“A new theory about ancient Mars puts some fizz back in the idea that the red planet was once warm, wet and potentially habitable.

Many studies have suggested that early Mars was covered by large oceans and blanketed by a thick atmosphere rich in carbon dioxide — the stuff that puts the bubbly zing in soda. But if that’s all true, then when the oceans evaporated a lot of the carbon dioxide should have turned into what scientists call carbonates, which should be strewn all over the place.

Problem is, the carbonates aren’t there. One recent study found trace amounts in Martian dust, just enough to conclude that Mars probably didn’t have vast oceans.

The new model provides a way around this problem. It suggests the chemistry of Martian seas was different than has been assumed, so the clues have been missed.

If the seas of Mars were moderately acidic — in scientific terms, a pH under 6 instead of the pH of 8 common to terrestrial oceans — then carbonates could not have formed, says Alberto Fairen of the Universidad Autonoma de Madrid, in Spain.”