Rocks on Mars show signs of having been dissolved, with the most likely culprit being an acid fog created by vapors from volcanoes, scientists say.
Scarce, thin water vapor could be combining with the acidic gases from the volcanoes to form a corrosive fog clinging to rocks on the shaded side of hills, they say.
A similar phenomenon occurs on Earth with so-called Hawaiian "vog," a corrosive volcanic smog resulting in gases released from the Kilauea volcano.
"A lot of people have talked about weathering that would occur on Mars," says planetary scientist Ralph Milliken of Brown University, although he notes such erosion would take millions of years in the thin, dry atmosphere of Mars.
New research by planetary scientist Shoshanna Cole at Ithaca College in New York supports such a hypothesis.
She has used data collected by the now-defunct NASA Spirit rover to support the possibility that acidic vapors created a thin dissolved layer of rock "soup" on the surface of Martian rocks, changing their surface appearance.
"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," she says.
"This would have happened in tiny amounts over a very long time," she explains. "Nothing is being added or taken away, but it was changed."
Cole found that some rocks had been weathered by the acid fog to a greater extent than others.
That's likely due to their different locations, she suggests; rocks showing the most erosion were on steep, shady hillsides facing away from the sun, where water—and the acid fog—could persist for longer periods of time.
In an earlier study conducted in 2004, scientists exposed basalt rocks similar to rocks on Mars to sulfuric and hydrochloric acids, and found that the acid caused them to lose their crystalline structure, just as Cole observed in the Martian rocks, where the structure has essentially been melted into a gel or "soup."
The findings were reported Nov. 2 at the annual meeting of the Geological Society of America, held in Baltimore, Md.