Earth, our home planet, is a world unlike any other. The third planet
from the sun, Earth is the only place in the known universe confirmed
to host life.With a radius of 3,959 miles, Earth is the fifth
largest planet in our solar system, and it’s the only one known for sure
to have liquid water on its surface. Earth is also unique in terms of
monikers. Every other solar system planet was named for a Greek or Roman
deity, but for at least a thousand years, some cultures have described
our world using the Germanic word “earth,” which means simply “the
ground.”Our dance around the sunEarth orbits the sun once every
365.25 days. Since our calendar years have only 365 days, we add an
extra leap day every four years to account for the difference.Though
we can’t feel it, Earth zooms through its orbit at an average velocity
of 18.5 miles a second. During this circuit, our planet is an average of
93 million miles away from the sun, a distance that takes light about
eight minutes to traverse. Astronomers define this distance as one
astronomical unit (AU), a measure that serves as a handy cosmic
yardstick.Earth
rotates on its axis every 23.9 hours, defining day and night for
surface dwellers. This axis of rotation is tilted 23.4 degrees away from
the plane of Earth’s orbit around the sun, giving us seasons. Whichever
hemisphere is tilted closer to the sun experiences summer, while the
hemisphere tilted away gets winter. In the spring and fall, each
hemisphere receives similar amounts of light. On two specific dates each
year—called the equinoxes—both hemispheres get illuminated equally.Many layers, many featuresAbout
4.5 billion years ago, gravity coaxed Earth to form from the gaseous,
dusty disk that surrounded our young sun. Over time, Earth’s
interior—which is made mostly of silicate rocks and
metals—differentiated into four layers.At the planet’s heart lies
the inner core, a solid sphere of iron and nickel that’s 759 miles wide
and as hot as 9,800 degrees Fahrenheit. The inner core is surrounded by
the outer core, a 1,400-mile-thick band of iron and nickel fluids.
Beyond the outer core lies the mantle, a 1,800-mile-thick layer of
viscous molten rock on which Earth’s outermost layer, the crust, rests.
On land, the continental crust is an average of 19 miles thick, but the
oceanic crust that forms the seafloor is thinner—about three miles
thick—and denser.Like Venus and Mars, Earth has mountains, valleys, and volcanoes. But
unlike its rocky siblings, almost 70 percent of Earth’s surface is
covered in oceans of liquid water that average 2.5 miles deep. These
bodies of water contain 97 percent of Earth’s volcanoes and the mid-ocean ridge, a massive mountain range more than 40,000 miles long.Earth’s
crust and upper mantle are divided into massive plates that grind
against each other in slow motion. As these plates collide, tear apart,
or slide past each other, they give rise to our very active geology.
Earthquakes rumble as these plates snag and slip past each other. Many
volcanoes form as seafloor crust smashes into and slides beneath
continental crust. When plates of continental crust collide, mountain
ranges such as the Himalaya are pushed toward the skies.Protective fields and gasesEarth’s
atmosphere is 78 percent nitrogen, 21 percent oxygen, and one percent
other gases such as carbon dioxide, water vapor, and argon. Much like a
greenhouse, this blanket of gases absorbs and retains heat. On average,
Earth’s surface temperature is about 57 degrees Fahrenheit; without our
atmosphere, it’d be zero degrees. In the last two centuries, humans have added enough greenhouse gases to the atmosphere to raise Earth’s average temperature by 1.8 degrees Fahrenheit. This extra heat has altered Earth’s weather patterns in many ways.The
atmosphere not only nourishes life on Earth, but it also protects it:
It’s thick enough that many meteorites burn up before impact from
friction, and its gases—such as ozone—block DNA-damaging ultraviolet
light from reaching the surface. But for all that our atmosphere does,
it’s surprisingly thin. Ninety percent of Earth’s atmosphere lies within just 10 miles of the planet’s surface.We also enjoy protection from Earth’s magnetic field, generated by
our planet’s rotation and its iron-nickel core. This teardrop-shaped
field shields Earth from high-energy particles launched at us from the
sun and elsewhere in the cosmos. But due to the field’s structure, some
particles get funneled to Earth’s Poles and collide with our atmosphere,
yielding aurorae, the natural fireworks show known by some as the
northern lights.Spaceship EarthEarth is the planet we
have the best opportunity to understand in detail—helping us see how
other rocky planets behave, even those orbiting distant stars. As a
result, scientists are increasingly monitoring Earth from space. NASA
alone has dozens of missions dedicated to solving our planet’s mysteries.At
the same time, telescopes are gazing outward to find other Earths.
Thanks to instruments such as NASA’s Kepler Space Telescope, astronomers
have found more than 3,800 planets orbiting other stars, some of which are about the size of Earth,
and a handful of which orbit in the zones around their stars that are
just the right temperature to be potentially habitable. Other missions,
such as the Transiting Exoplanet Survey Satellite, are poised to find
even more. Source:National Geographic
Ovidio Universe: Planet Earth structure
