Earth DISTANCE FROM SUN: 93 million miles (159 million kilometers). REVOLUTION AROUND SUN: 365.3 days. ROTATION: 23.9 hours. DIAMETER: 7,926 miles (12,753 kilometers). DENSITY: 5.5 x that of water. SURFACE TEMPERATURE: -128°F (-89°C) to 136°F (57.7°C). Satellite: Moon Even an alien flying into the solar system would have to imagine Earth as the immediate choice for the planet most likely to harbor life. Its blue expanse of ocean, its thin yet dynamic veil of atmosphere, its brown and green jigsaw of continents—all provide essential clues to the possibilities presented by planet Earth. Unseen yet vital to Earth’s habitability is its molten nickel-iron core, which creates an extensive magnetic field. Along with the atmosphere, this shields the surface from nearly all harmful radiation from the sun. Life on Earth has adapted to thrive in its atmosphere, which is 78 percent nitrogen, and 21 percent oxygen. Early in its history, Earth looked much like its rocky neighbors Mercury, Venus, and Mars. But its specific distance from the sun, along with any number of yet-unknown variables, enabled it to evolve into a cradle of developing life. Earth is a planet in all kinds of motion. It glides along its orbit at 67,000 miles (108,000 kilometers) an hour, its upper-level winds can roar along at hundreds of miles an hour. Ocean currents carry warm water to cold climates, and cold water to warm climates. And the continents themselves are on their own slow march across the globe, moving at fractions of an inch per year. Movement of land masses on Earth has caused continents to collide, creating vast mountain ranges. Great oceans have opened between continents. Such activity has profoundly affected the development of life on the planet, as species separated by mountains and water bodies have developed significant differences.

LIFE GIVING WATERS Mars probably had it. So did Venus at some point. But of all the planets in the Solar System, only Earth kept its supply of liquid water. Essential to life on the planet, water fills its oceans and lakes, falls from the sky to clean the atmosphere, shapes the landscape, and even makes up most of the body weight of its living creatures. It is Earth’s hydrological cycle that drives its weather. Water in oceans and lakes evaporates and rises several miles high, condensing to form clouds. Floating above land masses, clouds provide cooling shade, and eventually create precipitation that flows back into lakes and oceans, ready to begin the cycle anew. High clouds transmit incoming solar radiation, and also trap outgoing radiation, providing a kind of blanket to help keep the planet warm.

SURFACE FEATURES Erosion from Earth’s dynamic winds and rains tends to erase even the largest land forms over time, but plate tectonics—the relentless drift of land masses across the Earth—along with other factors keep creating new features. Among the newest mountain ranges are the jagged Himalayas (shown here), between Tibet and Nepal. Formed as the Indian subcontinent crashed into Asia, the mountains (among them Everest, the highest mountain on Earth) have not yet been worn smooth by wind and rain.
Ice has done much to shape the face of Earth. Today the largest ice sheets are in Antarctica, Greenland, and the Arctic Ocean, but in eons past they marched halfway to the tropics, carving lakes and scouring mountain ranges to the ground.
Friction from Earth’s atmosphere disintegrates most incoming meteoroids before they even get close to the ground, but there are some surviving impact craters. The majority have been eroded and filled in by soil or water, but Arizona’s Meteor Crater still looks positively lunar-like thanks to its youth—just 50,000 years old.
Humankind has also done much to change the face of Earth. Dammed rivers give rise to some of the largest lakes on the planet. Highways cut through mountain ranges. And at night, the lights of Earth’s cities sparkle like reflected starlight.

PLATE TECTONICS There are fires below Earth’s surface; intense heat from the inner core causes rock in the molten outer core and mantle to move. This magma engine drives the tectonic plates that carry continents around the globe. Magma that pushes to the surface through volcanoes emerges as lava. Sometimes, as in Hawaii, the lava flows, fountain-like, to the surface. Or it can erupt explosively, as happened at Mount St. Helen’s in Washington state in 1980. When it’s not pushing continents into each other, tectonic action is tearing them apart. As if rending a garment, the Great Rift Valley is dividing the African continent. Rising at its side, a testament to the internal pressures at work, is the dormant volcano Kilimanjaro. Another massive rift is forming at the center of the Atlantic Ocean. There, upwelling magma is turning into new ocean floor and slowly pushing Europe and Africa away from North and South America.