a) Cylindrical map of the H₂O column density for data with a tangent altitude between 0.5 km and 1 km at any LST. VIRTIS-M pixels are identified on the surface by means of the position of the tangent point. Points having a column density exceeding the maximum value indicated by the colour ramp are marked in white. b) to d) The same map distribution for 1 ≤ tangent altitude ≤ 2.0 km, 2.0 ≤ tangent altitude ≤ 3.0 km and for tangent altitude ≥3.0 km is shown. e) True colour albedo map from Ciarniello et al. (2015), as a reference.