Because the sole source of the Moon's heat is derived from its illumination by the Sun, its mean temperature would be about that of the Earth except for the lack of atmosphere. Its extremes are very different. The Moon's surface at its equator can reach 130° C (266° F), but it cools off rapidly and by dawn descends to -173° C ( -280° F) - a range extending from above the temperature of boiling water to that of liquid air. These extremes are, however, attained only in the lunar "tropics" and only on the surface exposed to outer space. Because of the insulating properties of surface material, the effects of the daily heat or cold wave do not penetrate deeper than about half a meter (half a yard). Thermal radiation from these depths in the radio spectrum remains constant day and night and corresponds to a mean temperature of about -30° C ( -22° F).