Liquid water that once flowed on the surface of Mars could now be locked up deep in the planet's interior as an unusual form of ice, scientists reported earlier this month.

This high resolution picture was the first image received by Mars Global Surveyor scientists that began to hint at evidence of ancient oceans and lakes. Photo: NASA/JPL/MSSS.

In a paper published in the journal Nature December 14, Craig Bina of Northwestern University and Alexandra Navrotsky of the University of California at Davis said that water could be transported into the interiors of terrestrial planets, including the Earth and Mars, as "ice VII", a rare, dense form of water ice that forms at high pressures and low temperatures.

On Earth, such ice would form in deep ocean trenches, such as near the Pacific Ocean island of Tonga, where temperatures and pressures would permit the formation of ice VII. The ice would then be carried into the planet's interior in subduction zones, where one portion of the planet's crust is pushed beneath another.

In most cases the ice would melt as it was carried into warmer regions of the planet's interior and recycled back to the surface through volcanism. However, Bina said that as a planet cools, the ice would last longer and be carried to greater depths. "Some of the ice might never get warm enough to melt, and the ice could accumulate in the interior," he explained. "The 'doomsday scenario' would be a planet so cold that all of the water eventually accumulated as ice in the interior, leaving no water behind on the surface."

A gully on the south-facing wall of an impact crater in the southern hemisphere of Mars. Photo: NASA/JPL/Malin Space Science Systems

While the Earth remains active enough that ice VII could not accumulate within its interior, this process could explain what happened to the liquid water that once existed on the surface of Mars. That planet was once geologically active, but its surface became dormant over the course of billions of years as its interior cooled. As the case for a wet early Mars grows -- including the release earlier this month of Mars Global Surveyor images that show evidence of lakes that once existed on the planet -- scientists are struggling to understand what happened to the water.

Planetary scientists had believed that Mars' water either was lost to space through bombardment or thermal escape, or was locked up near the surface in the form of permafrost or ordinary ice deposits. Bina believes that it's possible some or all of the water could be in the form of ice VII deep within the planet.

"Water for forming gullies could be stored at shallow levels beneath the Martian surface, perhaps as normal ice," he said. "Our work suggests that there could also be water stored in the deep interior as high-pressure ice, which could be released to drive volcanic activity."

Determining whether any Martian water is stored within the planet as ice VII may require detailed seismological or geochemical studies. "One could look for it with seismology, as ice will have a different seismic velocity than rock," said Bina.