Supersaturated Water Vapor in Martian Atmosphere
ScienceDaily (Oct. 6, 2011) — Analysis of data collected by the European
Space Agency's Mars Express spacecraft leaves no room for doubt: the Martian
atmosphere of contains water vapor in a supersaturated state. This
surprising finding will enable scientists to better understand the water cycle
on
Mars, as well as the evolution of its atmosphere
The research was led by a team from the Laboratoire Atmosphères, Milieux,
Observations Spatiales (LATMOS, CNRS / UPMC / UVSQ), in collaboration with
Russian and French colleagues(1), and received support from CNES. It is
published in Sept. 30, 2011 issue of the journal Science.
On Earth, water vapor tends to condense, i.e. turn into a liquid, when the
temperature falls below dew point. The atmosphere is said to be 'saturated'
since it cannot hold any more moisture at that temperature and pressure.
The excess water vapor then condenses around suspended particles and dust,
forming precipitation. However, condensation may sometimes be much slower,
especially when particles and dust are scarce. Unable to condense, the
excess water vapor therefore remains in the gaseous state: this is known as
supersaturation. Until now, it was assumed that this phenomenon could not
occur
in the Martian atmosphere, although this had never been proved.
While several spacecraft have visited Mars since the 1970s, most of their
instruments were focused on surface data. Because of this, they only
observed the horizontal component of the Martian atmosphere. The way in which
water content on Mars varies with height remained largely unexplored. The surv
ey carried out by the SPICAM(2) spectrometer on board the Mars Express
spacecraft has now made it possible to fill this gap. SPICAM can establish
vertical profiles of the atmosphere using solar occultation, i.e. by observing
light from the Sun as it travels through the Martian atmosphere at sunrise
and sunset.
Contrary to previous belief, the researchers discovered that water vapor
supersaturation is a frequent phenomenon on Mars. They even observed very
high levels of supersaturation in the Martian atmosphere, up to ten times
greater than those found on Earth. "This ability of water vapor to exist in a
highly supersaturated state would, for example, allow [it] to supply the
southern hemisphere of Mars with water, far more efficiently than models
currently predict," points out Franck Montmessin, CNRS researcher at LATMOS
and
SPICAM(3) project leader. Moreover, a far greater quantity of water vapor
than thought may be transported high enough in the atmosphere to be
destroyed by photodissociation(4). If confirmed, this phenomenon would have
consequences for the whole issue of Martian water, a significant fraction of
which
is known to have continually escaped to space for billions of years, which
partly explains today's low abundance of water on the planet(5).
The vertical distribution of water vapor is key to the study of the
hydrological cycle on Mars. The hypothesis according to which the amount of
water
in the Martian atmosphere is limited by the saturation process therefore
needs revising. This finding has major implications for the current
understanding of both the climate and water transport on Mars.
Notes:
1. François Forget, CNRS researcher at the Laboratoire de Météorologie
Dynamique (LMD, CNRS/ENS Paris/UPMC/Ecole Polytechnique) took part in this
work. Both his laboratory and LATMOS belong to the Institut Pierre-Simon
Laplace.
2. This instrument is a dual ultraviolet and near infrared
spectrometer, designed and produced by three laboratories (LATMOS, the
Institut
d'Aéronomie Spatiale in Brussels and the Space Research Institute (IKI) in
Moscow), with funding from CNES.
3. Luca Maltagliati, the lead author of this study, received a CNES
grant during his post-doctorate at LATMOS.
4. Solar radiation breaks up water molecules, releasing atoms of
oxygen and hydrogen, which are then light enough to escape to interplanetary
space.
5. On Earth, the amount of water is estimated to be equivalent to a 3
kilometer-deep layer of liquid water over the whole surface of the planet.
Estimates for Mars are considerably lower, although
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