Conditions for the appearance of life
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Updated June 26, 2013
Our galaxy has about 200 billion stars and the statistics tell us that it contains 2,000 billion planets. It seems sufficient to say that life has certainly found many places among the habitable zones of these stars billion to develop. However, we must meet so favorable conditions that it severely restricts the possibilities. The first essential condition is the presence of water in the liquid state. Water is abundant in the material Universe, which consists of 74% hydrogen, 24% helium, 1% oxygen and other elements combined represent only 1% of the matter. Scientists think that liquid water is vital because of its role in biochemical reactions. It is even considered as an essential element to a healthy ecosystem because it helps enormously transport of the materials required for biochemical activity. A planet must have liquid water and keep it long enough, billions of years, to have a chance to preserve life. Nobody knows how life appears, the transition from inanimate to the animate is still a mystery, but life as observed on Earth is based on carbon chemistry in solution in liquid water. If life exists elsewhere it must be based on carbon chemistry, life has not taken the path of silicon much more present on Earth than carbon. Scholars have tried to imagine life in a different chemistry than the carbon and all say it is much more complicated because the carbon (C) is the essential component of organic compounds, it is the basis of many compounds and combines very well with other atoms, in particular with hydrogen, oxygen, nitrogen, phosphorus and sulfur. Since in the universe the water molecule (H2O) is present everywhere, all the planets must have. The challenge will be to keep the water on the surface in a liquid state for billions of years and for that the habitable planet must meet very many favorable stability conditions. First, it is necessary that the planet is orbiting in a habitable zone.
How big is this area of life?
Numerical models show that if the Earth from the Sun is 12% away, it will receive only 79% of the heat of the Sun and the Earth's climate gets carried, it quickly covered with ice, a few tens of years. But the Earth has always been habitable despite the evolution of heat from the Sun. At first the Sun was 25% less bright today and yet there has always been liquid water on the surface of the Earth with an average climate warmer than today. This is the paradox of young low Sun. This old conundrum of 1972, was raised by astronomers Carl Sagan and George Mullen when evidence of liquid water and life in the form of bacteria were found in geological strata, from the beginning of formation of the Earth. Sagan and Mullen suggested at the time that greenhouse three times greater than today, have existed with ammonia and methane. With this important greenhouse effect, the Earth has retained the low heat emitted by the young Sun. Then when the sun shone more and more, the carbonate silicate cycle took the relay to stabilize the Earth's climate. If the planet is capable of stabilizing the climate, the upper limit of the habitable zone fell to 1.6 AU. The inner limit of the habitable zone was calculated by computer models to 5% (0.95 AU). At this point, closer to the sun, the climate system gets carried away. The temperature increases, the greenhouse effect increases and oceans evaporate. The connections of the water molecules are broken into the atmosphere, and the hydrogen, very lightweight disappears into space. Gradually the planet loses all its water. This is between 0.95 and 1.65 AU the Earth can keep liquid water on its surface. But it will have to stay very long in this area to allow the evolution of life, It is thanks to carbonate silicate cycle and plate tectonics that the Earth was able to keep its atmosphere.
image: The habitable zone is not a fixed area, it changes according to the age, the temperature of the star and the presence of CO2 in the atmosphere of the Earth. During their evolution stars becoming more brilliant and hotter and hotter, the habitable zone is thus logically away from the star. A planet will therefore remain very long, billions of years in this area, to synthesize all the molecules necessary for life form. We do not know why it takes so long. Bacterial life appeared there are 3.8 billion years multicellular life 1.4 Ma ago, the first animals there are 600 million years. In summary to maintain liquid water must stay very long in a habitable zone but also keep an atmosphere. In the picture, the green area is the living area, tapering terribly over time, too close to the star it is too hot, too far away it is too cold.
Carbonate silicate cycle and plate tectonics
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The carbonate silicate cycle partly stabilize the Earth's climate long time scales. The cycle begins when atmospheric CO2 dissolves in rainwater, forming carbonic acid, H2CO3. The products of this alteration will end up in the oceans. There, the organisms that live on the surface of the ocean use to make shells of calcium carbonate (CaCO3). When organisms die, they fall to the ocean floor. Then plate tectonics recycle CO2 in subduction zones and releases gaseous by discharging into the atmosphere by volcanic vents. If no volcanoes spewed CO2 there would be no more CO2 after 400 000 years. Then the cycle begins again when the CO2 emitted by volcanoes is naturally dissolved by flushing water (water cycle) which brings the CO2 in the deep ocean. It is this process that made it possible to operate the "climate system". In summary to keep water in the liquid state, we need a living planet with intense geological activity. However the phenomenon of plate tectonics is absent on other planets in the solar system.
It seems as if the planet is too small as Mars, it can not have plate tectonics, but if the planet is bigger (super earth), convection is less efficient, there will be one big plate. Yet Venus, which is the same size as the Earth, does not benefit from plate tectonics!
nota: The circumstellar habitable zone or ecosphere is a theoretical circular tube surrounding a star where the temperature on the surface of planets in orbit would allow the appearance of liquid water. A habitable zone may harbor life, but favorable conditions for it it develops are so numerous opportunities that are extremely reduced.
Image: Diagram illustrating the carbonate-silicate cycle.
credit: J. F. Kasting, Science Spectra, 1995, Issue 2, p. 32-36. Adapted from J. F Kasting, 1993.
Astronomical requirements for a planet keeps its liquid water
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Another important requirement to maintain liquid water, concern the astronomical properties of the star around which the planet orbits. Again there are limited by the fact that 60% of the stars are double stars, which is not very favorable to the emergence of life as the orbits of the planets in these systems are so irregular and chaotic. The eccentricity of the habitable planet must remain low in the range of 0 (circular orbit). The Earth (0.01) is ideal. The observations of exoplanets show an average eccentricity of 0.29, which is huge. When the eccentricity is large, the orbit of the planet is unstable because it is exposed to the vagaries of gravitational forces of other planets (Jacques Lascar work) and thus the temperature stability is also random. Among the observed solar systems, very few offer their planets, nearly circular orbits as in our solar system.
Another important requirement to maintain liquid water, concern the mass of the star. The stars have a mass of between 1:100 and 100 solar masses. Larger stars than the Sun, between 1.2 and 1.5 solar masses emit too much ultraviolet light, which is not conducive to the emergence of life, in addition, they have a shelf life much too short for life it develops.
The smaller star than the Sun emit a lot of X-rays and harmful particles for life. 75% of the stars have a mass close to 0.5 solar masses. These stars shine very little and therefore the habitable zone is very close. Because of their proximity to habitable planets synchronize their rotation with their star by tidal effects, so they have a cold side and one burning. Although the border there may be a "temperate region," the situation is not very favorable to keep water in the liquid state. In addition to the distance they have no magnetic field as the fact of being synchronous prevents the differential rotation of the core of the planet. Which exposes them even more to solar radiation. In summary, it is around 0.9 stars 1.2 solar masses and that we can find life. There is nothing surprising in the simulations of computer models because they describe the ideal conditions that correspond to our system, however it allows researchers to understand better the complex conditions of life appeared.
nota: The eccentricity defined the form of an elliptical orbit, it varies between 0 and 1. 0 for circular orbits. High eccentricity decreases the smallest axis (perihelion) and increases the larger axis (aphelion), but does not change the major axis.
Image: The Sun is of spectral type G. A star is characterized by its color (its spectral type), and its luminosity. For a given spectral type, more the star is greater, more its heat is increased and more its brightness is high. The O and B stars are blue, the A stars are white, F and G stars are yellow, the K stars are orange, M stars are red.
States of water or phase transitions
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Pure water exists under a single phase, solid, liquid or gas, for a pressure and temperature. The exception is the triple point (see diagram opposite), where the three phases coexist at a particular temperature and pressure. A torque pressure, temperature, corresponding to a phase transition, that is, a change of state between a solid phase and a liquid phase (fusion) between a liquid phase and a solid phase (solidification), between a solid phase and a gaseous phase (sublimation) between a gas phase and a solid phase (condensation) between a liquid phase and a gas phase (vaporization), between a gas phase and a liquid phase (liquefaction). Above the critical transition point between the liquid and gas, the water reaches the fluid phase, in both gas and liquid at a pressure of 218 atmospheres and at a temperature of 374 ° Celsius.
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Image: The water remains liquid when the atmospheric pressure is above the triple point that is greater than 6.1 mb. The temperature must be above 0 ° C, and finally it must be below the boiling point, which depends on the pressure points. Just above the triple point (0.006 atm) just a few degrees so that the water remains liquid. Earth is between 0 ° C and 100 ° C. If there is sufficient pressure (218 atm), water remains liquid up to 374 ° C. State transition diagram or phase transition of pure water. The triple point, the 3 phases coexist in a temperature and pressure. The critical point (218 atm, 314 ° C) between the gas and liquid transition, the water reaches the fluid phase, the gas and liquid time.