Mars's atmosphere is composed primarily of carbon dioxide. During summer in Mars's southern hemisphere, global temperatures are high enough that most carbon dioxide is in the gas form in the atmosphere. Temperatures in the polar north, however, drop low enough that some of this carbon dioxide freezes, snows out, and collects in the seasonal polar ice cap. In the harsher southern winter, however, the southern ice cap grows large enough that roughly half of Mars's atmosphere disappears! For more on Mars's polar ice caps click here.