One planet ---EARTH Article Abstract
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Published: February 26, 2008
Earth (planet), third planet in distance from the Sun 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’s 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’s 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—Jupiter, Saturn, Uranus, Neptune, and the tiny dwarf planet Pluto.
Earth’s 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’s 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—462°C (864°F)—hot 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.