Mars once had a body of water bigger than Earth's Arctic Ocean and vast enough to cover the entire surface of the planet, NASA scientists said in a study released on Thursday.
The new research, providing estimates on the quantity of water on the Red Planet, is based upon detailed observations of two slightly different forms of water in Mars' atmosphere, and was published in the journal Science.
The study said that the Red Planet was once covered in a liquid layer 450 feet (137 meters) deep, occupying almost half of Mars' northern hemisphere. In some regions, water depths were greater than a mile (1.6 kilometers), NASA scientists said.
The study also showed that the vast majority of Martian water — 87 percent — has been lost to space.
"Our study provides a solid estimate of how much water Mars once had," said Geronimo Villanueva, chief author of the study.
"With this work, we can better understand the history of water on Mars," said Villanueva, a scientist at NASA's Goddard Space Flight Center in Greenbelt, Maryland.
Michael Mumma, a senior scientist at Goddard who was also a lead author, added: "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."
NASA scientists said new estimates of volume of water on Mars, and the differing chemical signatures of the two types of water, were discovered using the Keck Observatory's 10-meter Keck II telescope, NASA's Infrared Telescope Facility and ESO's Very Large Telescope located in Chile.
The new estimate is based on detailed observations of two slightly different forms of water in Mars' atmosphere.
One is the familiar H2O, made with two hydrogen atoms and one oxygen.
The other is HDO, a naturally occurring variation in which one hydrogen is replaced by a heavier form, called deuterium.
By comparing the ratio of HDO to H2O, scientists were able to measure the enrichment and determine how much water has escaped into space.
NASA said its researchers were especially interested in regions near the north and south poles, because the polar ice caps are the planet's largest known reservoir of water.