Polar ice caps form largely in response to the decreased solar energy received at the two poles of a planet. On Earth, the North and South Poles are oriented more vertically on the Solar Plane; this means that the planet's equator region receives the largest amount of solar radiation, while the poles receive the smallest amount. Aside from creating climates of intense cold (lack of heat), this causes evaporated atmospheric water to fall in the form of snow and sleet, creating large regions of ice that do not melt quickly. The more snow falls on the region, the more ice is formed and collected; the end result, after several thousand years, is that the North and South Poles are covered by a solid layer of "phase matter;" on Earth the matter is water and the phase is frozen. The North Pole has no landmass underneath; it is a free-floating mass of solid ice "between 9 and 12 million km^2" (Wikipedia). The South Pole has landmass underneath, making it more stable and significantly larger, at about 27 million km^2. Since ice is composed of fresh water (as opposed to salt water, in which it may rest), the South Pole contains over 70% of the Earth's total fresh water mass. Polar ice caps on other planets may be composed of other matter in a frozen phase; Mars has polar ice caps composed mostly of solid carbon dioxide ice.