Making the case for Martian oceans

15 June 2007

Space scientists say their new research provides yet more evidence that there was once a massive ocean on Mars. Their findings refuel a hot debate and open up the possibility of life on the Red Planet. Antenna investigates...

This research was published in the journal Nature on 14 June 2007.
Water is essential for life as we know it. So finding evidence of the wet stuff on Mars could provide a vital clue about whether life has ever existed there.
Although layers of ice have recently been found around the planet's poles, the idea that there were once oceans of liquid water on Mars is still hotly contested. Now space scientists in the US and Canada have worked out a new theory that backs up the ocean idea.

This picture of Mars was taken by the Hubble Space Telescope. You can see the icy regions in white at the poles.

Image: NASA

What's the debate?
In the 1980s, pictures of the Martian landscape seemed to show ancient coastal shorelines that could be traced for vast distances over the planet's surface. This led some space scientists to believe there may once have been a huge ocean that covered a third of the planet's surface.

Mars may have looked like this 2 billion years ago. An ocean this size could have covered a whopping 48 million square kilometres and held 10 times as much water as all the ice found on Mars so far.

Image: Taylor Perron

But the experts soon realised that their theory had a hitch. When the surface landscape of Mars was measured ten years later, they discovered there were large bumps in these shorelines up to 2 km high. Shorelines should always be level with the sea.

An artist's impression of the Martian surface.

Image: NASA Ames Research Center (NASA-ARC)

'These findings put a chink in the armour of the ocean hypothesis,' says lead researcher Taylor Perron. 'But we started to wonder whether there could have been a large-scale process that reshaped the shorelines on Mars over the last 2 billion years.'
'We knew that a process called "true polar wander" has caused changes in sea level on Earth over time, so we decided to investigate whether this process might have caused these changes on Mars too.'

Taylor Perron, space expert, Harvard University.

Image: Harvard University

What is true polar wander?
True polar wander is when a planet's rotation axis shifts, causing its surface landscape to warp. Now scientists think that when the oceans first formed on Mars, the planet was spinning on a different axis than at present. What caused the change?

This is Mars' present-day north pole. The large blue area might once have been filled by an ocean.

Image: MOLA Science Team

Large events, such as giant volcanic eruptions, can cause the distribution of mass on a planet's surface or in its interior to change radically. Because a planet is most stable if large land masses are closer to the equator, such events can cause an axis shift as the planet attempts to keep on an even keel.
How did they work this out?
The team used a mathematical model to work out where Mars's original poles would have been for the shorelines to be level. They found a match about 50 degrees and 3000 km away from the current pole position, exactly where they would have expected it to be if it was caused by true polar wander.

The team think the ancient Martian poles fell along the line marked in white.

Image: Taylor Perron

Their findings have excited extraterrestrial experts, because if the ocean existed, this much water couldn't all have evaporated into space. This means most of Mars's ocean water should still be there, hidden deep under the planet's crust.
'If there has been liquid water deep beneath the crust it does raise questions about whether there were long-term potential habitats for life that survived,' explains Taylor. 'There are certain bacteria that live deep under the Earth's surface, so it's possible that there could be similar life on Mars.'

Bacteria like this can survive in extreme conditions several kilometres underground.

Image: Maryland Astrobiology Consortium/NASA/STScI

'This is the first piece of evidence for a while that speaks of the existence of oceans on Mars. We're now a step closer to understanding where the water we see on Mars came from.'
Doug Millard, Senior Curator of Space at the Science Museum, agrees that these results could be exciting. 'These findings are clearly going to keep this issue of whether there were oceans on Mars alive, although it'd be rash to say that we've reached any definite conclusions yet.'
'We suspect there's ice under the surface of Mars and there may even be liquid water underground too. The life forms we recognise require liquid water, so this all helps answer the question of whether extraterrestrial life ever existed on Mars or is even still there.'

Doug Millard, Senior Curator of Space at the Science Museum.