Bottom spectrum: (50000) Quaoar (Q2, solid line) with Spitzer photometry data (triangle and diamond symbols) obtained at two different epochs plotted with the corresponding best-fitting spectral model (dashed line, parameters are listed in Table 3, column Spitzer) that includes CO diluted in N₂ and CO₂. The best-fitting model for the spectral dataset without CO and CO₂ is also overlapped and shown for comparison (grey dotted line, parameters are listed in Table 3, column Q2). Both models, convolved by the Spitzer IRAC filter bandpasses (horizontal segments), are shown as crosses (black and grey, respectively). Top spectrum: (50000) Quaoar (Q4, solid line) and corresponding best-fitting spectral model (dotted line, parameters are listed in Table 3, column Q4). The model, convolved by the Spitzer IRAC filter bandpasses (horizontal segments), is shown as black crosses. The top spectrum has been shifted by +0.1 units for clarity.