Life on Mars mystery deepens

By SIMON COLLINS

Just when Mars is closer to us than it has been for 60,000 years, scientists have scaled back their hopes that it may once have sustained life.

Chemical analysis of Martian rocks by an orbiting American spacecraft has dashed theories that the red planet was once covered by Earth-like oceans in which life might have evolved.

Less than a year ago, new fossil evidence also overturned theories that primitive bacteria which breathed out oxygen evolved on Earth between 3.8 billion and 4 billion years ago, when this planet was as dangerous and inhospitable as Mars is now.

It now seems that oxygen-emitting bacteria may not have appeared here until a billion years later, when meteorites had largely stopped bombarding us and the environment was much more suitable for life than Mars has ever been.

"The assumption has been that life developed in the oceans," says Dr Grant Christie of Auckland's Stardome Observatory.

"When you look at Mars and see [layers of rock like those laid down by oceans on Earth], you can put two and two together and figure that maybe Mars evolved life and then lost its atmosphere and doesn't have life any more.

"But the new stuff - the fossils on Earth and this announcement that these layers are probably not oceanic or sedimentary layers - now puts a big question mark on that."

It does not prove that there has never been life on Mars.

Auckland University geologist Dr Kathy Campbell says it is still possible that simple non-oxygen-emitting bacteria may have evolved deep inside the planet.

"Here on Earth if you drill down you can isolate bacteria at 3km down living without any oxygen," she says. "So it is possible there is a subsurface life on other planets too."

But it will be at least several years before humans work out how to send a spacecraft to Mars that can drill 3km down to look for it.

Richard Hall of Wellington's Carter Observatory says the red planet has always fascinated humans because it is more similar to the Earth than any other part of the solar system.

It is only about half the size of Earth, and takes roughly twice as long (687 days) to circle the sun.

Its distance from the sun varies between about 205 and 250 million kilometres, compared with the Earth's distance from the sun of 150 million kilometres.

It is cold (average temperature -55C), with a thin atmosphere that is 95 per cent carbon dioxide and shrouded in frequent violent dust storms that give the planet its red colour.

The famous Martian "canals" are now known to be unrelated dark spots which were put together as continuous features in the minds of credulous human observers.

Yet, despite the latest evidence that there may never have been oceans, Mars clearly has frozen water at its poles and almost certainly once had water.

Kathy Campbell, an American who worked for the National Aeronautic and Space Administration (Nasa) before moving to New Zealand six years ago, says only water could have carved out Mars' gigantic canyons which are bigger than America's Grand Canyon.

"All the surface terrain of Mars in certain places tells us there must have been water running there at some time," she says.

The latest evidence about Martian rocks is from the Mars Global Explorer, a Nasa spacecraft which has been orbiting Mars since 1997.

Last week it reported that it could find only tiny traces of calcium carbonates (limestone) in rocks that were thought to have been laid down under ancient oceans.

"In oceans on Earth you get heaps of it. They are just finding it in trace amounts on Mars," says Campbell.

"So large bodies of liquid water may never have existed there - or may not have been Earth-like anyway."

Until last year, geologists also believed that oxygen-producing bacteria might have evolved on Mars because fossils of similar bacteria had been dated in rocks in Australia and Greenland to between 3.8 and 4 billion years ago.

But late last year two separate studies in Australia and Greenland found that the chemical "signature" in the rocks that was thought to indicate photosynthesis producing oxygen was more likely caused by other, non-organic processes.

This discovery came too late for three other spacecraft which are now heading towards Mars and due to land early next year - two Nasa "Rover" missions with vehicles which will detach from the spacecraft and explore the surface of the planet, and a European mission named "Beagle II" after the ship on which Charles Darwin explored the South Pacific.

All three craft have been programmed to land in places where the chemical signature was thought to indicate past oceans and therefore the possibility of past life. That signature now looks a lot less certain.

Campbell is still excited, because the spacecraft will be the first of any kind to have landed on Mars with detailed scientific missions.

But the chances that these missions could find life beyond Earth are now less than they seemed just a few months ago.