If a massive solar storm struck the Earth today, it could wipe out our technology and hurl us back to the dark ages. Lucky for us, events like this are quite rare. But four billion years ago, extreme space weather was probably the norm. And rather than bringing the apocalypse, it might have kickstarted life. A large, X-class solar flare erupting on the surface of the sun in October 2014. Image: NASA Solar Dynamics Laboratory
That's the startling conclusion of research published in Nature Geoscience today, which builds on an earlier discovery about young, sun-like stars made with NASA's Kepler Space Telescope. Baby suns, it turns out, are extremely eruptive, releasing mind-boggling amounts of energy during "solar superflares" that make our wildest space weather look like drizzle.
Now, NASA's Vladimir Airapetian has shown that if our sun was equally active four billion years ago, it could have made the Earth more habitable. According to Airapetian's models, as solar superflares pounded our atmosphere, they initiated chemical reactions that yielded climate-warming greenhouse gases and other essential ingredients for life.
"The Earth should have been in a deep freeze four billion years ago," Airapetian told Gizmodo, referring to the "faint young sun paradox" first raised by Carl Sagan and George Mullen in 1972. The paradox came about when Sagan and Mullen realised that Earth had signs of liquid water as early as four billion years ago, while the sun was only 70 per cent as bright as it is today. "The only way [to explain this] is to somehow incorporate a greenhouse effect," Airapetian said.
Another early Earth puzzle is how the first biological molecules — DNA, RNA and proteins — scavenged enough nitrogen in order to form. Similar to today, the ancient Earth's atmosphere was composed primarily of inert nitrogen gas (N2). While specialised bacteria called "nitrogen fixers" eventually figured out how to break N2 and turn it into ammonia (NH4), early biology lacked this ability.
The new study offers an elegant solution to both of these problems in the form of space weather. The research began several years back, when Airapetian was studying the magnetic activity of stars in NASA's Kepler database. He discovered that G-type stars (stars like our sun) are like dynamite in their youth, frequently releasing pulses of energy equivalent to over 100 trillion atomic bombs. The most powerful solar storm ever experienced by humans, the 1859 Carrington event that caused worldwide power outages, pales in comparison.
"It is a crazy amount of energy. I can hardly comprehend it myself," Ramses Ramirez — an astrobiologist at Cornell University who was not involved with the study but collaborates with Airapetian — told Gizmodo.
It soon occurred to Airapetian that he could use this discovery to peer back into the early history of our solar system. He calculated that four billion years ago, our sun could have been releasing dozens of superflares every few hours, with one or more grazing the Earth's magnetic field every single day. "Basically, the Earth was under constant attack from super Carrington-sized events," he said.
Using numerical models, Airapetian then showed that solar superflares would be strong enough to dramatically compress Earth's magnetosphere, the magnetic shield that encircles our planet. Not only that, charged solar particles would bust a hole clean through the magnetosphere near our planet's poles, entering the atmosphere and colliding with nitrogen, carbon dioxide and methane. "So now you have these particles interacting with molecules in the atmosphere and creating new molecules — like a chain reaction," Airapetian said.
Artist's concept depicting energetic particles from solar superflares raining down on the early Earth. Image via Vladimir Airapetian
These solar-atmospheric interactions produce nitrous oxide, a greenhouse gas with 300 times the global warming potential of CO2. Airapetian's models suggest that enough nitrous oxide could have been produced to dramatically warm the planet. Another product of the endless solar storm, hydrogen cyanide (HCN), could have fertilised the surface with the nitrogen needed to form the early building blocks of life.
"People have looked at lightning and falling meteorites as ways to initiate nitrogen chemistry," Ramirez said. "I think the coolest thing about this paper is that nobody had really thought about looking at solar storms."
It will be up to biologists to determine whether the exact mix of molecules produced via superflares would have been enough to jumpstart life. That investigation is already underway. Researchers at the Earth Life Sciences Institute in Tokyo and elsewhere are now using Airapetian's models to devise new experiments that simulate conditions on the ancient Earth. If those experiments can produce amino acids and RNA building blocks, that would go a long way toward supporting the idea that space weather helped get life started.
In addition to helping piece together our origin story, Airapetian's models could shed light on the past habitability of Mars, which appears to have also been wet four billion years ago despite receiving even less radiation from the young sun. The study could have implications for life beyond our solar system, too.
We're just starting to figure out what constitutes a star's "habitable zone", where planets with liquid water oceans might exist. But the current habitable zone definition only factors in the brightness of the parent star. With more detailed information on a star's explosive activity, we might be able to glean more about the chemistry of exoplanet atmospheres, and the potential for a strong greenhouse effect to take hold.
"Ultimately, this will inform us whether the energy from a star is available in a way that can create the chemistry to create biomolecules," Airapetian said. "Without that, it would be a miracle to have life."