Question

### Origin of the Ancient Lunar Magnetic Field

Although the Moon currently lacks a global magnetic field, analysis of lunar rock samples returned by the Apollo missions indicates that a strong magnetic field existed on the Moon between $4.25$ and $3.56$ billion years ago. Two scientists discuss the potential mechanisms that generated this ancient lunar dynamo.

Scientist 1
The ancient lunar dynamo was driven by thermal convection within the Moon's liquid metallic core. Early in the Moon's history, the core was extremely hot and surrounded by a cooler mantle. This steep temperature gradient caused heat to flow rapidly outward, creating buoyant plumes of hot, liquid iron that rose through the outer core while cooler liquid sank. This thermal convection, combined with the Moon's rotation, generated a global magnetic field. The dynamo ceased approximately $3.5$ billion years ago because the core cooled to a point where the temperature gradient was too low to sustain convection, and the outer core began to solidify.

Scientist 2
The Moon's core is too small to have sustained a thermal convection dynamo. Instead, the dynamo was driven by precession-induced mechanical stirring. Billions of years ago, the Moon was much closer to Earth, and Earth's gravitational tidal forces exerted a powerful torque on the Moon's mantle. This torque caused the mantle to rotate along a slightly different axis than the liquid core (precession). The friction and velocity difference at the boundary between the wobbling mantle and the liquid core mechanically stirred the liquid iron. This mechanical motion, rather than thermal convection, powered the dynamo. The magnetic field shut down as the Moon drifted further from Earth, weakening the gravitational torque and ending the precession-induced stirring.

Based on the passage, Scientist 2 claims that the ancient lunar dynamo was powered by which of the following processes?