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The impact of a Mars-sized protoplanet vaporized proto-Earth’s upper layers and ejected large amounts of solid debris that later accreted to form the Moon. This impact also caused Earth’s axis to be tilted at an angle that is responsible for Earth’s seasons.
The proto-Earth accreted about 4.56 billion years ago in the protoplanetary disk, and the Moon-forming event occurred about 50 million years later. Molten rock covered the fresh new Earth, but it cooled quickly. Analysis of ancient gems called zircons shows that a water ocean existed by 4.3 billion years ago.
MOLTEN ROCK COVERED THE NEW EARTH: This image depicts Earth’s magma ocean after the Moon-forming event: exposed molten rock is yellow or red, while cooler solidified lava crust is black. No life could exist then.
Detailed simulations suggest that much of Earth’s original atmosphere was likely lost in the impact event, and more was later blown off because of the high heat. This primordial atmosphere contained almost no oxygen and would have been toxic to humans and most modern life.
This iconic object represents a collision between the proto-Earth and a Mars-sized protoplanet. The ejected material formed an Earth-orbiting debris disk that later accreted into the Moon.
After the Moon-forming impact, bombardment continued for the next billion years. Over time, these cosmic impacts became less frequent, allowing the planet to cool and form a solid crust. However, geological activities and erosion by water and weather erased Earth’s record of this early period. How do scientists learn about the events of this period? Our Moon lacks an atmosphere and plate tectonics, so it preserves a record of impacts experienced during the first one billion years and later periods. That is one reason why we explore the Moon: it holds the key to understanding early Solar System and Earth history.
IMPACTS PUMMELED THE EARTH AND MOON: The record of this intense bombardment is preserved on the Moon but not on Earth. Apollo samples and lunar meteorites inform us of the ages of major impact events, while crater counting reveals the number of impacts over time.
OUR MANY-FACED MOON: This color-coded elevation map of the Moon’s surface shows the effect of early bombardment on planetary crusts in the inner Solar System, including Earth’s. Most craters on this map are over 20 kilometers in diameter. The near side (left) and far side (right) present very different cratering records, and thus appearances. These images were made using data from NASA’s Lunar Reconnaissance Orbiter, built at Goddard.
Astronaut Harrison Schmitt collecting lunar rake samples during the Apollo 17 mission.
Scientists learned about Earth’s early bombardment history by counting lunar craters of various sizes and measuring the ages of Moon rocks brought back by the Apollo astronauts. The Moon preserves a good record of these impacts.