Polar heating and the shape of Venus

Observations of an increase of the brightness temperature of Venus towards the poles are reported. Measurements of the deviations from circular symmetry of radio emission at 6 cm were made with a two-element interferometer near a time of inferior conjunction with Venus. The projected baseline lengths of the planet at the first null of the visibility function imply that the planet is either larger (by about 7 km) or brighter (by about 5 K) at the poles than at the equator. Possible causes of polar heating include a strong downwelling of the Venusian atmosphere at the poles and a slight topographical flattening relative to the gravitational equipotentials. Calculations of brightness temperature variations for a planet with a topographical oblateness of about 0.6 km are shown to agree with the observations, however the possibility of this oblateness being due to a fossil rotational bulge is argued against.