Giant flare-ups from the young sun might have kept early Earth warm – and any life nicely fertilised. By splitting inert nitrogen molecules in the atmosphere, charged particles from the sun could have sparked chemical reactions that heated the planet and could be the precursor for life.
This suggestion is the latest attempt to solve a famous paradox known as the “faint young sun” problem. About 4 billion years ago, the sun was only 70 per cent as bright as it is today, which should have made the Earth a frozen snowball. But geological evidence shows that ancient Earth was warm enough for liquid water. The same holds true for Mars.
Now, Vladimir Airapetian of NASA’s Goddard Space Flight Center in Maryland suggests that blasts of protons from the hyperactive young sun could be the answer.
Airapetian was modelling what would happen if a solar storm as big as the record-setting 1859 Carrington event hit Earth today. It was so powerful that auroras – the so-called northern lights – were visible in the sky in near-tropical latitudes. Such big storms used to be frequent: data from young stars seen by the Kepler spacecraft suggests that the early sun could have launched flares stronger than the Carrington event as often as 250 times per day.
The Earth’s magnetic field usually protects it from assaults by the sun, but Airapetian’s models showed that these energetic solar particles could easily punch through the magnetic field at the poles.
“One night it occurred to me, wow, these particles now have a great pathway to get to the atmosphere,” he says. There they could do something useful: destroy molecular nitrogen.
Nitrogen is an essential component for life on Earth, but the young Earth probably only had its molecular form, N2, which is useless for life. Solar particles from flares could split these molecules apart, allowing nitrogen to take more biologically useful configurations. Nitrous oxide, a more potent greenhouse gas than carbon dioxide, could have kept the climate cosy, for example.
As a bonus, similar reactions would have also made hydrogen cyanide, which can further react to form organic molecules like amino acids.
It’s a compelling suggestion, says James Kasting of Penn State University, but it needs to be confirmed with more sophisticated atmospheric models. Kasting thinks that ultraviolet light from the sun might have destroyed nitrous oxide before it could mix into the atmosphere.
“It would take a convoluted mechanism to produce that high up in the atmosphere and then get enough of it into the lower atmosphere to produce a good greenhouse effect,” he says.
Airapetian hopes to address that charge and others with a forthcoming paper that shows particles from flares can penetrate almost to the surface, where rain could carry the chemical products they make the rest of the way down. Another pending work will apply the same process to explain liquid water on early Mars, he says.
Journal reference: Nature Geoscience, DOI: 10.1038/ngeo2719
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