Ratios among CO, CH₄, and C₂H₆, the three most volatile species systematically targeted in comets, compared to comets measured within R_h = 2 au, for which H₂O sublimation is expected to be fully activated. The position of 13P results from its unusually large CO and CH₄ abundances relative to C₂H₆ among comets (see Table 3 and related text in Section 4.4.1). The color bar at right shows that most comets measured within 2 au have lower CO abundance ratios relative to H₂O (e.g., CO/H₂O < 10%), as evidenced by their dark-blue to black colors. For completeness, two comets measured beyond 2 au are labeled and their positions marked “X.”