Spectral indicators: VNIR constraints and modelled ones. Constraints from VNIR observations as given in Table 1 correspond to the shaded zones. Modelled indicators are plotted as a function of grain size a, which is composed of an intra-molecular mixture of water ice, either crystalline hexagonal (Ih, solid line) or amorphous (AmI, dashed line), with AmC and Th, with a fraction of f = 0.05% (red) or 0.1% (black). The size is regularly indicated (in μm) along the curves: (a) BD2 versus BD1; (b) VIS2 versus VIS1; (c) VIS3 versus BD1; (d) A versus a. Bolometric Bond albedo constraints inferred from IR observations by Howett et al. (2014) are also plotted (dotted line).