A comparison of laboratory absorbance by H₂O + HDO from Figure 4 and H₂O + SO₂ at 100 and 50 K from M. J. Loeffler & R. L. Hudson (2010). SO₂ absorption does not overlap the HDO absorption. Carbonates absorb at shorter wavelengths than SO₂, see, e.g., Figure 3 in R. J. Cartwright et al. (2020), so they too do not contribute to the 4.14 μm absorption in our Titania spectrum. Because of the abundance of water on the surface of Titania, it is likely that the absorption in our spectrum of Titania is exclusively due to HDO. The spectra are vertically offset for clarity.