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High-Silica ‘Halos’ Found in Gale Crater Shed Light on Wet Ancient Mars

Fracture-associated ‘halos’ of lighter-toned bedrock have been found on the lower north slope of Aeolis Mons (Mount Sharp) in Gale crater, Mars, indicating that the Red Planet had liquid water much longer than previously believed.

Pale zones called ‘halos’ border bedrock fractures visible in this 2015 image (colorized) from the Curiosity rover. Measurements overlaid on the image offer a sense of scale for the size of these fractures. The rover team determined that the halos are rich in silica, a clue to the duration of wet environmental conditions long ago. Image credit: NASA / JPL-Caltech.

Pale zones called ‘halos’ border bedrock fractures visible in this 2015 image (colorized) from the Curiosity rover. Measurements overlaid on the image offer a sense of scale for the size of these fractures. The rover team determined that the halos are rich in silica, a clue to the duration of wet environmental conditions long ago. Image credit: NASA / JPL-Caltech.

“The concentration of silica is very high at the centerlines of these halos,” said Dr. Jens Frydenvang, a scientist at Los Alamos National Laboratory and the University of Copenhagen.

“What we’re seeing is that silica appears to have migrated between very old sedimentary bedrock and into younger overlying rocks.”

“The goal of NASA’s Curiosity rover mission has been to find out if Mars was ever habitable, and it has been very successful in showing that Gale crater once held a lake with water that we would even have been able to drink, but we still don’t know how long this habitable environment endured,” he said.

“What this finding tells us is that, even when the lake eventually evaporated, substantial amounts of groundwater were present for much longer than we previously thought — thus further expanding the window for when life might have existed on Mars.”

Whether this groundwater could have sustained life remains to be seen. But the new study buttresses recent findings by another research team who found boron on Mars, which also indicates the potential for long-term habitable groundwater in the planet’s past.

The halos were first analyzed in 2015 with Curiosity’s science-instrument payload, including the laser-shooting ChemCam instrument.

The rover has traveled more than 10 miles (16 km) over more than 1,700 sols (Martian days) as it has traveled from the bottom of Gale crater part way up Mount Sharp in the center of the crater.

The elevated silica in halos was found over 65 to 100 feet (20-30 m) in elevation near a rock-layer of ancient lake sediments that had a high silica content.

“This tells us that the silica found in halos in younger rocks close by was likely remobilized from the old sedimentary rocks by water flowing through the fractures,” Dr. Frydenvang said.

“Specifically, some of the rocks containing the halos were deposited by wind, likely as dunes. Such dunes would only exist after the lake had dried up.”

“The presence of halos in rocks formed long after the lake dried out indicates that groundwater was still flowing within the rocks more recently than previously known.”

The research was published online this week in the journal Geophysical Research Letters.

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J. Frydenvang et al. Diagenetic silica enrichment and late-stage groundwater activity. Geophysical Research Letters, published online May 30, 2017; doi: 10.1002/2017GL073323