Cores of Terrestrial Planets

Abstract

ONE feature of Prof. H. C. Urey's new theory¹ of the development of the planets is that it leads him to conclude on several grounds that the earth's central core has an iron composition. This conclusion is in conflict with Ramsey's proposal² that the density throughout the earth's deep interior, at least below a depth of 1,000 km., is essentially determined by pressure right through to the earth's centre rather than by change of chemical composition. The conclusion as stated is also in conflict, though to a less degree, with my proposals, which differ from Ramsey's through finding a balance of evidence³ in favour of a separate chemical composition, such as nickel–iron with possibly some denser material, for the earth's inner core, but agree with Ramsey's in postulating a modified ultrabasic rock composition for the outer part of the core, that is, between depths of 2,900 km. and 4,980 km.—the region that I have referred to⁴ as ‘E’; the latter postulate will be referred to as α. On the further hypothesis, β say, that the terrestrial planets have a common primitive composition, Ramsey's theory implies that neither Venus, Mars nor Mercury has an inner core, whereas my work favours the view that all three have inner cores.