Russian and German physicists have offered an explanation for the new data obtained by Martian satellites, capturing the "escape" of hydrogen atoms from the upper Martian atmosphere into outer space. The developed model fits well with the observations and explains a number of puzzling phenomena related to the atmosphere of Mars. The research was published in the journal Geographical Research Letters.
The atmosphere of Mars is cold and rarefied, like the Earth atmosphere at high altitudes. Under such conditions, there is no liquid water, but rather clouds consisting of tiny ice crystals. On Earth, such clouds -- called "feathery" -- are formed at 6 kilometers above the surface. As the ice crystals are rather heavy, the bulk of the water is contained in the lower atmospheric layer, approximately 60 kilometers thick. However, the data obtained from the U.S. satellite MAVEN (short for "Mars Atmosphere and Volatile EvolutioN") and the Hubble Space Telescope evidences a periodic stream of hydrogen atoms escaping the planet. Their only source may be water dissociating into oxygen and hydrogen in the upper atmospheric layers (70-80 kilometers from the ground) as a result of exposure to ultraviolet radiation.
The researchers have ventured a guess as to how water is "launched" to this seemingly unreachable altitude.
According to observations, the number of hydrogen atoms flying away into outer space increases during the time of summer solstice in the Southern Hemisphere and during dust storms. Moreover, fluctuations of water concentration in the upper and lower atmosphere occur simultaneously. This led the physicists to put forward a hypothesis that some sort of a "pump" must be driving the water up. The team used numerical modeling to explain the nature of this process.
The basis for the 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 water transfer from the ground and into the thermosphere (the atmosphere layer where temperature declines with height), and takes into account the impact of dust storms. Previously, MIPT researchers and their German and Japanese co-authors presented a model describing the distribution of water vapor and ice in the Martian atmosphere over the course of a year. The model came to be a part of a broader description of the processes taking place on Mars. Unlike that earlier research, the new model takes water photodissociation into account.
Carried away by the wind
As the processes occurring in the atmosphere of Mars are clearly seasonal, it is often necessary to identify the time frame in which a certain event occurs. On Earth, we would have simply named a date -- for example, March 20, the spring equinox day. But even though a calendar of its own has been developed for Mars (the Darian calendar), consisting of 24 months, each 27-28 days long, it is not very convenient. It is not that easy to figure out from the phrase "day 20 of the Pisces month" which season in which hemisphere is meant. In practice, it is much easier to pin a point in the orbit where the planet is. For this purpose, heliographic longitude is used (figure 1).
Modeling has demonstrated that water concentration in the atmosphere changes significantly over the year, reaching its maximum at the heliographic longitudes of 200° to 300°. In this time, the planet passes perihelion, the point in the orbit where Mars is closest to the sun (figure 2).
"Water vapor flows are at their maximum at Ls = 260°, which corresponds to the southern summer, when average planet temperature is also at its maximum. During the period from Ls = 220° to 300°, the ice on the surface of Mars at southern latitudes intensely sublimates, and at altitudes below 40 kilometers, the resulting water exists as water vapor, while higher up, it forms ice clouds," explains Dmitry Shaposhnikov, the lead author of the paper and a researcher at the MIPT Laboratory of Applied Infrared Spectroscopy.
Seasonal winds blowing along meridians carry heat and moisture from the "summer" hemisphere over to the "winter" hemisphere. The distribution of flows at altitudes above 120 kilometers evidences that there are also other winds existing in the lower and the adjacent middle latitudes, but their contribution to the overall layout is not that significant.
The bulk of the water is concentrated in the lower atmosphere, below 30 kilometers, but calculations have shown that water can "seep" into upper atmosphere layers, caught up in a small upward flow of water vapor between 20° and 70° southern latitude that only exists during perihelion -- a bottleneck of sorts (figure 2c). If the water manages to pass it, seasonal winds carry it toward the North Pole. Along the way, some of the H2O breaks down, affected by UV rays, into hydrogen and oxygen, while most of it, together with the cooling air, descends back into lower atmospheric layers, condensing around the North Pole area. That way, the Martian northern polar cap is formed (the southern one is much smaller).
Dusty and foggy
Dust storms, sometimes engulfing the entire planet, naturally have an impact on the circulation of water, but in a way that is far from obvious. First, dust-laden air heats up more, which prevents water condensation. Second, dust particles promote ice crystal formation (the dust provides a nucleus for ice formation), leading to more clouds. Third, the storms affect the circulation of air streams along meridians.
In order to study the impact of strong dust storms, the parameters were taken of the global dust storm that occurred during perihelion in Martian year 28 (calculated from April 11, 1955), that is in the years 2006-2007 on Earth. Modeling has shown that the temperature increased by over 20 degrees Celsius at the South Pole, and by over 45 C at the North Pole. Winds blowing from pole to pole have also become stronger.
Dmitry Shaposhnikov explains: "More atmospheric heating in the north is due to the fact that the airflow arriving from the south cools off, intensely descending onto the planet's surface and transferring to the surface the energy that becomes thermal energy [see figure 3]. Our calculations have shown that a higher temperature during a dust storm causes an increase in water vapor concentration and a higher intensity of airflow circulation."
Higher water content causes the hydrosphere thickness to increase from 60 to 70 kilometers. Ice clouds become denser and relocate 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 down. Due to this, ice clouds are located higher in a storm, locking in more moisture. Therefore, a higher content of dust in the air helps water pass through the bottleneck and get into higher atmospheric layers.
Is it the sun that rules the tides?
The moon is responsible for the tides on Earth. On Mars, the satellites Phobos and Deimos are too small to have any significant impact. The sun has the strongest influence on the planet, its gravitation also affecting water vapors. As a result, during the day, there is an "ebb tide" observed -- the formation of an upward water vapor flow -- while in the evening, there is a "high tide", when a downward flow is formed (figure 4).
"The sun is operating as a pump, which 'activates' in the daytime and helps water reach heights of over 60 kilometers above the ground. During a dust storm, the concentration of moisture in the air and airflow speed are higher, and therefore, the 'pump' is able to lift water higher up," Dmitry Shaposhnikov explained.
All theory, dear friend, is gray ...
In order to verify model validity, the authors have compared the obtained results with the data collected by Mars Reconnaissance Orbiter, MRO, in the 28th Martian year. Both modeling and experimentation have shown increased water content in the atmosphere during the perihelion (figure 5). Unfortunately, the measurements conducted by MRO in the very dust storm and at heights exceeding 80 kilometers proved unsuccessful. However, at the highest altitude accessible for measurement using this apparatus (about 70 to 80 km), the measured and calculated values of water vapor content have turned out to be almost the same: approximately 70 to 80 cubic centimeters per cubic meter.
The results of night measurements directly before a global dust storm (Ls between 200-250°) also fit well with the modeling, demonstrating increased water content in a downward water vapor flow. However, according to the MRO data, water content maxes out at the height of 40 to 50 kilometers, while the model provides for lower water content as the height decreases. This could perhaps result from the fact that the distribution of dust particles by size preset in the model is different from the actual distribution. The model also predicts a sharp drop in water content in the atmosphere after Ls = 330°, which is not supported by experimental data.
However, the experimental and calculated season-dependent distributions of water content are quite similar (see figure 5). Both demonstrate the existence of a bottleneck in the Martian water circulation, which may only be passed by water at a certain time during perihelion. Water is also more likely to successfully pass the bottleneck if perihelion coincides with a dust storm.
"The new model aligns well with the observations and allows to explain a number of phenomena in the Martian atmosphere (the presence of water vapors at altitudes above 80 kilometers, the seasonal fluctuations, the impact of dust storms and solar tides) and may be used to test new hypotheses," Alexander Rodin commented. He is one of the authors of the study and the head of the MIPT Laboratory of Applied Infrared Spectroscopy.
Rodin added: "We are very much looking forward to receiving the data provided by the Russian ACS spectrometer unit within the ExoMars global project, whose capabilities are much broader than those of the MRO instruments whose data we relied on. Moreover, the research demonstrates just how significant the processes localized in the polar areas of a planet may be for its global climate. By the way, this applies to the Earth, too."
All model data are available online at https://mars.mipt.ru/. The research was partially supported by the Russian Science Foundation (RSF) and German Science Foundation (DFG).
Geophysical Research Letters