The Martian landscape is being dissolved -- in some places -- by acid fogs, according to new clues drawn from bringing together data from all of the instruments on NASA's (now defunct) Spirit rover. The fogs were likely created by the scarce water in Mars atmosphere combining with the acidic vapors from volcanoes and then clinging to the shady sides of rocks and hills.

On Earth the closest analogy is Hawaiian vog -- acidic volcanic smog caused by gaseous releases from the Kilauea volcano. On Mars, however, the acid weathering process has been much more subtle and slow -- taking place over hundreds of millions of years in the thin, dry frigid Martian air.

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The acid fog on Mars appears to have created a gel on the rock surfaces that has essentially melted the neat crystalline texture of the rocks into a sort of messy rock soup.

“A lot of people have talked about weathering that would occur on Mars,” said planetary scientist Ralph Milliken of Brown University. And those researchers have developed models for such things as acid fogs eating away at Martian rocks over the eons, although evidence for such processes has been scarce. “This (new work) is consistent with some of these models.”

The new work is that of planetary scientist Shoshanna Cole, who has assembled a strong case for acidic vapors making a thin rock soup on the surfaces of Martian rocks in a 100-acre area on Husband Hill, in the Columbia Hills of Gusev Crater. Cole integrated data from all the instruments on NASA's Mars Exploration Rover Spirit to reveal patterns that no single instrument could have detected. She presented her work on Monday in Baltimore at the annual meeting of the Geological Society of America.

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“I look at the geology with all of the instruments data sets,” said Cole, an assistant professor at Ithaca College who started working on the project for her Ph.D. thesis while she was at Cornell University. “Different instruments give different information.”

Cole studied the knobby rocks on Cumberland Ridge and the Husband Hill summit that are called the the 'Watchtower Class' outcrops, she said. These rocks seem to form the bedrock of the area -- which has been around for billions of years, she said.

You don't need to know anything about iron geochemistry to know that the stuff represented by the pie charts varies greatly across this scene, which is about 1/3 the size of a football field. Also, the 1.2m scale bar is the distance between the rover's right and left wheel tracks. S. Cole. /NASA/JPL/Cornell/Arizona State University

To get the chemical composition of these rocks, Cole looked up the work done with Spirit's Alpha Proton X-ray Spectrometer (APXS). She found that the Watch Tower Class rocks in the area had the identical chemical composition is the same, despite different appearances. The Mössbauer Spectrometer revealed a range in the proportion of oxidized iron to total iron, which suggests that something had chemically reacted to different degrees with different rocks.

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The iron oxidation ratio ranged over Cumberland Ridge from 0.43 to 0.94 across a span of only 30 meters. Over the same stretch of ground the Miniature Thermal Emission Spectrometer (Mini-TES) and the Mössbauer Spectrometer showed that the crystalline minerals in the rocks lost their structure, becoming less crystalline as the iron oxidation state changed.

Capping it off is the fact that the knobby protrusions, or agglomerations, in the rocks vary in size right along with the other trends, according to images from Spirit's Pancam and Microscopic Imager.

“We can see the agglomerations progress in size from west to east and the iron changes in the same way,” Cole said. “It was super cool.”

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It looks like the rocks all started out as basically the same. Then the rocks were changed by acidic water vapor from volcanic eruptions. That acid fog stuck to the rocks and dissolved some minerals, forming a gel. The water then dried up and left behind a residue that formed the agglomerations.

“This would have happened in tiny amounts over a very long time,” said Cole. “There's even one place where you see the cementing agent healing a fracture. It's pretty awesome. I was pretty happy when I found that one.”

She also has an explanation for why some rocks are more weathered by the acid fog than others. When she mapped out the locations of the most altered rocks with the biggest agglomerations, she found they were on shady, steeper hillsides facing away from the sun, where water can persist longer. The rocks that were least affected by the acid fog were on lower relief areas where the sun shines a lot, Cole explained.

“What I really like about Shoshanna's work is that it's a nice integration of all the instruments,” said Milliken, who was not directly involved in this study. “It's exactly what a geologist would do today if they went out in the field.”