Billions of years ago, Mars had water on its surface and was a place not too far removed from potentially livable. So what happened?

Scientists haven’t found the big answer just yet, but a new analysis has eliminated one study — which puts NASA one step closer to figuring it out. According to the space agency, a new examination of the largest known deposit of carbonate minerals on Mars suggests that the original Martian atmosphere may have already lost most of its carbon dioxide by the era of valley network formation.

"The biggest carbonate deposit on Mars has, at most, twice as much carbon in it as the current Mars atmosphere," Bethany Ehlmann of the California Institute of Technology and NASA Jet Propulsion Laboratory said. "Even if you combined all known carbon reservoirs together, it is still nowhere near enough to sequester the thick atmosphere that has been proposed for the time when there were rivers flowing on the Martian surface."

The majority of Mars’ atmosphere is made up of carbon dioxide, and that gas can be pulled out of the air and sequestered or pulled into the ground by chemical reactions with rocks to form carbonate minerals. Before we landed rovers on Mars and started digging around, scientists expected to find large Martian deposits of carbonates holding much of the carbon from the planet's original atmosphere. Instead, these missions have found low concentrations of carbonate distributed widely, and only a few concentrated deposits.

So, what options are still on the table? Mars’ current atmosphere is too weak for liquid water on the surface, but a denser atmosphere in the past could have allowed for surface water and temperatures level enough not to freeze or evaporate it. Scientists are now digging deeper into a theory that posits Mars did have a much denser atmosphere during its flowing-rivers period, and then lost most of it to outer space from the top of the atmosphere, as opposed to sequestration.

(Via NASA)