Scientists say the first building blocks of organic life, amino acids and carboxylic acids, may have been formed from solar particles of solar eruptions colliding with gases in Earth's early atmosphere.
In the late 1800s, scientists speculated about the origins of life to have begun in a "warm little pond": A soup of chemicals, energized by lightning, heat, and other energy sources, that could mix together in concentrated amounts to form organic molecules.
When these conditions were recreated in a lab at the University of Chicago, US, in 1953, scientists were able to find that 20 different amino acids had formed.
"From the basic components of early Earth's atmosphere, you can synthesize these complex organic molecules," said Vladimir Airapetian, a stellar astrophysicist at NASA's Goddard Space Flight Center in Greenbelt, Maryland, and coauthor of this new paper published in the journal Life.
70 years since, scientists now believe ammonia (NH3) and methane (CH4) were far less abundant; instead, Earth's air was filled with carbon dioxide (CO2) and molecular nitrogen (N2), which require more energy to break down. These gases can still yield amino acids, but in greatly reduced quantities.
Seeking alternative energy sources, Airapatian, using data from NASA's Kepler mission, pointed to a new idea: energetic particles from our Sun.
In 2016, Airapetian published a study suggesting that during Earth's first 100 million years, while the Sun was about 30 per cent dimmer, solar "superflares" - powerful eruptions seen every 100 years or so today - would have erupted once every 3-10 days.
These superflares launch near-light speed particles, regularly colliding with our atmosphere and kickstarting chemical reactions.
So after publishing, Airapetian was contacted by the Yokohama National University team from Japan.
Dr. Kobayashi, a professor of chemistry there, was trying to understand how galactic cosmic rays - incoming particles from outside our solar system - could have affected early Earth's atmosphere.
To understand this, Airapetian, Kobayashi, and their collaborators created a mixture of gases matching early Earth's atmosphere as we understand it today.
They combined carbon dioxide, molecular nitrogen, water, and a variable amount of methane, considered to be low in Earth's early atmosphere. They shot the gas mixtures with protons (simulating solar particles) or ignited them with spark discharges (simulating lightning), replicating the University of Chicago experiment for comparison.
They found that as long as the methane proportion was over 0.5 per cent, the mixtures shot by protons (solar particles) produced detectable amounts of amino acids and carboxylic acids.
But the spark discharges (lightning) required about a 15 per cent methane concentration before any amino acids formed at all.
"And even at 15% methane, the production rate of the amino acids by lightning is a million times less than by protons," Airapetian added. Protons also tended to produce more carboxylic acids (a precursor of amino acids) than those ignited by spark discharges.
These experiments suggested our active young Sun could have catalysed the precursors of life more easily, and perhaps earlier, than previously assumed.