Comparison between the four grand average Umbriel spectra shown in Figure 3 (black, A–D) compared to laboratory spectra (LS) and synthetic spectra (SS) (gray, numbered 1–20) of different candidate constituents. The candidate constituents shown here are summarized in Table 3 using the same numerical sequence (1–20): (1) LS of mixed NH₃-hydrates, (2) LS of flash-frozen NH₃–H₂O solutions, (3) SS including amorphous NH₃, (4, 5) SS including crystalline NH₃ ice, (6-8) LS of NH₄ carbonate, (9, 10) LS of NH₄ bicarbonate, (11, 12) LS of NH₄ chloride, (13) LS of thermonatrite, (14) SS including kaolinite, (15) SS including methylamine, (16) SS including ethylamine, (17) SS including propionitrile, (18) SS including ethylene, (19) SS including propyne, and (20) SS including methanol. The band centers of the 2.14 μm, 2.2 μm, 2.22 μm, and 2.24 μm bands (Tables A2–A5) are highlighted by the purple, blue, green, and red zones, respectively. All spectra were normalized to unity between 1.74 and 1.77 μm and offset vertically for clarity. Some of the laboratory spectra were vertically scaled for easier presentation in panels II and III. The synthetic spectra were generated with the previously described Hapke-Mie modeling program (model components, grain sizes, and mixing ratios are summarized in Table 4).