Density maps of the VIR visible data set used in the study. Panel A corresponds to the VSS mission phase; panel B to the VTH; panel C to VSH; panel D to VH2; and panel E regroups every four. For details about the mission phases, see Table 1 and Sect. 2.2. Each map is built with TOPCAT with a Plate Carrée projection (see Sect. 2.5), and observations are represented as points. The scale corresponds to the square root of the observation density.