Carl Sagan's article on Mars in the December, 1967 NGM
Is another classic to find. The only space mission to return any data on the Red Planet when this was written was Mariner 4. Viking was still known as Voyager. The canals were still considered as possible real objects on the planet's surface (but not made by intelligent beings).
Men were considered ready to land on Mars in the 1980s.
Some nice artwork (see the speculation on large Martian life forms - pure Sagan) and of course Sagan's excellent prose, even at that early stage of his career. The only article he ever wrote for NG.