"Perhaps about 4.3 billion years ago, Mars would have had enough water to cover the entire surface in a liquid layer about 450 feet (137 metres) deep," Xinhua reported on Thursday, citing a NASA statement.
"More likely, the water would have formed an ocean occupying almost half of Mars' northern hemisphere, in some regions reaching depths greater than a mile (1.6 kilometres)," it added.
In all, the red planet's early ocean would have contained 20 million cubic kilometres of water, but since then, 87 percent of that water has been lost to space.
The new findings, published in the US journal Science, were based on six-year observations of two slightly different forms of water in Mars' atmosphere using the most powerful telescopes on Earth including the W. M. Keck Observatory on Hawaii.
Mars' early ocean would have contained 20 million cubic kilometres of water
By comparing the ratio of "heavy" water containing deuterium, a heavier form of hydrogen, with regular water, scientists believed that Mars must have lost a volume of water 6.5 times larger than the amount trapped in the present polar caps.
An early ocean on Mars containing the lost water would have covered 19 percent of the planet's surface, they said. By comparison, the Atlantic Ocean occupies 17 percent of the Earth's surface.
Based on the surface of Mars today, a likely location for this water would be in the Northern Plains, which has long been considered a good candidate because of the low-lying ground.
"With Mars losing that much water, the planet was very likely wet for a longer period of time than was previously thought, suggesting the planet might have been habitable for longer," said author of the study Michael Mumma, a senior scientist at NASA's Goddard Space Flight Center.