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."