Scientists announced their findings in an article on NASA's website today, which estimates that the ocean covered 19 per cent of the Red Planet's surface, near its northern hemisphere, around 4.3 billion years ago.
Studies of the topography of Mars have long fuelled speculation about just how much water once existed on the planet's surface, and whether the planet was "wet" enough to sustain life.
Ocean covered Red Planet's north
"In the ancient past, we have some indications that water was flowing on the surface, but how much water was there?" planetary scientist Geronimo Villanueva explained in a video released by NASA. "Are we talking about oceans, are we talking about small rivers, or a little rain?"
To figure this out, three of the world's most powerful telescopes — the European Southern Observatory’s Very Large Telescope in Chile, and the W.M. Keck Observatory and NASA Infrared Telescope Facility in Hawaii — were pointed at Mars to measure leftover water molecules in the planet's atmosphere.
The molecules in the atmosphere, known as HDO, are slightly heavier than the more commonly known H20, because they contain a heavier isotope of hydrogen, known as deuterium.Both H20 and HDO existed on Earth and Mars, but the lighter H20 has since evaporated from Mars's surface due to its weaker gravity than the Earth's.
Researchers found that most of the HDO is concentrated near the planet's northern hemisphere, a low-lying area that could have held a large body of liquid water.
Ocean was about 1.6 km deep
That ocean could have held up to 20 million cubic kilometres of water — more than 6 1/2 times the amount of water currently locked in the planet's north and south polar ice caps. That's more water than the Earth's Arctic Ocean.
"This ocean has a maximum depth of around 5,000 feet, or around one mile [1.6 km] deep. It's not as deep as the deepest points of our oceans, but comparable to the average depth of the Mediterranean Sea," says Villanueva.
The findings suggest that Mars has lost about 87 per cent of its water content over billions of years. Only 13 per cent of it remains in the ice caps. But it takes a long, long time for that much water to evaporate into the atmosphere, suggesting that the planet was "wet" for far longer than scientists previously estimated.
“With Mars losing that much water, the planet was very likely wet for a longer period of time than was previously thought, suggesting it might have been habitable for longer,” said Michael Mumma, a senior scientist at Goddard and author on the paper.