A study in Nature Geoscience explains how atmospheric circulation creates a previously undetected ozone layer above the Red Planet's southern winter pole.
On our planet, ozone is a pollutant at ground level, but at higher altitudes it provides an essential protective layer against harmful solar ultraviolet light. However, ozone molecules are easily destroyed by solar ultraviolet light and by chemical reactions with hydrogen radicals, which are released by photolysis of water molecules. The role of pollution in its destruction has been a major focus of attention since the mid-1980s, when a hole in the ozone layer was discovered above Antarctica.
Ozone was detected on Mars in 1971. The ozone concentration on the planet is typically 300 times thinner than on Earth, although it varies greatly with location and time. In recent years, the SPICAM UV spectrometer on board Mars Express has shown the presence of two distinct ozone layers at low-to-mid latitudes.
In the new study, Dr Franck Montmessin and Dr Franck Lefèvre, both from LATMOS in Guyancourt, France, have analyzed about 3,000 occultation sequences and vertical ozone profiles collected by SPICAM on the night side of Mars and then compared the data with a global climate model, LMD, to detect a previously unknown ozone layer located at heights of 35 – 70 km, with a peak concentration at 50 km. This layer shows an abrupt decrease in elevation between 75 and 50 degrees South.
The third ozone layer was found to exist only above the winter pole. SPICAM detected a gradual increase in ozone concentration at 50 km until midwinter, after which it slowly decreased to very low concentrations, with no layer perceptible above 35 km.
Dr Montmessin and Dr Lefèvre believe that the observed polar ozone layers are the result of the same atmospheric circulation pattern that creates a distinct oxygen emission recently identified in the polar night. This circulation takes the form of a huge Hadley cell in which warmer air rises and travels poleward before cooling and sinking at higher latitudes.
“This process consists of deep vertical downwelling of oxygen-rich air which has been transported from the summer hemisphere,” said first author Dr Franck Montmessin.
“Oxygen atoms produced by CO2 photolysis in the upper branch of the Hadley cell eventually recombine in the polar night to form molecular oxygen and ozone. The concentration of ozone gas at night is dependent upon the supply of oxygen and the rate of destruction due to hydrogen radicals.”
“This ozone-forming process has no counterpart on the Earth, so Mars provides an example of how diverse and complex chemical processes can be in the atmospheres of terrestrial planets and how they may potentially operate on exoplanets.”