Earth make-up differs from Sun

For a century, scientists have assumed that the Earth has the same chemical makeup as the Sun. However, scientists at The Australian National University have challenged this belief. Professors Ian Campbell and Hugh O’Neill from the Research School of Earth Sciences at ANU said their research shakes up our understanding of the Earth’s chemistry – right to the core. “For decades, it has been assumed that the Earth had the same composition as the Sun, as long as the most volatile elements, such as hydrogen, were excluded. This theory is based on the idea that everything in the solar system generally has the same composition. Since the Sun comprises 99 per cent of the solar system, this composition is essentially that of the Sun,” Professor O’Neill said. As it is easier to measure the chemical makeup of chondritic meteorites, planetary geologists have long used these to more precisely determine the Sun’s composition – and, therefore, the composition of the Earth. From this, scientists have concluded that the Earth has a ‘chondritic’ composition. Professor Campbell said this thesis has been challenged again and again. “Recent discoveries have shown that the ratio of two rare earth elements in Earth’s volcanic rocks is higher than in chondritic meteorites. Many scientists have explained this by arguing that these elements must have a hidden reservoir near the Earth's centre to balance this ratio out. This reservoir would also be enriched in the heat-producing elements uranium, thorium and potassium,” he said. Professor Campbell spent twenty years researching mantle plumes – columns of hot rock that rise from the boundary of the Earth’s core and are the mechanism that removes heat from the Earth’s centre. “The problem with the idea of a hidden reservoir is that although these elements could be hidden, we would be able to detect the heat they produce,” he said. “However, mantle plumes simply don’t release enough heat for these reservoirs to exist. Consequently, the Earth is not the same composition as chondrites or the Sun.” Professor O’Neill has developed an explanation as to why the Earth’s composition may differ from chondrites. “The Earth is thought to have formed by collision of planetary bodies of increasing size. In our research, we suggest that by the time these planetary bodies had reached a moderate size, they developed an outer shell that contained a significant amount of heat-producing elements,” he said. “During the final stages of the Earth’s formation, this outer shell was lost by a process called ‘collisional erosion’. This produced an Earth that has fewer heat-producing elements than chondritic meteorites, which explains why the Earth doesn’t have the same chemical composition as chondritic meteorites.” The research has been published in Nature. A copy is available from the ANU media office. Editor's Note: Original news release can be found here.