The high surface temperature of Venus was confirmed in the 1950s by radio astronomers and later by the Mariner 2 spacecrafts and Venera probes of the 1960s and 1970s. It would ordinarily be expected that the surface temperature of Venus should be within the range of that calculated from solar heating; which is generally below 200K. Yet the Soviet Venera craft which first landed on the surface of Venus found a planetary environment with a surface pressure of 90 bars and a temperature well over 700K. Scientists were for some time puzzled by this observation, but they soon found the answer in the greenhouse effect.