Early Earth Was No Inviting Blue Planet—It Was More Like Venus

New insights on how, and when, terrestrial planets become habitable.

magma planet.jpg
Earth's early magma ocean gave our planet its first atmosphere.

Experiments conducted by a scientific team led by Paolo Sossi from ETH Zurich in Switzerland indicate that the atmosphere of Earth shortly after its origin was like the atmosphere of Venus today. That is, it consisted mostly of carbon dioxide and nitrogen, and was about 100 times denser than it is now.

Sossi simulated the early magma planet that was Earth in the laboratory and measured how the magma evolved chemically over time. The gasses fuming from that lava ocean would become Earth’s primordial atmosphere, and would have resembled the thick, hot Venusian atmosphere of today. If so, the two planetary neighbors started out very similar.

But while Venus changed very little, Earth changed dramatically. The prolonged presence of water on our planet would have dissolved all that carbon dioxide in sea water, and eventually buried it under the surface thanks to Earth’s major recycling process—plate tectonics. That scrubbed most of the carbon dioxide from our atmosphere. Later, after life arose, even more was removed by plants and other photosynthesizing organisms. Today Earth’s atmosphere is dominated by nitrogen and oxygen, and much thinner than it used to be.

The authors point out that the ratio of carbon dioxide to nitrogen gas is the same on Venus as it is on Mars: 97 to 3. That, they suggest, may be the starting point for terrestrial planets, assuming they had a similar initial endowment of gases. Since the ratio hasn’t really changed, it would also mean that both those planets remained dry for a very long time and lacked an efficient recycling mechanism. Otherwise their atmospheres should have evolved very differently.

If the authors are correct, and Earth’s early atmosphere was dominated by carbon dioxide and nitrogen (other lines of evidence support that notion as well), it’s bad news for the origin-of-life theory suggested by the famous Miller-Urey Experiment, which requires significant concentrations of methane and ammonia to produce amino acids, the building blocks of life.

Those compounds would have been unlikely to exist on early Earth, according to Sossi’s work. If they were present, it would have been only for a short period, after a large impact produced a lot of water vapor. The window could have stayed open just a few million years, although even that may have been long enough for life to arise. In a seminal paper, Antonio Lazcano and Stanley Miller—the same one who conducted the Miller-Urey experiment—estimated that the critical period necessary for life to take hold could have been as short as a few thousand years. So the methane-ammonia origin theory is not fully off the table after all.

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