A Martian crater may have once held a vast lake of water, a new research has claimed.
A new analysis by a team led by Steve Ruff from Arizona State University suggests floodwaters entered the crater through the huge valley that breaches 'Lake Gusev' 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, researchers said.
In 2004, NASA landed Spirit, one of its two Mars Exploration Rovers, inside 160-km-wide Gusev Crater, because from orbit, Gusev, with its southern rim breached by a meandering river channel, looked 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 3.2km 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.
However, 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, researchers said.
While Comanche's carbonate minerals were originally attributed to hydrothermal activity, the team's new analysis points to a different origin.
"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," said Ruff.
Comanche started out as a volcanic ash deposit known as tephra that originally covered the Columbia Hills and adjacent plains. This material, Ruff said, 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, researchers said.
"The lake didn't have to be big. 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," Ruff said.
The research was published in the journal Geology.
