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Clay minerals point to vast Martian lakes

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A study published today in Nature indicates that large swathes of the ancient Martian highlands, comprising about half the planet, contain clay-like minerals which can only form in the presence of water, demonstrating that the Red Planet once hosted "vast lakes, flowing rivers and a variety of other wet environments that had the potential to support life", as NASA puts it.

Said minerals, called phyllosilicates, were spied by the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) and other instruments aboard NASA's Mars Reconnaissance Orbiter. They "preserve a record of the interaction of water with rocks dating back to what is called the Noachian period of Mars' history, approximately 4.6bn to 3.8bn years ago", and while they were buried by subsequent volcanic activity during drier periods, impact craters have exposed them to view.

John Mustard, a member of the CRISM team from Brown University, and lead author of the Nature study, explained: "The minerals present in Mars' ancient crust show a variety of wet environments. In most locations the rocks are lightly altered by liquid water, but in a few locations they have been so altered that a great deal of water must have flushed though the rocks and soil. This is really exciting because we're finding dozens of sites where future missions can land to understand if Mars was ever habitable and if so, to look for signs of past life."

The latest results back an earlier CRISM study published in the 2 June issue of Nature Geosciences which demonstrated that Mars was not awash with water, but that the liquid was present on the surface during some considerable time.

A close look at a northern-Mars impact basin called the Jezero Crater using the CRISM and High Resolution Imaging Science Experiment identified "three principal classes of water-related minerals dating to the early Noachian period", viz: aluminum-phyllosilicates; hydrated silica or opal; and the more common and widespread iron/magnesium-phyllosilicates. Significantly, the variations in the minerals "suggest that different processes, or different types of watery environments, created them".

NASA elaborates: "Thousands to millions of years after the clays formed, a system of river channels eroded them out of the highlands and concentrated them in a delta where the river emptied into a crater lake slightly larger than California's Lake Tahoe, approximately 25 miles in diameter."

NASA's image of the Jerezo Crater shows that "ancient rivers ferried clay-like minerals (shown in green) into the lake, forming the delta":

NASA's colour-enhanced image of the delta in the Jezero Crater

Bethany Ehlmann, another member of the CRISM team from Brown and lead author of the Jerezo study, concluded: "The distribution of clays inside the ancient lakebed shows that standing water must have persisted for thousands of years. Clays are wonderful at trapping and preserving organic matter, so if life ever existed in this region, there's a chance of its chemistry being preserved in the delta." ®

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