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Ice

Ice

Structure of ice

Physical properties of ice

Natural ice occurrence

Current glaciology research

Resources

Ice is the solid state of water, and the amount of water on the surface of Earth includes ice in the Polar regions and at high elevations. The relative proportion of each of the three states of water on Earth is the consequence of a balanced equilibrium controlled by the amount of incoming solar energy and the amount of reflection, known as albedo, from clouds, water, ice caps, etc. The amount of ice at any location on Earth varies seasonally, over the long term with climatic change, and even with movements of tectonic plates. One of the most abundant of Earths substances, ice manifests itself in a variety of forms including snow, hail, glaciers, icebergs, and sea ice, along with the artificially produced ice cube. Ice and water behave differently from other materials in a number of important ways.

Structure of ice

Because they share a common composition with their liquid state, ice molecules also consist of the same 2 to 1 ratio of hydrogen and oxygen atoms, the H2O molecule. The shape of this molecule, the oxygen atom at the center with the two hydrogen atoms separated by an angle of 104.52°, dictates the structure of ice. All naturally occurring ice crystals are hexagonal in shape and all snowflakes reflect this six-sided crystal habit. The crystal lattice consists of linked hexagonal rings of water molecules with considerable open space in the center of the ring.

Under artificial laboratory conditions of very high pressures and low temperatures, ice can be forced to crystallize in a number of allotropic forms that are stable only under those particular conditions. Crystallization can occur in these laboratory situations in one of several non-hexagonal forms. This is similar to the way that carbon atoms may crystallize to form graphite or, under more extreme conditions, diamond. The conditions under which the alternate forms might be created do not occur naturally on Earth. They may, however, be present on other bodies in space.

The crystalline structure of ice may be deformed by stress, such as the weight of overlying ice on the deeper portions of a glacier. One type of deformation involves shearing of the crystal lattice along parallel planes. Recrystallization, a different kind of deformation, entails the change in the shape and orientation of crystals within the solid. Both of these processes produce the phenomenon known as creep or creeping flow, responsible for the flowing motion of massive ice bodies such as glaciers.

Physical properties of ice

Pure liquid water is transformed to its solid state, ice, at a temperature of 32°F (0°C) when the pressure is at one atmosphere. The density of liquid water at the freezing point is 62.418 lb/ft3 (0.99984 g/cm3) but decreases to 57.23 lb/ft3 (0.9168 g/cm3) when that water organizes itself into crystalline ice at 32° F (0°C). This density difference is due the large open spaces within the crystal lattice of ice. The increased volume of the solid lattice causes pure water to expand by approximately 9% upon freezing, which can rupture pipes or damage engines if the expanding water has no outlet. Ice is one of a few solid substances that is lower in density than the corresponding liquid state. Surface ice floating on a lake or pond helps to insulate the water below, reduces mixing, and can prevent the water body from freezing solid. This fact has often been cited as an important factor in the development and evolution of life in freshwater.

The freezing point of water containing dissolved solids decreases proportionately to the amount of solute. For example, as the salinity of the water increases its freezing point decreases. This is the principle behind the practice of road and sidewalk salting in some areas during winter. Salt lowers the freezing point of water and, at temperatures near the freezing point of pure water, can cause snow or ice to melt.

When pressure is exerted on ice crystals at temperatures near the melting point, the edges of those crystals may melt. When that pressure is released, the water refreezes. This process, called regelation, may be familiar to those that have formed snowballs. The loose snowflakes are partially melted by the pressure of the hands. When the pressure is released, the refreezing water hardens and causes the cohesion of the flakes into a ball. On very cold days, however, the pressure that can be exerted by the hands is insufficient to cause melting, and the snowball is more difficult to form.

Natural ice occurrence

Most of the natural ice on Earth occurs at extreme latitudes, for example the Greenland ice sheet and sea

ice at the North Pole and the Antarctic ice sheet. Sea ice, massive ice sheets, valley and mountain glaciers all combine to form the polar ice caps. Large areas of the polar and subpolar regions are underlain by permafrost. Polar ice caps and glaciers contain a large proportion of Earths freshwater resource. Over 75% of all freshwater, or 2.15% of all water on Earth, presently exists in the form of ice. This proportion was significantly greater during past glacial epochs.

Glacial ice is a particularly sensitive indicator of climatic change. The rapid retreat of mountain glaciers has been cited as evidence of global warming. If all ice at the poles and in glaciers melted, sea level would rise approximately 260 ft (80 m).

Ice is known to occur on a variety of bodies in space. The origin of water on Earth has been postulated to be a result of collisions with comets and/or meteors containing ice. The presence of ice has been confirmed at the poles of the moon and the planet Mars. The existence of ice on Mars may be an indicator of the potential for the existence of life forms in the warmer and wetter past of that planet. The rings of Saturn and nebulae outside our solar system are thought to contain ice. Europa, a moon of Jupiter, is thought to have a liquid-water ocean beneath a crust of ice. Scientists have also suggested that ice on such bodies might be used to supply the water for manned space missions, as well as being split into its component gases and used for fuel.

Current glaciology research

Current research is focused on reducing the impact of ice on modern society. Ice causes damage to pipes in homes, damages crops, restricts ability to travel, breaks power lines and other property, interferes with the function of airplanes and ships, along with other human considerations, such as contributing to accidental injuries. Engineers study ice to better prepare to build structures that interact with it, such as airplanes, ships, and even oil platforms on the ocean. Climatologists and environmental scientists are working to understand the effects of global warming on the polar ice caps. Meteorologists study the formation of ice in the atmosphere. Other scientists are looking for improved methods by which ice can be controlled on roads. Biologists work to develop

KEY TERMS

Albedo The fraction of sunlight that a surface reflects. An albedo of zero indicates complete absorption, while an albedo of unity indicates total reflection.

Allotropic Said of substances that take multiple forms, such as graphite and diamond, usually in the same phase.

Freezing point The temperature at which a liquid solidifies, 32°F (0°C) for water.

Glaciology The study of all aspects of ice and its associated processes.

Hexagonal crystal system One of six crystal systems. Characterized by one axis that is of unequal length to three identical perpendicular axes, commonly displaying three- or six-fold symmetry.

Melting point The temperature at which a solid becomes liquid, 32°F (0°C) for ice.

Permafrost Permanently frozen soil or subsoil.

Recrystallization The formation of new crystals, while in the solid state.

Regelation A two-fold process involving the melting of ice under pressure and the refreezing of the melt water upon the release of that pressure.

methods of protecting crops from frost damage. Physicists and engineers try to improve understanding of the properties of ice in order to improve the performance of sports equipment such as snow skis and ice skates. Geologists are studying the formation of ice volcanoes along the shores of the Great Lakes. Also, space scientists are looking for additional ice in our solar system and beyond, and planning new techniques and equipment that will allow man to someday utilize that ice in the exploration of other worlds.

See also Ice ages; Icebergs.

Resources

BOOKS

Gosnell, M. Ice: The Nature, the History, and the Uses of an Astonishing Substance. New York: Knopf, 2005.

Hambrey, M., and J. Alean. Glaciers. Cambridge, United Kingdom: Cambridge University Press, 2004.

Tarbuck, E.J., F.K. Lutgens, and D. Tasa. Earth: An Introduction to Physical Geology. Upper Saddle River, NJ: Prentice Hall, 2004.

David B. Goings

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