for National Geographic News
A sediment-filled Martian crater holds evidence of an ancient lake that could reveal whether the red planet was once habitable, scientists say.
The find is based on new data from the Mars-orbiting HiRISE camera, which revealed layers of fine, clay-bearing sediments inside a feature known as Holden Crater. (See some of the first HiRISE color images of Mars.)
On Earth such sediments are produced by placid, long-lived lakes, a type of environment that scientists say is most likely to preserve signatures of livable conditions—or even life itself—from the ancient past.
"We don't know if the lake was habitable or whether there was ever any biological activity," said HiRISE co-investigator John Grant, a geologist with the Smithsonian's National Air and Space Museum.
"But you've got [a lake] that focuses a sediment from a broad area into an environment where it can be preserved," he said.
"So if you want to go and look for evidence of past habitability or even life, Holden Crater would be a good place to go."
The 88-mile-wide (142-kilometer-wide) Holden Crater formed three to four billion years ago when an enormous impact smashed inside an existing Martian basin.
The collision sent up a spray of shattered boulders and debris, including blocks of rock as large as 165 feet (50 meters) across.
The rubble that fell back into the crater, known as megabreccia, is likely some of the oldest exposed rock on the red planet.
The ancient debris, embedded in the clay-bearing sediment layers, dates to Mars's earliest geologic period, the Noachian, when scientists think the planet was warmer and wetter than it is today.
In fact, the presence of liquid water on early Mars may be the reason the rocks are now exposed instead of being hidden deep within younger soil layers.
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