I was reading on the 'net earlier about Hans Hörbiger. He was an Austrian pseudo-scientist who after observing the Moon became convinced that it was made of ice. He then developed his "World Ice" theory (Welteislehre), which was basically that ice had determined everything in the universe. He developed it into a scientific, philosophical, poetic and artistic world view. After Hörbiger's death the Nazis adopted Welteislehre as their cosmological viewpoint, a Nazi alternative to the 'Jewish' theory of relativity.
The Wiki page on Welteislehre
mentions that Willy Ley (a famous German rocket expert who had moved to the US before the war, and probably one of the big influences on Hergé's research), had once tried to point out to Hörbiger that the daytime surface temperature of the Moon was far to high for the existence of ice, but Hörbiger responded with "either you believe in me and learn, or you will be treated as the enemy." I guess he wasn't very open to criticism then....
Anyway, this at least proves there was some debate that there was ice on the Moon (or that it was made of the stuff in this case) around at the time Hergé was creating his book - it might perhaps have influenced Hergé's thinking, even if indirectly.