# What causes the seasonal change in size of the Martian polar caps? During what season would the largest amount of dry ice be present at the polar cap?

$C {O}_{2}$ from the Martian atmosphere condenses upon cooling, then evaporates upon warming. Thus the maximum amount of solid $C {O}_{2}$ at either pole occurs during that hemisphere's winter.
Mars has a thin atmosphere, but the atmosphere it does have is mostly $C {O}_{2}$ gas. On Earth this would remain gaseous year-round but Mars is cold enough to allow the $C {O}_{2}$ to condense during winter, forming "dry ice".
As we see on Earth, winter alternates between hemispheres, and for the same reason. Mars rotates around a tilted axis, which brings alternately more warmth from the Sun to the northern hemisphere and less to the southern hemisphere, then the reverse. The polar $C {O}_{2}$ ice caps grow and shrink accordingly.