Fields of Old

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Science  25 Feb 2011:
Vol. 331, Issue 6020, pp. 989
DOI: 10.1126/science.331.6020.989-b

Earth's magnetic field is sustained by fluid motion in the planet's liquid iron outer core, spurred today by latent heat released as the solid inner core grows. The timing of the field's appearance has been uncertain, as back calculations imply that close to 2 billion years may have been needed for Earth's internal temperatures to cool sufficiently to allow a solid core to form. Biggin et al. report paleomagnetic measurements, and improved new dates, from rocks in South Africa that provide further evidence that Earth had a stable magnetic field by 3.5 billion years ago. As a check that the original magnetic signature had not been reset, the authors showed that cobbles in a nearby conglomerate preserved random magnetic orientations. These data thus further strengthen the inference that early Earth had a field and that some other process aside from solidification of the inner core was driving the geodynamo then. The data further hint that the early field was capable of reversing.

Earth Planet. Sci. Lett. 302, 314 (2011).

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