Permafrost is permanently frozen ground, typical of the treeless plains of Siberia (tundra), though common through arctic, sub-arctic, and antarctic regions on Earth to a depth of as much as 600 m. While the top layer of soil heats up in the summer, permafrost exists in deeper layers where the summer heating fails to melt the frozen ground. A continuous layer of permafrost is found in areas where the annual mean temperature is below about -5°C (23°F).
The atmospheric climate is the main factor determining the existence of permafrost. However, the spatial distribution, thickness and temperature of permafrost is highly dependent on the temperature at the ground surface. The temperature at the ground surface, although strongly related to climate, is influenced by several other environmental factors such as vegetation type and density, snow cover, drainage, and soil type.
Permafrost also exists in the upper part of the crust on Mars.