Warning: Wikipedia contains spoilers.
For book publication, the stories were loosely woven together and rewritten, in some cases, to jibe with the new storyline. The chapter titled "The Second Expedition", for example, was originally a short story titled "Mars is Heaven", set in the 1960s and dealing with characters nostalgic for their childhoods in the midwestern United States in the 1920s. In the Chronicles version, which takes place forty years later but which still relies upon the 1920s nostalgia, the story contains a brief paragraph about medical treatments that slow the aging process, so that the characters can be traveling to Mars in the 2000s but still remember the 1920s.