Spread over an area the size of the Atlantic Ocean, it would have straddled the north pole and contained the equivalent of a tenth of the water on Earth.

For decades scientists have argued as to whether the Red Planet once harboured bodies of water big enough to help nourish a true hydrological cycle marked by evaporation and rainfall.

Recent evidence suggests as much, but doubts remained.

To dig deeper, Gaetano Di Achille and Brian Hynek of the University of Colorodo in Boulder sifted through huge stores of images collected by NASA's Mars Orbiter Laser Altimeter (MOLA) in the late 1990s and other more recent European and US satellite-based monitoring systems.

The data was not new, but the researchers were the first to link up all available information on Mars' terrain into a single computer-driven model.

The study, published in Nature Geoscience, found 52 river-delta deposits scattered across the planet.

More than half occurred at about the same elevation, and thus probably marked the boundary of the once-massive sea.

All of these would have been connected either directly to the ocean, or to its groundwater table along with several large, adjacent lakes.

The scientists calculated that the ancient sea covered 36 percent of the planet's surface and contained about 124 million cubic kilometres (30 million cubic miles) of water.

Even as single-cell life forms were emerging on our planet some 3.5 billion years ago, Mars probably had an Earth-like water cycle including precipitation, runoff, cloud formation, ice formation and groundwater accumulation, they conclude.

In a parallel study published in the Journal of Geophysical Research (Planets), Hynek and colleagues catalogued some 40,000 river valleys on Mars, four times the number previously suspected.

"The abundance of these river valleys required a significant amount of precipitation," Hynek said.

"This effectively puts the nail in the coffin regarding the presence of past rainfall on Mars."

Many puzzles, however, still remain.

"One of the main questions we would like to answer is where all of the water on Mars went," said Di Achille.

Future Mars missions -- including NASA's 485 million dollar (400 million euro) Mars Atmosphere and Volatile Evolution mission (MAVEN), slated for launch in 2013 -- may provide clues.

The new studies also provide critical leads on where to look for signs of early Martian life.

"On Earth, deltas and lakes are excellent collectors and preservers of signs of past life," said Di Achille.

"If life ever arose on Mars, deltas may be the key to unlocking Mars' biological past."

Hynek also noted that long-lived oceans may have provided an environment for microbial life to take hold on Mars.

The European Space Agency (ESA) and NASA have separately forecast a manned flight to Mars in about three decades.

The Red Planet's distance from Earth varies between 55 million kilometres (34 million miles) and more than 400 million kilometres.

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