Segregation of materials in cosmogony

Abstract

Were the raw material for forming planets to have the chemical composition usually assumed, considerable segregation of light and heavy elements must have occurred to an extent that varied from one planet to another. The problem of such segregation is examined in the case of a body of the raw material acted upon by its own gravitation. If the material is entirely gaseous, segregation could not occur in any acceptable time. This shows that some other phase must have played a part, and so confirms the view that the planets were formed 'cold'. Dust-grains provide the phase most likely to be required, but grains of normal interstellar dust would settle too slowly; there must have been some adhesion of such grains to form at any rate a relatively small number of somewhat larger grains. After some discussion of the resistance offered by a gas to a grain moving through it, formulae are given for characteristic times associated with various cases of such motion. These are applied to a proto-planet of the sort contemplated in one particular theory of the origin of the solar system (McCrea 1960). If some adhesion of grains results from encounters between them produced by thermal agitation, then more considerable adhesion or accretion will occur when an enlarged grain falls towards the centre of such a proto-planet. It is shown that this accounts for: (a) the existence of planetesimals having diameter of order 1 m near the centre of the proto-planet, (b) the aggregation of such planetesimals into a body of planetary mass, (c) the release of energy that could account for the dispersal of the light materials. These processes could occur in an acceptably short time interval. The formulae are also applied, more briefly, to a body of material similar to a 'primary object' of the sort considered by Urey (1964). They indicate that there should be even less difficulty in accounting for segregation of heavy material in this case. However, if segregation could occur as suggested in the first application, it may be difficult to see how material could reach the greater density in the second case without some segregation having already taken place. If this work is correct, a solid body like the Earth can come into being only because of the prior existence of the smallest known naturally occurring bodies in the cosmos, the interstellar grains; an Earth exists because a galaxy is slightly dusty.