tokens: HWHM versus albedo. The modeled HWHM for 67P is located close to satellites of Mars, Phobos and Deimos, suggesting a similarity in the optical properties and structure of their surfaces, such as size of particles and porosity. The variation of the enhancement factor, against geometric albedo does not follow a recognizable trend (lower panel of Fig. 5 ). However, it shows two collections of data points, the values of the enhancement factor for all bright bodies studied in Rosenbush et al. (2002) are lower than 1.6, while for all dark objects they do not go beyond 1.8. It is notable that Phobos and Deimos including 67P do not resemble any group of data points. Another comparative study can be done for 67P and existing data for Centaurs and Kuiper belts objects (KBOs) ( Belskaya et al. 2008 ). Since these bodies are observed in the small phase angle range, for the comparison we only adopt the local phase curve of 67P at nm in the same phase angle range as those objects. The best-fit value of the phase coefficient in mag/deg for 67P is plotted together with the measured phase coefficients of KBOs and Centaurs (Table 1 of Belskaya et al. 2008 and references therein) with respect to the albedo (see Fig. 6 ). Fig. 6 Phase coefficient versus albedo for Centaurs, classical Kuiper belt objects (KBOs), and trans-Neptunian objects in band together with comet 67P/CG at nm. Figure 6 shows that 67P with the phase coefficient, , is located in Centaurs regions. The two particular Centaurs with similar phase coefficient to 67P are (54 598) Bienor and 1999 TD10. Centaurs are believed to have a greater steepness in their phase curve than other solar system bodies ( Bauer et al. 2003 ; Rousselot et al. 2005 ). 5.2. Comparison with laboratory measurements of the phase curve at small phase angles We also compared our results with laboratory measurements that study the dependence of the photometric characteristics of particulate surfaces on the surface properties, that is, albedo, grain size distribution, and porosity. There are not many experimental studies at very small phase angles. The measurements that are made in the opposition-effect region were carried out by Jost et al. (2016) for icy particles with a high-albedo close to unity and by Psarev et al. (2007) for low- and high-albedo samples. We found that the laboratory reflectance data from carbon soot at the wavelength of 630 nm in the phase angle range of 0.008–1.5 ( Psarev et al. 2007 ) show a behavior that is very similar to the measured local phase curve from the Imhotep-Ash region of 67P at nm. The local phase curve we extracted was normalized at to make it comparable with normalized laboratory measurements (see Fig. 7 ). In Fig. 7 two samples, the ultra-dispersed quartz and the carbon soot, behave similarly for the phase angles >0.2 (the limit we determined for the local phase curve of 67P from OSIRIS close-flyby images). However, light scattering by quartz sample is defined by its very special
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