Washington, June 29 : The cores of Earth and Mars formed under very different conditions, according to a new study that has compared silicon samples from Earth, meteorites and planetary materials.

The research has also shown that the Earth and the moon have the same silicon isotopic composition supporting the theory that atoms from the two mixed in the early stages of their development.

"We were quite startled at our results which showed that the heavier isotopes from silicate Earth samples contained increased proportions of the heavier isotopes of silicon. This is quite different from meteorites from the silicate portions of Mars and the large Asteroid Vesta - which do not display such an effect even though these bodies also have an iron core," said Professor Alex Halliday from Oxford University.

Jointly carried out by an international team of scientists from Oxford University, University of California, Los Angeles (UCLA) and the Swiss Federal Institute of Technology in Zurich (ETH), this is the first time that isotopes have been used in this way to investigate how Earth's core formed.

On Earth rocks that make up volcanoes and mountain ranges and underlie the ocean floor are made of silicate - compounds made of silicon and oxygen linked with other kinds of atoms.

Silicate is found down to a depth of 2,900 km - roughly half way to the centre of the Earth.

At this point there is an abrupt boundary with the dense metallic iron core. Studies in the 1950's had demonstrated that the outer core had a density too low to be made of pure iron and that it must also be made up of some lighter elements.

Silicate samples from Mars and Vesta are identical to a primitive class of meteorites called chondrites that represent average solar system material from small "planetesimals" that never underwent core formation.

According to Professor Halliday, "the most likely explanation is that, unlike Mars and Vesta, the Earth's silicon has been divided into two sorts - a portion that became a light element in the Earth's core dissolved in metal and the greater proportion which formed the silicon-oxygen bonded silicate of the Earth's mantle and crust."

At depths the silicates change structure to denser forms so the isotopic make-up depends on the pressure at which metal and silicate separate, he said.

According to the team, this research has provided new evidence that the Earth's core formed under different conditions from those that existed on Mars.

"This could be explained in part by the difference in mass between the two planets. With Earth being eight times larger than Mars the pressure of core formation could be higher and different silicate phases may have been involved. The mass of a planet also affects the energy that is released as it accretes (or grows)," they said.

The research appears in Nature magazine.