Dr Shoshanna Cole, a planetary researcher at Ithaca College, has pieced together a compelling story about how acidic water vapors may have eaten at the rocks in an area on Husband Hill in the Columbia Hills of Gusev Crater on Mars.
This mini-panorama was taken by NASA’s Spirit rover on August 23, 2005, just as the rover finally completed its intrepid climb up Husband Hill. The summit appears to be a windswept plateau of scattered rocks, little sand dunes and small exposures of outcrop. The approximate true-color view spans about 90 degrees and consists of images obtained by the rover’s panoramic camera during 18 individual pointings. Image credit: NASA / JPL-Caltech / Cornell.
Dr Cole used a variety of data gathered by several instruments on NASA’s Mars rover Spirit to tease out information from exposures of the ancient bedrock.
“The work focused on the Watchtower Class outcrops on Cumberland Ridge and the Husband Hill summit,” Dr Cole explained.
Spirit examined Watchtower Class rocks at several locations spanning about 650 feet (200 m) along Cumberland Ridge and the Husband Hill summit.
The chemical composition of these rocks, as determined by Spirit’s Alpha Proton X-ray Spectrometer, is the same, but the rocks looked different to all of the other instruments.
Across Cumberland Ridge, Spirit’s Mössbauer Spectrometer – a tool for identifying the types of iron-bearing minerals in rocks and soil – showed there was a wide range in the proportion of oxidized iron to total iron, as if something had reacted with the iron in these rocks to different degrees.
This iron oxidation state ranges from 0.43 to 0.94 across a span of only 100 feet (30 m).
Meanwhile, data from both the Mössbauer Spectrometer and the Miniature Thermal Emission Spectrometer (Mini-TES) showed that the minerals within the rocks changed and lost their structure, becoming less crystalline and more amorphous.
And these trends match the size of small bumps (agglomerations) seen images of the rocks taken by Spirit’s Pancam and Microscopic Imager.
“So we can see the agglomerations progress in size from west to east and the iron changes in the same way. It was super cool,” Dr Cole said.
But the fact that the rocks were otherwise the same in composition indicates that they were originally identical.
False-color mosaic of Cumberland Ridge, with superimposed pie charts representing iron-bearing mineralogy. Image credit: Shoshanna B. Cole / NASA / JPL / Cornell / Arizona State University / Morris et al., 10.1029/2008JE003201.
Dr Cole hypothesizes that the rocks were exposed to acidic water vapor from volcanic eruptions, similar to the corrosive volcanic smog (vog).
“We propose that the knobby protuberances are agglomerations of preexisting grains, formed from the interaction of acid fog and the host rock,” Dr Cole and co-authors explained.
“In this alteration scenario, acid fog condensed on the outcrop surfaces, dissolving material at the condensation-surface interface and forming a gel, which desiccated as the adsorbed water evaporated.”
“This would have happened in tiny amounts over a very long time. There’s even one place where you see the cementing agent healing a fracture,” Dr Cole.
“It’s pretty awesome. I was pretty happy when I found that one.”
Dr Cole Marchalant and her team reported their results today at the annual meeting of the Geological Society of America in Baltimore, Maryland.
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Shoshanna B. Cole et al. 2015. In-situ Evidence for Alteration by Acid Fog on Husband Hill, Gusev Crater, Mars. Geological Society of America Abstracts with Programs, vol. 47, no. 7, p. 268; paper 94-10
