The population of the Earth is constantly increasing: according to various forecasts, by 2050 it could reach from 8 to 13 billion people. It is unknown how long our planet will be able to support such a horde. Science fiction writers have seen the colonization of other planets in the solar system as a solution to the problem for a very long time - practically from the beginning of the 20th century. Let's try to figure out how realistic such a perspective is.
A native land - forever beloved, where can you find another one like this?
Before talking about the prospects for the development of other worlds, it is worth understanding what made possible the emergence of life on Earth.
First, the Earth (which is natural) is a terrestrial planet - that is, a rocky celestial body, consisting mainly of metals and silicon.
Secondly, the Earth is located in the so-called "habitable zone" - in other words, it is not too close to the Sun and not too far from it. Due to this, the Sun has the ability to warm up our planet, but not to a crisp crust.
Third, the Earth is a geologically active world. This is important for several reasons at once. The presence of a liquid outer core, consisting of molten metals, provides the Earth with a magnetic field, which, in turn, protects the planet's surface from harmful solar radiation and from atmospheric erosion by the so-called solar wind (that is, a stream of ionized particles emitted by the Sun). The geological activity of the earth's crust also made it possible to block a large part of the carbon in the rocks and thereby avoid too strong a greenhouse effect.
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Fourthly (and this partly follows from the “third”), the Earth has a breathable atmosphere and a large amount of water, the presence of which is a prerequisite for maintaining protein life.
Alien worlds
Now let's take a look at other planets in the solar system and compare them with the Earth.
From the point of view of habitability, you can immediately discard the so-called outer planets - that is, Jupiter, Saturn, Uranus and Neptune. They are too far from the Sun, are predominantly gas (which is why they are called "gas giants") and are too massive. The satellites of the giant planets are also not suitable for life, although some of them (for example, on Enceladus) even contain water in the form of a liquid.
With the inner planets (except for the Earth), everything is also complicated. Mercury is definitely not suitable for life. It is too close to the Sun, its small mass did not allow it to hold the atmosphere, and all geological activity has long ceased as a result of cooling. In other words, Mercury is a dead piece of rock with no perspective. The same can be said for the Moon. But on Mars and Venus it is worth dwelling in more detail.
Red Planet
In many science fiction novels, Mars has figured either as an object of colonization or as a source of trouble in the form of aggressive aliens. The red planet is indeed in many ways similar to Earth, and about 3 billion years ago this similarity was even more striking: the planet had a dense atmosphere and a large amount of liquid water, rivers flowed along the continents, and the depressions were seas. What has happened since then?
First, due to its small size and mass (about 11% of the Earth's mass), Mars completely cooled down, which led to the cessation of geological activity and the loss of the magnetosphere. Due to the lack of geological activity, the planet's atmosphere has ceased to be replenished; due to the small planetary mass and the influence of the solar wind, the existing atmosphere gradually evaporated. This led to the fact that the water on the planet was partially sublimated into a gaseous form, and partially froze due to the cooling accompanying the rarefaction of the atmosphere. Water molecules that entered the atmosphere of Mars, in turn, were destroyed by ionized particles, which led to the loss of a large part of the planet's hydrogen reserves.
Thus, terraforming Mars seems to be a very time-consuming, one might even say - almost impossible task, because for this you have to recreate the planet's atmosphere and either protect it from erosion by the solar wind, or ensure its continuous replenishment. The absence of a magnetosphere will also cause the surface of Mars to be bombarded with deadly solar radiation. In addition, Mars is far enough from the Sun, so that even with a dense atmosphere and a concomitant greenhouse effect, the temperature on the planet's surface may not be high enough for comfortable life. On the other hand, a considerable part of these problems can be solved by placing huge mirrors at Lagrange points around the planet - they can protect Mars from the solar wind,in addition, with their help it will be possible to organize "external heating" of the surface.
In favor of Mars as the future abode of mankind is the fact that the duration of the day on the red planet practically coincides with that of the earth, in addition, there is an alternation of seasons, since the angle of inclination of the planet's axis is close to the Earth. In general, life on Mars is quite possible - but only under hermetically sealed domes. By the way, NASA is already going to carry out such an experiment and grow a plant on Mars in a miniature greenhouse.
morning Star
Another promising planet is Venus, which is often called "Earth's twin". Like Earth, Venus is located in the habitable zone, in addition, it is almost identical to our planet in size and mass.
Unlike Mars, Venus has a totally luxurious atmosphere. Unfortunately, this atmosphere makes the planet even less hospitable than its absence. It is composed primarily of carbon dioxide. As a result, due to the greenhouse effect, the temperature on the surface of Venus is 467 degrees Celsius, and the pressure due to the high density of the atmosphere is about 93 bar (that is, 93 times more than the atmospheric pressure on Earth at sea level). The atmosphere constantly contains dense clouds consisting of gaseous sulfuric acid. Since Venus, like Mars, does not have a magnetosphere, light gases, including water vapor, are constantly blown out by the solar wind. Finally, the Venusian day is 116 days 18 hours long. All in all, an inhospitable place.
Terraforming Venus also looks like a time consuming task - even more time consuming than terraforming Mars. Unlike Mars, Venus does not need to be heated, but cooled - and this is always an energetically more expensive process. The current atmosphere will have to get rid of for the most part, which means - somewhere to put a monstrous amount of carbon dioxide. Again, you have to somehow solve the problem of protection from the solar wind. Finally, Venus will have to be unwound in order to bring the length of the Venusian day to some reasonable value. As a result, the energy budget for this event will be inflated to absolutely unimaginable proportions. According to various estimates, the complete terraforming of Venus may require up to 1040J, which is six orders of magnitude more than the annual amount of energy produced by the Sun.
However, there is some good news. On Venus, it is quite possible to build "flying cities": a sealed bubble filled with terrestrial air in Venusian conditions will naturally float at an altitude of 55-65 km above the planet's surface. And since our city flies anyway, it is quite possible to make it fly around the planet with a frequency corresponding to the earth's day.
Conclusion
Unfortunately, the solar system - with the exception of the Earth - is a very inhospitable place, so humans can live on Mars and Venus only in closed colonies, which clearly cannot be a good home for millions (or even billions) of Homo sapiens. In this regard, humanity's only hope for a full-fledged space colonization is terrestrial exoplanets - like the recently discovered Kepler-186f - in combination with the development of interstellar travel technologies. At least as of today it looks more realistic.