[meteorite-list] A Seasonal Ozone Layer Over The Martian South Pole (Mars Express)

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Wed, 2 Oct 2013 16:17:28 -0700 (PDT)
Message-ID: <201310022317.r92NHSdg004285_at_zagami.jpl.nasa.gov>

http://sci.esa.int/mars-express/52881-a-seasonal-ozone-layer-over-the-martian-south-pole/

A seasonal ozone layer over the Martian south pole
European Space Agency
29 September 2013

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 (O_3 ) 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 Lef??vre, two scientists from LATMOS in
Guyancourt, France, have analysed approximately 3000 occultation
<http://sci.esa.int/venus-express/49415> 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
<http://sci.esa.int/mars-express/46488> 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
<http://sci.esa.int/mars-express/50198> 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 CO_2 photolysis in the upper branch of
the Hadley cell eventually recombine in the polar night to form
molecular oxygen (O_2) 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
<http://exploration.esa.int/mars/48523-trace-gas-orbiter-instruments/?fbodylongid=2187>
spectrometer on board the ExoMars Trace Gas Orbiter will take over
the task of atmospheric profiling."


Background information

The results described in this article are reported in
"Transport-driven formation of a polar ozone layer on Mars
<http://dx.doi.org/10.1038/ngeo1957>/", by Franck Montmessin and
Franck Lef??vre, published online on 29 September 2013 in Nature
Geoscience; doi: 10.1038/ngeo1957

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 M??t??orologie Dynamique (LMD)
and LATMOS. This model computes the evolution of 16 gaseous
species by means of a comprehensive description of the Martian
photochemistry,

Mars Express was launched in June 2003 and became ESA's first
visit to another planet in the Solar System. The scientific
payload, provided by research institutes throughout Europe,
consists of seven instruments that provide remote sensing
measurements of the atmosphere, ground and below the surface.
Since arrival in orbit around Mars in December 2003, Mars Express
has been helping to answer fundamental questions about the
geology, atmosphere, surface environment, history of water and
potential for life on Mars.


Contacts

Franck Montmessin
Laboratoire Atmospheres, Milieux, Observations Spatiales (LATMOS)
Guyancourt, France
Email: franck.montmessin at latmos.ipsl.fr
Phone: +33-1-80-28-52-85

Olivier Witasse
Mars Express Project Scientist
Research and Scientific Support Department
Directorate of Science and Robotic Exploration
ESA, The Netherlands
Email: Olivier.Witasse at esa.int
Phone: +31-71-5658015
 
Received on Wed 02 Oct 2013 07:17:28 PM PDT