How does the sun pump the water of Mars into space

Russian and German physicists explained the new data obtained by Martian satellites, capturing the "escape" of hydrogen atoms from the upper Martian atmosphere into outer space. -atmospheric. The model developed fits well with the observations and explains a number of confusing phenomena related to the atmosphere of Mars. The research was published in the journal Geographical Research Letters.

The atmosphere of Mars is cold and thin, as the atmosphere of the Earth at high altitude. Under these conditions, there is no liquid water, but rather clouds composed of tiny ice crystals. On Earth, such clouds – called "feathers" – are formed 6 kilometers above the surface. Because ice crystals are rather heavy, most of the water is contained in the lower atmospheric layer, about 60 kilometers thick. However, data obtained from the American MAVEN satellite (abbreviation for "Mars Atmosphere and Volatile EvolutioN") and the Hubble Space Telescope highlight a periodic stream of hydrogen atoms escaping from the planet. Their only source may be water dissociated in oxygen and hydrogen in the upper layers of the atmosphere (70-80 km from the ground) as a result of ultraviolet radiation exposure.

The researchers tried to guess how the water was "launched" at this seemingly inaccessible altitude.

According to observations, the number of hydrogen atoms flying into outer space increases during the summer solstice in the southern hemisphere and during storms of dust. In addition, fluctuations in the water concentration in the upper and lower atmosphere occur simultaneously. This has led physicists to hypothesize that a kind of "pump" must raise the waters. The team used numerical modeling to explain the nature of this process.

The basis of this study was provided by the Martian General Circulation Model (MGCM) developed at the Max Planck Institute in Germany. The model provides a detailed description of the transfer of soil water to the thermosphere (the atmosphere layer in which temperature decreases with altitude) and takes into account the impact of storms from dust. Previously, MIPT researchers and their German and Japanese co-authors had presented a model describing the distribution of water vapor and ice in the Martian atmosphere over the course of a year. The model has been integrated with a broader description of ongoing processes on Mars. Unlike the previous research, the new model takes into account the photodissociation of water.

Carried away by the wind

Since the processes occurring in the Mars atmosphere are clearly seasonal, it is often necessary to identify the time frame in which a given event occurs. On Earth, we would have simply named a date – for example, March 20, the day of the spring equinox. But even though a full calendar has been developed for Mars (the Darian calendar), consisting of 24 months, lasting from 27 to 28 days, it is not very practical. It is not easy to understand, from the expression "day 20 of the month of Pisces", the season to which the hemisphere is destined. In practice, it is much easier to identify a point in the planet's orbit. For this purpose, heliographic longitude is used (Figure 1).

The modeling has shown that the concentration of water in the atmosphere changes significantly during the year, reaching its maximum at the heliographic longitudes of 200 ° to 300 °. Meanwhile, the planet goes to perihelion, the point in the orbit where Mars is closest to the sun (Figure 2).

"The water vapor flows reach their maximum at 260 ° Ls, which corresponds to the austral summer, when the average temperature of the planet is also maximum.In the period of Ls = 220 ° to 300 °, ice on the surface of Mars at the southern latitudes, intensely sublimated and at altitudes below 40 km, the resulting water is in the form of water vapor, while it forms ice clouds higher ", says Dmitry Shaposhnikov, lead author of the paper and researcher at the MIPT Laboratory of Applied Infrared Spectroscopy.

Seasonal winds blowing along the meridians carry heat and moisture from the "summer" hemisphere to the "winter" hemisphere. The distribution of flows at altitudes above 120 km shows that there are also other winds in the lower and adjacent mid-latitudes, but their contribution to the overall pattern is not so significant.

Most of the water is concentrated in the lower atmosphere, less than 30 km away, but calculations have shown that water can "seep" into the upper atmosphere layers, entrained by a small ascending flow of water vapor between 20 ° and 70 ° south latitude that exists only during perihelion – a bottleneck (Figure 2c). If the water manages to cross it, the seasonal winds transport it to the North Pole. Along the way, part of the H2O breaks down into hydrogen and oxygen, under the effect of ultraviolet rays, while most of it, as well as cooling air, goes down into the layers of the lower atmosphere. , condensing around the North Pole. In this way, the Nordic polar ice cap is formed (the southern cap is much smaller).

Dusty and foggy

Dust storms, which sometimes invade the entire planet, naturally have an impact on the flow of water, but in a way that is far from obvious. First, the dust – laden air heats up further, preventing condensation of water. Secondly, dust particles favor the formation of ice crystals (dust provides a nucleus for ice formation), leading to more clouds. Third, storms affect the flow of air currents along the meridians.

In order to study the impact of the strong dust storms, we took the parameters of the global dust storm that occurred during the perihelion of the 28th Martian year (calculated from April 11, 1955), that is from the years 2006-2007 on Earth. The modeling showed that the temperature had increased more than 20 degrees Celsius at the South Pole and more than 45 C at the North Pole. Winds blowing from one pole to the other have also become stronger.

Dmitry Shaposhnikov explains: "More heat in the north is due to the fact that the air coming from the south cools down, descending intensely to the surface of the planet and transferring energy to the surface transformed into energy. thermal. [see figure 3]. Our calculations showed that a higher temperature during a dust storm resulted in an increase in the concentration of water vapor and an increased intensity of air circulation. "

The higher water content increases the thickness of the hydrosphere from 60 to 70 km. Ice clouds become denser and move to higher altitudes. A higher content of dust particles in the air promotes the formation of a large number of tiny ice crystals, which take longer than usual to settle. For this reason, ice clouds are located higher in a storm, trapping more moisture. As a result, a greater amount of dust in the air helps the water to cross the bottleneck and to penetrate into the upper atmospheric layers.

Is it the sun that rules the tides?

The moon is responsible for the tides on Earth. On Mars, the Phobos and Deimos satellites are too small to have a significant impact. The sun has the greatest influence on the planet, its gravitation also affecting the water vapor. As a result, during the day, we observe a "tide down" – the formation of a flow of steam upward – while in the evening there is a "high tide" ", when a downward flow is formed (Figure 4). ).

"The sun works as a pump that" gets active "during the day and helps the water to reach heights of over 60 km above the ground.When a dust storm, the concentration of humidity in the air and the speed of the air flow are higher, the "pump" is able to raise the water higher, "said Dmitry Shaposhnikov.

The whole theory, dear friend, is gray …

In order to verify the validity of the model, the authors compared the results obtained with the data collected by Mars Reconnaissance Orbiter, MRO, during the 28th Martian year. Modeling and experimentation showed an increase in water content in the atmosphere during perihelion (Figure 5). Unfortunately, the measurements made by the MRO in the dust storm and at heights greater than 80 km were unsuccessful. However, at the highest accessible altitude for measurements using this device (approximately 70 to 80 km), the measured and calculated values ​​of the water vapor content were found almost identical: about 70 to 80 cubic centimeters per cubic meter.

The results of nocturnal measurements taken just prior to a global dust storm (200-250 ° L) fit well with the modeling, demonstrating an increase in water content in a downward flow of water vapor. However, according to the MRO data, the water content reaches its maximum between 40 and 50 km altitude, while the model predicts a lower water content when the height decreases. This may be due to the fact that the predefined size distribution of dust particles in the model differs from the actual distribution. The model also predicts a sharp drop in the water content of the atmosphere after Ls = 330 °, which is not corroborated by the experimental data.

However, experimental and calculated distributions of moisture content by season are quite similar (see Figure 5). Both demonstrate the existence of a bottleneck in the circulation of Martian water, which can only be traversed by water at some point during perihelion. Water is also more likely to pass the bottleneck if perihelion coincides with a dust storm.

"The new model fits the observations well and allows to explain a number of phenomena in the Martian atmosphere (the presence of water vapor at an altitude higher than 80 km, seasonal fluctuations, the Impact of dust storms and solar tides) and can be used to test new hypotheses, "commented Alexander Rodin. He is one of the authors of the study and the head of the MIPT Applied Infrared Spectroscopy Laboratory.

Rodin added: "We look forward to receiving the data provided by the Russian ACS spectrometer unit as part of the global ExoMars project, which has much more capabilities than the MRO instruments on which we have been working. The importance of processes located in the polar areas of a planet for the climate of the planet, and this also applies to the Earth. "

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All model data is available online at https: //March.mipt.ru /.
The research was partly funded by the Russian Scientific Foundation (RSF) and the German Science Foundation (DFG).