What type of ice primarily composes the South Polar Ice Cap?

The South Polar Ice Cap on Mars is primarily composed of carbon dioxide ice, commonly referred to as "dry ice." During the Martian winter, temperatures drop significantly, allowing carbon dioxide to freeze and form an extensive polar ice cap that can vary in thickness and size with the seasons. In the summer, much of this carbon dioxide sublimates back into the atmosphere, demonstrating the unique seasonal changes found on Mars that aren't present on Earth.

While water ice is also present on Mars, especially in polar regions and beneath the surface, the distinctive feature of the South Polar Ice Cap during periods of freezing is its composition of carbon dioxide ice. This fact highlights the difference in environmental conditions and the types of ice that can form on Mars compared to Earth. The presence of the other options, such as ammonia and methane ice, is not typical in Martian polar regions and thus not responsible for the formation of the ice cap in the south, further solidifying carbon dioxide ice as the correct answer.