This panorama was acquired by the Spirit Rver from a position in the "Columbia Hills" near the summit of "Husband Hill."
This panorama was acquired by the Spirit Rver from a position in the "Columbia Hills" near the summit of "Husband Hill."

Mars’ rugged terrain may have been carved out by acid fog, according to a new study.

In a paper presented this week to the Geological Society of America, researchers used data from the 2003 Mars Spirit rover to analyze Martian bedrock to show that acid vapor may have “eaten at the rocks” in a large area of 'Watchtower Class' outcrops.

"The special thing about Watchtower Class is that it's very widespread and we see it in different locations," said Shoshanna Cole, who is an assistant professor at Ithaca College and who is studying the area. “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 said the rocks, located on Husband Hill in the Columbia Hills of Gusev Crater on Mars, have the same chemical composition, but looked different to some of the instruments aboard the Spirit rover.

The instruments show that something had reacted with iron in the rocks to “different degrees.” Another instrument showed that the “minerals within the rocks changed and lost their structure, becoming less crystalline and more amorphous.”

Along the studied area, the rocks showed different ranges of iron oxidation in a small space of 30 meters. Small bumps on the rocks matched the difference in oxidation, said Cole.

"So we can see the [bumps] progress in size from west to east and the iron changes in the same way," Cole said. "It was super cool. 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.”

Cole thinks acid fog could explain the differences.

When exposed to the fog, some of the rocks’ minerals were dissolved, “forming a gel,” and when the water from the fog evaporated, it left behind a “cementing agent” leading to the formation of the bumps.

"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."

Cole’s findings build on a 2004 study that showed mock Martian rocks lose their crystalline structure when exposed to acid.

Why was there such a difference over such a relatively small span? Cole believes microclimates are the cause.

The amount of time the gel-like substance was on the rocks depended on wind and sun exposure. The most chemically altered rocks were not as exposed to the sun, while those showing less changes were in sunnier areas, Cole said.

Robotic Spirit’s mission to Mars ended in 2010.