The Case for Mars: The Plan to Settle the Red Planet and Why We Must is a nonfiction science book by Robert Zubrin, first published in 1996, and revised and updated in 2011.

The book details Zubrin's Mars Direct plan to make the first human landing on Mars. The plan focuses on keeping costs down by making use of automated systems and available materials on Mars to manufacture the return journey's fuel in situ. The book also reveals possible Mars colony designs and weighs the prospects for a colony's material self-sufficiency and for the terraforming of Mars.

The Mars Direct plan was originally detailed by Zubrin and David Baker in 1990. The Case for Mars is, according to Zubrin, a comprehensive condensation for laymen of many years' work and research. Chapters 1 and 4 deal primarily with Mars Direct.

For Robert Zubrin, the attractiveness of Mars Direct does not rest on a single cost-effective mission. He envisions a series of regular Martian missions with the ultimate goal of colonization, which he details in the seventh through ninth chapters. As initial explorers leave hab-structures on the planet, subsequent missions become easier to undertake.

Large mall like structures buried in regolith, pressurized habitats would be the first step toward human settlement; the book suggests they can be built as Roman-style atria on the surface and then be buried with regolith, with easily produced Martian brick. During and after this initial phase of habitat construction, hard-plastic radiation- and abrasion-resistant geodesic domes could be deployed on the surface for eventual habitation and crop growth. Nascent industry would begin using indigenous resources: the manufacture of plastics, ceramics and glass.

The larger work of terraforming requires an initial phase of global warming to release atmosphere from the regolith and to create a water cycle. Three methods of global warming are described in the work and, Zubrin suggests, are probably best deployed in tandem: orbital mirrors to heat the surface; factories on the surface to pump halocarbons such as perfluromethane into the atmosphere; and the seeding of bacteria which can metabolize water, nitrogen and carbon to produce ammonia and methane (these would aid in global warming). While the work of warming Mars is on-going, true colonization can begin.

The Case for Mars acknowledges that any Martian colony will be partially Earth-dependent for centuries. However, it suggests that Mars may be a profitable place for two reasons. First, it may contain concentrated supplies of metals of equal or greater value to silver which have not been subjected to millennia of human scavenging and may be sold on Earth for profit. Secondly, the concentration of deuterium – a possible fuel for commercial nuclear fusion – is five times greater on Mars. Humans emigrating to Mars thus have an assured industry and the planet will be a magnet for settlers as wage costs will be high. The book asserts that “the labor shortage that will prevail on Mars will drive Martian civilization toward both technological and social advances.”

While detailing the exploration and colonization, The Case for Mars also addresses a number of attendant scientific and political factors.