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."

Mars Meteorites, Martian Rocks Have Same Origin

The rear of the stone from the Tissint Martian meteorite is almost completely covered with a glossy black fusion crust.

CREDIT: Image © Natural History Museum, London

Scientists are a step closer to reconciling a mystery on Mars, a cosmic oddity centered on Martian rocks and pieces of the Red Planet discovered on Earth.

The composition of meteorites long suspected to come from Mars have confounded scientists for a long time.

Planetary scientists know that rocks sampled from the Martian surface are high in nickel, yet the Martian meteorites (known as the SNC meteorites) happen to have significantly less nickel than those other sampled rocks.

Bernard Wood

Now, a new study unveiled today (June 19) may help explain why the rocks are chemically different yet still hail from the same planet.

"The Spirit rover in the Gusev crater found nickel concentrations five times as high in the crater than in the meteorites," Bernard Wood, a geologist at the University of Oxford and lead author of the study, said.

A study published in November 2012 that analyzed Martian meteorites found that Earth and the Red Planet share similar formation histories.

CREDIT: NASA

Wood and his team found that oxygen is a key element that could explain the chemical components of these rocks.

The older rocks sampled by the Spirit rover (in operation on Mars until 2010) formed under more oxygen-rich conditions, while the young meteorites were crafted in a low-oxygen environment, according to Wood's model.

Hap McSween

"[In Wood's model] the upper mantle of Mars was more oxidized than the lower mantel, so when you partially melt the upper mantle, you get these ancient rock compositions and when you partially melt the less oxidized lower mantel, you get the Martian meteorite compositions," said Hap McSween, a planetary geologist at the University of Tennessee who is unaffiliated with the study.

When the volcanic liquids that produced the SNC meteorites were formed under low-oxygen conditions in Mars' interior, sulfides remained behind as the liquids rose, leaving nickel trapped in the deep interior.

The volcanic rocks were therefore low in nickel, Wood said.

The surface rocks, found in the Gusev crater, were formed in a high-oxygen environment in Mars' interior where the sulfides — together with their nickel — dissolved in the volcanic liquid.

The rocks are therefore nickel-rich.

This piece of hardened lava came from Mars. 

After being knocked off the Martian surface by an asteroid or comet, it drifted in space for millions of years, until it reached Earth and fell to the ground as a meteorite.

CREDIT: AMNH/D. Finnin

The rocks in the Gusev crater formed more than 3.7 billion years ago while the SNC meteorites date back 118 million to 1.3 billion years, Wood said. This plays into the theories scientists have about Mars' past.

"It's still consistent with one idea of Mars, which is that it's sort of wet and warm … and the atmosphere was oxidized very early on, that's certainly an idea that's been kicking around for a long time," Wood told reporters.

Wood applied his knowledge of Earth's geological processes to understand what might be happening on Mars.

"On Earth, we know that we cycle oxygen rich rocks into the Earth's interior through plate tectonics, through so-called subduction," Wood said.

"The oxidized surface materials are pushed down into the interior and so we argue that’s a plausible explanation for Mars."

Although that explanation could account for why the older but oxygen-rich rocks were found in the upper mantle while the oxygen-poor rocks came from a deeper part of Mars' interior, McSween doesn't think there is necessarily evidence to support a tectonic past on Mars.

"Although there are some suggestions that Mars might have had plate tectonics at some point, there really is no evidence for it, but this is at least a suggestion that something presumably cycled oxidized materials from the surface back into the upper mantle and maybe that's in the cards here," McSween told reporters.