May Editorial      

Earth's Formation

The formation of plant Earth happened through a process called 'accretion' which is believed to have taken between 30 and 40 million years. During this time smaller 'planetesimals' floating in space collided and fused together to create a larger object (a protoplanet). As this larger object swept in its orbit around the sun, it eventually collected enough material to 'accrete' to planet status, becoming the Earth as we know it today. Many of the planetesimals from which the Earth is formed had metallic iron cores, and during the growth of the Earth, this metal-rich core re-equilibrated with the Earth's silicate mantle. Today the Earth has an iron-rich core that accounts for about one-third of its mass. This core is surrounded by a rocky silica-rich mantle that constitutes most of the rest of the Earth, with the thin crust of the Earth's surface making up the rest. To follow the Earth's formation further, it was thought for many years that during the equilibration processes between the core and the mantle, siderophile elements (elements that have a weak affinity for oxygen and sulphur and are readily soluble in molten iron; siderophile elements include iron itself, nickel, cobalt, platinum, gold, tin, and tantalum, palladium etc.) were drawn into the iron-rich core, thus depleting the upper mantle of those elements. But this is where the story becomes more complicated. In contradiction to the above theory, the Earth's primitive upper mantle appears to have had a relative abundance of highly siderophile elements. To explain this discrepancy, a 'late-veneer hypothesis' has been put forward and for the last 30 years has been the dominant paradigm for understanding the Earth's early history.

According to the late-veneer hypothesis, most of the original siderophile, elements, such as gold, platinum, palladium and iridium, were drawn down to the Earth's core over tens of millions of years and thereby removed from the Earth's crust and mantle. The amounts of siderophile elements that we see today, therefore, must have been supplied after the core was formed. That is by a later meteorite bombardment. This bombardment also would have brought in water, carbon and other materials essential for life, the oceans and the atmosphere. This theory is supported by the fact that the level of siderophyile elements in chondrites (stony meteorites that have not been modified due to melting or differentiation of the parent body as the meterorite slammed through the atmosphere) is similar to that found in the Earth's mantle. Furthermore, this late chondritic material is also commonly believed to be the source of the Earth’s volatiles, such as water and carbon.

Recently the late-veneer hypothesis too has been queried. For a start, it has been shown that nickel and cobalt, which are moderate siderophile elements, are soluble in silicate liquids under high pressure (Origin of the Earth and Moon, 2000). This could suggest that the equilibrium between core-forming metals and the silicate mantle could have happened at the bottom of a magma ocean. But there still remains the question of why high siderophile elements, such as platinum, palladium, rhenium, iridium, ruthenium and osmium are still found on the earth's surface. The experiments to establish why this is have been slow in coming because it is very difficult to recreate the exact conditions of early Earth. But Munir Hymayun, a Florida State University researcher working with colleagues from NASA, showed in his recent Nature Geoscience (Published online: 13 April 2008) paper that they were able to do just that. They used an 880-ton press to compress samples of rock containing palladium. This created conditions of extremely high-pressure-and-temperature equal to those found at a depth of about 300 miles inside the Earth. The samples were then analysed under a highly sensitive coupled plasma mass spectrometer to measure the distribution of palladium within the sample. Their analysis has shown that under these extreme conditions the distribution of palladium was in the same relative proportions between rock and metal as is observed in the natural world. So putting it another way, under sufficiently high pressure and temperature, the siderophile elements could be retained in the mantle during the equilibrium process.

Although they do not postulate that meteorite bombardment did not contribute to the presence of siderophile elements in the upper mantle, the researchers suggest that there are other means which could explain the presence of these elements. They are hoping that their work will stimulate new thinking about core formation, about the core's present relation to the mantle, and the meteor bombardment history of the early Earth.