What is Snowball Earth?
Planet Earth covered by ice from pole to pole for long periods in the geological past.

Snowball earth describes the coldest global climate imaginable - a planet covered by glacial ice from pole to pole. The global mean temperature would be about -50°C (-74°F) because most of the Sun's (Solar) radiation would be reflected back to space by the icy surface. [The fraction of radiation reflected is termed albedo (a) and it ranges widely from ~0.1 for liquid water, ~0.3 for bare land, ~0.45-0.65 for bare ice depending on the bubble content, to ~0.9 for fresh snow.] The average equatorial temperature would be about -20°C (-10°F), roughly similar to present Antarctica. Without the moderating effect of the oceans, temperature fluctuations associated with the day-night and seasonal cycles would be greatly enhanced. Because of its solid surface, the climate on a snowball earth would have much in common with present Mars. Despite the cold and dry climate, the atmosphere would still transport some water vapor from areas of sublimation (direct change from solid to vapor) to areas of condensation. Given sufficient time, glacial ice would thicken and flow in the opposite direction. Glacial flowage results in sedimentary deposits (glacial erratics, tills, moraines, eskers, ice-rafted debris, etc.) that fingerprint the glacial activity long after the ice has disappeared.