When Mars had its oceans

WE CALL MARS the Red Planet these days, but 3.5 billion years ago calling it the Wet Planet would have been more accurate. New research suggests that Mars then had a vast ocean covering more than a third of its entire surface, writes DICK AHLSTROMScience Editor

Scientists have always been interested in the presence of water on Mars – hidden underground today or on the surface in times past. Water indicates possible life and a watery history may mean that Mars supported early life in the same way as its near neighbour Earth.

Now researchers have a more obvious place to look for past life, in the hundreds of river deltas that ring the fringes of the putative Martian ocean. The team from the University of Colorado at Boulder published their findings earlier this week in the journal Nature Geoscience.

The study is the first to integrate a wide range of topographical information and water-related features collected from past Nasa and European Space Agency orbiting missions to Mars dating back to 2001.

The team, with funding from Nasa, looked at delta deposits and thousands of river valleys, valley networks and the elevation of adjacent features in an effort to establish where a coastline might have defined either a vast lake or the limits of an ocean.

They were not disappointed. More than half the 52 river delta deposits identified by the Colorado group, each fed in turn by many separate river valleys, all shared a single elevation. They ringed the edges of the proposed ocean, according to lead author Dr Gaetano Di Achille of Boulder’s Laboratory for Atmospheric and Space Physics.

He and co-author Prof Brian Hynek estimate that the ocean would most likely have covered 36 per cent of the planet, spread out over the northern hemisphere including the planet’s north pole. It would have contained about 124 million cubic kilometres of water, enough to cover a smooth surfaced planet of the same size with a layer of water 550 metres deep.

This ancient ocean would have contained 10 times less than the volume of the oceans on Earth today, but still represents a vast amount of water given Mars is only just over half the size of Earth, Hynek says.

The typographical surveys also show large lakes at elevations above the ocean, likely filled by the transport of groundwater between the lakes and the sea, but also possibly by rainfall.

The planet has about 40,000 river valleys, according to a second study led by Hynek published in the Journal of Geophysical Research-Planets. "The abundance of these river valleys required a significant amount of precipitation," says Hynek.

This means Mars likely had a water transport system much the same as Earth’s, with evaporation from the ocean’s vast surface moving towards land where it condensed to form clouds and rainfall. So the Martian skies overhead more than three billion years ago would have looked quite familiar, dotted with clouds and with occasional rain and storms.

“Collectively, these results support the existing theories regarding the extent and formation time of an ancient ocean on Mars and imply the surface conditions during the time probably allowed the occurrence of a global and active hydrosphere integrating valley networks, deltas and a vast ocean as major components of an Earth-like hydrologic cycle,” the authors write.

This raises an obvious issue, Di Achille adds. “One of the main questions we would like to answer is where all of the water on Mars went.”

The launch of a Nasa satellite, Maven-Mars Atmosphere and Volatile Evolution, in 2013 might answer this question, he suggests.

Studies have already shown that at least some water exists as ice just under the planet’s dusty surface. Much greater reserves may exist deeper down.

The research also gives scientists useful places to look next for signs of life on the Red Planet. “On Earth, deltas and lakes are excellent collectors and preservers of signs of past life,” Di Achille says. “If life ever arose on Mars, deltas may be the key to unlocking Mars’ biological past.”

Twitter: @dickahlstrom