NASA scientists have published details of a vast ocean that once covered half of Mars’ northern hemisphere, but has been lost to space over millennia.
In the journal Science, the astronomers explained how they used the ratio of two different types of water to estimate how wet the red planet was 4.5 billion years ago. “Early Mars (4.5 billion years ago) had a globally equivalent water layer at least 137 meters deep,” they say.
Martian water molecules are just like those on Earth: they are made of one oxygen and two hydrogen atoms. But there are two types of hydrogen: the normal type and a slightly heavier one called deuterium.
Because of this difference in mass, water normally containing hydrogen is lost to space more quickly than the type containing deuterium. That means that the current ratio of normal water to deuterated water can be used to calculate how much water was on the planet far back in time.
The researchers used NASA’s Keck II telescope and Infrared Telescope Facility in Hawaii, as well as ESO’s Very Large Telescope in Chile, to map the Martian atmosphere over six years. They found very high concentrations of deuterium water in the planet’s ice caps.
That suggests Mars must have had an enormous amount of water at some point in the past — as much as six times what it does now. The researchers calculated that if the surface were perfectly flat, the resulting ocean would have been 137 meters deep.
But like Earth, Mars is not completely flat. Based on today’s topography, the scientists estimate it likely merged into a Northern Hemisphere ocean that was nearly two kilometers deep in places and covered about one-fifth of the Earth’s surface.
It took millions of years, but as Mars’ atmosphere thinned, it will have slowly evaporated into space. The thinning of the atmosphere also removed the greenhouse effect that kept temperatures on the surface high enough for liquid water, so that the roughly 13 percent that remained froze in the planet’s ice sheets, where it remains today.
The big question is whether that ocean was home to life, but that’s impossible to answer at this point. Curiosity has detected methane on the planet, which could have an organic or geological source.
The answer may become clearer in 2018, when the European Space Agency will send a rover called Exomars to the planet to hunt for the chemical signatures of microbes living below the surface.
Science2015. DOI: 10.1126/science.aaa3630 (About DOIs).
This story originally appeared on Wired UK.