Therefore, considering the absorption characteristics of the atmosphere and the emissivity factor derived from earlier JPL radar experiments, a fairly uniform 800 degrees F was estimated as a preliminary temperature figure for the entire surface. Venus is, indeed, a very hot planet. CLOUD TEMPERATURES: THE INFRARED READINGS Mariner II took a close look at Venus’ clouds with its infrared radiometer during its 35-minute encounter with the planet. This instrument was firmly attached to the microwave radiometer so the two devices would scan the same areas of Venus at the same rate and the data would be closely correlated. This arrangement was necessary to produce in effect a stereoscopic view of the planet from two different regions of the spectrum. Because astronomers have long conjectured about the irregular dark spots discernible on the surface of Venus’ atmosphere, data to resolve these questions would be of great scientific interest. If the spots were indeed breaks in the clouds, they would stand out with much better definition in the infrared spectrum. If the radiation came from the cloud tops, there would be no breaks and the temperatures at both frequencies measured by the infrared radiometer would follow essentially the same pattern. The Venusian atmosphere is transparent to the 8-micron region of the spectrum except for clouds. In the 10-micron range, the lower atmosphere would be hidden by carbon dioxide. If cloud breaks existed, the 8-micron emissions would come from a much lower point, since the lower atmosphere is fairly transparent at this wavelength. If increasing temperatures were shown in this region, it might mean that some radiation was coming up from the surface. As a result of the Mariner II mission, scientists have hypothecated that the cold cloud cover could be about 15 miles thick, with the lower base beginning about 45 miles above the surface, and the top occurring at 60 miles. In this case, the bottom of the cloud layer could be approximately 200 degrees F; at the top, the readings vary from about minus 30 degrees F in the center of the planet to temperatures of perhaps minus 60 degrees to minus 70 degrees F along the edges. This temperature gradient would verify the limb-darkening effect seen by the microwave radiometer. At the center of Venus, the radiometer saw a thicker, brighter, hotter part of the cloud layer; at the limbs, it could not see so deeply and the colder upper layers were visible. Furthermore, the temperatures along the cloud tops were approximately equally distributed, indicating that both 8- and 10-micron “channels” penetrated to the same depth and that both were looking at thick, dense clouds quite opaque to infrared radiation.