Question

Read the passage below:

For decades, planetary geologists attributed the high-albedo markings on the lunar surface known as lunar swirls exclusively to recent cometary impacts that scoured the uppermost layer of regolith. However, this impact-scouring hypothesis failed to account for why these bright optical anomalies coincided precisely with localized crustal magnetic anomalies. Recent magnetohydrodynamic simulations offer an alternative mechanism: strong localized magnetic fields deflect the solar wind, thereby shielding the underlying regolith from space weathering induced by solar proton bombardment. Because unshielded regolith darkens over time due to the reduction of iron minerals into nanophase metallic iron ($Fe^0$), the magnetically protected regions retain their pristine, high-reflectivity surface. Crucially, spectrographic measurements of swirl interiors revealed a pronounced deficiency in hydroxyl ($OH^-$) absorption features, confirming that hydrogen ions from the solar wind are excluded from interacting with oxygen in the lunar soil. Consequently, the prevailing consensus has shifted toward solar wind deflection as the primary mechanism governing swirl formation, although the origin of the crustal magnetic anomalies themselves remains a subject of ongoing debate.

Which sentence in the passage provides empirical evidence directly validating the physical mechanism responsible for shielding the regolith from chemical alteration?