Alain Meunier of the University of Poitiers and his colleagues studied rocks at Mururoa Atoll in French Polynesia, the site of French nuclear testing from the 1960s to the 1990s, that contain clay minerals that appear similar to some observed on Mars.
Some scientists say the great number of such clay deposits observed from orbit by satellites is evidence the Red Planet had abundant water, perhaps at its surface, more than 3.75 billion years ago.
But Meunier says the clays at Mururoa precipitated directly from water-rich molten rock as it cooled, suggesting the clay deposits on Mars -- if formed in the same manner -- don't necessary prove there was free-flowing water in any great amount on the planet.
"Mars was not as warm and wet in its earliest time as some have suggested. I do not believe in an early ocean on Mars," Meunier told BBC News.
However, a U.S. researcher who has studied the martian satellite clay data extensively argues the Mururoan rocks don't explain the large amounts of clays seen in some regions of Mars.
"The question is: how do you generate thick sequences of this stuff?" asked John Mustard of Brown University.
"Their model cannot, I don't think, explain a Mawrth Vallis and other thick sections where we can quite clearly demonstrate many hundreds of meters, if not more, of clay formation."
Mustard says he believes many of the clays were produced below the martian surface by warm water interacting with rocks for long periods in hydrothermal systems.
One of the science missions of the Curiosity rover recently landed on Mars is to drive to and investigate the base of a mountain where clays have been detected by satellite.
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