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’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 tiny 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.
For thousands of years,
human beings could only wonder about Earth and the other observable planets in
the solar system. Many early ideas—for example, that the Earth was a sphere and
that it traveled around the Sun—were based on brilliant reasoning. However, it
was only with the development of the scientific method and scientific
instruments, especially in the 18th and 19th centuries, that humans began to
gather data that could be used to verify theories about Earth and the rest of the
solar system. By studying fossils found in rock layers, for example, scientists
realized that the Earth was much older than previously believed. And with the
use of telescopes, new planets such as Uranus, Neptune, and Pluto were
discovered.
In the second half of
the 20th century, more advances in the study of Earth and the solar system
occurred due to the development of rockets that could send spacecraft beyond
Earth. Human beings were able to study and observe Earth from space with
satellites equipped with scientific instruments. Astronauts landed on the Moon
and gathered ancient rocks that revealed much about the early solar system.
During this remarkable advancement in human history, humans also sent unmanned
spacecraft to the other planets and their moons. Spacecraft have now visited
all of the planets except Pluto. The study of other planets and moons has
provided new insights about Earth, just as the study of the Sun and other stars
like it has helped shape new theories about how Earth and the rest of the solar
system formed.
As a result of this recent
space exploration, we now know that Earth is one of the most geologically
active of all the planets and moons in the solar system. Earth is constantly
changing. Over long periods of time land is built up and worn away, oceans are
formed and re-formed, and continents move around, break up, and merge.
Life itself contributes to
changes on Earth, especially in the way living things can alter Earth’s
atmosphere. For example, Earth at one time had the same amount of carbon
dioxide in its atmosphere as Venus now has, but early forms of life helped
remove this carbon dioxide over millions of years. These life forms also added
oxygen to Earth’s atmosphere and made it possible for animal life to evolve on
land.
A variety of scientific
fields have broadened our knowledge about Earth, including biogeography,
climatology, geology, geophysics, hydrology, meteorology, oceanography, and
zoogeography. Collectively, these fields are known as Earth science. By studying
Earth’s atmosphere, its surface, and its interior and by studying the Sun and
the rest of the solar system, scientists have learned much about how Earth came
into existence, how it changed, and why it continues to change.
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