Word: infra
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Dates: during 1960-1969
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...scrutinize its target at the end of its long voyage, the spacecraft carried two radiometers, one of them sensitive to radio microwaves, the other to infra-red rays. Each type of radiation behaves differently when passing through clouds or gases; the frequencies were selected to tell as much as possible about the temperature of Venus and the nature of its atmosphere...
Readings on the two infra-red wave lengths were essentially identical. This could be interpreted to mean that there were no breaks in the Venusian clouds and that the infra-red waves came from a high, opaque cloud deck. The amount of carbon dioxide above this deck was too small to be detected...
...Infra-red readings also showed the temperature on top of the cloud deck to be about -30° F. It varied little over the planet except in one spot at the southern end of the boundary between light and dark, where it fell to about -50° F. No one is sure what this means. Perhaps the clouds are higher or more opaque at this point. Perhaps a surface feature, such as a high mountain, forces them upward...
...instruments made another important observation: the limb (edge) of the Venusian disk appears darker than the center. This "limb darkening" means that microwaves and infra-red radiation really originate at the surface or at some level below the top of the atmosphere. Rays coming from the limb must pass slantwise through a greater thickness of atmosphere, and so appear weaker. This rules out one of the leading theories about the Venusian atmosphere: that it is highly ionized on top and therefore glows, making the planet appear hotter than it really is. Such an atmosphere would be brighter at the edges...
Physicist Louis D. Kaplan of the University of Nevada and JPL, who helped design Mariner II's infra-red experiment, thinks that at ground level, Venus' atmospheric pressure may be 10 to 20 times that of Earth. Its dry, unbreathable air contains perhaps 10% carbon dioxide (v. .03% for Earth) and probably a little nitrogen. The clouds are so dense that the surface is probably dark. Radar waves bounced off Venus indicate rather uncertainly that there may be both mountains and smooth places, as on the earth...