Measured spectral trends from on-swirl to off-swirl locations for spectral pairs representing a spectral trend consistent with reduced space-weathering and increased soil compaction, respectively. Panels a, b: mare swirl Reiner Gamma. Panels c, d: highland swirl near Dufay. The footprint size of the spectra is 700 × 700 m. Each series of spectra was extracted at seven equidistant points on the straight line connecting the on-swirl location with the corresponding off-swirl location. The spectra which are brightest and darkest at 1.579 μm, respectively, were used to compute the compaction-significance spectral index (see Sect. 2). The wavelength range is 0.661 μm to 2.577 μm.