Mars’ atmosphere supersaturated with water vapour
Washington: The upper atmosphere of Mars contains up to a hundred times more water vapour than anyone had ever imagined, according to a new study.
It seems that previous models have greatly underestimated the quantities of water vapour at heights of 20–50 km.
This surprising discovery has major implications for understanding the Martian water cycle and the historical evolution of the atmosphere, researchers say.
Using data from the European Space Agency's Mars Express probe and NASA's Mars Reconnaissance Orbiter, astronomers determined that water in the upper Martian atmosphere undergoes a process known as supersaturation — something astronomers thought could not occur on the dry, dusty planet.
Under normal conditions on Earth, water vapour condenses around tiny dust or aerosol particles or salts when the atmospheric temperature drops below a certain “dew point”.
The atmosphere is then said to be “saturated”, since it cannot hold any more moisture at that temperature and pressure. Any water vapour in excess of the “dew point” will normally condense to form droplets or icy crystals.
However, supersaturation may occur when some of the water vapour remains in the atmosphere, instead of condensing or freezing. When condensation nuclei (assumed to be dust aerosols on Mars) are too rare, condensation is impeded, leaving substantial amounts of excess vapour.
Until now, it was generally assumed that such supersaturation cannot exist in the cold Martian atmosphere.
However, the SPICAM data have revealed that supersaturation occurs frequently in the middle atmosphere – at altitudes of up to 50 km above the surface – during the aphelion season, the period when Mars is near its farthest point from the Sun.
The study has been published in the journal Science.