Mars wasn’t always the cold desert we see today. There’s increasing evidence that water once flowed on the Red Planet’s surface billions of years ago. And if there was water, there must also have been a thick atmosphere to keep that water from freezing. But sometime around 3.5 billion years ago, the water dried up and the air, once heavy with carbon dioxide, dramatically thinned leaving only the wisp of an atmosphere that clings to the planet today. Where exactly did the Martian atmosphere go? This question has been a central mystery of the planet’s 4.6 billion year history. For two MIT geologists, the answer may lie in the planet’s clay. They propose that much of the missing atmosphere could be locked up in the planet’s clay surface as methane — a form of carbon that could be stored undisturbed for eons.