This graphic shows the Mariner 2 infrared temperature study of the clouds of Venus.  It was long believed that Venus, similar in size and in a close orbit to Earth, was Earth-like, and could possibly even support life. By the time of the Mariner mission, ground-based observations showed that the planet was too hot, and its CO2 atmosphere would not support life. Data from Mariner confirmed this, and also revealed that Venus has no appreciable magnetic field or radiation belts, as Earth has.