Thursday, 29 September 2011

New Research Sheds Light Upon The Early Evolution Of The Earth's Atmosphere

Serpentine minerals under a microscope
When the Earth formed, it was a ball of molten rock with a small metallic core kept solid by intense pressure, despite the heat. This new planet would had had but a trace of an atmosphere, containing a lot of water vapour and the noble gases. When the Earth cooled and continents formed, the water vapour would condense and fall to form the oceans, lakes and rivers. Yet where did the more abundant gases such as oxygen and nitrogen come from?

Scientists long believed that these were brought to Earth by a vast meteorite shower from 4.4 to 3.8 billion years ago called the Late Heavy Bombardment. While the sheer number of meteorites needed would have been incredible, it is still a plausible theory and the best one available. Yet a new study of interactions between the atmosphere and the layers of the Earth suggests that this long standing theory is wrong.

The study, conducted by the University of Melbourne's School for Earth Sciences led by Dr Mark Kendrick, shows that atmospheric gases are mixed into the molten rock of the mantle when a continent is subducted under another.'This finding is important because it was previously believed that inert gases inside the Earth had primordial origins and were trapped during the formation of the solar system. Our findings throw into uncertainty a recent conclusion that gases throughout the Earth were solely delivered by meteorites crashing into the planet.'

Previous theories stated that gases could not follow the subduction line down into the Earth's mantle. Yet  analysis carried out upon rock samples has shown that such a process is possible and has happened in the past, at least during a time of intense geological activity.

The team collected samples composed of serpentine minerals from mountain belts in Italy and Spain, which once existed at the bottom of a tropical ocean and were raised to their current elevation by the collision of the African and European plates. By analysing the inert gases and halogens trapped in these rocks, the team was able to show gases are incompletely removed by the mineral transformations that affect serpentinites during the subduction process. 

'The serpentinite rocks are special because they trap large amounts of seawater in their crystal structure and can be transported to great depths in the Earth's mantle by subduction,' Kendrick said. This proves that they once existed on the surface. This conclusion provides new insights into the role of these trapped gases in the evolution of the planet. Apart from affecting the chemistry of the crust while in the mantle, the gases eventually were brought to the surface by the currents within the magma and became a part of the atmosphere.