Evaporation or Sublimation? Water or CO2? (approximate colors; credits: NASA)

Caption NASA:" Compare these two close-up pictures taken on Sol 20 (left) and Sol 24 of a trench dug in the Martian surface by NASA's Phoenix Lander. Those Soles of the Phoenix Mission, correspond to June 15 and 18, 2008, on planet Earth. Light-colored, dice-sized chunks, visible in the lower left shadow region of the trench in the Sol 20 image have vanished by Sol 24 -- a strong indication that the chunks were ice uncovered by digging the shallow trench. The vanishing act likely demonstrates the sublimation of ice in the trench, a process similar to evaporation, in which the ice went directly from solid to gas after it was exposed to sunlight and the thin, dry Martian atmosphere".