Question

Ancient Lunar Magnetic Field Models

The Moon currently lacks a global magnetic field, but magnetized crustal rocks indicate it possessed one between $4.25$ and $3.56\text{ billion years ago}$ ($\text{Ga}$). Two models are proposed to explain this ancient lunar dynamo:

*Model 1 (Thermal-Compositional Dynamo)*
This model proposes that the lunar dynamo was driven by thermal and compositional convection within a liquid metallic core. As the core cooled, solid iron crystallized at the center, releasing lighter elements (such as sulfur) into the remaining liquid outer core. The buoyant rise of these lighter elements, combined with heat loss, drove fluid convection that maintained a continuous, stable magnetic field of approximately $100\text{ }\mu\text{T}$ for nearly $700\text{ million years}$, ending only when core crystallization was complete.

*Model 2 (Mechanical Impact Dynamo)*
This model proposes that the lunar dynamo was not continuously active but was periodically restarted by massive basin-forming impacts. A giant impactor would transfer immense angular momentum to the Moon’s mantle, temporarily altering its rotation rate relative to the liquid core. This differential rotation at the core-mantle boundary generated shear and turbulence in the liquid core, initiating a dynamo. Each dynamo event lasted only $10\text{ to }20\text{ million years}$ before friction synchronized the rotation of the mantle and core, extinguishing the magnetic field until the next major impact.

Statement: According to Model 2, a lunar crustal rock that crystallized continuously over a $50\text{-million-year}$ period of impact quiescence (a time with no large impacts) would record a strong, steadily active global magnetic field throughout its entire formation.