Visions of the Cosmos

Planetary Science

The Solar System

The Late Heavy Bombardment

Shortly after the time of the creation of the Earth and the other terrestrial planets the planets were subjected to fierce bombardments from comets and meteorites which died down to a relatively low value.  However evidence shows that around 700 million years after their creation the planets were subjected to a second bout of violent bombardment by asteroids and/or comets.New dating of lunar rocks add to a growing body of evidence that the Moon was pelted by a flurry of large asteroids and/or comets during a relatively brief geologic time span about 3.9 billion years ago. It is thought that this same bombardment affected the entire inner solar system, producing thousands of impact craters on Mercury, Venus, the Earth, the Moon and Mars.

It is generally known as the Late Heavy bombardment or LHB.   It is sometimes also referred to as the lunar catastrophe since the rocks on the moon appeared to have been melted during this brief period and many of the important large craters were produced during this time.  It has been estimated that this lunar catastrophe would have lasted only about 200,000 years but, in that time, nearly 2,000 large craters would have been formed as well as many of the Moon's giant impact basins.


Many scientists think that the Earth would have been bombarded to a far greater extent and that the frequent impacts could have delayed the emergence of life on the primitive world.    On Earth, at least 22,000 impact craters with diameters greater than 20 kilometres were produced, including about 40 impact basins with diameters of about 1000 kilometres in diameter. Several impact craters of about 5,000 kilometres were created as well - each one exceeding the dimensions of Australia, Europe, Antarctica or South America. The thousands of impacts occurred in a very short period of time, potentially producing a globally-significant environmental change at an average rate of once per 100 years.

Most of the craters in the southern hemisphere of Mars were produced during this event which possibly includes the massive Hellas Basin - the lowest large area on the planet.

The explanations advanced for the reasons why the Late Heavy Bombardment occurred read like something out of science fiction.   Computer simulations indicate that there is a strong possibility that, about 700 million years after the creation of the Earth, the outer planets moved their orbits into their present positions and that Uranus and particularly Neptune migrated outwards from the Sun which destabilised the asteroid and /or the Kuiper Belt comets and sent them into a host of chaotic orbits to collide with the planets of the Inner Solar System.  Saturn is also thought to have moved whereas Jupiter moved in a bit.  All the work is relatively new but the various theories as to the cause of the LHB gives us plenty of food for thought.  An interesting fact is that in many other star systems a great number of large gas giant planets have been found to be in orbits very close to their parent star.  These so called 'Hot Jupiters' are thought to have been formed much further out and to have migrated inwards towards their parent star early in the history of these other' solar systems'.  


Solar System