This thread over in /r/RetroFuturism got me wondering about when planetary scientists actually knew things about the surface of the other bodies in our solar system that were accurate. In particular Venus. The linked paintings are from 1939-1940 and couldn't be more wrong about the surface of Venus. I realize this is from a science fiction publication, but were they just far off from contemporary science? Or was it anyone's guess at that point? In particular, was the surface condition of Venus understood before the Venera missions of the USSR in the 1970s?
Venus was discovered to have an atmosphere as early as the 18th c. during the 1761 transit. Not much more information could be gleaned until radar investigations in the 1960's, which showed that Venus had interesting topographical features (particularly compared to the moon). But, Soviet and American flybys in the early 60's began to establish its harsh and unearth-like climate; Mariner 2 measured temperatures of around 500 K.
The belief that Venus had a lush, earth-like atmosphere was very common up until then. This belief is tied to the concept of Venus as earth's 'sister planet', derived from speculation based on the only facts available to early astronomers: that it is the closest planet to earth, is similar in mass (about 80% that of earth), and has an atmosphere. That last point may have led to speculation that there was water on Venus, possibly gathered in steamy oceans and large jungles. A perfect setting for cheap scifi!
I can offer only one datum: John Campbell's Solar System is a re-publication of "an eighteen-part series for Astounding [magazine] on the latest developments in solar system science", from 1936 and 1937, just before he took over the editorship. For Venus, he admits how mysterious is, but he still confidently lists facts, and more interestingly, some of the data that support his assertions. Some of his notions are true (long day), some false (the composition of the clouds, temperature).