Secular variation of a metallic asteroid dynamo

Abstract:

The mechanisms by which inward core solidification may drive dynamo activity, and the properties of any fields that may result from this process, are highly uncertain. The fast cooling rates of the IVA iron meteorites suggest that their parent core had its silicate mantle removed by planetary collisions during the early solar system. Due to the resulting rapid radiative surface cooling, the IVA parent core solidified from the top-down, permitting a cold metallic crust that feasibly experienced fields generated by the hot interior liquid as it inwardly solidified. The IVA meteorites therefore potentially contain unique paleomagnetic information regarding top-down solidification. Through x-ray microscopy of the cloudy zone in the Steinbach and Chinautla meteorites and traditional paleomagnetic measurements on silicates extracted from the Steinbach, Bishop Canyon and São João Nepomuceno meteorites, we argue that the IVA parent core generated an intense (>100 µT) and secularly varying (time-scale <100 kyr) field during top-down solidification. These results show that certain iron meteorites are capable of having experienced dynamo fields, and that asteroids can generate directionally varying magnetic activity, strengthening claims that the fundamentals of dynamo activity are consistent across small and large bodies. Models of the thermochemical evolution and solidification of an unmantled core suggest that this field resulted from liquid motion induced by the repeated delamination and sinking of material from the base of the inwardly solidifying crust. This efficient dynamo generation mechanism was likely capable of readily creating magnetic activity at the slow cooling rates expected within mantled, inwardly solidifying cores (e.g., Mercury, Ganymede, many asteroids). Combining this observation with that of efficient solidification-driven dynamos during bottom-up asteroid core solidification, it is likely that magnetic activity was widespread in the early solar system.