Mosaic of optical images of the residues OR7 (a.) and OR8 (c.), formed on Silicon windows during the 2021 session. Images (b.) and (d.) are close-up images. Residue OR7 and OR8 were formed during the same session from N₂-CH₄ and a NH₃-CH₄ ice mixtures, respectively. The different structures of the residues are most probably due to the different chemical composition of the starting ice (see text).