Global Atmospheric Circulation Creates Seasonal Ozone Layer Over Martian South Pole

For the past decade, ESA's Mars Express orbiter has been observing atmospheric structure on the Red Planet. Among its discoveries is the presence of three separate ozone layers, each with its own characteristics. A new comparison of spacecraft data with computer models explains how global atmospheric circulation creates a layer of ozone above the planet's southern winter pole.

Ozone (O3) is a form of oxygen gas which contains three atoms, rather than two. On Earth, ozone is a pollutant at ground level, but at higher altitudes it provides an essential protective layer against harmful solar ultraviolet (UV) light.

However, ozone molecules are easily destroyed by solar ultraviolet light and by chemical reactions with hydrogen radicals, which are released by photolysis (splitting) 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.

Until the early 1970s, no one could be sure whether ozone existed on any of the other planets. Ozone was then detected on Mars and it has since been discovered on Venus by ESA's Venus Express mission. On Mars, the ozone concentration 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. These comprise a persistent, near-surface layer below an altitude of 30 km, and a separate layer, which is only present in northern spring and summer, and whose altitude varies from 30 to 60 km.

In recent years, SPICAM has also provided evidence for the existence of a third ozone layer which exists 40-60 km above the southern winter pole, with no counterpart above the North Pole.

In a paper published in the journal Nature Geoscience, Franck Montmessin and Franck Lefèvre, two scientists from LATMOS in Guyancourt, France, have analysed approximately 3000 occultation sequences and vertical ozone profiles collected by SPICAM on the night side of Mars.

This 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.

"Oxygen atoms produced by CO2 photolysis in the upper branch of the Hadley cell eventually recombine in the polar night to form molecular oxygen (O2) and ozone. 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."

"The study of ozone on Mars is fundamental in understanding the photochemical processes that control the chemical reactions which recycle carbon dioxide, the main gas in the Martian atmosphere," said Olivier Witasse, ESA's Mars Express Project Scientist. "This recycling ensures the long-term stability of an atmosphere around Mars." -- ESA