PerspectivePlanetary Science

Ancient planetary dynamos, take two

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Science  31 Jul 2015:
Vol. 349, Issue 6247, pp. 475-476
DOI: 10.1126/science.aac6224

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Within our solar system, the Earth and Mercury are in a class of their own, being the only rocky planets presently possessing global magnetic fields of internal origin. Given how different these fields are (see the figure), how these two planets got into this situation is a complicated story. With the recent discovery by Johnson et al. (1) of ancient remanent magnetism trapped in crustal rocks dating as far back as 3.9 billion years ago on Mercury, and, on page 521 of this issue, by Tarduno et al. (2) of a remanent magnetism for Earth dating back 4.2 billion years ago, the plot may thicken even more. Or could a tape rewind provide us with the clues we may have missed so far to explain the observations?