Origin of water on the Earth

There is 4,5 billion years, the Ground was formed by accretion of silicated dust covered with a fine film of Eau. It was not whereas a hostile and deserted planet, which had a considerable energy accumulated at the time of accretion, as well as many elements in a gas state. With the wire of time, the Earth became that which we call today the blue planet, because it has water in very great quantity in the liquid state.

Source of water on Earth

Today still, the scientists are not unanimous as for the origin of the presence of water in gas form in the atmosphere.

the assumption more the appraisal of the scientists is that of the carbonaceous Chondrite S which arrived on Earth at the end of accretion and which transported water.

Of others thinks that water comes from the Comet S, which, after the period of accretion, were crushed on Earth. Indeed, the comets are celestial bodies resulting from the Ceinture of Kuiper or Nuage of Oort; they have a diameter in general lower than twenty kilometers, and are made up to 80% of ice.

Some lean for the theory of degasification: i.e. they think that after the formation of our planet, there is 4,5 billion years, water was present in gas form in the various envelopes of the Earth.

There exists also a fourth assumption, little privileged scientists, contribution of water by the Micrométéorite S, whose diameter is about the micrometer, which is very frequent.

However, of recent discovered seem to suggest with the scientists that there would not be only one, but at least two phenomena at the origin of water. Indeed, by evaluating the report/ratio of deuterium on hydrogen present in Halley's Comet thanks to measurements of the Probe Giotto and by comparing this relationship with that measured on Earth, the astrophysicists noted a difference which pushed them to think that water did not come solely from meteoritic or cometary impacts, as some thought it, but that it also came from another source, surely the water of the degasification of the sphere. Indeed, they obtained a report/ratio of some 3.10^-4 against 1,5.10^-4 in the terrestrial oceans. But, by some means that it is, the water, once arrived on Earth, in gas or solid form according to the stated cases, is, on the assumption of the meteorites and comets, evaporated because of the conditions of temperature; it was then retained by the protective atmosphere. On the assumption of degasification, the energy provided by the Earth involved an important volcanicity which made it possible water to escape from the coat. Then, the temperature cooling, the steam present in the atmosphere condensed little by little and was at the origin of a thick vapor cloud around planet. Also, during million years, a flood of torrential rains fell down on the Earth, person in charge of the appearance of the oceans. The variable climate little, these oceans was maintained and when the life appeared, they absorbed most of the $\ mathrm {CO} _2$ . The temperature then decreased so that water in the form of ice finally could be maintained on Earth. Thus water was from now on present under its three states.

Water in the liquid state

One can distinguish five great factors (hypothetical classified by decreasing order of importance) which made it possible water to be maintained under its three states:

the disintegration of radioactive elements contained in the coat (since accretion) contributed to increase in an important way the terrestrial temperature. This phenomenon is at the origin of the degasification of certain elements contained within the ground towards its periphery.

the Earth is located at an ideal place in our Solar system since it is neither too near nor too far from the Sun. Its temperature, although a little too low, makes it possible water to be maintained in another form that the ice at the hot places of the sphere. But thanks to the greenhouse effect (due to the presence of $\ mathrm {CO} _2$ ) the conditions become ideal so that water is present in abundance in the liquid state.

the Earth, like all the bodies, attracts many elements: this is due to a phenomenon called the gravitational attraction. According to this phenomenon, all the material bodies attract each other reciprocally because direct of their mass and reverse of the square of their distance according to the formula: $P=G*\frac{m_1\times m_2}{d^2}$ . Thus by its relatively important mass the Earth should prevent that the gas elements that it releases do not leave in space. But actually at the molecular level, it is more complicated than that: the retention of the atmosphere results in a fight between the gravitation which tends to retain the molecules, and the speed of the molecules which tends to release them from the gravitational field; a detailed study of the involved factors reveals that the mean velocity of a particular type of molecule is proportional to the square root of the temperature, and inversely proportional to the square root of the mass of the molecule. To know if the gravitation of the Earth is able to retain a given gas, it is necessary to compare the speed of this one with the Escape velocity which defines necessary minimal speed so that a particle is released forever from a field of gravitation. The escape velocity on the surface of the Earth is of 11,2km/s (to be supplemented with editor equation and continuation of the book). By retaining these elements, the Earth constitutes an atmosphere. Thus released water is not lost for the Earth.

the presence of a satellite , the Moon, in rotation around the Earth, allowed the stability of the terrestrial axis of rotation. Consequently the terrestrial climate (related to the axis of rotation), or little was not modified during time. This climatic characteristic contributed to maintain a strong presence of liquid water. And with the development of the life there is 3,5 billion years, the oceans became thanks to the first bacteria of the “Puits of carbon”, i.e. thanks to the presence of these micro-organisms the ocean was able to absorb a great quantity of carbon dioxide, about the hundred million tons per annum. Thus the being reduced Greenhouse effect, the average temperature of the Earth dropped to be stabilized with approximately 15°C (the current average terrestrial temperature).

In the external core of the Earth, regarded as liquid since the study of the seismic waves, the temperature is higher than 5000 °C, which has due ionization to the elements contained in the core. The Earth being in rotation on itself, the external core is moving, and consequently, according to the model of the Dynamo, there is production of gigantic running: the Magnetic field. This one prevents in particular the Solar wind arriving on Earth and thus to disintegrate certain molecules such as water or from dispersing the atmosphere. Moreover one such field to the gravitational exhaust is opposed while acting on the ionized layers which constitute the higher layers of the atmosphere.

Telluric another planets

Foreword

Water, whatever its state, is not only present on Earth as we saw previously. Indeed, water appeared on another planets of the solar system of way similar to the Earth. However it was not maintained there.

In the telluric planets, water is present in the atmosphere or the ground. A priori , it should take comparable forms there since these planets are relatively similar the ones to the others. Thus, the density of Mars is only two to three times lower than that of the Earth; that of Venus in is close, so much so that the evening star is often called the “binocular of the Earth”.

In spite of that, any exchange from one planet to another so much the atmospheric conditions vary there. The atmospheres of Mars and Venus especially consist of carbon dioxide (approximately 95% in volume) and of nitrogen, with water and oxygen, carbon monoxide traces, whereas the Earth has a very different atmospheric composition. The terrestrial atmosphere as well as the temperature would not be the same ones without the conditions previously described. Thus contrary to the Earth, the axes of rotation of Venus and Mars undoubtedly have undergoes great fluctuations, which in particular modified their climate.

The Earth, Mars and Venus however had at the beginning of the atmospheres which differed very little, as well by their chemical composition as by the conditions of temperature and of pressure which reigned there. At all events the state of water on each of three telluric planets differs henceforth.

Current conditions of telluric planets

Venus

The remote observation of water on Venus is difficult because of the thick layer of clouds, localized to approximately about fifty kilometers of the surface of this planet and which masks its surface. If the Venusian atmosphere contains approximately carbon dioxide 96,5% and nitrogen 3,5%, one finds there also steam in negligible quantity. Its surface, directly observable in the visible field, was abundantly charted by the planetary probes and the Hubble telescope. The average temperature on the Venus ground is of 460°C what thus does not contribute to the liquid presence of water.

March

Over Mars, the atmospheric composition resembles that of Venus: about 95% of carbon dioxide 3% of nitrogen and approximately argon 2%. The steam accounts for only 0,001% of the atmosphere. However with the difference of Venus, water is present over Mars at the same time in solid and gas form, and this in spite of a temperature which does not exceed - 60°C on average on the surface. The conditions of pressure and temperature on the surface of Mars do not allow the liquid presence of water. Water is especially present in the form of ice in the polar icecaps, which condense and are sublimated with the north pole and the south pole according to a seasonal rate/rhythm. Sometimes one observes cloudy water small formations, in particular near the volcanos. If the water of the caps were distributed on all planet, it would form a total ocean about a score or of about thirty meters thickness; on Earth, such a total ocean would have a 2,7 kilometers depth. As for the pressure partial of water over Mars, it does not exceed some ten thousandth atmospheric pressure total and present of strong fluctuations related to the seasonal cycle of condensation and sublimation of the polar icecaps.

Mercury

Mercure is not massive enough and is too close to the Sun to retain an important atmosphere; the water molecules are quickly destroyed under the influence of the ultraviolet rays.

The origin of this diversity

The water contained in the atmosphere

Since they started from quasi-similar initial conditions, why these three large planets did have such divergent destinies? The answer is due to the history of water over Mars and Venus. Water seems to have been more abundant in the past on the sisters of the Earth. The value of the report/ratio of the abundance of heavy water ( $\ mathrm {HDO}$ ) and of water ( $\ mathrm {H} _2 \ mathrm {O}$ ) indicates, for example, that there was in the beginning much more steam in the atmospheres Martian and Venusian. In the oceans and the terrestrial atmosphere, the report/ratio $\ mathrm {HDO}$ / $\ mathrm {H} _2 \ mathrm {O}$ is worth approximately 1,5.10^-4, value whose planetologists think that it changed forever since the creation of the Earth. On the basis of the principle which this value characterized all the telluric planets at their beginning, they measured the report/ratio $\ mathrm {HDO}$ / $\ mathrm {H} _2 \ mathrm {O}$ in the atmospheres of Venus and Mars. Their results reveal a strong enrichment out of deuterium of the atmospheres Martian and Venusian. The steam which mixes with the Martian atmosphere would contain 5 times more deuterium than that of the Earth and that of Venus nearly 120 times more. The planetologists deduce from these observations that the atmospheric steam was present in quite higher quantity over Mars and especially on Venus, in the past. How did it disappear? Atmospheric enrichment out of deuterium is explained by the mechanism of the gravitational exhaust which privileges the exhaust in the space of the lightest molecules. This is why it supports the ejection of ordinary water compared to that of heavy water, which explains why the latter concentrated in the atmospheres of Mars and Venus.

The water contained in the ground

If water were definitely more abundant in the atmospheres of Mars and Venus, was it also on their ground? In the case of Mars, we have traces of presence, on the surface, great quantities of water (perhaps liquid) at the beginning of the history of planet. The first is the existence of ramified valleys which furrow the old grounds of the southern hemisphere and which go back to more 3 billion years; they give the impression that liquid water ran out in quantity on planet, which indicates that at that time existed a dense and hot atmosphere. The second index is the probable presence of an ocean which would have covered the large plains with North there are 2 to 3 billion years. Recent radar measurements of the mission Mars Global Surveyor, reinforced this assumption, put forth during the first measurements of the Sonde Viking. They revealed the presence of long lines several thousands of kilometers and whose altitude is constant. Would they be banks? If such an ocean existed, the water which it contained would have formed a total ocean of at least thirty meters thickness.

Origin of this disappearance

But why did the water of Venus and Mars, which seemed present in abundant quantity, disappear? Being given the pressures which reigned on the surface of three planets at the beginning of their history, water undoubtedly existed rather in gas form on Venus, liquid form on the Earth and solid form over Mars.

Venus

On Venus, the presence in great quantity of carbon dioxide and water in the form of vapor caused an greenhouse effect which quickly developed, so that the temperature on the Venus surface rose little by little until the 730 K currently recorded. In the absence of the greenhouse effect (taking into account the distance from Venus to the Sun), can its temperature of surface should rather be about 300 K. the pressure of carbon dioxide on the Venus surface having remained constant, how one explain the disappearance of water present at the origin and whose presence in the atmosphere is attested by enrichment out of deuterium of the residual vapor? According to the planetologists the steam would have been dissociated by the solar Rayonnement, then would have escaped in space. The planet Mars has two characteristics: on the one hand, it is further away from the Sun, consequently colder than telluric another planets; in addition, it is notably smaller and less dense. Thus, its mass represents only the tenth of the terrestrial mass. These characteristics mean that at its origin, Mars contained much less heavy and radioactive elements likely to contribute to its internal energy.

March

At the beginning of the Martian history, the pressure on the surface of planet was undoubtedly lower than those which reigned on the surface of the Earth and Venus; however, the primitive atmosphere of red planet was denser than that which reigns today. The probable presence of water in liquid form is an index. Various recent discoveries provided others of them. Thus the Magnétomètre of the Mars Global Surveyor probe recently discovered a fossil magnetic field in the grounds of the southern hemisphere of planet. This field would be the print left by an old magnetic field, which would have existed at the time of the first hundreds of million years of planet. Thus Mars would have had an energy interns higher than that of today. The internal energy of Mars would have increased the volcanic activity and facilitates the formation of an atmosphere by degasification. The planetologists, estimated the density of this atmosphere by studying the isotopic reports/ratios of the nitrogen and the rare gases which are good indicators of the atmospheric exhaust. The relationship between the Isotope S of the nitrogen ¹⁵N and ¹⁴N, in particular, is higher than that of the atmosphere of the Earth of a factor 1,7. Such a value indicates that the primitive atmosphere of Mars had an atmospheric pressure near to the tenth of that of the Earth. However the magnetic field of Mars seems to be extinct at the end of a billion years, undoubtedly because of the low mass of planet. The Martian atmosphere would have then more easily escaped in the absence of such a magnetic field and would have been disintegrated by the ultraviolet rays. The free water sank then under Martian surface and solidified in the crust by freezing, probably to a few hundred meters of depth, this free water corresponds to that which one can observe today.

Thus water disappeared little by little from Mars and Venus.

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