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).