Structure of the Terrestrial Planets

Abstract

BULLEN¹ has suggested that to reconcile known data for Venus, Earth and Mars with the hypothesis that they have the same overall composition, the outer core of the Earth may be composed of Fe₂O. The principal reason seems to be the resolution of an alleged discrepancy between the modern solid radius of Venus determined by radar², 6,053.7±2.2 km and the value of 6,270 km computed by Bullen on the phase-change hypothesis for the core³. But this latter value was arrived at on the basis of somewhat approximate calculations, whereas more accurate calculations⁴ find no discrepancy of any significance in interpreting Venus as possessed of three main zones with mechanical-elastic properties the same as in the corresponding zones of the Earth. In such calculations some allowance for the existence in Venus of a small inner core could readily be made, but it is clear that no adjustment for such an inner core, which in the Earth has volume only 0.76% of the whole, could seriously affect the computed value of the overall radius. For if the inner core were completely annihilated and the Earth contracted down geometrically without change of volume of the remainder, the outer radius would shrink only by 16 km, so that averaging of its properties with those of the main bulk of the core would produce negligible change of radius, whereas Bullen's discrepancy is over 200 km.