User:Spiggot/Origin of water on earth

4.5 billion years ago the earth was formed by the consolidation (accrétion) of silicate dust covered with a fine film of water. At that time it was a hostile desert planet with a lot of energy released during the consolidation (accrétion) and many elements were present in gaseous form. Over time earth has become what we now call the blue planet, because it has a great deal of water in its liquid form.

The source of water on earth

Even today scientific theories differ on the origin of the water vapour present in the earth's atmosphere.

• The most commonly held theory is that carbonaceous chondrites arriving on earth at the end of the consolidation brought water with them.
• Others think that water was brought by comets striking the earth after the consolidation. Comets are Kuiper belt or Oort cloud objects normally less than 20km in diameter, formed of ice (~80%) and rock.
• Still others tend to the out-gassing theory; that after the creation of the earth water was present in gaseous form in the various envelopes (layers?) surrounding the earth.
• A fourth theory, with narrow if any scientific support, is mentioned for competeness: that micrometeorites (a very common form of meteorite having a diameter of a few microns) were responsible for bringing water to the earth.

Nevertheless, recent research seems to point not to one single phenomenon being responsible for the origin of water on earth, but to at least two. Study of the ratio of deuterium to hydrogen in Halley's comet using data provided by the Giotto probe (1:3×10^-4) has shown that there is a difference compared to the ratio measured in the earth's oceans (1:1,5×10^-4). These differences have led astrophysicists to believe that comets and meteorites are not the only source of water on earth, as some suggest.

By whatever method water arrived on earth, in gaseous or in solid form depending on the theory, it evaporated, either due to the prevailing temperature in the case of comet or meteorite delivery, or, in the case of degassing, due to volcanic activity caused by the energy released by the earth in consolidation, allowing water to escape from the mantle. Cooling of the earth allowed the water vapour to condense little by little causing a thick cloud layer to form around the planet. During millions of years torrential rainstorms beat upon the earth causing the creation of the oceans. Climatic conditions having varied little, these oceans persisted. The appearance of life led to the absorption of a great deal of carbon dioxide ($\mathrm {CO} _{2}$), leading to a farther drop in temperature; enough for ice to form on earth, leading to its existence in its three states.

Water in its liquid state

Five main factors, listed below in descending order of importance, are though to have led to the conservation of water in its three states on the earth:

• The breakdown of radioactive elements in the earth's mantle (since consolidation) has contributed to a significant increase in terrestrial temperature. This phenomenon led to the degassing of certain elements contained in the earth into the atmosphere.

• Earth is positioned perfectly in the solar system, neither too far nor too close to the sun. Its temperature, albeit slightly too low, allows water to exist in forms other than ice in the warmer parts. The greenhouse effect, due to the carbon dioxide in the atmosphere, leads to ideal conditions for water to exist in its liquid state in abundance on the surface.

• The earth, as is the case with all massive bodies, has a gravitational well. Gravity acts in relation the mass of the two objects, and in the inverse of the distance squared between them:

$P={\frac {m_{1}\times m_{2}}{d^{2}}}$

where:

• $P$ is the gravitational force
• $m_{1}$ and $m_{2}$ are the masses involved, and
• $d$ is the distance between them

Due to the relatively great mass of the earth, it will tend to prevent molecules of gas escaping from the atmosphere into space. Bodies must reach a certain velocity to escape the gravitational pull of a massive body; earth's escape velocity is around 11.2m/s. In the case of gaseous molecules it is a little more complicated: the average velocity of molecules in a gas is dependent on the square root of the temperature of the gas and inversely proportional to the square root of the mass of the molecules. To find if the earth is capable of retaining a certain gas the average velocity of the gas molecules should be compared to the escape velocity for the earth. Due to its ambient temperature and the mass of the water molecule, the earth has managed to retain water in its gaseous form in its atmosphere.

• The existence of a satellite orbiting around the earth, the moon, has led to a stabilisation of the earth's rotational axis. As a result the earth's climatic conditions (dependent on the rotational axis) have also remained relatively stable over time. This stabilising characteristic has meant that liquid water has remained on earth's surface in great quantity. The appearance of life 3.5 billion years ago led to to the oceans absorbing large amounts of carbon dioxide, in the order of several hundred of millions of tonnes per year. This in turn led to a reduction in the greenhouse effect, and earth's temperature reduced to the current average of about 15°C.

• The outer core, believed to be liquid following seismic studies, is at a temperature of 5000°C, causing the ionisation of elements in the core. As the earth rotates on its axis the core too rotates, leading to a dynamo effect and a huge magnetic field. One effect of this magnetic field is that it deflects the solar wind from the earth, and therefore prevents certain molecules (for example water vapor) from being vaporized from the atmosphere into space. Additionally the magnetic field opposes the escape of molecules by acting on the ionized molecules that form the upper atmosphere.

The other earth-like planets

Introduction

Whatever its state, water is not restricted to the earth; it appeared on other planets in the solar system in much the same ways, however it did not remain.

On earth-like planets water is found in the atmosphere and in the ground. It could be assumed that it would be found in much the same states on all these planets as they are relatively similar; Mars' density is only two or three times less than the earth's; and Venus' is close enough for it to be often nicknamed "Earth's twin".

Despite this, everything is different in terms of atmospheric conditions. Mars and Venus' atmospheres are mainly carbon dioxide (about 95% in volume) and nitrogen, with traces carbon monoxide, oxygen and water. Earth's atmosphere is very different. Earth's atmosphere and temperature would not be what they are today were it not for the conditions described above. As opposed to the Earth, Venus and Mars' orbital axes have no doubt varied greatly, which notably has modified their climates.

In the beginning the Earth, Mars and Venus had very similar atmospheres, both in terms of the constituent elements and in terms of their temperature and pressure. However, the state of water on each of these planets differed from then on.

Current state of the earth-like planets

Venus

The distant observation of water on Venus is difficult because of a thick layer of clouds roughly fifty kilometers above the surface. The atmosphere of Venus is made up of about 96.5% carbon dioxide and 3.5% nitrogen, water vaper is also present in trace amounts. Its surface has been thoroughly mapped by planetary probes and by the Hubble Telescope. The average surface temperature of Venus is 460°C which contributes to the lack of water in liquid form.

Mars

The atmospheric composition of Mars resembles that of Venus: nearly 95% carbon dioxide, 3% nitrogen, and around 2% argon. Water vapor represents only 0.001% of the atmosphere. However, unlike Venus, water is present in its solid and gaseous forms despite a surface temperature that usually does not exceed -60°C. The pressure and temperature on the surface of Mars do not permit liquid water. Water is mostly found in solid form in the polar caps of the planet, that condense and sublimate seasonally. If the water in the poles were distributed throughout the planet, it would form a global ocean that would completely cover the surface and be about 20 to 40 meters deep; on earth, such a global ocean would have a depth of 2.7 kilometers. With regard to the partial pressure of water on Mars, it does not excede roughly ten thousandths of the total atmospheric pressure and is subject to strong fluctuations linked to the seasonal cycle of condensation and sublimation of the polar caps.

Mercury

Mercury is both too small and too close to the Sun to retain an atmosphere of any significance; its water molecules were quickly destroyed by the prevalent ultraviolet rays.

The origin of this diversity

Water contained in the atmosphere

Since the planets had such similar initial conditions, why did these three large planets have such divergent destinies? The history of water on Mars and Venus holds the answer. Water seems to have been more abundant in the past on the sisters of earth. The value of the ratio between heavy water ($\mathrm {HDO}$) and water ($\mathrm {H} _{2}\mathrm {O}$) indicates that there was originally much more water vapor in the martian and venusian atmospheres. In earth's oceans and atmosphere, the ratio $\mathrm {HDO}$/$\mathrm {H} _{2}\mathrm {O}$ about 1,5.10^-4, because of this, planetologists believe that it has not changed since the creation of earth. Working on the assumption that this value characterizes every terrestrial planet at their creation, they have measured the ratio $\mathrm {HDO}$/$\mathrm {H} _{2}\mathrm {O}$ in the atmospheres of Venus and Mars. Their results reveal a strong enrichment of deuterium of the martian and venusian atmospheres. The water vapor in the martian atmosphere would contain 5 times more deuterium than that of earth and the atmosphere of venus would contain close to 120 times more. The planetologists deduce from these findings that the atmospheric water vapor was present in much greater quantities on mars and even more so on venus, in the past. How did the water disappear? The atmospheric enrichment of deuterium is explained by the mechanism of gravitationel escape that favors the escape of the lightest molecules. It favors the ejection of ordinary water in ratio to that of heavy water, this explains why the later is concentrated in the atmospheres of Mars and Venus

Water on the ground

If water was decisively more abundant in the atmospheres of Mars and Venus, was there a similar abundance in their soils? In the case of Mars, we have some clues of the presence, on the surface, of large quantities of water (maybe liquid) in the beginning of its history. The first is the existence of branched valleys that furrowed the ancient terrains of the southern hemisphere and date back to more than 3 billion years; these valleys give the impression that liquid water flowed in significant quantities on the planet, this indicates that at this time there existed a dense, warm atmosphere. The second indication of the presence of water is the probable presence of an ocean that would have covered great plains in the north of the planet 2 to 3 billion years ago. The recent radar mesurements of the mission Mars Global Surveyor, have reinforced this hypothesis, issued at the same time of the mesurements of the Viking probe. These mesurements have revealed the presence of long lines of several thousands of kilometers in a constant atmosphere. Were these made by ocean shores? If such an ocean existed, the water that it contained would have formed a global ocean at least thirty meters deep.

Origin of the disappearance

But why did the water on Venus and Mars, that appeared to be present in such vast amounts, disappear? Given the pressure present on the surface of the three planets at the beginning of their lives, without a doubt, water existed in its gaseous form on Venus, in its liquid form on Earth, and in its solid form on Mars.

Venus

The presence of a large quantity of carbon dioxide and water vapor created a rapidly intensifying greenhouse effect, of the sort that the surface temperature rose steadily until its currently recorded 730 K. In the absence of this greenhouse effect (given the distance of Venus from the Sun), the surface temperature would have been roughly 300 K. The surface carbon dioxide pressure being constant, how can one explain the disappearance of the water present at the origin whose presence in the atmosphere was confirmed by the enrichment of the residual water vapor in deuterium? According to palaeontologists, the water vapor would have been broken up by the Sun's radiation, then escaped into space. The planet Mars has two characteristics: firstly, it is farther away from the Sun; secondly, it is notably smaller and less dense. Mars' mass is equal to about a tenth of that of Earth. These characteristics signify that a its origin, Mars contained much less heavy or radioactive elements that would contribute to its internal energy.

Mars

At the beginning of its life, the pressure on the surface of Mars was, without a doubt, lower than that of Venus or Earth; nevertheless, the primitive atmosphere of the red planet was denser than prevails today. The probably presence of liquid water is an indicator; diverse recent discoveries have provided others. The magnetometer of the Mars Global Surveyor probe recently discovered a faint magnetic field in the southern hemisphere. This field could be a trace left by an former strong magnetic field that at the time of the first few hundred millions years of the planet's existence. This would indicate that Mars had a greater internal energy than is has today. This would have been accompanied by increased volcanic activity, and the formation of an atmosphere by gaseous emissions of the volcanoes. Planetologists have estimated the density of that atmosphere by studying the isotopic ratios between nitrogen and rare gases that are good indicators of atmospheric escape. The ratio between the densities of nitrogen isotopes ¹⁵N and ¹⁴N, in particular , is higher than that in the Earth's atmosphere by 70 percent. Such a value indicates that Mars' primitive atmosphere had an atmospheric pressure close to a tenth of Earth's. However, the magnetic field of Mars seams to have become extinct after a few billion of years, probably because of the small mass of the planet. The Martian atmosphere would then have more easily escaped in the absence of a magnetic field and would have been disintegrated by ultraviolet rays. The remaining water has, then, has sunk through the Martian surface and has frozen in the crust as frost, probably at a depth of several hundred meters; this remaining water is observable today.

That is how water has, bit by bit, disappeared from Mars and Venus.

Notes

at what height above the earth's surface is this escape velocity to be measured.

See also

• water — about water in everyday life
• water molecule — technical discussion of the chemistry of water

Category:Water Category:Cosmology Category:Earth