The normalized reflectivity gradient of interstellar comet 2I/2019 Q4 (Borisov) across the g and i bands as a function of time (a) and the heliocentric distance (b). No significant variation is seen. The horizontal dashed–dotted line is the mean value of the normalized reflectivity gradient, with the gray zone representing the associated ﹩\pm 1\sigma ﹩ uncertainty. The vertical gray dotted lines in panels (a) and (b) mark the perihelion epoch and distance of the comet, respectively.