Astounding Science Fiction, British Edition — page 75: what you’re looking at

Machine-transcribed from the original scan — historical spelling and the odd misread are preserved.

VENUS OF TROY may be noted that this inner core has about twice the diameter of the moon. The average density of the earth’s outer crust is about 2.67; but the density of the core varies from 12.2 in the center to 10.1 at the transition level; the average density of the earth as a whole is about 5.52. In the past, the existence of this dense inner core has been explained as being due to a separation and concen- tration of the densest materials, pre- sumably at some time when the earth was in a liquid state. Dr. Ramsey thinks that this is not necessarily the case. If olivine, the principal con- stituent of the intermediate layer, were to collapse its molecular struc- ture at a critical pressure of about one million four hundred thousand atmospheres to give a new phase with about twice the density, this would account for nearly all of the data which we have at present on the con- ditions near the center of the earth. As we have already seen, this is not an unwarranted assumption, since many other materials collapse at much lower pressures. A few moments’ thought at this point will show you that a certain critical mass is required before the central core is produced, since if an arbitrary Planet X is too small, the internal pressure never builds up to the necessary level to produce the denser phase. It should be noted, however, that the critical pressure would depend, not only on the total mass. of the planet, but also on its internal temperature, since all known phase transitions are dependent on both factors. Mercury, Mars, and our moon are apparently in the lower range, existing without cores; each of the other major planets, it is believed, may support one. Dr. Ramsey and Dr. J. M. Lighthill have considered at some length a number of the pre- dictable consequences of such a process 73 of core formation. Perhaps the most interesting situation of all is that of a planet which is just slightly smaller than the critical size. For simplicity in his calculations, he has used the pres- sure at the center of the body as the independent variable, with its mass as the dependent variable. The accom- panying graph—taken from Nature, 168, p. 677 (1951)—shows the com- plex relationship which connects the two at a particular temperature. Point Y represents the critical pressure P, produced by the critical mass M,,. From known values of density and pressures, et cetera, inside the earth, it may be calculated that point Y corresponds to a mass 0.80 times that of the earth, with a radius of about 3900 miles. To the left of Y, a smooth descending curve is obtained by straightforward and conventional cal- culations. The right hand portion of the curve shows the behavior of the mass-pressure relationship if we as- sume that the collapsed ore material connicloooks.c© iim