Illustration of the different spectral trends for two on-swirl and off-swirl pairs of spectra from Reiner Gamma. Panels a–c: pair of spectra with a low CSSI, where the space-weathering model provides a better fit. Panels d–e: pair of spectra with a high CSSI, where the soil compaction model provides a better fit. The best-fit spectra for a compacted off-swirl spectrum are shown in panels a and d. On-swirl spectra that have been artificially space-weathered according to Wohlfarth et al. (2019) to obtain a best fit to the measured off-swirl spectrum are shown in panels b and e. The locations of the spectra are displayed in Fig. 1a. The exact locations, best-fit model parameter values and CSSI values are listed in Table 3. The best-fit root mean squared errors (RMSE) are listed in the captions of the individual diagrams.