Space agencies, huge corporations, and media-savvy billionaires — ahem, Elon Musk — have all promised that building colonies on the surface of Mars “will fix just about everything" and give humanity a chance "to try out something completely new and leave all the bad stuff behind," Kelly and Zach Weinersmith write in their new book, “A City on Mars: Can We Settle Space, Should We Settle Space, and Have We Really Thought This Through?” (Penguin Press).