Mixing ratios of primary volatiles in 21P/Giacobini–Zinner. In this graphic, temporal behavior of molecular mixing ratios in 21P/G-Z are shown. In each panel mixing ratios relative to measured water are shown in different colors and are compared with the median reference value for JFCs (black line) and median reference value among all sampled comets (blue line). On the x-axis, both days to perihelion passage and observation dates are reported. On the y-axis molecular abundances of each detected species are reported. Panel (a) shows CO mixing ratios measured with iSHELL (green dots), the weighted mean value and error (green line and shadow) across the observing campaign, and also reports previous measurements performed by Mumma et al. (2000) and Weaver et al. (1999) for comparison (triangles and squares, respectively). Panel (b) shows ethane mixing ratios measured with iSHELL (purple dots), the weighted mean value and error (purple line and shadow), and previous measurements reported by DiSanti et al. (2013) and Mumma et al. (2000; diamonds and triangles, respectively). Panel (c) shows methanol mixing ratios measured with iSHELL (yellow dots) and their possible seasonal trend (yellow line), along with the previous measurements reported by (DiSanti et al. 2013) and Weaver et al. (1999; diamonds and squares, respectively). Finally, panels (d)–(h) report hydrogen cyanide, acetylene, formaldehyde mixing ratios (or upper limits) measured with iSHELL (orange, pink, cyan, red, and light purple dots, respectively), and previous upper limits (squares) reported by Weaver et al. (1999).