Dense elements, such as gold and platinum, are formed when stars explode into supernovae. On being formed these stellar metals are ejected into space at super-fast velocities in multiple directions. Over the course of time, the metal fragments collide with bodies, like planets.
The long-standing reason why planet Earth has such a high abundance is thought to be related to bombardment, over the course of millions of years, with space rock containing the metals (what are known as siderophile elements).
This multiple-bombardment theory by small rocks (or ‘planetesimals’) is now being challenged by a theory which postulates that the precious metals on Earth originate from just one collision from one very big rock, and this rock brought virtually all of the precious metals to the planet.
The collision could also be the one that gave rise to the Moon (the generally accepted theory of the Moon is that it was once part of Earth). This theory has been put forward by scientists from Japan’s Tokyo Institute of Technology. The foundation of the theory is computer modelling, where the outcome is that there is little evidence of the Earth being repeatedly hit by space rocks and there is more evidence of on major collision. The single-collision argument is thought to account for the precious metals being buried deep into the Earth’s layers and at a time before the Earth’s crust had fully formed. This would place the collision some 4.45 billion years ago.
According to the lead researcher, Dr Ramon Brasser a strong gravitational force from Jupiter pulled various items of space debris in the solar system. The force of this pull caused various objects to impact, and one such object struck the Earth. Of course this remains a hypothetical theory (one dubbed the called the Grand Tack hypothesis), but one which is also worthy of further study.
The new theory is described in the journal Earth and Planetary Science Letters. The research paper is called “Late veneer and late accretion to the terrestrial planets.”
