Both Mars and Earth experience acid fog, and new research suggests this phenomenon may have eaten away at rocks on the Red Planet.

The researchers focused on the 'Watchtower Class' outcrops on Cumberland Ridge and the Husband Hill summit, and found evidence that acidic vapors once swept the region, the Geological Society of America reported.

"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," said planetary scientist Shoshanna Cole.

This means the rocks could tell a story of what Mars looked like billions of years ago. The researchers compared a number of data sets, bringing fascinating patterns to light. The Watchtower Class rocks at a dozen locations along Cumberland Ridge and the Husband Hill summit were determined to have similar chemical compositions, but looked significantly different from each other. Across Cumberland Ridge there appeared to be a wide range of proportions of oxidized iron to total iron in the ancient rocks. This suggests something once reacted with the iron to different degrees. Minerals in the rocks also appeared to have lost their structure, becoming more amorphous than crystalline. The patterns match the size of small bumps, dubbed agglomerations, that have been identified in Pancam and Microscopic Imager pictures 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."

Since the rocks were the same in composition aside from these small deviations, the researchers believe they were once "made of the same stuff" before something caused them to change. One possibility is that the rocks were exposed to acidic water vapor from volcanic eruptions. This phenomenon would have been similar to corrosive volcanic smog that has caused health concerns in Hawaii today. If this "vog" were to have been released on Mars billions of years ago it would have dissolved minerals in the rocks, forming a "gel." The water would have also evaporated, leaving behind a cementing agent that caused 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."

Past studies have provided evidence of the possibility of acid fog on Mars by exposing mock Martian basalt rocks to sulfuric and hydrochloric acids. The findings showed the process caused the rocks to lose their crystalline structure, which appears to have been what happened to the actual Martian rocks. Microclimates (how much sunshine and wind the rocks were exposed to) could explain why the rocks showed a trend in the iron and the degree to which they were changed.  More dramatically altered rocks with larger agglomerations are seen on slopes facing away from the Sun, while less altered rocks with smaller agglomerations are in sunnier regions.

The findings were presented at the annual meeting of the Geological Society of America.