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The Detection of Water

In ancient times, water was a precious liquid, but nobody knew of what it was really made. It took chemists like Lavoisier to discover the composition of water in the 18th century.

The water molecule

The water molecule consists of two hydrogen atoms linked to one oxygen atom. Its chemical formula is H20, and is the most abundant substance on the surface of the Earth. It is found in all three natural states, solid, liquid and gaseous. We are familiar with it as ice or snow in the solid state and it is also present in innumerable minerals as natural hydrates.

Natural water is always in the form a mixtures; as the oxide of hydrogen it contains mineral and organic impurities. Water vapour is an important constituent of the atmosphere.

Moreover, water is still the reference element for the definition of the Celsius thermometric scale: the temperatures of melting ice and condensing steam at the normal atmospheric pressure of 1013.25 hPa are defined as 0°C and 100°C).

As a liquid, water has unusual thermodynamic properties that determine our physical and biological world.


The Kelvin (K) thermodynamic unit of temperature is defined by assigning the value of 273.16 K to the thermodynamic temperature of the triple point of water. Zero degree (°C) Celsius is based on the same scale, but the Celsius zero corresponds to 273.16 K on the thermodynamic scale. The triple point of water is the temperature at which ice, liquid water and steam are simultaneously in equilibrium; the presence of all other substances must be excluded. This triple point for pure water is taken as a fixed point on the temperature scale.

Triple point of water

[Phase Diagrams of H2O and CO2]

The choice of the value, 273.16 K was not arbitrary: the 0° and 1000° points on the Celsius scale correspond to the melting and boiling points of water at normal atmospheric pressure, respectively; these were the earlier, less accurate standards that were used to define the centigrade scale. The change of scale was made without changing the value of the basic degree unit. However, it is extremely difficult to obtain and preserve completely pure water: the sealed container is itself a source of impurities. In addition, the isotopic composition of water, which is variable and depends on the origin, is not that accurately defined.

The accuracy in thermometry is above all limited by the impossibility of obtaining a perfect equilibrium between the thermometer, no matter what kind is used, and the substance whose temperature is to be measured (water at its triple point for example). As a result, the accuracy is limited to about one thousandth of a Kelvin at ambient temperature, and is even less at extreme temperatures: about 0.1 K at around 1000°C (the melting point of gold) and at 14 K (the boiling point of hydrogen).

Mankind has known and given names to water in its solid form as long as language has been used. Accordingly, we have a great many different names for water in a great many languages: "glance, ghiaccio, hielo", (Latin languages); "is, Eis, ice,..." (Germanic languages); "led, lod,..." (Slavonic languages), "tskhali" (Caucasian languages), to give just a few examples.

Southern Greenland This image, facing 60 Degrees North - is coutesy of NASA. Fjords may be seen, carved by glaciers. In the Center of Greenland, the icesheet has accumulated to 3 km thickness. Icebergs can be seen floating offshore in the left foreground.

However, if water had been discovered by naturalists, there would have been two names only all over the world: one to denote mineral ice, the other rock ice - rock formed almost exclusively from an assembly of crystals of irregular, non-geometrically shaped mineral ice, in much the same way as we use the word quartz for the mineral and quartzite for quartz in the form of rock.

Mineral ice has ten different crystalline forms of which one only, Ih ice, is normally found on the Earth. The hexagonal symmetry of the crystal system of this variety can be seen if we examine a crystal of snow formed in the Earth's atmosphere out of water vapour. The triple point of ordinary ice (i.e., Ih ice) is the fixed point on the international temperature scale.

Rock ice is a polycrystalline ice that is virtually impermeable to water; this does not include snow, firn and frost. It varies considerably according to its origin and the metamorphoses it has undergone. By studying its texture (size, shape, layout of the individual crystals), the way it was made (statistical orientation of the axes of symmetry of the hexagons), its gas content (air bubbles), its liquid content (more or less salty drops in ice at melting point, called temperate ice), it is possible to distinguish several different types:

Today, ice is considered to be that solid crystallised form with the best known laws of deformation in all their complexity. These laws vary with the type of rock ice, and type is its turn a function of other factors. Nevertheless, certain types of ice which can still not be made in the laboratory can still be found in the polar ice-caps. In addition, at the ultra-low temperatures where they are found, transitions are so slow that they cannot be studied experimentally.

Water in the Universe

Since the Space Age opened on October 4, 1957, great progress has been made in the study of water in space. Satellites have the possibility of looking both at the Earth and at the Universe. The detection of water detection has become a common exercise for scientists, even if is sometimes quite difficult. The ultimate aim is to identify water and to locate it both on earth and in the Universe.

To achieve this, they use meteorological satellites (observing in different spectral regions or channels, like "V", "Infrared" and "Water vapour"), telescopes and radio telescopes.

For the observations of the Earth, they use ESA ERS images , as well as METEOSAT data to identify water. In Space, some of the effort is directed towards the Moon, Mars, Europa, as well as comets, interstellar clouds, etc.

Water in the solar system

If we examine Earth from space, we discover an abundance of fluid media. There is an atmosphere with that is continuous in motion (convection currents), and there are oceans with similar movements covering 75% of the Earth's surface.

If we plot temperature against pressure for the three states of water, and mark the conditions found on the surfaces of all the planets on the diagram, we will see that conditions allowing the presence of water in all its three forms are only found on Earth.

This is due to the special situation of the Earth:

The study of the atmospheres and surfaces on the other planets shows that a plentiful supply of water in the form of an ocean exposed to the atmosphere only occurs on Earth.

The history of the atmospheres and hydrospheres (watery bodies) of the different planets in the solar system and the actual data on the increase in the greenhouse effect on our planet permit us to forecast the likely future of the Earth (provided the current development does not change at some time in the future). Here is a table in which you will find the values of the gravity, the pressure and the temperature at the surface, and also the chemical composition of the atmosphere. Note that water is only present on the surface of Earth in such great quantities! We are indeed lucky - or could it be that we are here because of this fact....!!??

Planetary Atmospheres




440 (167°C)

He (42%), Na (42%), O (11.5%)

VENUS 888 90 730 (457°C) CO2 (96%), N2 (3.5%)


288 (15°C) N2 (77%), O2 (21%), H2O (1%), Ar (0.93%)
MARS 373 0.0007 218 (-55°C) CO2(95%), N2 (2.7%), Ar (1.6%)
JUPITER Planets consisting primarily of gas: ground level cannot be defined. The temperature is the theoretical temperature immediately above the cloud cover. 124 H2 (90%), He (10%), CH4 (0.2%)
SATURN 95 H2(97%), He(3%)
TITAN   1 70 N2 (82-9%), CH4 (1-6%), Ar (0-12%)

Here are some comments to this table:

On our sister planet, Venus, there is still some water in a dense atmosphere, but the main constituent is CO2, carbon dioxide. This has produced a "suffocating" greenhouse effect. The surface of Venus is a very inhospitable place, indeed!

The disappearance of water is due to the photodissociation of H2O (i.e. when the strong ultraviolet light from the Sun "breaks" the water molecule into hydrogen and oxygen). The hydrogen then escapes from Venus' gravitational field and the oxygen is absorbed into the planet's crust. The present value of the rate of escape is of the order of 107 atoms per cm2/sec. This a very small rate of escape, and assuming that it was constant throughout the ages, the total quantity of water lost since Venus was formed is only equivalent to an ocean with a depth of a few metres over the entire planet. Venus must therefore always have been extremely dry.

But the problem with this consideration is that it does not explain how the greenhouse effect originally started. The present high abundance of CO2, H2O and SO2, only explains how the effect is now maintained. It is not yet known which gas was first present in sufficient quantities and also capable of absorbing the infrared radiation from the ground (when Venus was still a young planet) to start the greenhouse effect. It is believed that this gas may have been water vapour, if Venus had comparable reserves of water to those on Earth at the beginning.

On Mars, solid water (ice) is trapped in the ground and within the icy polar regions (the ice caps). Many fossil traces of water can be seen on the surface and the Mars Pathfinder landed near one of these areas. This liquid water has since disappeared because of the weak gravitational field and the low temperatures at surface level. In the 19th century, several astronomers thought that there were "canals" on Mars and some also believed that they were big canals with water. The spacecraft that flew by Mars in the 1960s and 1970s found that the surface of Mars is much like that of the Moon.

On the Moon, very fine traces of water appear in the rocks in certain regions. Very recently, more information about this has become available from the NASA Lunar Prospector spacecraft. ESA is also planning to send a spacecraft (EUROMOON 2000) to the Moon's southpole to investigate the presence of water there.

In ancient times, naked-eye observers looking at the Moon imagined the existence of "lunar seas" similar to the oceans on the earth. With the evolution of astronomical instrumentation, however, the possibilities of observing planets in great showed that they were not seas of water, but dry crater plains. The spacecraft in the 1960s that were sent to the Moon from the US and the Soviet Union found a dusty surface and the Apollo astronauts who walked on the Moon did not find any water either.

There is a lot of water in the nuclei of comets, perhaps some 80% of their total mass. They are actually considered to be large "dirty snowballs" of dust and ice. When they are near the Sun (within about 3 astronomical Units, or 450 million kilometres) the water on the surafce of the nucleus is transformed directly from solid state to gas (sublimation). This is how a cloud of gas and dust appears around the nucleus from which the tail later develops.

The NASA spacecraft Galileo, is now exporing the giant planet Jupiter and its many moons. Of particular interest is the large moon, Europa, on which there seems to be a frozen sea of water. Perhaps there is water in liquid form deep below the surface?.

The European space probe, Huygens, which left for Saturn and its satellite Titan in 1997, will enter the latter's atmosphere in 2004. It flies on the Cassini spacecraft and together they will give us a better understanding and much new information about the atmospheres of this outer planet and its moons. Will Huygens find water at Titan?

Here are some interesting links concerning planets via the EAAE Hot Links Page.

The Water Molecule in the Distant Universe

Recently, astronomers have found water in many different places outside the solar system. This progress is mainly due to new and better instruments, in particular such that observe the infrared radiation from space. ESA's orbiting Infrared Observatory, ISO, has made many such observations during the past years.

Thanks to improved spectroscopic techniques, numerous molecules have been discovered in the Universe, many of which are carbon-rich (organic). Here are a few of the first molecules that were detected during astronomical research and the year they were discovered.

Chemicals discovered in Space

Cyanogen CN 1940


OH 1963
Water H2O 1968


NH3 1968

Molecular hydrogen

H2 1970

Carbon monoxide

CO 1970
Ethyl alcohol CH3CH2OH 1974
Hydrogen chloride HCl 1985

Water is present in interstellar grains that are formed in atmospheres of rather "cool" stars. That only means that they are colder than the Sun (surface temperature about 5700 K). When the star loses mass during the late phases of its life, these grains are blown into space and form large clouds. New stars are born from such clouds, and it is likely that some of the water in the solar system, in particular that that is still present in the interior of the nuclei of comets, is unchanged "interstellar water ice". That is why it would be so intersting to fly to a comet. The ESA space mission, Rosetta attempts to do exactly that, some years from now.


On the Earth, fogs, mists and clouds are visible signs of a massive condensation of water vapour in the atmosphere; dew and frost are similar phenomena, but limited to the surface of vegetation and other objects in contact with or near ground level.

Water vapour is a completely invisible gas which can only be "seen" optically via its absorption bands, particularly numerous and significant at wavelengths longer than those of visible light (infrared radiation, submillimetre and millimetre radiowaves). What is commonly called "water vapour" is in fact water in liquid phase appearing in the form of microdroplets as condensation takes place in a saturated atmosphere.

The change into water ice requires that 333.6 joules per gram of water at 0°C to be removed. Contrarily, 333.6 joules of energy must be available for one gram of ice at 0°C before it can be transformed into water.

Heat exchanges play an important role in the development of certain clouds where a process of convection is taking place, particularly when the temperatures are initially positive and there are significant quantities of condensed water per cubic metre of air. The only way of causing condensation of water vapour at non-saturating pressures is to cool the air to bring it to a temperature at which the number of water molecules it contains corresponds precisely with that required to achieve saturation. In nature, the temperature is lowered in several different ways: for clouds and mists, much the same process applies.

Under a clear sky at night with no clouds to act as a screen, the soil and vegetation emit heat in much the same way as a "black body", the maximum emission occurs at a wavelength of about 10 micrometers, exactly where an important transmission window exists in the water vapour spectrum. The air in direct contact with the soil will cool down and its stability at some centimetres or so above the soil - provided the wind is not strong - protects it from turbulent exchanges with the hotter air in higher layers. The result is the formation of dew.

Frost is formed under somewhat similar conditions, but obviously, at negative temperatures. Frost is not frozen dew; it is the direct condensation of water vapour in the form of ice on the vegetation and on other objects on the ground.

Clouds appear in the atmosphere in those regions where, as a result of certain processes, air masses have cooled sufficiently to reach the dew point (that is, temperature at which the water vapour in the atmosphere reaches the pressure required to produce saturation).

This cooling process occurs at altitude during the ascension of air masses, given the lowering of the temperature which accompanies (the adiabatic expansion of) the air as it rises. The most efficient condensation nuclei will play their role and will initiate liquid nuclei. Clouds droplets will be formed around these nuclei and will continue growing. Once the liquid phase has begun, over-saturation is bound to occur. The least "receptive" condensation nuclei will remain inactive and will, during the formation of stable clouds, end up being captured mechanically as a result of various mechanisms such as Brownian movement and diffusiophoresis.

If the cloud formed at its condensation level and continues rising, its temperature and saturation pressure will fall again and fresh water vapour molecules will joint the droplets that have already been formed, increasing their size.

Now try your skills again!

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