Despite increasing evidence that water flowed on Mars billions of years ago, scientists have been mystified by what happened to the thick, carbon dioxiderich atmosphere that must have once kept that water from freezing.Now two MIT geologists think they know. Geology professor Oliver Jagoutz and Joshua Murray, PhD 24, propose that much of this missing atmosphere could be locked up in the planets clay-covered crust.While water was present on Mars, they suggest, the liquid could have trickled through certain rock types and set off a slow chain of reactions that progressively drew carbon dioxide out of the atmosphere and converted it into methane, a form of carbon that could be stored in the clay for eons.This schematic illustrates the progressive alteration of iron-rich rocks on Mars as the rocks interact with water containing CO2 from the atmosphere. Over several billion years, this process could have stored enough CO2 in the clay surface, in the form of methane, to explain most of the CO2 that went missing from the planets early atmosphere.COURTESY OF THE RESEARCHERSThe researchers applied their knowledge of interactions between rocks and gases on Earth to how similar processes could play out on Mars. They found that the quantity of clay covering the Martian surface could hold up to 1.7 bar of CO2, which would be equivalent to around 80% of the planets early atmosphere. In some ways, Marss missing atmosphere could be hiding in plain sight, Murray says.The researchers think its possible that this sequestered carbon could one day be recovered and converted into propellant to fuel future missions between Mars and Earth.