Approximately every two Earth years, when summer arrives in the southern hemisphere of Mars, a window opens in its atmosphere, through which water vapor can rise from the lower layers of the planet’s gas to the upper layers. Most of it is carried away by the winds to the north pole, where it settles on the surface, but a certain amount of water vapor still disintegrates and disappears into space.
Scientists from the Moscow Institute of Physics and Technology and the Institute of Solar System Studies describes this unusual Martian water cycle in a study presented in the journal Geophysical Research Letters. Their computer simulation shows how water vapor overcomes the cold air barrier in the middle atmosphere of Mars and reaches higher layers. This, according to the authors of the work, will help to understand why Mars, unlike Earth, has lost most of its water.
Billions of years ago, Mars was a planet rich in water, rivers flowed on it and oceans raged. Since then, it has changed a lot: today there are very few areas with frozen water on its surface, and in the atmosphere water vapor is found only in trace amounts. In general, Mars may have lost at least 80 percent of its original water.
The reason for this lies in the fact that in the upper atmosphere of the Red Planet, the ultraviolet radiation of the Sun splits water molecules into hydrogen (H) and hydroxyl radicals (OH). From there, hydrogen irrevocably evaporates into space. Measurements using probes and space telescopes show that even today water continues to be lost in this way. But how is this possible? The layer of the middle atmosphere of Mars, like the tropopause of the Earth, should practically block the escape of gas, since this region is usually so cold that water vapor turns into ice.
In order to reveal this secret, Russian and German researchers conducted a simulation that revealed a previously unknown mechanism resembling a pump. Their simulation comprehensively describes the flows in the entire atmosphere enveloping Mars: from the surface to an altitude of 160 kilometers. Calculations show that the icy middle layer of the gas envelope becomes permeable to water vapor twice a day, but only in a certain place and at a certain time of the year.
The orbit of Mars plays a crucial role in this process: the path of the planet around the Sun, which lasts about two Earth years, is much more elliptical than that of the Earth. At the point closest to the Sun (approximately coincides with summer in the southern hemisphere) Mars is approximately 42 million kilometers closer to it than at the farthest point of the orbit, therefore summer in the southern hemisphere is noticeably warmer than in the northern.
When summer arrives in the southern hemisphere, water vapor can rise locally with warmer air masses at certain times of the day and reach the upper layers of the atmosphere. There the air flows carry gas to the north pole, where it cools again and settles. However, part of the water vapor is excluded from this cycle: under the influence of solar radiation, water molecules disintegrate, and hydrogen escapes into space.
-Paul Hartog, co-author of the work of the Institute for Solar System Research
This unusual hydrological cycle is enhanced by another feature of the Red Planet: huge dust storms that cover all of Mars at intervals of several years. The last such storms occurred in 2007 and 2018 and were extensively documented by orbital probes.
The amount of dust circulating in the atmosphere during such a storm facilitates the transport of water vapor to the upper layers of the atmosphere.
-Alexander Medvedev, co-author of the work of the Institute for Solar System Research
Scientists estimate that during the 2007 dust storm, twice as much water vapor fell into the upper atmosphere than it does in calm times for Mars. As the dust particles absorb sunlight and thus heat up, the temperature in the entire atmosphere rises to 30 degrees.
Our model with unprecedented accuracy shows how dust in the atmosphere affects the microphysical processes associated with the transformation of ice into water vapor.
-Dmitry Shaposhnikov, lead author of a study from the Moscow Institute of Physics and Technology
The authors of the paper conclude that, apparently, the atmosphere of Mars is more permeable to water vapor than the Earth’s, and the opened seasonal water cycle largely contributes to the continuing loss of water by the Red Planet.
How mars loses water
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