Without the Moon, the Earth would be a very different place. For starters, instead of the usual 24 hours, a day would have just 10—just as the Moon’s gravity gives us tides, it also has been slowing Earth’s rotation for billions of years. The Moon also stabilizes the Earth’s axis at an inclination of around 23 degrees; without it, the Earth’s axis would wobble like Mars’ axis, which varies between 13 and 40 degrees over the course of 10 to 20 million years.
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The first great discovery of the space age, the Van Allen belts are giant, donut-shaped bands surrounding the Earth. They contain charged particles from cosmic rays or the solar wind, a stream of particles constantly emanating from the Sun. These particles are trapped by the Earth’s magnetic fields. The Van Allen belts can swell dramatically in response to geomagnetic storms, typically caused by shock waves in the solar wind. If an orbiting satellite is engulfed by the belts during such stormy space weather, it can be knocked out of action by radiation damage.
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We live in a very violent universe, as demonstrated by the fact that the Earth is being constantly bombarded by a rain of supernova debris—subatomic particles accelerated to almost the speed of light. These cosmic rays are mostly individual protons, along with some helium nuclei, electrons, and a smattering of heavier elements. Although some rays do reach the surface of the Earth, most of them smash into oxygen or nitrogen molecules like a ripe tomato hitting concrete, producing a cascade of exotic particles that often reaches the surface.
About 8 percent of our normal background radiation exposure comes from cosmic rays, and they are responsible for creating a reservoir of radioactive carbon-14 in the atmosphere. Because living organisms stop taking in carbon-14 when they die, measuring the level of carbon-14 in their remains is the basis of carbon dating.
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Scientists have long known that the surface of the Earth is teeming with bacteria, from the cleanest kitchen to the murkiest depths of the ocean. In the last few decades, it’s become clear that bacterial territory also extends deep underground and high into the stratosphere. Even more recently, scientists have come to realize that the movement of these microbes can have large-scale effects.
For example, we would have a lot less precipitation without bacteria, as they promote the formation of ice crystals or rain drops within clouds. It’s been hypothesized that the bacteria have adapted to using clouds for long-distance transport to new sources of food, riding the rain back down to the surface.
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This small bird makes the longest known migration of any animal, on average flying more than 40,000 miles per year, and in some cases more than 50,000 miles. The Arctic tern arrives in the Northern Hemisphere to breed in May, at latitudes between Alaska’s and Massachusetts'. In August, it flies to the Southern Ocean that surrounds Antarctica.
Apart from a very dedicated, sun-loving, jet-setting human, the Arctic tern probably experiences more daylight than any other creature on Earth, fitting in two summers a year. Over an average lifespan of 13 years, a tern will rack up 570,000 miles—nearly 2.5 times the distance from the Earth to the Moon.
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Earth’s atmosphere includes six giant circulating cells, three in the Northern hemisphere and three in the Southern. Each hemisphere has a polar cell, a Hadley cell near the equator, and a mid-latitude cell, turning like a cog wheel between the others.
The circulation of the Hadley cells is the reason the great deserts of the world, such as the Sahara or the Australian Outback, are located 30 degrees north and south of the equator—the returning descending air is very dry. Jet streams—high altitude, high speed, narrow currents of air that flow west to east—form at the cell boundaries at 30 and 60 degrees. Airlines use jet streams to reduce flight times when going east, while west-bound flights avoid jet streams.
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When the glaciers pushed across Northern America and Eurasia during the last ice age, their tremendous weight pushed the underlying land down into the mantle, like someone pressing down on a rubber duck floating in a bathtub. When the glaciers melted about 11,000 years ago, that weight was removed, and the land began to bounce back to its pre-glacial level. In fact, it still hasn’t finished rebounding. The effect is most notable in places like the Kvarken Archipelago in Finland, where the rise of the land out of the sea can be measured on a year-to-year basis, and coastal stone age settlements are now located 500 feet above sea level.
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The oceans may be Earth’s most distinctive feature—140 million square miles of open, liquid water. The oceans have been here in some form almost since the beginning of the Earth, dating back 4.4 billion years. But, geologically speaking, they’re nearing the end of their existence—the sun is slowly getting brighter, and the extra heat will evaporate the oceans into space, and turn the Earth into an endless desert, in about a billion years’ time.
Today’s oceans are linked by a globe-spanning system of currents known as the thermohaline circulation, of which the Gulf Stream is just one branch: Warm water flows along near-surface currents towards the poles; it then cools and sinks. Cold, deep currents flow around the globe and rise up around Antarctica, due to a difference in salinity. This process mixes the Earth’s oceans, and likely plays an important role in maintaining our current climate.
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Floating on top of the mantle is Earth’s thin crust—approximately 6 miles thick beneath the ocean floors, and between 6 and 30 miles thick in the continents, although there are a few places where the mantle is exposed. In proportion to the 7,900-mile diameter of the Earth, the crust is about as thin as the skin of an apple.
The continents are continually merging into one giant supercontinent, and then breaking apart again in a cycle that takes up to 500 million years. Supercontinents have a huge impact on the Earth’s climate, producing vast interior deserts and provoking intense periods of global glaciation. These changes in turn have a deep impact on the course of evolution, and may even have been responsible for the emergence of multicellular organisms.
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The mantle is mostly solid rock. Huge reserves of radioactive elements in the deep mantle, such as uranium and thorium, produce about 20 terawatts of heat. (For comparison, total global electrical generation capacity is about 5 terawatts.) This heat, along with tremendous pressure due to the depth, makes the solid rock in the upper layers of the mantle flow like a very viscous fluid, forming convection cells that drive the motion of Earth’s tectonic plates.
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A subterranean ocean of molten iron and nickel forms the outer core. Because the bottom layers of the outer core are hotter than the upper layers, the electrically conducting liquid metal is constantly circulated by convection currents. Like a giant electromagnet, the circulating metal creates the Earth’s magnetic field, which protects us from dangerous radiation from space.
For reasons not well understood, the Earth’s north and south magnetic poles swap position on average roughly every 200,000 to 300,000 years, although it has been more then 780,000 years since the last reversal.
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With a diameter that's about 85 percent of the Moon’s, the inner core is a solid and slightly lopsided ball of nickel and iron. It is super-rotating—spinning slightly faster than the rest of the Earth—so it laps the surface between every 1,200 and 1,800 years. As the Earth slowly cools, the inner core is growing: The innermost part of the liquid outer core solidifies and becomes part of the inner core at a rate of about 1 millimeter per year.