The process is happening far more quickly than our current understanding of the red planet would suggest, say scientists. It is also not in line with what past observations would indicate should happen in the future, they say.
The gradual disappearance of the water happens when sunlight and chemistry turn water molecules into the hydrogen and oxygen atoms that they are made up of. When they are broken down, Mars's weak gravity is unable to keep hold of them and they disappear off into space.
The speed of that process suggests that Mars could lose its water more quickly than previously thought.
The planet was once flooded with flowing water, which has largely disappeared in its more recent history, previous studies have showed.
The new discovery is published in the journal Science. It was found using the Trace Gas Orbiter probe that was sent to the red planet on board the ExoMars mission, run by the European Space Agency and its Russian counterpart Roscosmos.
Instead of staying as expected on the Martian surface, the water is being carried up in much larger than projected proportions to an altitude of more than 80km, hanging in the planet's atmosphere.
That atmosphere contains up to 100 times more water vapour than its temperature should theoretically allow, the researchers write in the newly observed paper.
Water is also much more able to escape during the planet's warm and stormy seasons, researchers found using the same data.
Nowadays, Mars is largely dry: what water does exist is frozen in its ice caps. Though various formations on Mars show that it was once much wetter, that water was lost into space, and Mars gradually turned into the largely arid place it is today.
But there is some water left on its surface, which could be key both for the discovery of life already there and the chances of humanity moving to live on Mars.
While the water in the atmosphere represents only a tiny amount of the water on the planet, the researchers say it could potentially find its way into space if it floats high enough up in the atmosphere, and that could lead the planet to dry out yet further.
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