How can you tell what the atmosphere of Earth was like four billion years ago? The answer is simple, although technically difficult to do – read the rocks. Geologists and now astrogeologists and astrobiologists go back to the question of what the atmosphere was like during the early history of Earth because it is one of the key ingredients in the explanation for how life formed. To get their answers they have become very clever at reading the rocks, or in this case the zircon.
Zircon is a very common trace mineral in many kinds of rocks and soils. It’s relatively hard, crystalline material that, among other things, often contains trace amounts of radioactive elements uranium or thorium. The radioactivity has made it possible to date zircon with considerable precision, leading to the discovery that some zircons were formed about 4.4 billion years ago, the oldest known minerals.
Scientists at the New York Center for Astrobiology at Renssalaer Polytechnic Institute reasoned that zircon might also be used to determine what kind of gasses were present in the magma that formed the zircons. That, in turn, could reveal what gasses were escaping from magma that reached the Earth’s surface and were contributing to the formation of the atmosphere. Their results, published in Nature [30 November 2011, paywalled, The oxidation state of Hadean magmas and implications for early Earth’s atmosphere] may overturn fundamental assumptions about Earth’s early atmosphere.
The heart of the research was to create zircons in the laboratory, in essence making lava with various compositions and particularly with various levels of oxygen. The key to the research was using a rare earth metal, cerium, as a component of the zircon. Cerium is found in two oxidation states (containing different quantities of oxygen molecules). The more of the cerium with higher oxygen content found in zircon, the more likely the zircon was formed in magma with higher oxygen content. Since it is a long-standing hypothesis that most of Earth’s atmosphere was formed by outgassing from magma at the surface; demonstration that magmas of higher oxygen content would produce atmosphere with more oxygen could change long held beliefs about the early Earth atmosphere.
According to the Renssalaer researchers, Dustin Trail, E. Bruce Watson and Nicholas Tailby, zircon with the higher oxygen content was prevalent during the Hadean eon (4.7 – 3.8 billion years ago), and by their calculations this indicates that Earth’s atmosphere at the time contained more oxygen than previously thought. If it holds up under further testing, this is a significant finding that could change how astrobiologists view the conditions for the formation of life. Oxygen is a key component of organic material, and in the current notion of primordial atmosphere it was in short supply. It has long been assumed that the early atmosphere was mostly methane, carbon monoxide, hydrogen sulphide and ammonia – not the best mix for life. Now with the possibility that there was far more oxygen available in the crust of the Earth and in the atmosphere, the view on the formation of water and life could be pushed much closer to the origin of the Earth. As researcher Bruce Watson put it:
“Our planet is the stage on which all of life has played out,” Watson said. “We can’t even begin to talk about life on Earth until we know what that stage is. And oxygen conditions were vitally important because of how they affect the types of organic molecules that can be formed.”
Despite being the atmosphere that life currently breathes, lives, and thrives on, our current oxidized atmosphere is not currently understood to be a great starting point for life. Methane and its oxygen-poor counterparts have much more biologic potential to jump from inorganic compounds to life-supporting amino acids and DNA. As such, Watson thinks the discovery of his group may reinvigorate theories that perhaps those building blocks for life were not created on Earth, but delivered from elsewhere in the galaxy.
This sort of hypothesis will be controversial, but as is the case with novel but plausible research, it will be tested.