Viewed as a complete planetary body, Earth has a distinctive formation history, structure, and motion. Examine its age, shape, mass, gravity, rotation, orbit, axial tilt, and the properties governing its behavior in space.
No surviving rock records the instant Earth became a planet. The surface has been melted, weathered, buried, metamorphosed, and recycled by plate tectonics for billions of years. Scientists nevertheless place Earth’s age at about 4.54 billion years because several independent records can be...
The early inner Solar System was struck far more often than it is today. The disputed point is not whether large impacts occurred, but how their frequency changed. Did bombardment decline gradually as leftover planet-forming debris was removed, or did a delayed dynamical disturbance produce a brief...
The Hadean Eon begins with the formation of the Solar System about 4.567 billion years ago and ends just over 4 billion years ago. Its name evokes an inferno, and portions of early Earth genuinely matched that image. Accretionary impacts released immense energy, dense metal descended toward the...
Planetary growth compresses an enormous amount of change into a deceptively simple verb: accrete. Earth accumulated matter, but it did not do so at a constant rate or by one collision mechanism. Dust, pebbles, planetesimals, planetary embryos, and late impactors participated at different stages....
Earth did not begin as a small version of the familiar blue planet. Its ingredients passed through several radically different states: microscopic grains orbiting a young star, kilometer-scale bodies colliding in a disk, a growing world repeatedly melted by impacts, and finally a layered planet with...