Band peak location and equivalent band depth of the main absorption features. Band of methane ice is plotted in red, that of N₂ and CO, in purple and green, respectively. The results for data at sub-Earth east longitude of 5°, 41°, and 111° are represented with dots, diamonds, and crosses, respectively. Vertical dotted and full lines represent the computed mean band peak position and the position of pure methane ice as reported by Quirico & Schmitt (1997b) when applicable, respectively. The horizontal dashed line provides the computed mean equivalent absorption band depth. M. and M.P. give the mean fractional band depth and the mean band peak position, respectively.