Victoria Jaggard
National Geographic News

November 25, 2009

Mars may have once hosted a body of water roughly the size of Lake Michigan, say researchers who have found a telltale "bathtub ring" of minerals inside an ancient Martian impact crater.

The find means that Columbus crater, in Mars's southern hemisphere, is the best place yet to study the chemistry of so-called fossil lakes on the red planet, the scientists say.

Hundreds of Martian craters have been identified as possible fossil lakes, based on the presence of now dry channels or sediments deposited at former deltas, said lead study author James Wray of Cornell University.

But new pictures from NASA's Mars Reconnaissance Orbiter have revealed that Columbus crater has alternating layers of hydrated minerals—clays and sulfates known to form only in the presence of water.

"Some lakes in western Australia that are relatively acidic and pretty salty show similar minerals to what we see in Columbus crater," Wray said.

What's more, the crater is one of the few proposed fossil lakes thought to have been fed entirely by groundwater, Wray added.

"If [the water] had come from rain, we would expect to see channels," Wray said. "But we don't."

Mars Crater Lake Born of Volcanic Warping?

Columbus crater dates back to Mars's Noachian epoch, a warm, wet period that lasted from about 4.6 to 3.5 billion years ago.

Previously, researchers had thought another Noachian impact basin, Gusev crater, was the best example of a fossil lakebed, based on the crater's nearby channels and layered outcrops.

But when the Mars rover Spirit started exploring Gusev crater in 2004, the probe didn't find hydrated minerals, only volcanic basalt, Wray said.

No hydrated minerals means there's no hard proof water was ever in the crater, leading some scientists to question whether Gusev, and other craters like it, had ever hosted fossil lakes.

Using a near-infrared spectrometer, which reveals the types of minerals present based on the wavelengths of light they absorb and emit, Wray and colleagues have found clear layers of clay and sulfates in Columbus crater—just what would appear if a large lake had slowly evaporated.

Wray and colleagues think Columbus crater's lake might have formed when now inactive volcanoes in the nearby Tharsis Montes region were pouring out tons of lava.

"The weight of all that lava piling up warped the Martian surface," Wray said. This shifted the groundwater underneath, causing some of it to be pushed upward, filling the existing crater, he suggests.

Life's Chances on Mars

The types of minerals found inside Columbus indicate that, at least in the lake's early days, the water might have been hospitable for life. For instance, the crater's layers are full of gypsum, Wray said, which forms in relatively fresh water.

"That probably means the lake was initially not as salty," he said, "so that's a good thing for life," considering that too much salt can be toxic to life as we know it.

The new pictures, however, aren't enough for scientists to tell if the lake stayed liquid long enough for life to thrive or if the water rapidly froze.

During the Noachian, scientists think, Mars had a thicker atmosphere, which would have trapped enough heat for large bodies of water to be liquid. But over time, solar radiation stripped away Mars's atmosphere, creating the cold, dry world we see today.

Whether the Columbus crater lake was liquid or ice, it would have left behind the same types of minerals as it vanished, Wray said. In a frozen lake, life would have only been possible if heat from the ground had allowed pockets of liquid water along the lake bed, under the ice.

"If the lake froze, the ice would have been around for a long time, so any small pools of liquid left at the bottom could have lasted a long time," maybe long enough for life to have evolved, Wray said.

"But they'd be very cold, very salty pools of water."

http://news.nationalgeographic.com/news/2009/11/091125-mars-crater-lake-michigan-water.html

Cool thx