Choking London smog is bad enough - but it cannot compare with rock-devouring Martian vog.
Scientists have found evidence of acid fog caused by volcanic eruptions on the Red Planet.
A similar phenomenon is seen on Earth in Hawaii, where it is dubbed "vog".
The American space agency Nasa's Spirit rover discovered signs of acidic vapour eating away rocks in Gusev Crater on Mars.
Unusual rock outcrops across a feature called Cumberland Ridge bore small bumps, or agglomerations, thought to be due to minerals losing their structure and becoming less crystalline.
Instruments on the rover also revealed that something had reacted with iron in the rocks to varying degrees.
In other respects, the composition of the rocks was identical.
Planetary scientist Dr Shoshanna Cole, from Ithaca College, New York, said: "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."
Dr Cole believes the rocks were exposed to acidic water vapour from volcanic eruptions similar to the corrosive "vog" in Hawaii.
When the Martian vog touched the surface of the rocks, it dissolved away some of the minerals, forming a gel, according to the theory. Then the water evaporated leaving the agglomerations behind.
"Nothing is being added or taken away, but it was changed," Dr Cole added. "This would have happened in tiny amounts over a very long time."
The more altered rocks, with larger agglomerations, were found in shadier spots on steep slopes facing away from the sun.
Previous laboratory experiments had shown that when Martian basalt rocks are exposed to sulphuric and hydrochloric acids, they lose their crystalline structure in just the same way.
The findings were presented at the Geological Society of America's annual meeting in Baltimore.