the pressure- and density-distributions, for the overall radius, the moment of inertia, and the gravitational energy. Agreement with the observed radius requires a core-mass in the range of about 0.38–0.42 × 10 27 g, but it is not possible to settle the model more definitely because of the considerable uncertainties in the present observed mass, radius, and dynamical ellipticity. With rising internal temperature, the pressure at which the discontinuity between the inner and outer zones occurs may increase, and if so expansion of the planet might result by the order of 10 km in radius. This might produce rifts at the surface, but no mountains of terrestrial type produced by contraction would be expected on Mars. The quadratic law for k in terms of p , which less accurately can represent the seismic data for the Earth, has also been applied to Mars. The resulting model is so much more nearly uniform that it leads to an unacceptably low value for the dynamical ellipticity. The inference can be made that on present data the various properties of Mars are entirely consistent with its being of similar composition and structure to the Earth and that the elastic properties are in accordance with a linear relation between k and p . The linear law is also applied to determining the structure of the Moon, and leads to a radius about 1 km less than the observed value, which would be explicable by non-hydrostatic equilibrium of the outer layers. Adjustment by this amount brings the moment of inertia into close agreement with that based on purely dynamical theory.

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