Life most likely started during the Hadean Eon (4.5 to 4 billion years ago). However, the environmental conditions which contributed to the complexity of its chemistry are poorly known. A better understanding of various environmental conditions, including global and local, along with the internal dynamic conditions of the early Earth, are required to understand the origin of life. In new research, scientists examined the contributions of galactic cosmic rays and solar energetic particles associated with superflares from the young Sun to the formation of amino acids and carboxylic acids in weakly reduced gas mixtures representing the early Earth’s atmosphere.
An artist’s impression of the early Earth. Image credit: NASA.
To understand the origins of life, many scientists try to explain how amino acids, the raw materials from which proteins and all cellular life, were formed.
The best-known proposal originated in the late 1800s as scientists speculated that life might 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.
In 1953, University of Chicago researcher Stanley Miller and colleagues tried to recreate these primordial conditions in the lab.
They filled a closed chamber with methane, ammonia, water, and molecular hydrogen — gases thought to be prevalent in Earth’s early atmosphere — and repeatedly ignited an electrical spark to simulate lightning. A week later, they analyzed the chamber’s contents and found that 20 different amino acids had formed.
“That was a big revelation. From the basic components of early Earth’s atmosphere, you can synthesize these complex organic molecules,” said Dr. Vladimir Airapetian, an astrophysicist at NASA’s Goddard Space Flight Center.
But the last 70 years have complicated this interpretation. Scientists now believe ammonia and methane were far less abundant. Instead, Earth’s air was filled with carbon dioxide and molecular nitrogen, which require more energy to break down. These gases can still yield amino acids, but in greatly reduced quantities.
Seeking alternative energy sources, some scientists pointed to shockwaves from incoming meteors. Others cited solar ultraviolet radiation.
In 2016, Dr. Airapetian and colleagues published a study suggesting that during Earth’s first 100 million years, the Sun was about 30% dimmer.
But solar superflares would have erupted once every 3-10 days. These events launch near-light speed particles that would regularly collide with our atmosphere, kickstarting chemical reactions.
In their new study, the astronomers 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. The methane proportion in Earth’s early atmosphere is uncertain but thought to be low.
They shot the gas mixtures with protons (simulating solar particles) or ignited them with spark discharges (simulating lightning), replicating the Miller-Urey experiment for comparison.
As long as the methane proportion was over 0.5%, 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% 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,” Dr. Airapetian said.
“Protons also tended to produce more carboxylic acids (precursor of amino acids) than those ignited by spark discharges.”
“All else being equal, solar particles appear to be a more efficient energy source than lightning. But all else likely wasn’t equal.”
“Miller and Urey assumed that lightning was just as common at the time of the ‘warm little pond’ as it is today.”
“But lightning, which comes from thunderclouds formed by rising warm air, would have been rarer under a 30% dimmer Sun.”
“During cold conditions you never have lightning, and early Earth was under a pretty faint Sun.”
“That’s not saying that it couldn’t have come from lightning, but lightning seems less likely now, and solar particles seems more likely.”
“These experiments suggest our active young Sun could have catalyzed the precursors of life more easily, and perhaps earlier, than previously assumed.”
The study was published in the journal Life.
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Kensei Kobayashi et al. 2023. Formation of Amino Acids and Carboxylic Acids in Weakly Reducing Planetary Atmospheres by Solar Energetic Particles from the Young Sun. Life 13 (5): 1103; doi: 10.3390/life13051103
