Models for the Density and Elasticity of the Earth's Lower Core

Abstract

The possibility of lowering the estimated central density <it>ρ</it><inf>0</inf> of the Earth below the value of order 15g/cm3 arrived at in a previous paper has been examined in some detail, with the continued use of Bolt's seismic <it>P</it> velocity distribution for the core. It has been shown <it>ρ</it><inf>0</inf> can be as low as 12.6 g/cm3 if suitable <it>ad hoc</it> assumptions, compatible with the seismic data, are made on the variations of the incompressibility <it>k</it> and rigidity μ in the lower core. The value of <it>ρ</it><inf>0</inf> could be further lowered to 12.3 g/cm3 (in agreement with much earlier calculations) if the region <it>E</it>′ were chemically homogeneous, but this is improbable. Seven model representations, all compatible with the recent seismic data and closely concerned with the value of <it>ρ</it><inf>0</inf>, are presented of the variation with depth of <it>ρ</it> (the density), <it>k</it>, μ, and related properties, in the lower core. The models exhibit the consequences of particular assumptions on <it>k</it> and μ, as well as on <it>ρ</it><inf>0</inf>, and provide a basis for important immediate inferences and for constructing whole Earth models. A number of interesting conclusions can be drawn if evidence presented by Birch that <it>ρ</it><inf>0</inf> ≦ 13 be accepted. The first is that there must be rigidity, decreasing with increase of depth, in a substantial part of the volume occupied by the regions <it>F</it> and <it>G</it>. The formal calculations would require this to occur in both <it>F</it> and <it>G</it>, and so give rise to the suggestion that the Earth's core contains a <it>second</it> solid layer <it>F</it> outside the inner core <it>G</it>. Although the margin of probability favouring this suggestion is as yet fairly slender because of various background certainties, the suggestion receives some support on other grounds. Strong new support is provided for the long-standing inference that the inner core is solid, and the requirement of decreasing rigidity with depth throughout much of the lower core is also strongly indicated. The calculations imply that complete absence of rigidity in the Earth's core would require a central density of at least 14.7 g/cm3.