The Core Paradox

In a recent paper we noted the possibility that the liquid in the outer core was thermally stably stratified and that this stratification might prove a powerful inhibitor to circulation of the outer core fluid of the kind proposed for generation of the earth's magnetic field. Since the appearance of our original paper, a number of suggestions have been made for escaping this seeming paradox. The suggestions have been (1) that the liquid in the outer core is probably not pure iron; (2) that the liquid in the outer core consists of a slurry made up of liquid and fine iron particles in suspension; (3) that we are incorrect in our estimate of the melting curve of iron; or (4) that we are incorrect in our calculation of the adiabatic gradient. We have examined these suggestions and find that none of them seem likely to provide us with a reasonable escape from the core paradox. We note, however, that we previously overlooked the fact that the lower one-third of the fluid in the outer core may have a temperature distribution along an adiabat and might circulate freely. We do not know whether circulation of this fluid would be sufficient to produce an earth's magnetic field.