Alian Wang, Ph.D., makes adjustments to equipment in her laboratory. Wang and three others from the Department of Earth and Planetary Sciences in Arts & Sciences recently published the first solid set of evidence for water having existed on Mars at the Gusev crater, a site that the rover Spirit explored.

Alian Wang, Ph.D., senior research scientist in earth and planetary sciences in Arts & Sciences, and Larry A. Haskin, Ph.D., professor of earth and planetary sciences, who died March 24, used an array of sophisticated equipment on Spirit to find that the volcanic rocks at Gusev crater near the rover's landing site were much like the olivine-rich basaltic rocks on Earth.

The researchers also found that some of the rocks possessed a coating rich in sulfur, bromine, chlorine and hematite, or oxidized iron.

The team examined three rocks and found the most compelling evidence in a rock named "Mazatzal."

The rock evidence indicates a scenario where water froze and melted at some point in Martian history, dissolving the sulfur, chlorine and bromine elements in the soil.

The small amount of acidic fluids then react with the rocks buried in the soil and formed these highly oxidized coatings.

Trench-digging rover

During its traverse from landing site to the Columbia Hills, Spirit dug three trenches, allowing researchers to detect relatively high levels of magnesium sulfate comprising more than 20 percent of the regolith - soil containing pieces of small rocks - within one of the trenches, the Boroughs trench.

The tight correlation between magnesium and sulfur indicates an open hydrologic system - these ions had been carried by water to this site and deposited.

Spirit's fellow rover, Opportunity, earlier had detected a history of water at another site on Mars, Meridiani planum. This study (by Haskin et al.) covered the investigation of Spirit rover sols (a sol is a Martian day) 1 through 156, with the major discoveries occurring after sol 80.

After the findings were confirmed, Spirit traversed to the Columbian hills, where it found more evidence indicating water.

Today - sol 614 for Spirit on Mars, or 631 Earth days - the rover is continuing its investigation on the summit of Husband Hill, including some detailed mineralogy and geochemical studies on rock, soil and drift. Spirit is talking huge amount of photos in all directions, to take advantage of the rover now being at its highest elevation of the whole mission.

"We will stay on the summit for a few weeks to finish our desired investigations, then go downhill to explore the south inner basin, especially the so-called 'home-plate,' which could be a feature of older rock or a filled-in crater," Wang said.

"We will name a major geo-feature in the basin after Larry," she added.

Wang, Haskin, their WUSTL colleague Raymond E. Arvidson, Ph.D., the James S. McDonnell Distinguished University Professor and chair of earth and planetary sciences, and Bradley Jolliff, Ph.D., research associate professor in earth and planetary sciences, and more than two dozen collaborators from numerous institutions reported their findings in a recent issue of Nature.

The paper was the last one that lead author Haskin, a highly regarded NASA veteran and former chair of earth and planetary sciences at WUSTL, submitted before he died.

Soil involved with water

"We looked closely at the multiple layers on top of the rock Mazatzal because it had a very different geochemistry and mineralogy," Wang said. "This told us that the rock had been buried in the soil and exposed and then buried again several times over the history. There are chemical changes during the burial times, and those changes show that the soil had been involved with water.

"The telltale thing was a higher proportion of hematite in the coatings. We hadn't seen that in any previous Gusev rocks.

"Also, we saw very high chlorine in the coating and very high bromine levels inside the rock. The separation of the sulfur and chlorine tells us that the deposition of chlorine is affected by water."

While the multilayer coatings on rock Mazatzal indicates a temporal occurrence of low-quantity water associated with freezing and melting of water, the sulfate deposition at trench sites indicates the involvement of a large body of water.

"We examined the regolith at different depths within the Big Hole and the Boroughs trenches and saw an extremely tight correlation between magnesium and sulfur, which was not observed previously," Wang said. "This tells us that magnesium sulfate formed in these trench regoliths.

"The increasing bromine concentration and the separation of chlorine from sulfur also suggests the action of water. We don't know exactly how much water is combined with that.

"The fact that the magnesium sulfate is more than 20 percent of the examined regolith sample says that the magnesium and sulfur were carried by water to this area from another place, and then deposited as magnesium sulfate. A certain amount of water would be needed to accomplish that action."

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