A planetary scientist has pieced together a compelling story about how acidic vapours may have eaten at the rocks in a 100-acre area on Husband Hill in the Columbia Hills of Gusev Crater on Mars.
Shoshanna Cole, an assistant professor at Ithaca College in New York, used a variety of data gathered by multiple instruments on the 2003 Mars Exploration 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, said Cole, who began studying the area for her PhD thesis at Cornell University.
“The special thing about Watchtower Class is that it’s very widespread and we see it in different locations. As far as we can tell, it’s part of the ground there,” which means that these rocks record environments that existed on Mars billions of years ago, she said.
By combining data from previous studies of the area on Mars, Cole saw some intriguing patterns emerge. Spirit examined Watchtower Class rocks at a dozen locations spanning about 200 meters along Cumberland Ridge and the Husband Hill summit.
The chemical composition of these rocks, as determined by Spirit’s Alpha Proton X-ray Spectrometer (APXS), is the same, but the rocks looked different to all of the other instruments.
Across Cumberland Ridge – which is about 1/3 the size of a football field – the Mossbauer Spectrometer showed there was a surprisingly wide range in the proportion of oxidised 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 30 meters.
Meanwhile, data from both the Mossbauer 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, which Cole calls agglomerations, seen in Pancam and Microscopic Imager pictures of the rocks.
“But the fact that the rocks were otherwise the same in composition indicates that they were originally identical. That makes us think that they were made of the same stuff when they started out. Then something happened to make them different from each other,” she said.
Cole hypothesises that the rocks were exposed to acidic water vapour from volcanic eruptions, similar to the corrosive volcanic smog, or “vog,” that poses health hazards in Hawaii from the eruptions of Kilauea.
When the Martian vog landed on the surface of the rocks it dissolved some minerals, forming a gel. Then the water evaporated, leaving behind a cementing agent that resulted in the agglomerations.