EARTH, A PLANET 01

I INTRODUCTION Earth (planet), one of nine planets in the solar system, the only planet known to harbor life, and the "home" of human beings. From space Earth resembles a big blue marble with swirling white clouds floating above blue oceans. About 71 percent of Earth surface is covered by water, which is essential to life. The rest is land, mostly in the form of continents that rise above the oceans. Earth An oxygen-rich and protective atmosphere, moderate temperatures, abundant water, and a varied chemical composition enable Earth to support life, the only planet known to harbor life.

The planet is composed of rock and metal, which are present in molten form beneath its surface. The Apollo 17 spacecraft took this snapshot in 1972 of the Arabian Peninsula, the African continent, and Antarctica (most of the white area near the bottom). Earth surface is surrounded by a layer of gases known as the atmosphere, which extends upward from the surface, slowly thinning out into space. Below the surface is a hot interior of rocky material and two core layers composed of the metals nickel and iron in solid and liquid form. Unlike the other planets, Earth has a unique set of characteristics ideally suited to supporting life as we know it. It is neither too hot, like Mercury, the closest planet to the Sun, nor too cold, like distant Mars and the even more distant outer planets upiter, Saturn, Uranus, Neptune, and tiny Pluto. Earth atmosphere includes just the right amount of gases that trap heat from the Sun, resulting in a moderate climate suitable for water to exist in liquid form. The atmosphere also helps block radiation from the Sun that would be harmful to life. Earth atmosphere distinguishes it from the planet Venus, which is otherwise much like Earth. Venus is about the same size and mass as Earth and is also neither too near nor too far from the Sun.

But because Venus has too much heat-trapping carbon dioxide in its atmosphere, its surface is extremely hot ot enough to melt lead and too hot for life to exist. Although Earth is the only planet known to have life, scientists do not rule out the possibility that life may once have existed on other planets or their moons, or may exist today in primitive form. Mars, for example, has many features that resemble river channels, indicating that liquid water once flowed on its surface. If so, life may also have evolved there, and evidence for it may one day be found in fossil form. Water still exists on Mars, but it is frozen in polar ice caps, in permafrost, and possibly in rocks below the surface.