There are three planets in the solar system in the zone of life , where the planet is at a sufficient distance from its central star for th...
There are three planets in the solar system in the zone of life, where the planet is at a sufficient distance from its central star for the star's energy to cause the planet's surface to be at a temperature that is appropriate for the presence of liquid water to sustain life.
The Earth has indeed been in a state fit for life for billions of years, continuously. However, the surfaces of Venus and Mars have long had conditions that are not favorable to the persistence of liquid water, hence for life.
Why? What is the reason for this significant difference between planets in a similar zone and formed in a similar way?
Orbiting in the habitable zone is only one of the conditions for the presence of liquid water on the surface of a planet. For liquid water to be present, other conditions must be met.
For example, it needs a solid layer of crust. However, planets that form in the zone of life must be rocky planets, so a solid surface is a natural feature.
It is an obvious requirement also, that water as a substance must be present on the planet, too. The presence of water on these planets during formation is also very likely a property, since hydrogen and oxygen, from which water is formed, are common components of the universe, so when a planetary system formed, a significant amount of water must have accumulated and been present on appropriate celestial bodies. We see this on Earth (and on some of the moons of the giant planets, too).
Venus and Mars, however, are currently different. On Mars, there are traces of what once caused liquid water, and although we have no visible evidence for Venus, it would be harder to explain how the planet could have been composed without significant amounts of water on its surface at that time.
The presence of liquid water on the planet also requires a suitable atmosphere to protect the surface and to play a temperature-equalizing role in maintaining the liquid water.
The atmosphere is usually made up of a gaseous system of molecules of simple elements. These substances are also common components of the universe. A planet with sufficient minimum gravity to form in the zone of life will certainly have an atmosphere.
The Earth has an atmosphere suitable for supporting life. Venus and Mars have atmospheres, but they are not suitable for the presence of liquid water on the surface. However, it is very likely that all three planets had suitable atmospheres for the presence of liquid water when they formed.
Why are these planets, which were formed under such similar conditions, now so different on the surface?
We don't know for sure. We only have hypotheses about the difference. In general, and most of all, we suspect geological causes. However, among the possible hypotheses for the divergence, one has not yet emerged as the main cause: the possibility that the presence of life, life itself may have made the surface of the planet unfit for life. Perhaps the present state of Venus and Mars is evidence that life may once have existed on these planets.
What supports this idea?
During their formation, these planets must have formed with a similar material composition, so the materials on their surfaces must have been very similar also. It is safe to say that the material conditions on the surface of these three planets, and most likely their surface temperatures, must have been similar after the formation of the planets, but before the presence of life, at least to the extent that liquid water should have been existent on the surface.
We also know that life appeared very soon after the Earth formed, on the timescale of the Earth's existence. We don't know for sure how life comes into being, because we have not yet artificially created biological life, but we do know the conditions necessary for life on Earth and for life to emerge on Earth. These conditions could almost certainly have existed at the beginning on Venus and Mars.
If the emergence of life is not just a low-probability chance, but a probable event, perhaps a necessity if the right conditions are given, then primitive forms of life have emerged not only on Earth, but also on Venus and Mars. This reasoning is supported by the fact of the rapid emergence of life on Earth, and also by the hypothesis about the concept of life found in the thoughts.
Living matter, at its formation and even in its early stages, certainly and necessarily uses the resources of the inanimate environment for growth and reproduction. In the course of life's evolution, a certain time must pass before life is able to find a way to use living matter, or the byproducts of the processes of life as a resource.
We have evidence of such an initial state of life in the case of Earth. We have evidence that the Earth has passed through such a state. There was a time during the initial time of life on Earth when life primarily used the resources of the non-living environment, and in the process of using those resources, it transformed the surface on a global scale. Life on earth in its preliminary, non-equilibrium state, during the course of depleting inanimate resources and accumulating the products of metabolism, rendered the planet almost unfit for the existence of developing life.
Life on Earth, in the beginning, while using the inanimate environment and not the resources created by life, enriched its environment with oxygen, the metabolic product of life at that time, while certainly exhausting the environmental resources it needed. Primitive life at this time had not yet found a way to use the products of metabolism, so oxygen could eventually build up in the atmosphere, and the change in the composition of the atmosphere caused global change of climate, a significant drop in surface temperature, and resulted in the formation of the snowball Earth.
The transformation has caused the greatest extinction ever to affect the Earth. A state of runaway life has emerged. The Earth became almost unfit for life, and was transformed into a state of near lifelessness.
Life on Earth, perhaps in part through geological processes, but certainly through evolution, has found the way out of this situation, finding the solution to use oxygen, and ultimately creating a biosphere capable of sustaining environmental equilibrium.
Life continues to shape the Earth's surface ever today, while constantly transforming it.
Evolution is not, however, a required component or a strict consequence of the living state. Evolution is the product, the result of a suitably complex living state.
Evolution, and thus a biosphere that is capable of forming a complex equilibrium, requires the presence of hereditary material, but the living state does not necessarily require the function of a hereditary component, i.e. the ability to evolve.
Life on Earth essentially uses hereditary substances, hence, life on Earth is based on evolution. However, the formation of living matter does not necessarily require the recording of information, and therefore evolution, which is based on the recording of information, is not an obvious property.
If the initial conditions on Venus and Mars were suitable for life, life could have emerged on them. While life is in the initial stages of its formation, in the time when it is still using the resources of the planet available by the formation, and does not employ systems that allow information to be recorded, which would create the evolutionary adaptations to the changing environment that would make life capable of using the resources created by life, or perhaps, lacking the capability of evolution to find a way to use the changed environment as a new resource, or has insufficient time to adapt successfully, so that the process of life has failed to develop a biosphere capable of a sustained state of equilibrium, then the life that develops will inevitably shape the surface of the planet itself into a state unsuitable for life by depleting resources and consequently transforming surface conditions, creating a state of runaway life.
The surfaces of Venus, Earth and Mars are now very different from each other, while they may have formed under very similar conditions. Apart from geological differences and causes, it is perhaps, even most likely that the emergence of life on the surface and the way it operates has shaped the surfaces of these planets to be different and, in the case of Venus and Mars, unsuitable for life.
If indeed life eventually led to a dead end and transformed the surfaces of Venus and Mars into a form unfit for life (as it almost happened to Earth in the past), by now there may be little obvious trace of the primitive form of life remaining on the surfaces of these planets. The traces of former life may have been preserved not in the fossil imprint on Venus and Mars, but in the present state of the natural environment of the planetary surface, which has been rendered unfit for life.
On Earth, evolution has created a global biosphere that is capable of using almost anything on the surface, both existing and still non-existent, as a resource.
But evolution has its limits, especially, it has a specific speed of change. On Earth, through human activity, we are testing the flexibility of evolution, the resilience of the biosphere by disrupting the balanced environment. The new wave of mass extinctions has begun. The technological evolutionary development of intelligent species, at a rate that exceeds that of biological evolution, may offer hope of finding a way to restore the balance before the complex biosphere collapses, or, if we have enough time, to leave the environment that still allows life to persist.
But if we are not careful, or if we are not lucky enough, evolution will not be able to perform miracles, and life on Earth - along with humanity - may end up like Venus and Mars, where (human) life makes the planet unfit for life (at least for human life). Humankind itself can create a state of runaway life. We are close to the time to see, which destiny waits for the life-bearing Earth.
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