It's a longstanding mystery: how Mars lost the water that flowed across its surface billions of years ago.
Scientists now think they have an answer: much of it became trapped in the outer layer of Mars - its crust. They argue that a vast amount of Mars' ancient water is locked up in minerals below the surface of the Red Planet.
The findings have been discussed at the 52nd Lunar and Planetary Science Conference and are published in Science journal. The study used measurements gathered from Mars-orbiting spacecraft, rovers and meteorites. Researchers then developed a computer simulation of how water was lost over time.
More than four billion years ago, Mars was warmer and wetter - possibly with a thicker atmosphere. Water coursed through rivers, cutting channels in the rock, and pooled in impact craters. But by around a billion years later Mars had made a transition to the colder, desolate planet we recognise today.
Earth has a magnetic shield, or magnetosphere, that prevents the atmosphere from escaping. But Mars' magnetic shield is weak and could have allowed the elemental constituents of water to escape into space. But the rate at which hydrogen - one chemical constituent of water - escapes from that atmosphere today suggests this mechanism can't explain where all the water went.
Eva Linghan Scheller and colleagues from the California Institute of Technology (Caltech) in Pasadena now think they have come up with the answer. The results of their computer modelling work show that between 30% and 99% of Mars' initial water was incorporated into minerals and buried in the planet's crust.
Co-author Bethany Ehlmann, from Caltech explained that, "by studying data from Mars missions, It became clear that it was common - and not rare - to find evidence of water alteration". She continued: "When the crust becomes altered, it takes water - like liquid water - and sequesters it in a hydrated mineral that has water in its structure so that it is effectively trapped."
The authors suggest that most of the water was lost between about 4.1 and 3.7 billion years ago - during a stretch of Martian history known as the Noachian Period.