Mars May Have Hosted Potentially Habitable Lake
for National Geographic News
|March 10, 2008|
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.
"Many times in craters or places where you'd expect to find lake beds, the [evidence] would be buried by things that happened later over three or four billion years of geological activity," Grant said.
"[At Holden Crater] an unusual episode, a high-energy flood event, may have exposed some of these lake beds."
According to Grant's theory, the first lake inside the crater dried up, and later the plateau around the crater became inundated with water from Uzboi Vallis, a natural canal system.
The crater's rim eventually gave way to a torrential flood that shredded large blocks of sediment to expose the deeper lakebeds and megabeccia.
(Related news: "River-Size 'Flash Floods' May Have Carved Mars Craters" [February 20, 2008].) Grant and colleagues will present their research in an upcoming issue of the journal Geology.
But some experts, including University of Arizona astrophysicist Ethan Siegel, remain lukewarm on theories of a wet Mars.
"The geophysical community pretty much accepts that there was water on Mars, but where, when, and how much are open for debate," Siegel said.
"There is some evidence that there was water, like the meanders of what look like streambeds, and some evidence that there wasn't water, like the lack of carbonate deposits in those so-called streambeds."
On Earth carbonate minerals require liquid water to form, so their presence on the red planet would be the proverbial "squirting gun."
If the clay sediments in Holden Crater are found to contain carbonates, "that, indeed, may go a long way toward providing more evidence for a wet past on Mars," Siegel said.
Future missions, like the Martian Science Laboratory slated for launch in September 2009, could lay the matter to rest.
The lab's rovers are due to arrive on Mars in 2010, and Holden Crater is one of six potential landing sites now under evaluation.
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