Water Fact File for Earth

The Bottom Line:

Water, water everywhere...


Findings from Dr Kotwicki's 1991 Paper:

The Earth, a "water planet", contains some 0.07% water by mass or 0.4% by volume. Left to itself in space, this water would create a sphere 2400 km in diameter, big, but smaller than numerous icy bodies in the Solar System. Many publications quote, with small variations (except for the residence times which vary widely from author to author and  which recently receive more attention) the contents of Table 1 as the summary of water resources of the Earth, which is strictly speaking incorrect, as large quantities of water in the crust and mantle should also be taken into account if the water balance of the planet is contemplated. It is interesting to note that the first scientific reasoning on water balance of our globe came from the realms of celestial orbits: it can be traced to Copernicus (1543), who as well as being a keen astronomer, had also a good grasp of hydrology, contemplating in the opening chapters of his revolutionary book, how the Earth forms a single sphere with water and concluding that there is little water in comparison with land, even though more water perhaps appears on the surface.

Table 1. Water balance of the Earth's hydrosphere
Click here to view table.

Table 1, based here on Kalinin (1968), encompasses both the zones of active groundwater exchange and inactive groundwater, but it should be remembered that the total amount of water under our feet is actually much larger. Kalinin, well aware of this fact, quotes Vernadski who in 1936 estimated this quantity as 1.3 x 109 km3 for a 20-25 km crust  thickness, including water in various states and bonds with minerals, and Makarenko who in 1966 evaluated that a 5 km  crust contains 1.9% of free gravitational waters, 5.1% physically bound water and 5% chemically bound water. Both these  estimates represent approximately the volume of the World Ocean: Deles came to a similar figure of 1.2 x 109 km3for all water in the ground in 1861 (Pinneker, 1980). Fyfe et.al.(1978) maintain that the crust which has a mass of 2.3 x 1025 g must contain about half the mass of water as in the ocean and that the mantle (mass 4 x 1027 g) would need a water content of 0.03% to carry the equivalent of the hydrosphere mass. Thus, say they, the mass of water in the hydrosphere,  crust or mantle appears to be similar. Subduction carries water to depths of hundreds of kilometres at a rather fast rate: the  proportion of water returning to the surface or bound as the result of presumed cooling of the planet are presently  unknown. It seems possible (Van Andel, 1985) that the Earth's surface is losing water to the mantle through subduction of  oceanic sediments and crust. Geomorphologically, the circulating water is a powerful transporting agent, possibly critical for  ore deposition. The first direct evidence of that is the Kola Peninsula Bore, which at the depth of 12 km shows surprisingly large quantities of hot, highly mineralised water.

Table 2. Estimates of inactive water in the crust and the mantle of the Earth.
Click here to view table.

The origin of water on Earth is by no means certain and numerous mechanisms have been advocated. They fall into three basic groups: condensation of the primary atmosphere, outgassing of the interior, and extraterrestrial fallout.

The condensation theory, once universally sanctioned, has fallen into disfavour as current models of the primordial nebula and evolving Earth require that the primary atmosphere consisted mainly of hydrogen, helium, ammonia and methane. This  atmosphere would have been blown away by the intense solar wind during the so-called T-Tauri stage of the proto-Sun.  Adherents of the condensation theory tended also to forget that no Earth's atmosphere could hold all waters of the planet in suspension: for example now, the atmosphere holds only 0.001% of the World Ocean volume.

Present composition shows that our atmosphere is secondary, and suggests its both geological and biological origin.  Similarly, the hydrosphere is believed to be outgassed (Rubey, 1951) and condensed from the interior of our planet. However, this theory needs further investigation, as in fact there should be (Van Andel, 1985) some 20 to 40 times more  water on Earth, depending on which meteorite material would have been its main component. In this respect we should not  ask: "Where does the water come from?" but "Where is the missing water?". The latter is perhaps the question, which planetologists should ask more often.

Historically, the timing of emergence of the oceans was a matter of considerable dispute: according to Kuenen (1950) oceans were created early and rapidly in the Earth's history, Rubey (1951) promoted a continuous steady accumulation whereas Revelle (1955) was of opinion that they were formed late and rapidly. Water outgassed from the interior of our  planet comes from at least two sources: from surfacing terrestrial and oceanic basalts and from volcanic eruptions. Schopf (1980) calculates that some 0.25 x 109 km3of water was released from basalts during the last 3.5 billion years and  concludes that basalts alone cannot explain the emergence of oceans. Quoting existing evidence he says that the majority of outgassing happened between 4.6 and 2.5 billion years ago. New research (Stardacher and Allegre, 1985) indicate that the Earth outgassed rapidly, in some 50 million years after its accretion: further developments in this area are summarised in  Holland (1984b) and Kump (1989). Pinneker (1980, and references therein) estimates that 3.4 x 109 km3 of water  evaporated from the mantle, which he considers the source of all natural water on Earth.

As Meier (1983) recognises, the question of outgassing and of tectonic movement of water are of importance for  hydrologists in refining global water balance calculations. It is also of prime interest for planetologists: as Condie (1989) explains, the volatile contents and especially the water content of planetary mantles and the rate of volatile release are important in controlling the amount of melting, fractional crystallisation trends and the viscosity of planetary interiors which in turn affect the rate of convection and heat loss which are important in terms of evolutionary state.

Some 0.1 km3 year-1 of water (Ingmanson and Wallace, 1973) is thought to be presently outgassed by volcanic eruptions, which predominantly (Bullard, 1976) consist of water vapour: most of this water is, however, probably recirculated on a geological time scale (Holland, 1984a). It is known (Condie, 1989) that the 87Sr/86Sr ratio in marine carbonates varies with age and that the current 87Sr/86Sr ratio of seawater represents about a 4:1 mixture of river water and submarine volcanic water.

Some do not agree with the outgassing scenario altogether: Hoyle (1978), implicitly stating that water reside on the surface  of our planet only, expressed a view that the ocean and the carbon dioxide presented nowadays in the limestone rock have  not come from outgassing of the Earth. They were, argued he, the latter additions, a residue from accumulation of Uranus  and Neptune which happened to cross the Earth's orbit during some 300 million years after formation of the Solar System.

It is now well established that our planet both in geological and present times is exposed to various cosmic collisions  (Shoemaker, 1984, Alvarez, 1987). Extraterrestrial origin of the Earth's water is, therefore, possible, and comets (Whipple, 1976, 1978, Chyba, 1987) are commonly targeted as our potential water supplier. Estimates of quantity of water acquired in this way in the early stages of the evolution of our planet range from 4 to 40% (Chyba, 1987) or more (Hoyle, 1978) of the World Ocean volume. Some data (Frank et.al., 1986) seemed to suggest that this amount might have been further supplemented by some 1 km3 year-1in the form of as many as 107 mini-comets - each with the mass of about 105 g - hitting the Earth's atmosphere each year. This particular hypothesis has been disproved (Kerr, 1989). However, one should  not doubt that recent water acquisitions are plausible. Ahrens (1989) points out that the Earth continues to accrete material  containing water and puts the water budget of the mantle in the range of at least two World Oceans.

Considering other exotic sources of water, the Sun loses some 4 x 1012g s-1 of its matter in the form of solar wind whose ionic composition reflects probably that of the solar corona, which contains 0.77% of oxygen. This suggests that some 3 x 1010 g s-1of potential water is emitted into space: in a lifetime of this star it amounts to a mass equal for example to  some 10 billion 10 km diameter comets. As Taylor (1982) points out, hydrogen from the solar wind could be an additional source of water from reduction of FeO: this would apply to all terrestrial planets and the Moon which is often classified as a  terrestrial planet. Other possibility is (Pinneker, 1980) that water forms in the atmosphere where at a height of 250-300 km  atoms of hydrogen and oxygen may form molecules of water. A vastly greater amount is, however, probably lost from the Earth to interplanetary space.

How long the water on Earth will last? Whereas Kulp (1951) estimates that 1.0 x 109 km3 of water have dissociated into  hydrogen and oxygen and vanished into space so far, Shiklomanov and Sokolov (1983) state that at present there is no  grounds to speak about any significant positive or negative water exchange between the Earth's atmosphere and space. The  escape rate seems to be low indeed: Kasting (1989) using a one-dimensional globally-averaged model to calculate the  escape of hydrogen concludes that, the present atmosphere is marginally stable with respect to water and, with present  escape rate it will take some 7 billion years to lose the World Ocean. It corresponds to the water escape rate of 7 m3s-3.  As Kasting further explains, because the Sun is currently increasing its luminosity by about 1% every 100 million years, the critical solar flux for water loss could be reached within about one billion years, much shorter than the five billion years during which the Sun is expected to remain on the main sequence. It is not to say that the time is running out right now, but  ultimately, water loss will become a problem for the wellbeing of our planet and its inhabitants. 


Latest Findings about Water on Earth:



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