The paper discussed during this week’s Magnetic Personalities was Suavet et al. (2013; PNAS) “Persistence and origin of the lunar core dynamo”.
Research suggests that the Moon once had an active magnetic field, much like Earth’s magnetic field. Evidence comes from palaeomagnetic measurements made on lunar rocks collected during the Apollo missions. Based on new measurements on these ancient lunar lava flows, Suavet et al. present convincing evidence for a surprisingly intense magnetic field on the Moon around 3.56 billion years ago. This means that the Moon’s magnetic field may have been present at least 160 million years longer than previously thought.
Earth’s magnetic field is generated by what’s called a dynamo, driven by heat convection causing fluid motion of the conducting liquid iron alloy in the outer core. Like Earth, the Moon most likely also has a melted iron outer core. Studies have shown, however, that due to the smaller size of the Moon, compared to Earth, a similar heat convection driven dynamo would only have been able to persist up to about 4.1 billion years ago. Recent results suggesting an active dynamo well beyond this time would have required a very different power source.
A few alternative power sources have been considered, including a precession driven dynamo caused by the Moon’s core and its mantle rotating around slightly different axes. If the boundary between the core and the mantle is not quite spherical, their relative motion could stir up the liquid outer core enough to generate a magnetic field. Another possible explanation relates to large-scale meteorite impacts capable of temporarily changing the relative motion of the mantle with respect to the core. However, such dynamos are only expected be short-lived (<10,000 years). The last known impact large enough to produce such an effect (producing an impact crater diameter of around 300 km) is dated to 3.7 billion years ago. Such an impact-driven dynamo therefore seems a rather unlikely source for the magnetic field recorded in lava rocks on the Moon 160 million years later.
The source of the Moon’s dynamo and when or why it decayed remains a mystery, but by learning more about it we may also improve our understanding of Earth’s magnetic field.