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The Earth’s surface cooled quickly after the Moon-forming event, forming a hot solid crust. Condensing water vapor from volcanoes, together with water and chemicals delivered by asteroids and comets, produced the first oceans, where life likely began.
Earth’s oceans grew from small to global in scale in only 400 million years. Much of this water came from outer space, when migration of the giant planets scattered comets and asteroids throughout the Solar System in the Late Heavy Bombardment. These impactors brought water and organics (some of life’s ingredients) to the early Earth.
During this period, impacts on Earth created over 22,000 craters larger than 20 kilometers (km) in diameter, about 40 basins larger than 1,000 km, and several continent-sized basins larger than 5,000 km.
An ocean formed soon after the Moonforming impact, but life first appeared 800 million years later (3.7 billion years ago), after the Late Heavy Bombardment. Those impacts brought water and some of life’s ingredients, and created hydrothermal systems on Earth that were excellent incubators for life.
The icon above represents Earth during the Hadean period about 4.3 billion years ago. During this period, Earth had early oceans and dry continents with abundant old impact basins and fresh impact basins filled with lava.
TRADING PLACES: Shortly after their formation, the giant gas planets follow nearly circular orbits close to the Sun (lower left). About 3.8 billion years ago, Neptune and Uranus switch places and trigger major disruptions (center & top left). Then the giant planets settle into their current orbits, the Kuiper Belt and Oort Cloud comet reservoirs form, and the major bombardment ends (top right). Lower right: Detected Kuiper Belt objects.
The early Solar System probably started with the four giant planets in a compact configuration closer to the Sun, and in the order Jupiter, Saturn, Neptune and Uranus. Detailed simulations of planetary interactions suggest that when the orbits of Jupiter and Saturn fell into lockstep, their combined gravitational pull caused Neptune and Uranus to switch places and move outward. This triggered the dispersal of an outer disk of icy debris and 90% of a disk of rocky bodies between Mars and Jupiter, causing the Late Heavy Bombardment of Earth and the Moon. The remaining 10% form today’s asteroid belt.
Icy bodies are ejected from the Oort Cloud by passing stars and galactic tides, and from the Kuiper Belt by planetary perturbations. Each year, some icy bodies enter the inner Solar System and become active comets. Asteroids reside much closer to the Sun, in a stable belt between Jupiter and Mars.
Comets currently occupy at least two reservoirs: the Kuiper Belt, where Pluto resides, and the Oort Cloud. The Oort Cloud contains about 1,000 billion comets and extends halfway to the nearest star.