Data From Spirit Rover Provides Evidence for Acid Fog on Ancient Mars

False-color mosaic of Cumberland Ridge, with pie charts representing iron-bearing mineralogy. Image Credit: S. Cole, PhD thesis; background image: NASA/JPL/Cornell/Arizona State University; Moessbauer values from Morris et al. 2008 (doi: 10.1029/2008JE003201)

False-color mosaic of Cumberland Ridge, with pie charts representing iron-bearing mineralogy. Image Credit: S. Cole, PhD thesis; background image: NASA/JPL/Cornell/Arizona State University; Moessbauer values from Morris et al. 2008 (doi: 10.1029/2008JE003201)

The various rover and lander missions on Mars have provided unprecedented glimpses into the planet’s past, including geological history and environmental conditions. In many ways, ancient Mars was similar to Earth, with abundant water and volcanic activity. Now, new research has revealed that there was also another related Earth-like phenomenon: acid fog.

The new findings come from studies of data from the Spirit rover, which ceased operating in 2010 but sent back an enormous amount of information which will keep scientists busy for many years to come. The findings were presented yesterday, Nov. 2, in Baltimore, Md., at the annual meeting of the Geological Society of America by planetary scientist Shoshanna Cole from Ithaca College.

Cole and her team found evidence within a 100-acre region on Husband Hill, in the Columbia Hills of Gusev crater where Spirit landed in 2004. They focused on some rock outcrops called Watchtower Class, which are located on Cumberland Ridge and the summit of Husband Hill.

“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,” Cole said.

Microscopic Imager mosaic showing a a pre-existing crack being "healed over," which is evidence for the gel weathering alteration process. The image is from a Watchtower Class outcrop named Hillary on the Husband Hill summit. Image Credit: NASA/JPL/S. Cole

Microscopic Imager mosaic showing a a pre-existing crack being “healed over,” which is evidence for the gel weathering alteration process. The image is from a Watchtower Class outcrop named Hillary on the Husband Hill summit. Image Credit: NASA/JPL/S. Cole

When they combined the data from previous studies, they noticed some patterns emerge. Spirit studied the rocks at a dozen different locations along 200 meters. When the rover examined the chemical composition of the rocks with its Alpha Proton X-ray Spectrometer (APXS), they all appeared to be the same. But when other instruments were used, differences were noticed between the rocks. The Mössbauer Spectrometer for example showed a surprisingly wide range in the proportion of oxidized iron to total iron, which indicated that something had reacted with the iron, to varying degrees. Within only 30 meters, the iron oxidation state ranged from 0.43 to 0.94, a significant difference. Other instruments, such as the Mössbauer Spectrometer and the Miniature Thermal Emission Spectrometer (Mini-TES), indicated that minerals within the rocks had changed, losing their initial structure, becoming less crystalline and more amorphous. These changes even matched the size of small bumps on the rocks, called agglomerations, which had been observed in Pancam and Microscopic Imager images of the rocks.

“So 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.”

The Spirit rover examining rocks on Husband Hill in Sept. 2005. Image Credit: M. Di Lorenzo et al./NASA/JPL/Cornell Univ./File

The Spirit rover examining rocks on Husband Hill in Sept. 2005. Image Credit: M. Di Lorenzo et al./NASA/JPL/Cornell Univ./File

From the new paper:

“The Mars Exploration Rover Spirit observed evidence of this process in the Columbia Hills of Gusev Crater. Spirit examined a set of geochemically similar but mineralogically distinct outcrops on Cumberland Ridge and the Husband Hill summit. The iron oxidation state (Fe3+/Fetotal) of the ‘Watchtower Class’ exposures on Cumberland Ridge range from 0.43 to 0.94, across a distance of only ~30m. The outcrops’ microtextures exhibit knobby protuberances, whose size correlates directly with the iron oxidation state and abundance of nanophase Fe3+ oxides as measured by the Mössbauer Spectrometer, and the abundance of amorphous phases modeled through the deconvolution of Mini-TES thermal emission spectra. We propose that the knobby protuberances are agglomerations of preexisting grains, formed from the interaction of acid fog and the host rock. 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. Differences in condensation residence time, due to amount of insolation and/or winds, resulted in the oxidative, mineralogical, and microtextural variations Spirit observed on these targets.”

As Cole noted, it is thought that the rocks were all virtually identical initially. “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.”

So what caused the changes in the rocks? The leading hypothesis from the team is that the changes were the result of acidic water vapor being released from volcanic eruptions. The same kind of process can be seen on Earth, called volcanic smog or vog, which is common in Hawaii, occurring during eruptions of the Kilauea volcano. The acidic fog erodes the rocks, dissolving some of the minerals and forming a kind of gel. After the water evaporates, it leaves behind a cementing agent which forms the agglomerations.

“So nothing is being added or taken away, but it was changed,” Cole said. “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. It’s pretty awesome. I was pretty happy when I found that one.”

There is also additional evidence supporting the acid fog hypothesis, dating back to a different study conducted in 2004. Simulated Martian basaltic rocks were used in the experiments, based on data from the earlier Mars Pathfinder mission. When those rocks were exposed to sulfuric and hydrochloric acids, they lost their crystalline structure, similar to what happened in the Watchtower Class rocks.

Synthetic image of the Spirit rover on the flanks of Husband Hill. Image Credit: NASA/JPL-Solar System Visualization Team

Synthetic image of the Spirit rover on the flanks of Husband Hill. Image Credit: NASA/JPL-Solar System Visualization Team

Microclimates also play a role in how the rocks changed over time. Cole makes an analogy to a home garden, in that the alterations of various rocks depends on much sunshine and wind they individually received. The rocks which are more altered will have larger agglomerations, and are on steeper, shadier slopes with less sunlight, while the least altered rocks tend to be on sunnier and gentler slopes.

Spirit landed in Gusev crater on Mars on Jan. 4, 2004, and extensively explored the area until it was last heard from on March 22, 2010, after it became embedded in deep sand and was unable to escape. Even so, it lasted more than 20 times longer than its nominal mission of 90 days, and with its twin Opportunity (which landed on Jan. 25, 2004) still going today, is a testament to how well these rovers were designed and constructed.

“Spirit’s the rover who always had to try harder,” Cole said. “She was sent to Gusev Crater to look for lake deposits, but landed in lava field. She had to make a long trek to the Columbia Hills to find evidence of ancient watery environments. There’s still tons of data to analyze there, and that’s really nifty.”

More information about the Spirit rover mission is available here.

 

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