Weathering Phosphorus-Rich Rocks at Gusev Crater, Mars Might Have Protected Life From Mars' Acidic Waters

Weathering Profiles in Phosphorus-Rich Rocks at Gusev Crater, Mars, Suggest Dissolution of Phosphate Minerals into Potentially Habitable Near-Neutral Waters

Authors:

Adcock et al

Abstract:

Abundant evidence indicates that significant surface and near-surface liquid water has existed on Mars in the past. Evaluating the potential for habitable environments on Mars requires an understanding of the chemical and physical conditions that prevailed in such aqueous environments. Among the geological features that may hold evidence of past environmental conditions on Mars are weathering profiles, such as those in the phosphorus-rich Wishstone-class rocks in Gusev Crater. The weathering profiles in these rocks indicate that a Ca-phosphate mineral has been lost during past aqueous interactions. The high phosphorus content of these rocks and potential release of phosphorus during aqueous interactions also make them of astrobiological interest, as phosphorus is among the elements required for all known life. In this work, we used Mars mission data, laboratory-derived kinetic and thermodynamic data, and data from terrestrial analogues, including phosphorus-rich basalts from Idaho, to model a conceptualized Wishstone-class rock using the reactive transport code CrunchFlow. Modeling results most consistent with the weathering profiles in Wishstone-class rocks suggest a combination of chemical and physical erosion and past aqueous interactions with near-neutral waters. The modeling results also indicate that multiple Ca-phosphate minerals are likely in Wishstone-class rocks, consistent with observations of martian meteorites. These findings suggest that Gusev Crater experienced a near-neutral phosphate-bearing aqueous environment that may have been conducive to life on Mars in the past.

Mars' Gusev Crater Definitely had a Lake

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 neighbor 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 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."

link.

Mars' Gusev Crater had an Ephemeral Lake During the Noachian Age

Evidence for a Noachian-aged ephemeral lake in Gusev crater, Mars

Authors:

Ruff et al

Abstract:

Gusev crater has long been considered the site of a lake early in Martian history, but the Mars Exploration Rover Spirit found no apparent evidence of lake deposits along its 7 km traverse from 2004 to 2010. Although outcrops rich in Mg-Fe carbonate, dubbed Comanche, were discovered in the Noachian-aged Columbia Hills, they were inferred to result from volcanic hydrothermal activity. We now find evidence that the alteration of the Comanche outcrops is consistent with evaporative precipitation of low-temperature, near-surface solutions derived from limited water-rock interaction with rocks equivalent to nearby outcrops called Algonquin. Additional observations show that the Algonquin outcrops are remnants of volcanic tephra that covered the Columbia Hills and adjacent plains well before emplacement of basalt flows onto the floor of Gusev crater. Water-limited leaching of formerly widespread Algonquin-like tephra deposits by ephemeral waters, followed by transport and evaporative precipitation of the fluids into the Comanche outcrops, can explain their chemical, mineralogical, and textural characteristics.