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 Lefevre, 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.

The data were collected during three and a half Martian years (2004 - 2011), with greater sampling over the southern hemisphere due to the spacecraft's elliptical orbit and the requirement to obtain the majority of occultations on the planet's night side. They were then compared with the LMD global climate model (GCM), developed in France, which computes the evolution of 16 gas species by means of a comprehensive description of the Martian photochemistry.

When SPICAM observed regions poleward of 75 degrees South, which were experiencing continuous polar night, it detected a previously unknown layer of ozone located at heights of 35 - 70 km, with a peak concentration at 50 km. This third ozone layer shows an abrupt decrease in elevation between 75 and 50 degrees South.

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.

The authors of the paper in Nature Geoscience 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. (Earth's atmosphere has two Hadley cells between the equator and the subtropics.)

"This process consists of deep vertical downwelling of oxygen-rich air which has been transported from the summer hemisphere," explained Franck Montmessin, lead author of the paper.

"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. 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."

Despite SPICAM's coarser coverage of the northern polar region in autumn and winter, the scientists searched its data for evidence of a comparable layer of ozone in between 60 and 65 degrees North - but without success.

"At these latitudes, no polar ozone layer can definitively be identified from the SPICAM data," said Montmessin. "This implies that atmospheric chemistry and/or transport behave differently in the two hemispheres."

This dichotomy is confirmed by the GCM, which predicts no high-altitude ozone layer in the northern polar night region. Since the simulations show that Hadley circulation should be most active at the northern winter solstice, other processes besides transport must be considered.

The authors believe that the explanation lies in seasonal variations of temperature and water vapour, caused indirectly by the highly elliptical orbit of Mars and the planet's large axial tilt.

The southern summer takes place around perihelion, when Mars is more than 40 million km closer to the Sun than it is during the northern summer. As a result, the southern hemisphere has warmer summers than the northern hemisphere.

This temperature difference greatly influences the amount of water vapour in the atmosphere, since warmer air can contain more moisture. This, in turn, affects the production of ozone-destroying hydrogen radical molecules.

During the cooler northern summer, water vapour is essentially confined below 15 km. This vertical confinement reduces the transport of water from the north to the south.

Since hydrogen radical molecules can only be created by photolysis of water vapour above 25 km, few of these destructive radicals are produced in the northern hemisphere and transported southward. As a result, any ozone forming over the high southern latitudes remains nearly intact, allowing the creation of a polar ozone layer.

Conditions are very different during the southern summer. With Mars near perihelion and an increase of dust activity, the upper atmosphere becomes warmer. This warming raises the altitude at which the atmosphere becomes saturated with water to above 40 km and allows it to contain several times more water than around aphelion.

Enhanced hydrogen radical production from photolysis of water vapour results in a much stronger flow of ozone-destroying radicals to the north winter pole than occurs to the south winter pole in the aphelion season. This leads to a rate of ozone destruction that is about 100 times greater above the northern winter pole than above its southern counterpart.

"We believe this accounts for the different behaviour of the wintertime polar ozone layers on Mars," said Montmessin. "If there is an ozone layer above the northern winter pole, it must be very sparse compared with its southern counterpart."

"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."

"All being well, SPICAM observations of the planet's atmosphere will continue during the extended phase of the Mars Express mission, until the end of 2016, thanks to an orbit which is favourable for such measurements. From mid-2017 onwards, the NOMAD spectrometer on board the ExoMars Trace Gas Orbiter will take over the task of atmospheric profiling."

SPICAM (Spectroscopy for Investigation of Characteristics of the Atmosphere of Mars) enables scientists to derive vertical profiles of the Martian atmosphere to heights of well above 100 km. This is done by studying how light from bright stars is absorbed as it passes through the gases of the Martian atmosphere at different altitudes - a technique called stellar occultation.

The Global Climate Model (GCM) of Mars used for this study has been developed at the Laboratoire de Meteorologie Dynamique (LMD) and LATMOS. This model computes the evolution of 16 gaseous species by means of a comprehensive description of the Martian photochemistry, The results described in this article are reported in "Transport-driven formation of a polar ozone layer on Mars", by Franck Montmessin and Franck Lefevre, published online on 29 September 2013 in Nature Geoscience; doi: 10.1038/ngeo1957