Introduction to Oceanography/Water - origin and properties

The Origin of Water

As discussed before, about 70% percent of our planet is covered by water.
But what is water? Where does it all come from?

Had you visited our planet 5 billion years ago, you would have seen a completely different scene than today. Our lovely blue, brown and green planet was a molten ball of lava. During the geological time our planet has undergone considerable cooling that has allowed water to accumulate in the ocean basins. The relative occurrences of different gases in our atmosphere suggest that they have a different origin from that of the original gas cloud, the nebula, which eventually condensed to be our planet.

The accumulation of mainly silica compounds and iron oxides in this nebula caused an intensive heating. This heat was released as a result of lowering in the potential energy of those particles which accumulated in the early nebula, and also due to the condensation of outer particles, which accelerated increasing pressure on the inner particles. The decay of radioactive elements also contributed to the heat release. The rise of the temperature caused the partial melting of our planet. This melting has caused a differentiation of the elements abundant in the earth materials. Heavy particles like iron and nickel sunk into the core; light particles like aluminum, silicon, sodium, and volatile gases migrated to the outer mantle.

The massive eruptions of volcanoes through the early development of our planet released huge amounts of volatile gases from the molten rocks into the early atmosphere. It is presently thought that the composition of gases erupting from volcanoes today has changed little (or not at all) during this geological period. In the early stages of earth's history, its surface was too hot to allow water vapor to condense. Another thing worth mentioning is our great luck living on the third rock from the Sun. Venus has no atmosphere, nor water on its surface.

Mars, our second neighbor, has a very thin atmosphere, but no free water on its surface. We got lucky in that we are located right between those two, so our surface temperature - as we shall see in the next chapter, which is mainly determined by the amount of solar radiation the Earth receives - is neither too cold nor hot, and hence allows for most of the water to remain in its liquid state. But this process that filled the Earth's oceans started only after its surface cooled below a temperature of 100 degrees Celsius.

The Composition of Water

Water molecules are made of one oxygen atom and two molecules of hydrogen. The structure of the water molecule causes it to be electrically unbalanced. The oxygen atom at one end is much bigger than the hydrogen atoms and is thus more positively charged. The oxygen atom has six protons carrying six units of positive charge, compared to one positive charge of each hydrogen atom. Thus, the water molecule has an electric dipole.

Left: The molecular model of the Water molecule. Right: The distribution of electric charge on the molecule

The electric dipole of the water molecule is the main reason that water is a unique substance.
Among the special characteristics of water, we shall discuss the following:

Super-Solvent

Since water molecules have an electric dipole, they are the ultimate solvent and hence can dissolve almost every naturally occurring substance. Later on in this book we shall see how this fact contributes to the chemical properties of the ocean.

Large Heat Capacity

The dipole of water molecules causes them to electrically attract each other. Hence, liquid water tends to have micro-structures of molecules that group together and constantly break apart. These micro-structures are based on hydrogen bonds, which, again are based on the unique structure of the water molecules. The temperature of a substance is an expression of the vibrations of the molecules inside the substance. So if we heat water molecules, they do not easily shake, thanks again to hydrogen bonds.

Another important implication of the water dipole is the relatively high boiling point and high freezing point. All hydrogen bonds must break before water can boil. This also leads to a high energy of heating. As you probably already know, water boils at 100^oC and freezes at 0^oC. The reason water boils at such a high temperature is due to hydrogen bonds, which give water a large 'heat capacity.' 'Heat capacity' is the energy needed to raise the temperature of 1 cubic cm of water by 1 degree Celsius. For water we need to invest 4.18 K Jouls. This is known also as specific heat. Water has the highest specific heat of all solids and liquids (except of NH3 known as liquid ammonia). This causes the oceans to be a huge 'temperature buffer,' as we shall later see.

Anomaly of specific gravity

Specific gravity is the ratio of the density of a substance to the density (mass of the same unit volume) of a reference substance. The specific gravity of normal substances rises as their temperature is lowered. The specific gravity of water is 'lowered' when it is cooled down to 4 degrees Celsius. This is, again, due to its unique molecular structure.