In the 1940s, sci-fi as we know it was just beginning. Interest in the stars and what might dwell beyond our own atmosphere was paired with pop culture revolutions like the brand new comic book industry.
And so magazines like Fantastic Adventures were born. In 1940, the year that the second annual World Science Fiction Convention was held, Fantastic Adventures published a series of issues, the rear covers, of which were dedicated to extra-terrestrial life in our own solar system. Concepts of alien life were worked up based on the science of the time and were depicted by artist Frank R. Paul.
Check out his wonderful concepts of the beings that might have been. We here at DVICE particularly like his sentient race of Panda-critters living on Jupiter's moon Io. If only these had all turned out to be true.