What kept the Moon’s dynamo alive?

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 deduced that the rocks had been magnetized by a surface field of at least 13 μT, which is consistent with a strong dynamo. Although the age difference is just 4% between the rocks in the 2012 study and the younger rocks in the new study, one explanation for the prolonged life of the dipolar field beyond its expected span can be ruled out: an off-center hit by an asteroid that set the Moon rocking back and forth in its tidally locked orbit. No impacts big enough occurred that late. Another proposed mechanism remains in play: The chemically stratified layers that formed when the mantle crystallized could have become dynamically unstable and triggered a second round of convection. (C. Suavet et al., Proc. Natl. Acad. Sci. USA, in press, doi:10.1073/pnas.1300341110 .)