  Thanks to Rosetta orbiter's and Philae lander's data our knowledge of
cometary nuclei composition has experienced a great advancement. The properties
of 67P/CG nucleus are discussed and compared with other comets explored in the
past by space missions. Cometary nuclei are made by a collection of ices,
minerals, organic matter, and salts resulting in very dark and red-colored
surfaces. When far from the Sun, exposed water and carbon dioxide ices are
found only in few locations of 67P/CG where the exposure of pristine subsurface
layers or the recondensation of volatile species driven by the solar heating
and local terrain morphology can sustain their temporary presence on the
surface. The nucleus surface appears covered by a dust layer of variable
thickness. Dust grains appear mostly dehydrated and are made by an assemblage
of minerals, organic matter, and salts. Spectral analysis shows that the
mineral phase is dominated by silicates, fine-grained opaques and ammoniated
salts. Aliphatic and aromatic groups, with the presence of the strong hydroxyl
group, are identified within the organic matter. The surface composition of
cometary nuclei evolves with heliocentric distance and seasonal cycling:
approaching perihelion the increase of the solar flux boosts the activity
through the sublimation of volatiles which in turn causes the erosion of
surface layers, the exposure of ices, the activity in cliffs and pits, the
collapse of overhangs and walls, and the mobilization and redistribution of
dust. The evolution of color, composition, and texture changes occurring across
different morphological regions of the nucleus are correlated with these
processes. In this chapter we discuss 67P/CG nucleus composition and
evolutionary processes as observed by Rosetta mission in the context of other
comets previously explored by space missions or observed from Earth.
