How did the Moon's small core make a strong ancient field?
In plain words
Some old Moon rocks record a magnetic field about as strong as Earth's, though the Moon's iron core is tiny. Either an unusual power source existed or the measurements are misleading.
Precise statement
Apollo sample paleointensities suggest surface fields of roughly 0.2 to 1 G between about 4.0 and 3.5 Ga, difficult to power by thermal convection in a core of radius near 350 km. Determine whether these records reflect a core dynamo (driven by precession, mantle stirring by impacts, or a basal magma ocean), impact-generated transient fields, or overestimated paleointensities. An answer is a reconciled paleointensity record with a physically allowed source.
What would settle it
Paleointensity measurements on well-dated, unshocked lunar samples, including new returned samples, with energy-budget tests of candidate dynamos.
Status in the literature
Unverified note
Narrett and coauthors 2025 (Science Advances, https://doi.org/10.1126/sciadv.adr7401) show that basin-forming impact plasmas could amplify a weak dynamo field of about 0.02 G to about 0.4 G near the basin antipode, which may explain the strong paleointensities without a strong dynamo; contested.