Chemists at UC San Diego have uncovered a new chemical reaction on tiny particulates in the atmosphere that could allow scientists to gain a glimpse from ancient rocks of what the atmospheres of the Earth and Mars were like hundreds of millions years ago. Their discovery also provides a simple chemical explanation for the unusual carbonate inclusions found in a meteorite from Mars that was once thought by some scientists to be evidence of ancient Martian life.

“We never knew before how the atmosphere could be trapped in carbonate,” said Mark Thiemens, dean of UC San Diego’s Division of Physical Sciences who headed the team of scientists that detailed its discovery in this week’s early online edition of the Journal of the Proceedings of the National Academy of Sciences. “This chemical reaction, which takes place on the surface of aerosols in the atmosphere, not only provides us with an understanding of how these carbonates can form on the Earth and Mars. It gives us a new tool to better understand climate change, as our planet warms and becomes more dusty.”

Robina Shaheen, a postdoctoral researcher in Thiemens’ laboratory, discovered the chemical reaction and detailed its importance in the Earth’s atmosphere after four years of painstaking experiments in which she found a higher than expected proportion of oxygen 17 isotopes in the carbonates found on dust grains, aerosols and dirt from various parts of the world.

Martian meteorites, such as ALH84001, which was once thought to exhibit evidence of extraterrestrial life, have carbonates with similarly high oxygen 17 anomalies. Scientists have long attributed those anomalies to photochemical processes involving ozone and carbon dioxide in the thin atmosphere on Mars, which is bathed by intense ultraviolet radiation. But after finding similar anomalies on terrestrial carbonates formed in atmospheric aerosols, Shaheen surmised they might be the result of another chemical process more common to both planets.

She analyzed in painstaking detail in the laboratory and in the Earth’s atmosphere how ozone molecules interacted with oxygen-bearing mineral aerosols from dust, sea spray and other sources to form hydrogen peroxide and carbonates containing this same oxygen-isotope anomaly.