Most of the precious metals that exist on Earth today may have been brought here by comets and asteroids four billions of years ago. At that time, the entire inner solar system passed through a violent phase called the Late Heavy Bombardment.

During those times, vast numbers of comets, asteroids, meteorites and other space bodies battered the four rocky inner planets, causing massive amounts of damage. Most of the largest craters on Earth, Mercury, Mars, Venus and the Moon were produced during these times.

Earth's crust and mantle should not contain metals like gold, because while the planet was molten they would have sunk into the core. As a result, many geologists think they were supplied by one final meteor bombardment after the planet solidified.

A sticking point remains, though: if the storm gave Earth its precious metals, then sedimentary rocks laid down at the time should contain plenty of those metals, but they don't. There is little doubt of the bombardment but it may have brought in other substances. Some say that vast amounts of water were brought here via cometary ices, and that it was through this event that our planet's atmosphere and oceans formed.

Details of the study he and his team conducted appear in the September 8 issue of the top scientific journal Nature, Space reports.