![]() This is followed by the mesosphere, which extends up to 53 miles (85 km) in altitude (this is where meteors burn up), and the thermosphere, which reaches far into space, at least 373 miles (600 km) high. Above that is the stratosphere, where clouds and weather balloons fly, going up to 31 miles (50 km). Most human activity takes place in the lowest atmospheric layer, the troposphere, which extends 5 to 9 miles (8 to 14.5 km) over our heads, NASA says. Our planet's atmosphere is 78% nitrogen, 20% oxygen, 0.9% argon and 0.04% carbon dioxide, plus trace amounts of other gases, according to NASA. The area of Australia is 2.97 million square miles (7.66 million square km), according to the Australian government. North and South America together constitute 16.43 million square miles (42.55 million square km), according to the online encyclopedia Nations Online, while the frozen continent of Antarctica is 5.41 million square miles (14 million square km). The largest continent, which is sometimes known as Afro-Eurasia (though more commonly broken up into Africa, Europe and Asia), has a total area of 32.8 million square miles (84.95 million square km), according to the Encyclopedia of World Geography. ![]() In Earth's center is a nickel-iron core that is liquid on the outside, down to 1,400 miles (2,250 km), but is crushed by incredible pressures into a solid form at the lowest depths, according to the USGS.Įarth has several enormous landforms. A common misconception is that all the rock in the mantle is melted into magma in reality, most of it is in a highly viscous form that is so thick that it takes millions of years for its movement to become apparent. ![]() The mantle is the next layer down, extending to about 1,800 miles (2,900 km) below Earth's surface. Our planet's crust is a thin outer layer, containing mostly silicate and basaltic rocks, that extends on average around 18 miles (30 km) below the planet's surface, according to the U.S. Earth is composed of many elements, chief among them oxygen, silicon, magnesium, iron, aluminum and nickel, according to Caltech's Infrared Processing and Analysis Center. ![]()
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