Some people look at Mars and see boulders and dust. Others see stuff that isn’t there. Not yet, anyway. Colonies. Raccoons and maple trees. Astronauts chatting to robot assistants in sign language. Vegetable gardens, rabbit herds and fish farming made possible by, of all things, global warming. Can a Wal-Mart outside the Gusav Crater Gated Community be far behind? “The possibilities are endless,” said Kevin Sloan, 22, president of the Mars Society chapter on the Pennsylvania State University campus.