Fields of Old

摘要

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 forfarth's internal temperatures to cool sufficiently to allow a solid core to form. Biggin etal. 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.