ESA - Mars Express Mission patch.
Oct. 4, 2011
A new analysis of data sent by the SPICAM spectrometer aboard the Mars Express spacecraft of ESA, has revealed for the first time that the planet's atmosphere is supersaturated with water vapor. This surprising discovery with important implications for understanding the Martian water cycle as well as the evolution of its atmosphere is presented in an article in Science on September 29, 2011.
Although several satellites have visited Mars since the 70's, very few direct measurements of the vertical structure of the atmosphere of the planet were made.
Mars Express in orbit around Mars - Credit: ESA
Overall, most of the instruments that have operated in orbit to date have primarily looked at the surface. Only by studying the horizontal distribution of water in the atmosphere, they left the question of the vertical profile of water vapor in the atmosphere almost unexplored. Given this lack of direct measurements, the representation of the vertical distribution of water vapor, a key element in the hydrological cycle of Mars, is generally based on the theoretical predictions of climate models.
Finally a vertical profile of water vapor in the Martian atmosphere!
This data gap is now filled by the SPICAM spectrometer (Spectroscopy for Investigation of Characteristics of the Atmosphere of Mars) Mars Express. Used in occultation mode when it scans the sunlight through the atmosphere of the planet just before sunrise or sunset, the instrument records the measurements of various constituents of the atmosphere.
Vertical profiles of concentration can then be established, including water vapor. Or SPICAM data obtained in the infrared range in the spring and summer in the Northern Hemisphere indicate that the vertical distribution of water vapor in the Martian atmosphere is not compatible with models that Climate plan. These results provide the first evidence of the existence of water vapor in a state of supersaturation on Mars.
The atmosphere of Mars contains about 10,000 times less water vapor than that of Earth. If it condenses on the surface, the water in the Martian atmosphere would form a layer of 10 microns (1/100th of a mm) thick over the entire planet. However, water vapor is a gas trace marked by very strong momentum, proving to be one of Mars' atmospheric constituents the most variable (locally, its concentration can vary by a factor greater than 1000 during the year).
Diagram of water cycle in the Martian atmosphere - Credit: ESA / Medialab AOES
In standard terrestrial conditions, water vapor tends to condense around small airborne dust or aerosol particles or salts, when the temperature drops below the "dew point". The atmosphere is so-called "saturated", because it can hold more moisture at this temperature and the pressure. Any amount of water vapor in excess of the "dew point" condenses into droplets or ice crystals that come later rush.
However, when condensation nuclei (which are assumed to be mineral dust suspended in the atmosphere) are too rare, the condensation is very slow and does not allow water vapor to condense excess. This creates a supersaturation of water remaining in gaseous phase imbalance.
Frequent supersaturation
So far, it was assumed a priori that such a supersaturation could not exist in the Martian atmosphere, also very cold (- 100 ° C): any molecule of water beyond the saturation level was supposed to immediately transform the form of ice.
For Mars, the conditions of pressure and temperature allow only two states for water: solid and / or gas.
Nevertheless, the data revealed that the SPICAM supersaturation of water vapor is a common occurrence on Mars. Supersaturation levels are high have been found up to 10 times higher than those found on Earth.
Mars Express in orbit around Mars - Credit: ESA
In fact, there is much more water vapor in the upper part of the Martian atmosphere than anyone had imagined.
Climate models have apparently he greatly underestimated the concentration of water vapor at altitudes above 15 km, a level at which water vapor encounters a theoretical point of condensation, with 10 to 100 times more water than was originally planned.
"The vertical distribution of water vapor is a key factor in the hydrological cycle on Mars, and an old paradigm, which assumes that water is mainly controlled by the physics of saturation should now be completely revised" Luca Maltagliati notes, post-doctoral fellow at CNES LATMOS. "Our discovery has major implications for understanding the global climate and for the transport of water at large. Indeed, the capacity of the water vapor to exist in a state of high supersaturation allows him to power atmospheric layers dominated by north-south movements. With this, the Southern Hemisphere is supplied with water much more efficiently than predicted by current models. This mechanism has probably endured and will endure even over thousands of years. "
"The data show that SPICAM much larger amount of water vapor can be transported in the atmosphere high enough to be destroyed by photodissociation," said Franck Montmessin, SPICAM scientific leader and co-author of the article.
Image of the south pole of Mars made with data from Mars Express in January 2011. Credits: ESA
Solar radiation breaks the water molecules and produce oxygen atoms of hydrogen and light enough to then escape to interplanetary space. This has fundamental implications on the issue of water on Mars, known significant fraction continually escapes to space (through the hydrogen and oxygen) for billions of years, explaining part the low concentration of present water.
After these initial revelations, the further data analysis SPICAM could still deliver amazing results.
Article references:
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
Images (Mentioned), Text, Credits: CNES / ESA / Translation: Orbiter.ch.
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