Modelled brightness temperature T_b (K) of a regolith of grains with a given size of 10 µm (solid line), 50 µm (dashed line), or 500 µm (dotted line) at equilibrium temperature at a Jupiter distance of 4.96 AU in the wavelength range of the JWST-MIRI instrument. The grains are composed of pure crystalline ice (Ih90) with intra-mixed AmC with a fraction ƒ = 3%. The esa (black) or δ-Edd (red) diffraction corrections are used here, and ξ = 0. The solar contribution (orange) is plotted for all sizes in the case of an esa diffraction correction.