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 LHB) is a hypothetical event around 4.1 to 3.8 billion years ago (Ga). During this event a very large number of the impact craters on the Moon would have formed, and by inference on Earth, Mercury, Venus and Mars as well. (While the LHB is "late" in the Solar System's accretion period when Earth and the other rocky planets formed and gained most of their mass, it is early in Solar System history.) The evidence for this event comes primarily from the dating of lunar samples brought back by the Apollo astronauts, which indicates that most impact melt rocks formed in this rather narrow interval of time. While many hypotheses have been put forth to explain a spike in the flux of asteroids or comets in the inner Solar System at this time, no consensus yet exists as to its cause. The Nice model, popular among planetary scientists, postulates that the gas giant planets underwent orbital migration and forced objects in the asteroid belt and/or Kuiper belt on eccentric orbits that put them in the path of the terrestrial planets. Nevertheless, some 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.