Hearing the phrase "extraterrestrial life", most will think about some inhabitants of a distant, distant galaxy, or about aliens from another dimension; mankind is accustomed to thinking in stereotypes that are imposed by cinema and mass media. However, with the beginning of the space age, our knowledge of the near space has greatly expanded, and they allow, if not to assert, then with a high degree of probability to assume that extraterrestrial life may be very close. Not more than 50 years ago, a whole science appeared - astrobiology; it is a discipline related to astronomy, biology and geology. While these scientists are mainly engaged in theoretical research, however, the leading space agencies are seriously considering the inclusion of astrobiologists in future space missions. After all, someone should be studying extraterrestrial life on a professional level?
Naturally, we are talking about forms of life that are different from ours. Protein life of planet Earth - it is peculiar only to the Earth. Under no other conditions can this life exist. If we consider the conditions of our life, it turns out that we are very effeminate and unadapted to the conditions of the space around us. Take, for example, the temperature limits: the life temperatures of protein compounds are from 0 to 40 ° C; below - water freezes, above - protein denaturalizes. And there are also conditions for pressure, atmospheric composition, radiation, and more.
It is likely that other environmental conditions imply not only different laws of metabolism, but in general, a different basis for life. And here the ideas of most astrobiologists diverge in two mutually exclusive directions. The first implies that life in the Universe can only exist on a carbon basis, such scientists are jokingly called "carbon chauvinists"; representatives of the other direction say that under certain conditions the basis of life, in principle, can be any.
Why exactly carbon? What is special about this item? One might get the impression that in this way we are trying to point out our peculiarity: they say, we are made of carbon, which means that all life is possible only from carbon. In fact, such a ridiculous argument is not scientific. Carbon life should be very common in the Universe, since there are a number of objective reasons for this.
First, carbon is one of the most abundant elements in the universe. The evolution of most stars does not end with supernova explosions. The end product of the evolution of 99% of stars are white dwarfs, which include carbon cores. It is carbon that ends the last, fourth stage of nuclear reactions in the evolution of those same 99% of stars. Heavier elements are obtained exclusively from supernova explosions.
Secondly, the ability of carbon to attach to itself as many as four other atoms, due to the peculiarity of its outer electronic shells, distinguishes it from all other elements. Undoubtedly, there are many tetravalent metals and non-metals, but none of them can hold the neighboring atoms around it so firmly. The reason for this phenomenon is the very low atomic mass of carbon. It is the lightest tetravalent element, so its compounds are the strongest.
Only these two reasons allow us to observe more than 40 thousand substances that include carbon, and only 1.5-2 thousand substances that do not contain carbon. This is the reason for the division of the science of chemistry into organic (studying carbon compounds) and inorganic, which is engaged in the study of other elements.
However, no matter how firm the positions of the "carbon chauvinists" are, the thought does not stand still, and by the middle of the twentieth century, other ideas and concepts of building life were proposed. For example, scientists are quite allowed to use compounds in which silicon is present instead of carbon as an analogue of protein molecules. It, like carbon, is capable of binding up to four neighboring atoms, its compounds, similar to carbon compounds, are capable of polymerizing, etc. They also quickly found a replacement for water, the medium that ensures metabolism; it can be, for example, ammonia. Oxygen is the oxidizing agent in "carbon" life, and both oxygen and nitrogen can be used in silicon. And so on.
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Thus, we can assume the following: no matter on what basis life is made, it is important that organisms have the ability to take resources from the environment, convert them into energy and "building material" for their reproduction. Are there such places in the solar system? Yes, and there are many of them.
The first thing that comes to mind of any person is the planet Mars. Despite its seeming desolation, waterlessness and lack of atmosphere, there is plenty of evidence for the existence of life on Mars in past eras. Everything from the nature of surface erosion to the color of the Martian soil indicates this. In addition, water ice was recently discovered on Mars, located almost everywhere under the soil layer, and methane in a carbon dioxide atmosphere can be considered as a product of the vital activity of some Martian organisms.
However, the idea of the habitability of Mars is already boring enough for both the scientific world and the townsfolk. The main theories have long been formulated and are only awaiting verification by the explorers and colonizers of Mars.
Much more interesting phenomena await humanity a little further, beyond the asteroid belt. With the discovery of the atmosphere of some satellites of the giant planets, the idea of extraterrestrial life in the solar system took on a new form. Saturn's moons Titan and Enceladus have an atmosphere; moreover, there are seas and oceans on Titan, consisting not of water, but of natural gas. This satellite has the concept of weather and climate; research vehicles recorded daily and seasonal temperature fluctuations. No less interesting is the satellite of Jupiter, for example, Europa, covered with a thin layer of ice, under which there is a huge water ocean. The probability of life in the waters of this ocean is more than great.
Or maybe life on other planets of our system has existed for so long that the degree of development of the local civilizations allows them to completely hide the traces of their existence from us. After all, a civilization that has mastered interplanetary, and maybe interstellar flights, definitely has much greater skills and knowledge, and it is not difficult to deceive the naive inhabitants of the third planet, who have recently entered space. How can you explain the fact that UFO sightings became widespread after the end of World War II? Perhaps the aliens began to observe their "younger brothers" more closely after they discovered nuclear reactions? The answers to these and many other mysteries remain to be solved by astrobiologists.