Ancient minerals probe Earth's early atmosphere
DR EMILY BALDWIN
Posted: 01 December 2011
In the first study of its kind, scientists have used the oldest minerals on Earth to reconstruct the composition of our planet's atmosphere just 500 million years after its formation, finding that it shares much closer characteristics to present-day conditions that previously thought.
The widely accepted view of early Earth is that the atmosphere was burdened with noxious methane, carbon monoxide, hydrogen sulfide and ammonia, and that persistent oxidizing conditions only began when the Earth was 2.3 billion years old, around half its current age.
"It has always been understood that conditions on Earth were very reducing 'in the beginning' due to the initial intermingling of silicate materials (ultimately forming the mantle) and the elemental iron that forms the core," says Bruce Watson of the New York Center for Astrobiology at Rensselaer Polytechnic Institute, one of the authors presenting the new results in the 1 December issue of the journal Nature. "The presence of elemental (reduced) iron implies reducing conditions, and therefore reduced gases such as methane, hydrogen sulfide and ammonia." Scientists use the term "reduced" to describe conditions with limited oxygen.
Earth's thin, breathable atmosphere as seen from the International Space Station. Image: NASA/JSC.
Watson and his colleagues have now turned this assumption on its head by demonstrating that the early atmosphere was likely dominated by the more oxygen-rich compounds found within our current atmosphere – including water, carbon dioxide, and sulphur dioxide.
In keeping with the theory that Earth's atmosphere was formed by gases released from volcanic activity, the team recreated the conditions of lava formation in the laboratory to examine what gases are present in those magmas supplying the atmosphere. As magma moves through the Earth's interior it can either erupt onto the surface, or solidify underground. Research efforts into the magmatic conditions of early Earth are made all the more difficult by the fact that Earth constantly recycles its crust such that there is little evidence from the first half billion years of Earth history. Zircons, however, are the sole mineral survivors from this epoch at over 4 billion years in age, and can provide a window back into this time.
"We used a "proxy" for oxygen pressure in the form of the ratio of a rare-earth element called cerium," explains Watson. Cerium is a useful gauge of oxygen abundance because it can be found in two different oxidization states depending on the pressure of oxygen in the system. "The cerium ratio in zircon is what we calibrated in the lab as a function of oxygen pressure, then applied to the ancient zircons."
The study revealed an elevated amount of the more oxidized version of cerium, meaning that the volcanic gases of the early Earth were relatively oxidized, much like those emanating from volcanoes today. Watson adds, however, that their results do not imply large amounts of free oxygen in the early atmosphere; it took life – and 1.5 billion more years – to build this up to the present level.
Since an oxidized atmosphere is not thought to be a great starting point for life – methane and its oxygen-poor counterparts provide a much better stepping stone from inorganic compounds to amino acids and DNA – the new results have implications for understanding exactly how and when life began on this planet, and whether life's ingredients were in fact delivered from elsewhere in the Universe.
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