Ecology: interaction biology. II-16. (supplement) Search for life in the Solar System

II-16. (addition) Search for life in the Solar System
There are many other worlds or just one world? This is one of the biggest questions that encourages the study of Nature. Albert the Great. XIII century.
The main candidate in the search for extraterrestrial life is, of course, Mars. In the past, it was very similar to Earth, and even now, it probably preserves the conditions necessary for the existence of the simplest forms of life. But the search for life in the Solar System is not limited to this planet.
Titan, the largest satellite of Saturn (the size of this satellite is larger than the sizes of Pluto and Mercury), attracts attention. As evidenced by the results of the American station "Cassini" and the descended probe "Huygens", on the surface of Titan there are seas and rivers flow. In its dense atmosphere (pressure - 1.5 earth), there are clouds from which it rains. Unfortunately, Titan is too cold for water to exist in a liquid state: rivers on this celestial body are formed by methane and other substances that are gases in earthly conditions. But the rock in which the streams and rivers of Titan flow is water ice! For optimists who dream of life on a different basis than water-carbon, Titan provides a rich food for fantasy.
Europa, a satellite of Jupiter, is very interesting from the point of view of searching for life. Its surface is covered with ice, under which, apparently, there is an ocean of liquid water! This celestial body is slightly smaller than the Moon and is much farther from the Sun, but the heating of its interior may be facilitated by the influence of the giant Jupiter, which is located next to it. There may also be water on Enceladus, one of Saturn's satellites. However, it is likely that this celestial body does not have enough heat to support water in a liquid state.
Io, another satellite of Jupiter, is interesting from the point of view of its geology. This celestial body, not much larger than the Moon, has a metallic core and a silicate shell. Moreover, on its surface, traces of geological processes have been registered, including volcanic eruptions! Unfortunately, water reserves on Io have not been found.
Finally, a certain interest from the point of view of searching for life is also caused by Jupiter itself - a gas giant, the largest body in the Solar System, except for the Sun. Its atmosphere consists of hydrogen and helium with an admixture of methane, water, and ammonia. In its center, there is likely a relatively small solid core, above which there is a huge layer of atmosphere. It is likely that at one of the layers of the Jovian atmosphere, there are water-ice clouds. Perhaps life can exist in them?
Most modern efforts in the search for life are based on the idea that extraterrestrial life will be similar to Earth's. For example, from the point of view of the chemical basis of life, we can easily imagine its existence based on aqueous solutions and organic compounds and actually cannot - on any other basis. There is even such a concept - "water-carbon chauvinism" - the idea that alien life must have the same chemical basis as Earth's. Are other options possible? There is no answer yet.
And if the chemical basis of life is identical to Earth's, will its development follow the same path as on our planet, or will it be different? Modern science does not have established ideas about how regular the appearance of exactly such living organisms that we observe on Earth is in the course of evolution. If the assumption that many key stages of the evolution of Earth's life passed through several evolutionary branches at the same time is correct, it indicates the patterned character of evolution. We will not prove this statement in detail here, but we will point out that it is quite likely that vertebrates went ashore several times; several times in the course of evolution, feathered creatures capable of flight - birds - arose; several times, germinal membranes or flowers of flowering plants could arise... The commonality of the paths of evolution of Earth's organisms is a consequence of their similar "arrangement" and the same adaptation tasks that they have to solve in the course of their evolution.
But how will life develop on other planets in conditions that are incomparable to Earth's? We can only guess. As an example of a flight of fantasy, let's consider a possible variant of alien life invented by Carl Sagan (a well-known scientist who studied extraterrestrial life at NASA, the American space agency).
"On a giant planet, like Jupiter, with an atmosphere rich in hydrogen, helium, methane, water vapor, and ammonia, a solid surface is inaccessible, but there are rather dense cloud layers into which organic molecules can fall from the sky, like manna from heaven, as it turned out with the products of our laboratory experiments. There is also a characteristic obstacle for life on such a planet: the atmosphere is turbulent and in its lower layers heated to very high temperatures. Organisms must be careful not to be carried down and toasted.
To show that life is not excluded on such planets, which are quite different from Earth, my colleague from Cornell, E. E. Salpeter, and I performed some calculations. Of course, we cannot exactly know what life will be like in such a place, but we wanted to assume, within the framework of known laws of physics and chemistry, whether such a type of world can, in principle, be inhabited.
One way to preserve life in the described conditions is to give offspring before being toasted, and to hope that convection will carry some of your offspring to higher and colder layers of the atmosphere. Such organisms can be very small. We called them sinkers (from English sinker - sinker). However, you can also become a floater (from English float - to float) - a huge hydrogen balloon that pumps out helium and other heavier gases, leaving only the lightest gas - hydrogen inside; another option is a balloon with hot air, which maintains buoyancy by maintaining a high temperature inside, for which energy is spent, which is obtained from food. Like in the case of the air balloons familiar to us, the deeper the floater dives, the greater the lifting force that returns it to the upper, cooler, and safer regions of the atmosphere. The floater can feed on organic molecules that form in the atmosphere or produce them independently, using sunlight and air, like plants on Earth. It should be noted that the larger the floater is, the more viable it will be. Sagan and I imagined floaters several kilometers across - the size of a whole city, much larger than the largest whales that have ever existed.
Floaters can move in the atmosphere, releasing jets of air, like a jet plane or rocket. We imagined them gathered in huge lazy herds that stretch as far as the eye can see, with a characteristic protective coloration, which indicates that they also encounter problems. Because in the considered environment, there is at least one more ecological niche - hunting. Hunters (from English hunter - hunter) - creatures are fast and agile. They hunt floaters not only for their organic matter but also for the stored pure hydrogen. Hollow sinkers could evolve into the first floaters, and self-propelled floaters - into the first hunters. There cannot be too many hunters, since in this case, they would absorb all the floaters and die themselves.
Physics and chemistry allow the existence of such forms of life. Art gives them a certain charm. Nature, of course, is not obliged to follow our ideas. But if there are billions of inhabited worlds in the Galaxy, then perhaps among them, there will be several inhabited by sinkers, floaters, and hunters that we have invented, staying within the laws of physics and chemistry" (K. Sagan, 2005).

Additional materials:
Column: Aliens are nearby!