This is not a morality tale about planetary preservation – and what happens if you don’t take good care of your literal ground of being. But after years of assuming that Mars is and always had been inhospitable to life, scientists at NASA are now convinced that an ocean once covered 20% of the surface of Mars. In some places, the ocean was likely a mile in depth. And according to Charles Cockell, a professor of astrobiology at Edinburgh University, “The longer water persists on a planetary body in one location, particularly if there is geological turnover, the more likely it is that it would provide a habitable environment for a suitable duration for life to either originate or proliferate. An ocean would meet this need.”

The Martian atmosphere would have been warm and moist and there were probably streams, rivers, and lakes, as well. This more verdant view of Mars comes from observing the planet through three of the world’s most powerful infrared telescopes over a period of six years – in order to map the planet’s atmosphere. Scientists wanted to see, among other things, how water molecules in the Martian air differed over the seasons and in different locations.

Martian water, like water on Earth, is composed of two hydrogen atoms and one oxygen atom. But there is also another form of water made with a heavy isotope of hydrogen called deuterium. Over time, water containing normal hydrogen evaporates into space while the heavier form is left behind and increases proportionate to the amount of normal water lost. Thus, scientists can infer from the concentration of deuterium how much water once was present. Based on what they’ve observed, they estimate that six times more water was once present than is now frozen in the Martian icecaps.

The ocean likely lasted for millions of years. But as the atmosphere thinned and air pressure decreased, more of the ocean merged into space. Air temperatures dropped and now 13% of the water that once flowed there is frozen at the poles.

Given the new evidence, scientists now recognize that Mars once had the conditions capable of supporting life as we know it. In 2018, the European Space Agency will be sending its Exomars Rover in search of telltale signs.