Magmatic origin of the Dewar magnetic anomaly: Implications for an early lunar dynamo.

The Moon's ancient magnetic field provides critical insights into lunar thermal and magnetic evolution, yet the lifetime of the interior dynamo remains debated. Returned samples yield complex and contradictory paleomagnetic records, while orbital data reveal crustal magnetic anomalies of uncertain origin from either a core dynamo or transient impact-generated fields. Here, we jointly invert gravity and magnetic observations in the region around the Dewar swirl, a high-albedo feature associated with the Dewar magnetic anomaly. We identify a shallow, magnetized, high-density body consistent with buried mare basalt. Our estimates require a paleointensity exceeding 11 microtesla, suggesting an active lunar dynamo about 4.2 billion years ago, as constrained by the superposed basin ejecta. The results also show that swirl formation requires horizontal magnetization and iron oxide enrichment. These findings link a magnetic anomaly to its geologic origin and magnetizing field, providing constraints on the lunar magnetic and volcanic history.