Spectral gradient, estimated in the 550–800 nm range, of Phobos’ red unit (11%/(100 nm); red vertical line) derived from spectra presented in Fraeman et al. (2012) compared to the values reported by Fornasier et al. (2007) for cometary nuclei, Jupiter Trojans, and the different dynamical classes of the transneptunian population. Frequency refers to the number of objects in the different histograms. The figure was adapted from Fornasier et al. (2007). The blue star indicates the spectral slope of comet 67P determined at phase 1.3º (12.6%/(100 nm)) and at a heliocentric distance of 3.6 au by Fornasier et al. (2015).