The G/R intensity ratio as a function of the heliocentric distance (AU). The same symbol is used for the different spectra of each comet. The most distant comets have higher ratios. Other values from the literature are also plotted. The solid line represents the average value only for our sample of spectra, and the standard deviation (σ) range is denoted with horizontal dashed lines. If we only consider the data at r < 2 AU, the average value of the G/R ratio is 0.09 ± 0.02. The vertical dashed line at 3 AU represents the distance out to which the sublimation of water is strongly decreasing (Crovisier & Encrenaz 2000).