Measured wavelength shifts vs. averaged D_λ. The averaged D_λ is the average value as obtained from the Eris entries in Table 2, the error bar is just the standard deviation around the average. D_λ is used as proxy of real depth in the surface, as indicated by the legends on the top of the figure. Note that with exception of the XS spectrum, filled symbols are used for absorption bands in the visible and open ones for bands in the near-infrared (λ > 1000 nm).