A thick cloud layer covers the entire surface. The sunlight that reaches the surface is absorbed and re-emitted, heating up the clouds from below and producing a "greenhouse effect". The planet's surface temperature is approximately 900° F. The surface of Venus is smoother than the other terrestrial planets. There is less variation in altitude than is seen on Earth. Sixty per cent of Venusian surface is less than 1,600 feet high. This variation of altitude could be due to the fact that deeper low lands have been filled with sand and other wind blown materials. There is an upland region called Beta Regio with its a great shield volcanoes Rhea Mons and Theia Mons. These two sites are greater than the Great Shield Volcanoes of Hawaii. The lowest part on the Venusian surface is a canyon, Diana Chasma, although this place is much shallower than the corresponding lowest point on earth, the Marinas Trench.