Pioneer Venus observations of temperature and radiative fluxes are examined in an attempt to understand the thermal balance of the lower atmosphere. If all observations are correct and the probe sites are typical of the planet, the second law of thermodynamics requires that the bulk of the lower atmosphere heating must come from a source other than direct sunlight or a thermally driven atmospheric circulation. Neither the so-called greenhouse models nor the mechanical heating models are consistent with this interpretation of the observations. One possible interpretation is that two out of the three probe sites are atypical of the planet. Additional lower atmosphere heat sources provide another possible interpretation. These include a planetary heat flux that is 250 times the earth's, a secular cooling of the atmosphere, and a chemically energetic rain carrying solar energy from the clouds to the surface. Other data make these interpretations seem unlikely, so measurement eror remains a serious possibility.