For those hoping to find life on the Red Planet there is good news, and bad news. According to NASA, Mars once had an ocean that covered slightly more of its surface area than the Atlantic does on Earth. The bad news is that the Ocean disappeared billions of years ago and most of it was lost to space and did not go underground or get frozen into the ice caps.

A study published in the most recent edition of Science describes the researchers efforts to track the amount of water lost to space. Using ground-based observatories, the team attempted to construct estimates based on the water signatures in the planet’s atmosphere.

“Our study provides a solid estimate of how much water Mars once had, by determining how much water was lost to space. With this work, we can better understand the history of water on Mars,” said Geronimo Villanueva in a statement.

Villanueva is a scientist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, and lead author of the new paper.

According to the researchers, Mars once had enough surface water to cover the entire planet in 450 feet of water. Based on the geography however, it is more likely that the water formed a ocean which covered half of the northern hemisphere. In some regions the depth would have been more than a mile. That was about 4.3 billion years ago.

The estimate was made using observations from the the European Southern Observatory’s Very Large Telescope in Chile as well as Hawaii’s W.M. Keck Observatory and NASA Infrared Telescope Facility.

The instruments were used to measure chemical signatures of H2O and HDO in the Martian atmosphere. H2O is, of course, the same water we are familiar with on Earth. HDO is a variation of water in which the heavier deuterium replaces part of the hydrogen.

These readings were monitored for six years, or three martian years to create a unique map of each compound and its ratio. These maps show regional variations during various seasons, called microclimates which exist despite the current Martian climate which is an almost planet-wide desert.

The scientists compared their water signature readings with the ratio of water trapped in a meteorite which was knocked loose from mars 4.5 billion years ago and arrived later on Earth. By comparing the two researchers were able to measure atmospheric changes and determine how much water was lost to space.

Close attention was paid to the northern and southern polar regions of Mars which contain the largest known water reservoirs remaining on the planet.

“The water stored there is thought to capture the evolution of Mars’ water during the wet Noachian period, which ended about 3.7 billion years ago, to the present,” according to the team.

Based on their measurements, the researchers determined the relative amounts of the two types of water in the polar ice caps. They concluded that Mars lost a volume of water equal to 6.5 times that of the current polar caps which means that the ancient ocean would have had a volume of 5 million cubic miles.

The ocean, most likely in the planets north, would have covered 19 percent of the planets surface. On Earth, the Atlantic Ocean covers 17 percent of the surface.

“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 the second author on the paper.

A large amount of water, present for a very long time provides some hope for those hoping to find past or present life on Mars. That water, however, appears to have been gone for a vary long time and was lost in a way that would have been difficult for life to adapt to.