A picture of young Mars

By Mark Peplow

Reconstruction of the red planet's past reveals acid rain and briny seas.
What would Mars's oceans have looked like 4 billion years ago? Scientists have worked out the answer, and found a planet with a climate ideally suited to life.

Their model also answers a planetary puzzle: if Mars was once a warm, wet 'greenhouse' planet rich in carbon dioxide, why does its surface contain so few carbonate minerals?

Scientists believe that the martian atmosphere must once have been thick with carbon dioxide, a greenhouse gas that would have kept the young planet warm enough for liquid water to carve its mark so clearly on the landscape.

Some of this carbon dioxide should have been trapped in tell-tale traces of carbonate minerals such as siderite (iron carbonate) that solidified from the oceans. Geologists have seen this happening on Earth, but NASA's orbiting craft, the Mars Global Surveyor and Mars Odyssey, have found very little carbonate on the red planet's surface.

Alberto Fairen, a chemist from the Autonomous University of Madrid, Spain, and his colleagues have worked out what kind of conditions must have been present on Mars for there to be so much carbon dioxide but so little carbonate.

The answer, they conclude in an article published this week in Nature1, is that the oceans were acidic enough to stop any siderite solidifying. If Mars's oceans were richly salted with iron and sulphate ions, the seas' pH would have dropped to around 6.2; similar to some tap water, but not quite as acidic as vinegar. Earth's oceans today have a pH of about 8.

The rest is here: Nature