Manual Earth

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Earth is the third planet from the Sun and the only astronomical object known to harbor life. According to radiometric dating and other sources of evidence, Earth .
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Continental crust averages some 25 miles 40 km thick, although it can be thinner or thicker in some areas. Oceanic crust is usually only about 5 miles 8 km thick. Water fills in low areas of the basalt crust to form the world's oceans. Earth gets warmer toward its core. At the bottom of the continental crust, temperatures reach about 1, degrees Fahrenheit 1, degrees Celsius , increasing about 3 degrees F per mile 1 degree C per km below the crust. Geologists think the temperature of Earth's outer core is about 6, to 7, degrees F 3, to 4, degrees C and that the inner core may reach 12, degrees F 7, degrees C — hotter than the surface of the sun.

Earth's magnetic field is generated by currents flowing in Earth's outer core. The magnetic poles are always on the move, with the magnetic North Pole accelerating its northward motion to 24 miles 40 km annually since tracking began in the s. It will likely exit North America and reach Siberia in a matter of decades. Earth's magnetic field is changing in other ways, too.


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Globally, the magnetic field has weakened 10 percent since the 19th century , according to NASA. These changes are mild compared to what Earth's magnetic field has done in the past. A few times every million years or so, the field completely flips, with the North and the South poles swapping places.

The magnetic field can take anywhere from to 3, years to complete the flip. The strength of Earth's magnetic field decreased by about 90 percent when a field reversal occurred in ancient past, according to Andrew Roberts, a professor at the Australian National University. The drop makes the planet more vulnerable to solar storms and radiation, which can could significantly damage satellites and communication and electrical infrastructure. When charged particles from the sun get trapped in Earth's magnetic field, they smash into air molecules above the magnetic poles, causing them to glow.


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This phenomenon is known as the aurorae , the northern and southern lights. Nowhere else in the solar system is there an atmosphere loaded with free oxygen, which is vital to one of the other unique features of Earth: life. Air surrounds Earth and becomes thinner farther from the surface.

Roughly miles km above Earth, the air is so thin that satellites can zip through the atmosphere with little resistance. Still, traces of atmosphere can be found as high as miles km above the planet's surface. The lowest layer of the atmosphere is known as the troposphere, which is constantly in motion and why we have weather. Sunlight heats the planet's surface, causing warm air to rise into the troposphere. This air expands and cools as air pressure decreases, and because this cool air is denser than its surroundings, it then sinks and gets warmed by the Earth again.

Above the troposphere, some 30 miles 48 km above the Earth's surface, is the stratosphere. The still air of the stratosphere contains the ozone layer, which was created when ultraviolet light caused trios of oxygen atoms to bind together into ozone molecules. Ozone prevents most of the sun's harmful ultraviolet radiation from reaching Earth's surface, where it can damage and mutate life. Water vapor, carbon dioxide and other gases in the atmosphere trap heat from the sun, warming Earth.

Without this so-called " greenhouse effect ," Earth would probably be too cold for life to exist, although a runaway greenhouse effect led to the hellish conditions now seen on Venus. Earth-orbiting satellites have shown that the upper atmosphere actually expands during the day and contracts at night due to heating and cooling.

The second most abundant element is silicon , at 27 percent, followed by aluminum , at 8 percent; iron , at 5 percent; calcium , at 4 percent; and sodium , potassium and magnesium , at about 2 percent each.

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The mantle is made of iron and magnesium-rich silicate rocks. The combination of silicon and oxygen is known as silica, and minerals that contain silica are known as silicate minerals. Earth's moon is 2, miles 3, km wide, about one-fourth of Earth's diameter. Our planet has one moon, while Mercury and Venus have none and all the other planets in our solar system have two or more.

The leading explanation for how Earth's moon formed is that a giant impact knocked the raw ingredients for the moon off the primitive, molten Earth and into orbit. Scientists have suggested that the object that hit the planet had roughly 10 percent the mass of Earth, about the size of Mars.

Structure and Surface

Earth is the only planet in the universe known to possess life. The planet boasts several million species of life, living in habitats ranging from the bottom of the deepest ocean to a few miles into the atmosphere. And scientists think far more species remain to be discovered. A six-year-long, nationwide research project has concluded with solutions to help the dairy industry reduce greenhouse gas emissions while boosting profitability. By Monday, Sept. This site uses cookies to assist with navigation, analyse your use of our services, and provide content from third parties.

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