Typical configuration of Mars during summer in the Northern hemisphere (winter in the South). Oxygen atoms are released by the dissociation of carbon dioxide molecules exposed to UV radiations. The oxygen follows a path (indicated by the arrow) towards the South Pole, carried by the atmospheric circulation dominating during this season. Once arrived at the South Pole where a permanent night reigns, the oxygen atoms recombine to form the ozone layer observed by SPICAM at about 50 km.
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 winter poles.
Ozone molecules are easily destroyed by solar ultraviolet light and by chemical reactions with hydrogen radicals (HOx), which are released by photolysis (splitting) of water (H2O). 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 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 a paper published in the online version of the journal Nature Geoscience, Franck Montmessin and Franck Lefèvre, two scientists from LATMOS in Guyancourt, France, have analyzed approximately 3,000 occultation sequences and vertical ozone profiles collected by SPICAM on the night side of Mars.
They evidence for the existence of a third ozone layer which exists 40-60 km above one, or both, of the winter poles.
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.
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–in vain.
"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.
In fact, enhanced HOx production from photolysis of water vapour during summer in the southern hemisphere results in a much stronger flow of ozone-destroying radicals to the north winter pole than occurs to the south winter pole at the opposite 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. "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 to such measurements."
Laboratoire Atmosphères, Milieux, Observations Spatiales (LATMOS)
Email: franck.montmessin at latmos.ipsl.fr
Phone: +33 (0)1 80 28 52 85
Olivier Witasse, Mars Express Project Scientist
Research and Scientific Support Department
Directorate of Science & Robotic Exploration
ESA-ESTEC, The Netherlands
Email: Olivier.Witasse at esa.int
Phone: +31 71 5658015