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
survey 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:
- 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.
- 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.
- Luca Maltagliati, the lead author of this study,
received a CNES grant during his post-doctorate at LATMOS.
- Solar radiation breaks up water molecules, releasing
atoms of oxygen and hydrogen, which are then light enough
to escape to interplanetary space.
- 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
