PLANET EARTH - The Incredible Visual Guide Part 6 ppt

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The feature of planet Earth that makes it so special is liquid

water—the substance that is vital to life as we know it. As a

simple compound of hydrogen and oxygen, water is probably

common throughout the universe, but mainly in the form of

solid ice or gaseous water vapor. Both occur throughout the

solar system, but liquid water is rare, mainly because the

other planets are either too hot or too cold. Earth is unique

in the solar system in having temperatures that allow all

three forms of water to exist, sometimes in the same

place at the same time.

WATER AND ICE

￾ WATER IN SPACE

Water is constantly careening around the solar

system in the form of comets—“dirty snowballs” of

ice, dust, and rock fragments. It also occurs on other

planets, but mainly as water vapor or, as in this crater

near the north pole of Mars, as ice. However, liquid

water may exist beneath the icy surface of Europa,

one of the moons of Jupiter—and where there is

water, there may be life.

ATOMS AND MOLECULES

Water is a mass of molecules, each with two hydrogen atoms

and one oxygen atom. This explains its chemical formula, H2O.

The molecules of liquid water are loosely bound by electronic

forces, enabling them to move in relation to each other. When

water freezes, the molecules become locked together, and

when it evaporates they burst apart.

Ice If water freezes, the

water molecules lock

together in a “crystal

lattice” to form the solid

structure of ice.

Ice forms a thin crust on

the sand dunes of this

crater floor on Mars

Water In liquid form, the

water molecules cling

together, but are able to

move around each other

and flow.

Water vapor Heat

energy breaks the bonds

holding water molecules

together, so they move

apart to create a gas.

Ice has a regular geometrical

structure of water molecules

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