Signs of acid fog found on Mars

A planetary scientist has pieced together a compelling story about how acidic vapours may have eaten the rocks in a 100-acre area on Husband Hill in the Columbia Hills of Gusev Crater on Mars.

Shoshanna Cole from the Cornell University used 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.

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.

Across Cumberland Ridge the data showed there was a surprisingly wide range in the proportion of oxidized iron to total iron, as if something had reacted with the iron in these rocks to different degrees.

Meanwhile, further data showed that the minerals within the rocks changed and lost their structure, becoming less crystalline and more amorphous.

These trends match the size of small bumps, which Cole calls agglomerations, seen 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."

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 noted.

Cole hypothesises that the rocks were exposed to acidic water vapour from volcanic eruptions, similar to the corrosive volcanic smog that poses health hazards in Hawaii from the eruptions of Kilauea.

When the Martian smog 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.

She presented the findings at the annual meeting of the Geological Society of America in Baltimore, Maryland, on November 2.