The early Solar System experienced a period known as the Late Heavy Bombardment (LHB), or lunar cataclysm. This event, occurring approximately 4.1 to 3.8 billion years ago, was characterized by a high rate of asteroid and comet impacts across the inner Solar System, affecting bodies like Mercury, Venus, Earth, and Mars, including Earth’s Moon. The bombardment played a significant role in shaping the surfaces and compositions of these nascent planets.
The Turbulent Early Solar System
The formation of the Solar System began about 4.5 billion years ago from a rotating cloud of gas and dust known as the protoplanetary disk. Dust and gas within this disk coalesced into planetesimals, the building blocks of planets. Initially, the giant planets like Jupiter, Saturn, Uranus, and Neptune are thought to have formed in a more compact orbital configuration than their present-day positions.
A leading theory, the Nice model, suggests that the migration of these giant planets destabilized the asteroid and comet belts. As Jupiter and Saturn drifted apart, their orbits crossed a resonance, increasing their eccentricities. This instability scattered countless smaller objects from the outer Solar System, sending them hurtling inward. This reorganization of planetary orbits triggered the intense impacts of the LHB.
Planetary Transformation and Earth’s Water
The Late Heavy Bombardment left significant marks across the inner Solar System, most visibly as widespread cratering on airless bodies. The Moon, Mercury, and Mars bear extensive scars from this period, showcasing numerous large impact basins. On Earth, while geological processes like erosion and plate tectonics have erased most direct evidence, the planet also underwent significant transformation.
The intense impacts generated heat, potentially resurfacing early Earth. The LHB is also believed to have delivered substantial water and other volatile compounds to Earth. While comets were once considered a primary source, evidence now points to water-rich asteroids and meteorites as the main contributors to Earth’s oceans and early atmosphere. The Moon serves as a well-preserved record, retaining impact scars that offer insights into the bombardment that affected the inner Solar System.
Evidence from Space
Scientists have pieced together the history of the Late Heavy Bombardment through various lines of evidence. Lunar samples from the Apollo missions are a primary source. Radiometric dating of these rocks, particularly impact melt rocks, revealed a clustering of ages between 3.8 and 4.1 billion years ago, indicating a widespread and intense period of impacts.
Crater density and distribution on the Moon and other airless bodies like Mercury and Mars also provide a visual record of bombardment intensity. Studying these patterns helps infer the history of impacts. Analysis of meteorites offers clues about the early Solar System’s composition and dynamics, reinforcing understanding of the materials involved. This combined evidence allows scientists to confidently theorize about this distant and impactful period in our Solar System’s history.