Model fitting of one VIRTIS-H spectrum. (A) Minimization of χ² differences for a set of synthetic spectra obtained for 13 different values of atmospheric temperature in the UMLT region, T_dist stands for temperature (K) from a fixed value used as reference; (B) synthetic spectrum best fits an individual nadir spectrum taken by the instrument VIRTIS-H. The fit is carried out using frequencies ranging 4.20–4.35 μm. The first-step fit and the residuals are also shown with green and brown lines, respectively (see text.)