BioEd Online

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.

As the oceans receded, any dissolved carbon dioxide would have been lost back into the atmosphere, and eventually stripped away from the planet by the harsh stream of solar particles bombarding the planet.

There is evidence to support the scientists' scenario. NASA's exploration rover Opportunity recently found large quantities of sulphate minerals such as jarosite on Mars.

Acid rain

The team has used this assumption to paint a detailed picture of the young planet. Today's martian atmosphere is at less than one-hundredth the pressure of Earth's atmosphere. But the scientists say that 4 billion years ago, volcanic eruptions would have flooded Mars with sulphur dioxide and carbon dioxide, to create an atmosphere with four times the pressure of Earth's.

This atmosphere supplied a steady drizzle of acid rain, which dissolved iron, magnesium and other minerals as it trickled into the oceans, putting roughly a gram of iron in every 22 litres of seawater. The briny seas would also have contained the same concentration of sulphate that is formed by dissolving a teaspoon of Epsom salts (magnesium sulphate) in about five litres of water.

"We know from the geological history that there has been plenty of volcanism on Mars, so these concentrations are reasonable if there was also a huge carbon dioxide reservoir," says Baker.

"The resulting scenario is very exciting because the cycle allows long residence times for carbon dioxide in the atmosphere," adds Fairen. "Basically, the lack of carbonates in the martian surface could have helped to keep Mars warm for longer."

A warm planet is good news for the prospect that life once existed there. The team's martian model shares much of its chemistry with parts of the Rio Tinto, in south-west Spain. This acidic river, in which high concentrations of iron and sulphur are dissolved, teems with living creatures including bacteria, yeast and fungi.