One reference to Mars in the original pitch that I neglected to mention upthread
was not a literal reference.
Original Star Trek Pitch wrote:
The First Lieutenant --
[...] His name is "Mr. Spock". And the first view of him can be almost frightening -- a face so heavy-lidded and satanic you might almost expect him to have a forked tail. Probably half Martian, he has a slightly reddish complexion and semi-pointed ears. [...]
Clearly, at this stage of development, in early 1964, Roddenberry considered Mars to be habitable, or at least once to have been inhabited (or probably so).