About 4 to 3.8 billion years ago a period of intense comet and asteroid bombardment is thought to have peppered all the planets including the Earth. Many of the numerous craters found on the Moon and other bodies in the Solar System record this event.
One theory holds that a gravitational surge caused by the orbital interaction of Jupiter and Saturn sent Neptune careening into the ring of comets in the outer Solar System. The disrupted comets were sent in all directions and collided with the planets. These water-rich objects may have provided much of the water in the Earth's oceans.
Image: Artwork depicting the Late Heavy Bombardment (credit: Chris Butler/SPL)
Volcanoes and comets bring water to the Earth.
Dr Iain Stewart explains the theory that steam from volcanoes and water from comets filled the Earth's oceans.
Our planet develops its inner heat.
Dr Iain Stewart explains how the Earth developed its inner heat during a time known as the Hadean eon, about 4.5 billion years ago.
The early Solar System was a shooting gallery.
Professor Brian Cox explains how the orbiting gas giants may have caused an enormous asteroid and comet bombardment in the inner Solar System 3.6 billion years ago. Earth and the other planets were peppered by asteroids and comets.
The Late Heavy Bombardment (commonly referred to as the lunar cataclysm, or the LHB) is a hypothetical event thought to have occurred approximately 4.1 to 3.8 billion years ago (Ga). During this interval, a disproportionately large number of asteroids apparently collided with the early terrestrial planets in the inner solar system, including Mercury, Venus, Earth and Mars. The LHB happened "late" in the Solar System's accretion period when the Earth and other rocky planets had formed and accreted most of their mass; it is a period still early in the history of the solar system as a whole.
Evidence for the LHB event is primarily from dating lunar samples brought back by the Apollo astronauts, which (dates) imply that most impact melt rocks were formed in this rather narrow interval of time. Several hypotheses are now offered to explain the spike in the flux of asteroids or comets in the inner Solar System but no consensus yet exists. The Nice model is popular among planetary scientists; it postulates that the gas giant planets underwent orbital migration and forced objects in the asteroid and/or Kuiper belts into eccentric orbits, and thereby into the path of the terrestrial planets. Alternatively, other researchers argue that the lunar sample data do not require a cataclysmic cratering event near 3.9 Ga and that the apparent clustering of impact melt ages near this time is an artifact of sampling material affected by a single large impact basin. They note that the rate of impact cratering could be significantly different between the outer and inner zones of the Solar System.