We know that weather on Mars is vastly different than what we experience here on Earth. However, new findings show the Red Planet may experience a weird, Earth-like meteorological phenomenon: acid fog.  

Planetary scientists are like cosmic sleuths, searching for clues in the data and then piecing them together to determine what happened in a planet’s history. Shoshanna Cole, an assistant professor at Ithaca College, discovered evidence of acid fog eating away at bedrock on Mars when she was combing through data collected by the Spirit rover. She presented her findings on November 2 at the annual meeting of the Geological Society of America in Baltimore, Maryland.

The Spirit rover was sent to look for ancient lake beds on Mars, landing in Gusev Crater and traveling to Columbia Hills, Husband Hill, and Cumberland Ridge to study ancient bedrock. Cole studied these areas in great detail for her Ph.D. thesis, focusing on the "Watchtower Class" rock outcrops in the region. Using data collected by several of Spirit’s instruments back in 2003, she discovered evidence that acidic vapors appeared to have eaten away at the bedrock in a 100-acre area 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 which means that these rocks record environments that existed on Mars billions of years ago,” Cole explained in a statement.

Spirit studied Watchtower Class rocks at over a dozen locations over a 200-meter (656-foot) area along the Cumberland Ridge and Husband Hill summit. Spirit’s Alpha Proton X-ray Spectrometer (APXS) determined they all had the same chemical composition; however, the rocks appeared to look different to the rover’s other instruments.

Spirit’s Mössbauer Spectrometer showed there was a surprisingly wide variation in the ratio of oxidized iron to total iron in the Cumberland Ridge samples, ranging from 0.43 to 0.94 across a 30-meter (98-foot) section, suggesting that something had reacted with the iron in these rocks to varying degrees.

^{The pie charts show the variation across the region. Image from S. Cole, PhD thesis; background image: NASA/JPL/Cornell/Arizona State University; Moessbauer values from Morris et al. 2008 (doi: 10.1029/2008JE003201).}

Data collected from both the Mössbauer Spectrometer and the Miniature Thermal Emission Spectrometer (Mini-TES) showed that the minerals within the rocks changed: they lost their initial structure, becoming less crystalline and more amorphous. These changes corresponded with small bumps on the rocks called agglomerations, which were seen in the pancam 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 explains. Since the chemical composition of the rocks was the same, this indicates they were originally identical, leading scientists to believe that something happened to the rocks to make them different.

So what caused the changes in the rocks? Cole hypothesizes that acidic water vapor from volcanic eruptions is behind the changes. Similar to the volcanic smog, or “vog,” we see on Earth, when the corrosive Martian vog landed on the bedrock, it would have dissolved some of the minerals, forming a gel. Once the water vapor evaporated, a cement-like agent was left behind and resulted in 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."

In 2004, scientists conducted laboratory experiments exposing mock martian basalt rocks (based on data from the Mars Pathfinder mini-rover) to sulfuric and hydrochloric acids. The results showed that when exposed to acids, like would be present in the Martian vog, the rocks lost their crystalline structure – just like Cole proposed.

Microclimates also play a role. The time that the gel was present on the rocks was determined by how much sunshine and wind the rocks received. The least altered rocks were on sunnier, gentler slopes, and the large agglomerations were observed on the very steep, shady slopes.

Using Spirit data to piece together this Martian vog puzzle is a real testament to the little rover’s success. "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."