Mercury is the closest planet to the Sun. It belongs to the cosmic bodies of the terrestrial group and is located relatively close to us. However, comparatively little is known about Mercury today. Some time ago, it was considered the least explored planet. Various parameters (the nature of the surface, climate features, the presence of an atmosphere, its composition) of Mercury remained a mystery due to the extremely inconvenient position of the planet for observation and research using spacecraft. The reason for this is the proximity to the Sun, which spoils any equipment directed towards it or approaching it. Nevertheless, over the centuries of constant attempts at observation, impressive material was collected, which, after the beginning of the space age, was supplemented by data from interplanetary stations. The atmosphere of Mercury is included in the list of characteristics that were studied by Mariner 10 and Messenger. The thin air shell of the planet, like everything on it, is subject to the constant influence of the luminary. The Sun is the main factor that determines and shapes the features of Mercury's atmosphere.
Observation from Earth
It is inconvenient to admire Mercury from the surface of our planet because of its proximity to the Sun and the peculiarities of its orbit. It appears in the sky close enough to the horizon. And always during sunset or dawn. The observation time is negligible. Under the most favorable set of circumstances, this is about two hours before dawn and the same after sunset. In most cases, the duration of observation does not exceed 20-30 minutes.
Phases
Mercury has the same phases as the Moon. Flying around the Sun, it either turns into a narrow crescent, or becomes a full circle. In all its glory, the planet is visible when it is opposite the Earth, behind the Sun. At this time, for the observer comes the "full moon" of Mercury. At the same time, however, the planet is at its maximum distance from the Earth, and bright sunlight interferes with observation.
Moving around the star, Mercury begins to visually increase in size as it approaches us. At the same time, the illuminated surface area is reduced. In the end, the planet turns to us with its dark side and disappears from visibility. Once every few years at such a moment, Mercury passes exactly between the Sun and the Earth. Then you can observe its movement across the disk of the star.
Methods of observation
Mercury can be seen with the naked eye or observed through binoculars shortly before dawn and after sunset, that is, at dusk. With a small amateur telescopeit will be possible to notice the planet during the day, but it will not be possible to see any details. It is important during such observations - do not forget about safety. Mercury never moves far from the Sun, which means that both eyes and equipment must be protected from its rays.
The ideal place for observing the planet closest to the star is mountain observatories and low latitudes. Here, the astronomer comes to the aid of clean air, a cloudless sky, and a short duration of twilight.
It was earthly observations that helped establish the fact that Mercury has no atmosphere. Powerful telescopes made it possible to consider many features of the planet's surface topography and calculate the approximate difference in temperature on the illuminated and dark sides. However, only the flights of AMS (automatic interplanetary stations) were able to shed light on other characteristics of the planet and clarify the data already obtained.
Mariner 10
In the entire history of astronautics, only two vehicles were sent to Mercury. The reason is a complex and expensive maneuver, which is necessary for the station to enter the planet's orbit. Mariner 10 was the first to go to Mercury. In 1974-1975, he circled the planet closest to the Sun three times. The minimum distance that separated the apparatus and Mercury was 320 km. Mariner 10 transmitted several thousand images of the planet's surface to Earth. About 45% of Mercury was photographed. Mariner 10 measured the surface temperature on the illuminated and dark sides, as well as the planet's magnetic field. In addition, the apparatus found that the atmosphere of Mercury is practically absent,it is replaced by a thin air shell, which contains helium.
Messenger
The second AMS sent to Mercury was Messenger. It started in August 2004. He transmitted to Earth an image of that part of the surface that Mariner 10 did not capture, measured the landscape of the planet, looked into its craters and found spots of an incomprehensible dark substance (possibly marks from meteorite impacts), which is often found here. The device studied solar flares, Mercury's magnetosphere, its gaseous envelope.
Messenger completed its mission in 2015. It fell on Mercury, leaving a 15-meter-deep crater on the surface.
Is there an atmosphere on Mercury?
If you carefully reread the previous text, you can see a slight contradiction. On the one hand, ground-based observations testified to the absence of any kind of gas envelope. On the other hand, the Mariner-10 apparatus transmitted to Earth information, according to which the atmosphere of the planet Mercury still exists and contains helium. In the scientific community, this message also caused surprise. And it's not that it contradicted previous observations. It's just that Mercury does not have the characteristics that favor the formation of a gaseous envelope.
What is the atmosphere? This is a mixture of gases, volatile substances, which can only be held at the surface by gravity of a certain magnitude. Mercury, small by cosmic standards, cannot boast of such a characteristic.maybe. The force of gravity on its surface is three times less than on Earth. Thus, the planet is not able to hold not only helium and hydrogen, but also heavier gases. And yet it was helium that was discovered by Mariner 10.
Temperature
There is another factor that casts doubt on the presence of the atmosphere of Mercury. This is the surface temperature of the planet. Mercury is the champion in this regard. During daylight hours, the temperature on the surface sometimes reaches 420-450 ºС. At such high values, the molecules and atoms of the gas begin to move faster and faster and gradually reach the second cosmic velocity, that is, nothing can hold them near the surface. In the temperature conditions of Mercury, the same helium should be the first to “escape”. In theory, it should not be on the planet closest to the Sun at all, and almost from the moment of its formation.
Special situation
And yet the answer to the question of whether there is an atmosphere on Mercury is positive, although it is somewhat different from what is usually hidden behind this astronomical concept. The reason for such a fantastic and at the same time quite real state of affairs lies in the unique location of the planet. The proximity of the star determines many of the characteristics of this cosmic body, and the atmosphere of Mercury is no exception.
The gas shell of the planet is constantly exposed to the so-called solar wind. It originates in the corona of the star and is a stream of nuclei, protons and electrons of helium. With solar wind to Mercuryfresh portions of the volatile substance are delivered. Without such recharge, all helium would disappear from the surface of the planet in about two hundred days.
Mercury's atmosphere: composition
Careful research helped to discover other elements that make up the gaseous shell of the planet. Mercury's atmosphere also contains hydrogen, oxygen, potassium, calcium, and sodium. The percentage of these elements is very small. In addition, the atmosphere of the planet Mercury is characterized by the presence of traces of carbon dioxide.
The air shell is very rarefied. The gas molecules in it actually do not interact with each other, but only move along the surface without collisions and collisions. Scientists have been able to establish the factors that determine the presence of the atmosphere of Mercury. Hydrogen, like helium, is delivered to its surface by the solar wind. The source of other elements is the planet itself or meteorites falling on it. The atmosphere of Mercury, the composition of which is planned to be thoroughly studied in the near future, is presumably formed as a result of the evaporation of rocks under the influence of the solar wind or diffusion from the bowels of the planet. Most likely, each of these factors contributes.
So, what is the atmosphere of Mercury? Highly rarefied, consisting of helium, hydrogen, traces of alkali metals and carbon dioxide. Often in the scientific literature it is called the exosphere, which only emphasizes the strong difference between this shell and a similar formation, for example, on Earth.
Despite all the difficulties in the lists of space targetsresearch is still listed and the planet Mercury. The atmosphere and surface of this cosmic body will probably be studied more than once using various devices. Mercury still holds a lot of interesting and unknown things. In addition, the study of planets such as Venus, Mars or Mercury, devoid of atmosphere or not, sheds light on the history of the formation and development of the Earth.
