Comparison between the four grand average spectra (black, A–D) and some representative laboratory and synthetic spectra shown in Figure 4 (gray). The number above each spectrum corresponds to the same spectrum number as shown in Figure 4 and Tables 5 and 6. All spectra were normalized to unity between 1.74 and 1.77 μm and offset vertically for clarity. Wavelength regions that show subtle features in the Umbriel spectra are highlighted by pairs of dotted lines spanning 1.583–1.597, 1.622–1.637, and 1.691–1.731 μm. The laboratory spectra shown in Panel I are NH₄ chloride (11), NH₄ bicarbonate (10), and thermonatrite (13). The synthetic spectra shown in Panel II include kaolinite (14), amorphous NH₃ (3), NH₃ ice (4), methylamine (15), and ethylamine (16).