The Moon’s magnetic field used to have both the strength and the dipolar structure of a vigorous, dynamo-generated field like Earth’s. Now, however, the lunar field is weak and patchy. Accounting for the field’s enfeeblement might seem straightforward. As it aged, the Moon’s molten core cooled and shrank to the point at which it could no longer sustain a dynamo. The trouble is, whereas an analysis of Moon rocks published last year put that transformation at 3.7 billion years ago, models of thermal convection in the Moon’s core put it at 4.1 billion years ago. What kept the dynamo alive for the intervening 400 million years? To find out, MIT’s Clément Suavet and his collaborators recently subjected two Moon rocks, both 3.56 billion years old, to magnetic, thermal, and other tests. (The photo shows a 5-g sample of one of the rocks next to a 1-cm3 cube.) The researchers...

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