An analysis of old data from NASA’s Spirit Rover has revealed new evidence for an ancient lake in the Red Planet’s Gusev crater.

In 2004, Spirit landed inside the 160-km-wide Gusev crater.

The crater has long been considered the site of a lake early in Martian history.

Yet when Spirit began to explore it, researchers found the crater’s floor was paved not with lakebed sediments, but volcanic rocks.

Less than 3 km away however stood the Columbia Hills, 90 m high. When the rover drove up into them, it indeed discovered ancient rocks that had been altered by water.

But no lake sediments were among them. Instead, scientists discovered evidence of hydrothermal activity.

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. While Comanche’s carbonate minerals were originally attributed to hydrothermal activity, the 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 Dr Steve Ruff from Arizona State University, the lead author of the study published in the journal Geology.

Comanche started out as a volcanic ash deposit known as tephra that originally covered the Columbia Hills and adjacent plains.

“This material came from explosive eruptions somewhere within or around Gusev,” Dr Ruff said.

“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 neighbor rocks with carbonates.”

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

“Comanche and a neighbor 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,” Dr Ruff said.

______

Steven W. Ruff et al. 2014. Evidence for a Noachian-aged ephemeral lake in Gusev crater, Mars. Geology, vol. 42, no. 4, pp. 359-362; doi: 10.1130/G35508.1