Early Days on Planet Earth
Take a stroll on the early Earth, about 4 billion years ago, and you would have seen a very alien place indeed.
Our study of other Sun-like stars, on the other hand, shows us that our sun was likely in the midst of its own version of the "terrible twos"—it was only about 70% as luminous as it is today, and prone to some pretty serious outbursts.
We still catch a glimpse of tamer versions of these outbursts in the form of solar flares, and coronal mass ejections (CMEs), which send immense quantities of dangerous solar particles streaming through the Solar System.
Back then, our youthful Sun was particularly liable to giant eruptions called "superflares." Now, we only get about one superflare a century; however, when the Sun was at its fussiest, it could generate up to ten of them per day. And now scientists think these tantrums may have fueled the origin of life on Earth.
As Vladimir Airapetian, lead author of the new study published in Nature Geoscience, explains, "Back then [i.e., 4 billion years ago]…Earth should have been an icy ball. Instead, geological evidence says it was a warm globe with liquid water. We call this the Faint Young Sun Paradox. Our new research shows that solar storms could have been central to warming Earth."
Cooking Up the Ingredients of Life
For the study, the team of NASA scientists calculated what effect these superflares may have had upon the Earth. They found that, since the early Earth had a much weaker magnetic field than today, the energetic particles wafted from the Sun's roiling surface were well able to reach the Earth's atmosphere, slamming into its predominantly nitrogen molecules and photodissociating them into their constituent nitrogen atoms.
Like a game of billiards, these atoms then struck the carbon dioxide molecules, breaking them into carbon and oxygen.
At this point, the nitrogen and oxygen atoms combined to form a super-greenhouse gas, nitrous oxide, which is about 300 times more effective at planetary warming than carbon dioxide. The team found that even in concentrations as low as 1% of the young Earth's CO2 content, N2O was enough to keep the planet nice and toasty, despite the faint young Sun.
And the incoming solar particles likely had another, even more important side-effect—they may have been responsible for providing the energy to transmute the Earth's chemical sludge into the complex molecular equipment of life. Molecules like RNA and DNA, as well as certain proteins and lipids, needed this powerful solar radiation to be cooked up in the first place.
The work is important not only for understanding the conditions on the early Earth, and how life may have formed, but it can also be extrapolated to understanding which extrasolar planets around which stars have the greatest likelihood of harboring life.