The reservoir's existence also may be a key to understand climate history and potential for life on Mars, researchers said. "There have been hints of a third planetary water reservoir in previous studies of Martian meteorites, but our new data require the existence of a water or ice reservoir that also appears to have exchanged with a diverse set of Martian samples," said Tomohiro Usui of Tokyo Institute of Technology in Japan, lead author of the paper and a former NASA/Lunar and Planetary Institute postdoctoral fellow.

"Until this study there was no direct evidence for this surface reservoir or interaction of it with rocks that have landed on Earth from the surface of Mars," said Usui. The samples showed water comprised of hydrogen atoms that have a ratio of isotopes distinct from that found in water in the Red Planet's mantle and current atmosphere. While recent orbiter missions have confirmed the presence of subsurface ice, and melting ground-ice is believed to have formed some geomorphologic features on Mars, this study used meteorites of different ages to show that significant ground water-ice may have existed relatively intact over time.
Researchers emphasise that the distinct hydrogen isotopic signature of the water reservoir must be of sufficient size that it has not reached isotopic equilibrium with the atmosphere. Curiosity's observations in a lake bed, in an area called Mount Sharp, indicate Mars lost its water in a gradual process over a significant period of time.
In this investigation, scientists compared water, other volatile element concentrations and hydrogen isotopic compositions of glasses within the meteorites, which may have formed as the rocks erupted to the surface of Mars in ancient volcanic activity or by impact events that hit the Martian surface, knocking them off the planet.
"We examined two possibilities, that the signature for the newly identified hydrogen reservoir either reflects near surface ice interbedded with sediment or that it reflects hydrated rock near the top of the Martian crust," said coauthor and JSC cosmochemist Justin Simon.

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