Scientists have come up with fresh evidence supporting the theory that a vast ocean covered much of the Martian northern hemisphere 3.5 billion years ago.

The work paints a picture of a possible Earth-like planet, warm enough to have rivers, sea shore beaches and an ocean covering a third of the surface.

On Earth, marine deltas can have a range of different features, tides, morphologies and geological make up.

But the one thing they all have in common is that they are located at the same elevation, sea level.

Now a report in the journal Nature Geoscience, by Dr Bryan Hynek and Gaetano Di Achille from the University of Colorado, claims a survey of ancient delta deposits at similar elevations on Mars appear to show them draining into a huge ancient ocean.

For decades scientists have been debating whether or not a global ocean once covered the flat lowland regions of the northern Martian hemisphere.

Martian river deltas

"If Mars had an extensive hydrological cycle in the past, with rain, groundwater reservoirs, ice sheets and surface run-off towards lakes and possibly a northern hemisphere ocean, then there should be evidence of deltas ringing the margins of these lowlands at a common elevation," said Mr Di Achille.

"Likewise, river valleys draining into such an ocean should also flow down to the same elevation, and shouldn't be found below this level."

To test their theory Dr Hynek and Mr Di Achille used global databases of known deltaic deposits and valley networks, together with topography data from a laser altimeter aboard NASA's Mars Observer spacecraft which is orbiting the red planet.

They assumed most of the planet's current topographical features were in place by the end of the Martian Noachian era about 3.5 billion years ago.

As the researchers put their data together they found the shoreline they were drawing was broadly consistent with palaeoshorelines suggested by previous studies.

They also studied deltas formed in closed basins to look for any ancient aquifers saturating the crust and systematically emerging on the surface.

In all 52 delta formations were examined along the border between the northern lowlands and southern highlands.

Common elevation

"More than half of the elevations examined turned out being at the same level, and so based on that data, the findings support the idea of an ocean on Mars," Mr Di Achille said.

"While the findings provide a significant piece of the puzzle they don't paint the full picture. They don't confirm the existence of such an ocean, but simply provide further evidence supporting theories about a large ancient ocean."

He says the next question is what happened to all the water.

"Did it simply freeze in the Martian ice caps, did it soak down below ground, or was it all degassed into space?"

"The next mission to Mars in 2013 will try to understand how important the loss of gas from the atmosphere was for conditions on Mars. This will give us a better understanding of what happened to all the water 3.5 billion years ago."