Water is the most important substance on earth. Life began in water, most life today resides in water and water makes up about 75 per cent of the weight of a living organism. Water can exist in three states - liquid, solid and gaseous - and today I will talk mostly about ice, the solid state of water, writes Dr William Revie.
Earth is unusual in the solar system because the surface temperature of the planet allows water to exist in all three physical states. Earth is the only planet in the solar system to have large bodies of liquid water in direct contact with the atmosphere. Oceans cover more than 71 per cent of the surface of the earth.
The smallest portion of water that can exist is the molecule, which consists of two atoms of hydrogen and one atom of oxygen - H2O. The water molecule is said to be polar because each of the hydrogen atoms bears a slight positive electrical charge and the oxygen atom bears a slight negative charge. Opposite charges are attracted to each other, and this polar nature of water gives it many properties that are important for life.
As liquid, water molecules slide over each other randomly. Water solidifies into ice crystals when the temperature is reduced to zero degrees. In the crystals, the water molecules arrange themselves in a precise, geometrical, three-dimensional array in which each oxygen atom is surrounded by four hydrogen atoms - two of the hydrogen atoms are joined to the oxygen atom to form the molecule and the other two hydrogen atoms are on adjacent water molecules but attracted to the oxygen atom by the opposite partial charges.
Ice is a more open structure than liquid water and is therefore less dense. This is why ice floats in liquid water. The fact that ice floats on water is an essential protective mechanism for aquatic life in cold climates.
When the cold season comes, bodies of water first freeze at the surface. The surface layer of ice insulates the water below from the cold atmosphere above, ice being a poor conductor of heat. Consequently, most bodies of water remain liquid under the surface ice and life can continue happily through the cold season.
Two permanent ice caps cover the poles of the earth. At the South Pole, Antarctica, the fifth largest of the seven continents, is covered with ice over more than 95 per cent of its surface. On the other hand, most of the Arctic ice cap, at the North Pole, is over a central ocean almost entirely surrounded by land.
Consequently, the rise in sea levels that would result from melting of the polar ice caps, due to an enhanced greenhouse effect, would largely result from melting of the land-based Antarctic ice cap. You can easily check this out with a glass of water and an ice cube.
DESPITE the superabundance of liquid water on the planet, most of it is unfit for human consumption - less than three per cent of all water on earth is fresh. And three-quarters of this fresh water is not available for use because it is packed away in the permanent ice caps that cover 10 per cent of the planet's land surface.
Significant extended coolings of the atmosphere and oceans occur about every 150 million years, each lasting a few million years. These are the ice ages, when large areas of the surface of the globe are covered with ice.
Earth last entered an ice age about 2.5 million years ago, at the start of the Quaternary period. Within the ice ages there are cold and warm fluctuations, called glacials and interglacials, that recur every 100,000 years. We are in an interglacial period. At the end of the Pleistocene epoch, about 10,000 years ago, continental ice sheets retreated from North America and Europe.
It was long assumed that no water exists on the moon, but in 1996 evidence was reported of water ice at the south pole of the moon and data returned in 1998 indicates the presence of ice at both poles. There is thought to be 6.6 billion tonnes of it.
How can ice survive on the moon? The moon has no atmosphere and surface ice, directly exposed to the vacuum, would sublimate into water vapour. Because the moon has such low gravity, the water vapour would be quickly lost to space. Almost all of the moon is exposed to sunlight during the lunar day and surface temperatures in direct sunlight reach 122 degrees. Any ice exposed for a brief period would be lost.
So the only way ice could persist on the moon is buried underground or in a permanently shadowed area. Both poles of the moon have areas that are permanently shadowed. Ice water could persist for billions of years in craters in these areas, where the temperature never exceeds -173 degrees. The ice was probably deposited by some of the innumerable meteorites that have bombarded the moon.
The moon's deposits have been described as the most valuable piece of real estate in the solar system. It is planned to establish permanent manned bases on the moon in the future. Shipping water there would cost about €10,000 per kilogram, and a local supply would be invaluable. The ice could also serve as a source of oxygen and of hydrogen, which could be used as rocket fuel.
- William Reville is associate professor of biochemistry and director of electron microscopy at University College Cork