Ethane: A Fingerprint For Life On Exoplanets

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Before discussing the conclusions of this paper released this week, I’ll start with a pub-quiz style question. How much of Earth’s atmosphere has not been made by living things?
The answer is: less than 1%, which is mostly argon. The overwhelming majority is biogenic; the nitrogen is a product of denitrifying bacteria, the oxygen from plants, and the inconspicuous CO2 is produced by everything, but especially animals.
So, would an alien astronomer look at a spectrum of Earth, see this cocktail of gases, and think, “Hmmm. Clearly this planet is harbouring life?” The chances are that they would. Earth looks decidedly odd from a distance. It would show a combination of a water-rich atmosphere, combined with a strong ozone absorption; this ozone peak is basically a proxy for oxygen, and its presence implies a continuous biological source of oxygen – especially in combination with reduced species such as methane, nitrous oxide, and methyl chloride. Without being replenished, oxygen rapidly removes itself by reacting with other gases and things on the Earth (you should see my brother’s car!).

From Hanel et al 1975, Exploration of the Solar System by infrared imaging. Apologies for the poor quality photo of a textbook.

The problem is, oxygen can also be produced by non-biological processes, and moreover, life doesn’t necessarily produce oxygen anyway. In fact, life got on pretty much fine for a long period of time without oxygen before it was released en masse, in what you could call the greatest mass pollution event in Earth’s history. So long lived was this period that we might expect most alien life supporting planets to be harbouring single celled ETs…….Read more:

Written by physicsgg

June 17, 2011 at 8:30 am


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