Mars has permanent ice caps at both poles fourth composed mostly of solid carbon dioxide ("dry ice"). The ice caps exhibit a layered structure with alternating layers of ice with varying concentrations of dark dust. In the northern summer the carbon dioxide completely sublimes, leaving a residual layer of water ice. It's not known if a similar layer of water ice exists below the southern cap (left) since its carbon dioxide layer never completely disappears. The mechanism responsible for the layering is unknown but may be due to climatic changes related to long-term changes in the inclination of Mars' equator to the plane of its orbit. There may also be water ice hidden below the surface at lower latitudes. The seasonal changes in the extent of the polar caps changes the global atmospheric pressure by about 25% (as measured at the Viking lander sites).
These images show the planet during the transition between spring and summer in the northern hemisphere (summer solstice). The annual north-polar, carbon- dioxide frost (dry ice) cap is rapidly subliming, revealing the much smaller permanent water-ice cap, along with a few nearby detached regions of surface frost.
Deimos and Phobos are composed of carbon-rich rock like C-type asteroids and ice. Both are heavily cratered. Phobos and Deimos may be composed of carbon-rich rock like C-type asteroids. But their densities are so low that cannot be pure rock. They are more likely composed of a mixture of rock and ice. Both are heavily cratered. The Soviet spacecraft Phobos 2 detected a faint but steady outgassing from Phobos. Unfortunately, Phobos 2 died before it could determine the nature of the material; water is the best bet. The most prominent feature on Phobos is the large crater named Stickney (the maiden name of A. Hall's wife). Like Mimas's crater Herschel (on a smaller scale) the impact that created Stickney must have almost shattered Phobos. The grooves and streaks on the surface were probably also caused by the Stickney impact. Phobos and Deimos are widely believed to be captured asteroids. There is some speculation that they originated in the outer solar system rather than in the main asteroid belt. Phobos and Deimos may someday be useful as "space stations" from which to study Mars (Sw Mars) or as intermediate stops to and from the Martian surface; especially if the presence of ice is confirmed.