NASA Mars MSL: Gusev Crater once held a lake

The Comanche outcrop, seen in a mosaic of Panoramic Camera images from the Mars rover Spirit, holds key mineralogical evidence for an ancient lake in Gusev Crater. 

Credit: NASA /JPL-Caltech /Cornell University /Arizona State University

If desert mirages occur on Mars, "Lake Gusev" belongs among them. This come-and-go body of ancient water has come and gone more than once, at least in the eyes of Mars scientists.

Now, however, it's finally shifting into sharper focus, thanks to a new analysis of old data by a team led by Steve Ruff, associate research professor at Arizona State University's Mars Space Flight Facility in the School of Earth and Space Exploration.

The team's report was just published in the April 2014 issue of the journal Geology.

The story begins in early 2004, when NASA landed Spirit, one of its two Mars Exploration Rovers, inside 100-mile-wide Gusev Crater. Why Gusev?

Because from orbit, Gusev looked, with its southern rim breached by a meandering river channel, as if it once held a lake – and water-deposited rocks were the rover mission's focus.

Yet when Spirit began to explore, scientists found Gusev's floor was paved not with lakebed sediments, but volcanic rocks.

Less than two miles away however stood the Columbia Hills, 300 feet high. When Spirit drove up into them, it indeed discovered ancient rocks that had been altered by water.

But to scientists' chagrin, no lake sediments were among them. Instead, scientists discovered evidence of hydrothermal activity, essentially hot springs like those in Yellowstone National Park.

But there's hope yet for Lake Gusev, thanks to a Columbia Hills rock outcrop dubbed Comanche. This outcrop is unusually rich in magnesium-iron carbonate minerals, a discovery made in 2010 that Ruff played a major role in making.

While Comanche's carbonate minerals were originally attributed to hydrothermal activity, the team's new analysis points to a different origin.

Cool waters
Says Ruff, "We looked more closely at the composition and geologic setting of Comanche and nearby outcrops."

"There's good evidence that low temperature surface waters introduced the carbonates into Comanche rather than hot water rising from deep down."

Comanche started out as a volcanic ash deposit known as tephra that originally covered the Columbia Hills and adjacent plains. This material, Ruff explains, came from explosive eruptions somewhere within or around Gusev.

Then floodwaters entered the crater through the huge valley that breaches Gusev's southern rim. These floods appear to have ponded long enough to alter the tephra, producing briny solutions.

When the brines evaporated, they left behind residues of carbonate minerals. As the lake filled and dried, perhaps many times in succession, it loaded Comanche and its neighbour rocks with carbonates.

"The lake didn't have to be big," Ruff explains. "The Columbia Hills stand 300 feet high, but they're in the lowest part of Gusev. So a deep, crater-spanning lake wasn't needed."

Today, the Columbia Hills rise as an island of older terrain surrounded by younger lava flows, Ruff says.

"Comanche and a neighbour outcrop called Algonquin are remnants of the older and much more widespread tephra deposit. The wind has eroded most of that deposit, also carrying away much of the evidence for an ancient lake."