Around 4.5 billion years ago, Mars was host to a body of water that was larger than the Arctic Ocean.
At around .085 miles deep, the water would have covered Mars' surface, but it's more likely the pool was slightly greater than one mile deep. In that case, it would have covered around 20 percent of the planet.
The new findings come from a team of researchers who collected data via three large telescopes on Earth: the European Southern Observatory's Very Large Telescope in Chile, and the Keck II telescope and NASA Infrared Telescope Facility in Hawaii.
The new estimate was based on detailed observations of two slightly different forms of water in Mars's atmosphere. One was the familiar form of water, made with two hydrogen atoms and one oxygen, H2O. The other was HDO, or semi-heavy water, a naturally occurring variation in which one hydrogen atom was replaced by a heavier form, called deuterium.
By measuring the ratio of H2O to HDO against their atmospheric rates of escape over nearly six years, the researchers were able to determine the planet's water content billions of years in the past. The research also indicates that Mars once contained several regional atmospheres and a climate divided into seasons.
The team was especially interested in regions near the north and south poles, because the polar ice caps are the planet's largest known reservoir of water. The water stored there is thought to document the evolution of Mars's water from the wet Noachian period, which ended about 3.7 billion years ago, to the present.
The new results showed that atmospheric water in the near-polar region was enriched in HDO by a factor of seven relative to Earth's ocean water, implying that water in Mars's permanent ice caps was enriched eight-fold. Mars must have lost a volume of water 6.5 times larger than the present polar caps to provide such a high level of enrichment. The volume of Mars's early ocean must have been at least 20 million cubic kilometres.
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Based on the surface of Mars today, a likely location for this water would be the Northern Plains, which have long been considered a good candidate because of their low-lying ground. An ancient ocean there would have covered 19 percent of the planet's surface; by comparison, the Atlantic Ocean occupies 17 percent of the Earth's surface.
It's possible that Mars once had even more water, some of which might have been deposited below the surface. Because the new maps reveal microclimates and changes in the atmospheric water content over time, they might also prove to be useful in the continuing search for underground water.