Even if several satellites did visit Mars since the 70's, very few direct measurements of the vertical structure of the planet's atmosphere were performed.
Overall, most of the orbiting instruments mainly observed the surface. By only studying the horizontal distribution of water in the atmosphere, they left the question of water vapour vertical pattern in the atmosphere almost unexplored. Since only a few direct measurements are available, the vertical distribution of water vapour, key element in the Martian hydrological cycle study, is generally based on theoretical predictions of climate patterns.
Finally a water vapour vertical pattern in the Martian atmosphere!
This shortfall in data is now being filled up by the SPICAM spectrometer (Spectroscopy for Investigation of Characteristics of the Atmosphere of Mars) of Mars Express. The instrument records measurements of different components of the atmosphere in occultation mode when observing the Sun's light passing through the planet's atmosphere just before sunrise and sunset.
Vertical concentration profiles can thus be established, including for the water vapour. But some data in the infrared range obtained by SPICAM in spring and summer in the northern hemisphere show that the vertical distribution of water vapour in the Martian atmosphere is not compatible with the climate model's predictions. These results provide for the first time proof of the existence of water vapour in supersaturation on Mars.
The Mars' atmosphere contains 10,000 times less water vapour than the Earth's. If the whole water of the atmosphere condense, it would only form a 10 micrometers (a hundredth of millimeter) thick layer on the planet's surface. However, the water vapour is a trace gas marked by very strong dynamics being one of the Mars' most variable atmospheric components (locally, its concentration can vary by a factor greater than 1,000 during the year).
In standard terrestrial conditions, the water vapour tends to form around ambient little dusts, particles, aerosols or salts when the temperature drops below the "dew-point". The atmosphere is thus called "saturated" because it cannot hold more moisture for that temperature and pressure. All the water vapour beyond the dew-point condenses to form droplets or ice crystals which will then precipitate.
Nevertheless, when the condensation nuclei (which are supposed to be ambient mineral dusts in the atmosphere) are too rare, the condensation is highly slowed down and prevent the excess water vapour to condense. This phenomenon thus creates a supersaturation of water which remains in a phase unbalance gaseous state.
A frequent supersaturation
Until now, such supersaturation was not thought to exist within the very cold Martian atmosphere (-100°C): every water molecule beyond the saturation level was supposed to immediately turn into ice.
Because on Mars, the pressure and temperature conditions make that only two states of water are possible: solid and/or gaseous.
However, the data of SPICAM revealed that the supersaturation of water vapour is a frequent phenomenon on Mars. Supersaturation levels were found up to ten times higher than on Earth.
In fact, there is much more water vapour in the upper part of the Martian atmosphere than anybody ever expected.
It appears that the climate models have seriously underestimated the water vapour concentration at altitudes greater than 15 km, altitude after which the water vapour reaches its theoretical dew-point with 10 to 100 times more water than what was initially expected.
“The vertical distribution of water vapour is a key factor of the hydrological cycle on Mars and the old paradigm which supposed that water is mainly controlled by saturation physics has to be entirely revised” Luca Maltagliati, post-PhD fellow of CNES at LATMOS, said. “Our discovery presents major implications for the understanding of the planet's climate as well as for the water transmission in the broad sense.“ ”In fact, this capacity of the water vapour to subsist in high supersaturation state allows it to reach atmospheric layers dominated by North-South motions.“ ”Thanks to this, the southern hemisphere is much more efficiently water supplied than the current models predicted.“ ”This mechanism is likely to have persisted and persist longer for thousands years.”
“The SPICAM's data show that a much larger quantity of water vapour can be brought high enough into the atmosphere to be photodissociated”, Franck Montmessin, SPICAM's Principal Investigator and co-author of the article, added.
The solar radiation brakes the water molecules and produces oxygen and hydrogen atoms then light enough to escape to the interplanetary space. This has fundamental implications for the Martian water problematic and the significant fraction of it which has been continuously escaping to space (through hydrogen and oxygen) for billion years, partly explaining the current low water concentration.
After these first revelations, the analysis continuation of the SPICAM's data could provide even more astonishing results.
Evidence of Water Vapor in Excess of Saturation in the Atmosphere of Mars, L. Maltagliati, F. Montmessin, A. Fedorova, O. Korablev, F. Forget, and J.-L Bertaux, Science, September 2011
- Luca Maltagliati, post-PhD fellow of CNES at the Atmosphere Environments and Space Observation Laboratory (LATMOS), 78280 Guyancourt, France
- Franck Montmessin, Atmosphere Environments and Space Observation Laboratory (LATMOS), 78280 Guyancourt, France
- Olivier Witasse, Mars Express' Project Scientist, ESA
- Francis Rocard: Solar system program scientist at CNES.