Earth

• 1. Introduction

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  Earth is the third planet from the Sun, and the only place in the known universe where life has ever taken hold. At 4.5 billion years old, it formed from gas and dust swirling around a young star. What came next defies easy summary: a molten rock cooling into a world, an ocean materializing from volcanic steam and cometary ice, and then something that has never been found anywhere else, life. How does a planet go from a bare ball of iron and silicate to a world covered 70.8% in liquid water, its atmosphere reshaped by billions of years of living things? What forces inside the planet keep driving its surface apart and back together? And how much longer can this world sustain the creature that has come to dominate its surface? Those are the questions worth sitting with.

• 2. A Planet Forms

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  The oldest material found anywhere in the Solar System dates to 4.5682 billion years ago. From collapsing gas and dust, the primordial Earth took somewhere between 70 and 100 million years to assemble itself through a process called accretion, as planetesimals gathered mass under gravity.

  Earth did not arrive alone. A Mars-sized object called Theia, carrying roughly 10% of Earth's mass, struck the young planet with a glancing blow. Some of Theia's mass merged with Earth; the rest scattered into orbit and eventually coalesced into the Moon. Computer simulations suggest that two blob-like remnants of Theia may still lie deep within Earth today.

  Between 4.0 and 3.8 billion years ago, a period called the Late Heavy Bombardment pounded the inner Solar System with asteroids. Earth's magnetic field was established by 3.5 billion years ago, protecting the growing atmosphere from being stripped away by the solar wind. Zircon grains found in Western Australia, dating back as far as 4.4 billion years, provide the oldest direct evidence that liquid water and felsic continental crust existed within 140-160 million years of the planet's formation.

• 3. Life Reshapes the World

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  Chemical reactions produced the first self-replicating molecules roughly four billion years ago. The last common ancestor of all current life appeared about half a billion years after that. Among the earliest physical traces of life are microbial mat fossils found in 3.48 billion-year-old sandstone in Western Australia, and biogenic graphite in 3.7 billion-year-old metasedimentary rocks in Western Greenland.

  Photosynthesis, when it evolved, changed everything. Organisms began harvesting sunlight directly, and the oxygen they released accumulated in the atmosphere. That oxygen, reacting with ultraviolet radiation, built the ozone layer, shielding the surface from harmful radiation and allowing life to spread from the oceans onto land. The cumulative result was the Great Oxidation Event, roughly two billion years ago.

  During the Neoproterozoic period, around 1 billion years ago, much of Earth may have been locked under ice in what scientists call Snowball Earth. That deep freeze preceded the Cambrian explosion, 535 million years ago, when multicellular life dramatically increased in complexity. Since then, at least five major mass extinctions have occurred. The most recent, 66 million years ago, was triggered by an asteroid impact that killed the non-avian dinosaurs but spared small animals including mammals, insects, lizards, and birds. Several million years ago, an African ape species gained the ability to stand upright. Humans emerged 300,000 years ago in Africa.

• 4. A Restless Interior

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  Earth is the densest planet in the Solar System. Its mass is approximately 5.97 times ten to the 24th kilograms, composed mostly of iron at 32.1%, oxygen at 30.1%, silicon at 15.1%, and magnesium at 13.9%. The core is dominated by iron at 88.8%, with smaller amounts of nickel and sulfur.

  At the planet's center, the temperature may reach 6,000 degrees Celsius and the pressure 360 gigapascals. Two heat sources feed this interior: primordial heat left over from formation, and radiogenic heat produced by the decay of isotopes including potassium-40, uranium-238, and thorium-232. About 3 billion years ago, Earth was generating twice its present heat output, which drove faster mantle convection and produced rare igneous rocks called komatiites that are almost never formed today.

  That internal heat drives plate tectonics. The lithosphere, Earth's rigid outer layer, is divided into major plates including the Pacific, North American, Eurasian, African, Antarctic, Indo-Australian, and South American. The fastest-moving plate is the Cocos Plate, advancing at 75 mm per year. The slowest is the South American Plate at 10.6 mm per year. Because oceanic crust is continuously created at mid-ocean ridges and consumed at convergent boundaries, most of the ocean floor is less than 100 million years old. The oldest oceanic crust, in the western Pacific, is estimated at 200 million years old, while the oldest dated continental crust is 4,030 million years old.

• 5. The Ocean World

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  Water defines Earth more than any other feature. Liquid surface water covers 70.8% of the crust, a global ocean with an area of 361.8 million square kilometers and a mean depth of 3,682 meters. The oceans hold approximately 1.35 metric tons of water, about one four-thousandth four-hundredth of Earth's total mass.

  Of all the water on Earth, 97.5% is saline. Of the remaining 2.5% that is fresh, about 68.7% is locked in ice caps and glaciers. Sea ice in the Arctic covers an area roughly as large as the United States, though it is retreating due to climate change. The average salinity of the ocean is about 35 grams of salt per kilogram of seawater.

  The ocean functions as a vast heat reservoir, buffering temperature swings and driving global climate patterns through thermohaline circulation that moves thermal energy from equatorial regions to the poles. Shifts in oceanic temperature distribution can trigger large-scale weather events such as the El Nino-Southern Oscillation. Without the greenhouse effect that the ocean and atmosphere together maintain, the average surface temperature would be -18 degrees Celsius rather than the current average of roughly 15 degrees Celsius, and life as it exists today could not survive.

• 6. Orbit, Tilt, and Climate

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  Earth orbits the Sun at an average distance of about 150 million kilometers, completing one revolution every 365.2564 mean solar days. Its axial tilt of approximately 23.44 degrees is what produces the seasons: summer in the Northern Hemisphere occurs when the Tropic of Cancer faces the Sun, and winter when it faces away.

  That tilt is not entirely self-sustaining. Tidal interactions with the Moon help stabilize Earth's axial tilt against gravitational torques applied by the Sun and other planets. Without the Moon, the rotational axis might behave as Mars's does, shifting chaotically over millions of years. During the Ediacaran period, approximately 620 million years ago, there were 400 plus or minus 7 days in a year, with each day lasting just 21.9 plus or minus 0.4 hours. The Moon is gradually receding from Earth at about 38 mm per year, and Earth's day is lengthening by about 23 microseconds per year.

  Earth receives 1,361 watts per square meter of solar energy. The troposphere, the lowest 11 km of the atmosphere where weather occurs, is composed of 78.084% nitrogen, 20.946% oxygen, 0.934% argon, and trace amounts of other gases including carbon dioxide. Without greenhouse gases trapping heat, the current average surface temperature of 14.76 degrees Celsius could not be maintained.

• 7. Earth's Future

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  Over the next 1.1 billion years, solar luminosity will increase by 10%, and over the next 3.5 billion years by 40%. Rising temperatures will speed up the inorganic carbon cycle, potentially reducing atmospheric carbon dioxide to levels lethally low for current plants, roughly 10 parts per million for C4 photosynthesis, within approximately 100 million years. A collapse in plant life would drain oxygen from the atmosphere, making animal life impossible.

  Earth's mean temperature may reach 100 degrees Celsius within 1.5 billion years. All ocean water would then evaporate and be lost to space, likely triggering a runaway greenhouse effect within an estimated 1.6 to 3 billion years from now. Even before that point, some ocean water would descend into the mantle as mid-ocean ridges cool and reduce steam venting.

  In about 5 billion years the Sun will become a red giant, expanding to roughly 1 astronomical unit, about 250 times its present radius. Earth may migrate outward to an orbit 1.7 astronomical units from the Sun as the star sheds roughly 30% of its mass. Or tidal effects may drag Earth inward, into the Sun's atmosphere, where it would be vaporized. Today, as of the 2020s, the human population stands at eight billion and global temperatures in 2020 were already 1.2 degrees Celsius warmer than the preindustrial baseline. Of the nine identified planetary boundaries, five have already been crossed.