Speaking at the European Geophysical Union in Vienna this week, a team of scientists said they have found strong evidence for Mars once having a lot more oxygen than it does currently. The results, if confirmed, would have important implications for the potential ancient habitability of Mars.

As reported by New Scientist, the results were presented by Agnès Cousin from the Research Institute in Astrophysics and Planetology in Toulouse, France. The evidence stems from the presence of manganese oxide in rocks examined by the Curiosity rover in Gale crater. Oxygen on Mars has been hinted at before, but this may be some of the best evidence yet.

The discovery of manganese oxide is important because it requires large concentrations of oxygen – in addition to water – in order to form. That means these manganese oxide deposits, found in regions of the crater where a lake likely once existed, hint not only at a highly oxidized atmosphere, but also flowing water on Mars in the past.

“We found 3 percent of rocks have high manganese oxide content,” Cousin said. “That requires abundant water and strongly oxidizing conditions, so the atmosphere may have contained much more oxygen than we thought.”

The current atmosphere on Mars is 95 percent carbon dioxide and 3 percent nitrogen, with trace amounts of oxygen. This composition is poisonous to life as we know it. Earth’s, by comparison, is 78 percent nitrogen and 21 percent oxygen, with trace amounts of carbon dioxide.

But it’s thought Mars may once have had more oxygen, produced either by chemical reactions in the atmosphere or, perhaps, by primitive life on the surface. The presence of oxygen, therefore, may not have only meant a more habitable environment (although too much could have been deadly), but it may also have been indicative of the presence of life – something we are still unsure about today.

The discovery of manganese oxide is said to be the most direct evidence that Mars once had oxygen yet. However, it will take much more research – by Curiosity and future missions, such as ExoMars – to get to the bottom of it.