Example of the construction of a synthetic spectrum and the retrieval of parameters using 0D LTE slab model fits. The top panels show the temperature profile (left) and the column density profiles after 10⁴ yr (right) for small dust (black) and total (solid) and visible (dashed) H₂O (blue) and CO₂ (green). The light, vertical, dotted lines indicate the snow lines. The middle panel shows the synthetic spectra of each molecule (assuming a distance of 140 pc) while the bottom panels show the goodness of fit for a grid of slab models with varying temperature and column density. The white contours show the radius (in AU) of a circle whose area maximises the goodness of fit for each combination of temperature and column density. The overall best fit parameters are shown on the top panels as the crosses, with the vertical error bars representing the spacing of the grid of slab models.