Heterogeneous chemistry on Mars

A model shows that the heterogeneous chemistry of water-ice clouds has a key role in the composition of the martian atmosphere

Credit: © 2008 NPG

Spacecraft sent to investigate Mars provide invaluable data allowing scientists to create models that aid our understanding of planetary atmospheres. Now, Franck Lefèvre of the University of Paris and CNRS and colleagues in France and the USA have shown¹ that the chemistry of the martian atmosphere is significantly affected by the heterogeneous reactions occurring on the surface of water-ice cloud particles. The researchers created an atmospheric model by matching data sent back from Mars Express and the Mars Global Surveyor.

Hydrogen radicals have a central role in regulating the carbon-dioxide-dominated martian atmosphere. They also react readily with ozone, which can be measured easily through UV spectroscopy on board spacecraft, making it a sensitive tracer of atmospheric chemistry on Mars. Lefèvre and co-workers compared the seasonal changes in ozone — measured by the Mars Express — with a general circulation model that accounts for gas-phase chemistry only. The model gives a good qualitative agreement, but at various latitudes it predicts ozone losses in the martian springtime that are too rapid, and ozone increases in summertime that are too slow.

Lefèvre and colleagues considered several possible explanations to account for such inconsistencies, including possible discrepancies in the photochemical and gas-phase kinetic data. The seasonality and geography of the variations matched well with the inconsistencies being caused by water-ice clouds. Including the heterogeneous chemistry between HO_x species and ice in the model brought it into excellent agreement with the measured ozone levels. This accounted for the seasonal and latitudinal variations, suggesting that water-ice clouds are an important factor in the atmospheric chemistry of Mars.