Mars: The (formerly) wet planet

ZUI this NASA press release dated 16 July:

Two studies based on data from NASA's Mars Reconnaissance Orbiter have revealed that the Red Planet once hosted vast lakes, flowing rivers and a variety of other wet environments that had the potential to support life.

One study, published in the July 17 issue of Nature, shows that vast regions of the ancient highlands of Mars, which cover about half the planet, contain clay minerals, which can form only in the presence of water. Volcanic lavas buried the clay-rich regions during subsequent, drier periods of the planet's history, but impact craters later exposed them at thousands of locations across Mars. The data for the study derives from images taken by the Compact Reconnaissance Imaging Spectrometer for Mars, or CRISM, and other instruments on the orbiter.

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The clay-like minerals, called phyllosilicates, preserve a record of the interaction of water with rocks dating back to what is called the Noachian period of Mars' history, approximately 4.6 billion to 3.8 billion years ago. This period corresponds to the earliest years of the solar system, when Earth, the moon and Mars sustained a cosmic bombardment by comets and asteroids. Rocks of this age have largely been destroyed on Earth by plate tectonics. They are preserved on the moon, but were never exposed to liquid water. The phyllosilicate-containing rocks on Mars preserve a unique record of liquid water environments possibly suitable for life in the early solar system.

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Another study, published in the June 2 issue of Nature Geosciences, finds that the wet conditions on Mars persisted for a long time. 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.

"The distribution of clays inside the ancient lakebed shows that standing water must have persisted for thousands of years," says Bethany Ehlmann, another member of the CRISM team from Brown. Ehlmann is lead author of the study of an ancient lake within a northern-Mars impact basin called Jezero Crater. "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."