“Artist’s concept of Early Earth”

A stormy, active sun may have kickstarted life on Earth
by Miles Hatfield  /  May 2, 2023

“The first building blocks of life on Earth may have formed thanks to eruptions from our sun, a new study finds. A series of chemical experiments show how solar particles, colliding with gases in Earth’s early atmosphere, can form amino acids and carboxylic acids, the basic building blocks of proteins and organic life. The findings were published in the journal Life. 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, Stanley Miller of the University of Chicago tried to recreate these primordial conditions in the lab. Miller 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, Miller and his graduate advisor Harold Urey analyzed the chamber’s contents and found that 20 different amino acids had formed. “That was a big revelation,” said Vladimir Airapetian, a stellar astrophysicist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, and co-author of the new paper. “From the basic components of early Earth’s atmosphere, you can synthesize these complex organic molecules.”

But the last 70 years have complicated this interpretation. 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, some scientists pointed to shockwaves from incoming meteors. Others cited solar ultraviolet radiation. Airapatian, using data from NASA’s Kepler mission, pointed to a new idea: energetic particles from our sun. Kepler observed far-off stars at different stages in their lifecycle, but its data provides hints about our sun’s past.

In 2016, Airapetian published a study suggesting that during Earth’s first 100 million years, the sun was about 30% dimmer. But solar “superflares”—powerful eruptions we only see once every 100 years or so today—would have erupted once every 3-10 days. These superflares launch near-light speed particles that would regularly collide with our atmosphere, kickstarting chemical reactions. “As soon as I published that paper, the team from the Yokohama National University from Japan contacted me,” Airapetian said. Dr. Kensei Kobayashi, a professor of chemistry there, had spent the last 30 years studying prebiotic chemistry. He was trying to understand how galactic cosmic rays—incoming particles from outside our solar system—could have affected early Earth’s atmosphere.

“Most investigators ignore galactic cosmic rays because they require specialized equipment, like particle accelerators,” Kobayashi said. “I was fortunate enough to have access to several of them near our facilities.” Minor tweaks to Kobayashi’s experimental setup could put Airapatian’s ideas to the test. 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. (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.

“solar eruption, including solar flare, coronal mass ejection, and solar energetic particle event”

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,” Airapetian added. Protons also tended to produce more carboxylic acids (a 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, Airapetian suggested. 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,” Airapetian said. “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.”

More information:
Kensei Kobayashi et al, Formation of Amino Acids and Carboxylic Acids in Weakly Reducing Planetary Atmospheres by Solar Energetic Particles from the Young Sun, Life (2023). DOI: 10.3390/life13051103]

Early Earth’s atmosphere was less conducive to lightning
by Rebecca Dzombak, American Geophysical Union  /  April 11, 2022

“In 1952, Stanley Miller and Harold Urey made sparks fly in a gas-filled flask meant to reflect the composition of Earth’s atmosphere around 3.8 billion years ago. Their results suggested that lightning could have led to prebiotic molecules necessary for the evolution of life, such as amino acids. At the time, scientists thought the early atmosphere would have been primarily methane and ammonia, but by the 1990s, experts argued for an atmosphere filled with carbon dioxide and molecular nitrogen. Now, a new study suggests that the composition of Earth’s primordial atmosphere likely made it harder to generate lightning, which may have increased the time it took to generate and accumulate prebiotic molecules important for life.

Electrons behave differently in an atmosphere composed of methane and ammonia versus one made mostly of carbon dioxide and molecular nitrogen. It stands to reason lightning discharges would behave differently, too, which could affect the likelihood of prebiotic molecules forming on early Earth. Yet few people have modeled how lightning discharges vary in different atmospheric environments. To look at how often electrons and gas molecules would have collided in the two versions of early Earth atmospheres, Köhn et al. modeled the probability of discharge sparking—the first step to a lightning strike. They found that in the carbon dioxide–nitrogen atmosphere, it’s harder to get lightning to spark. “Basically, in the nitrogen- and carbon-rich atmosphere, you need stronger electric fields for a discharge to initiate,” said Christoph Köhn, a scientist at the National Space Institute at the Technical University of Denmark, who led the study.

The models revealed that the carbon dioxide and nitrogen atmosphere needed about a 28% stronger electric field for streamers—the precursors of lightning—to discharge, because gas molecules and electrons are less likely to collide and build up electrical charges that can generate lightning strikes. Scaling up over space and time suggests there may have been fewer lightning strikes early in Earth’s history, therefore shrinking the odds of generating prebiotic molecules. “If lightning discharges were responsible for the production of prebiotic molecules, it’s important to get a very good theoretical understanding of what happened,” said Köhn. “The big question is still, Where do all these prebiotic molecules come from?” The study strictly modeled the earliest stages of a lightning strike—the sparks that start strikes—so for Köhn and colleagues, the next steps are to model whole lightning strikes and couple that with models of atmospheric chemistry. Together these studies could give a more complete look into how lightning may have been linked to prebiotic molecules.”

More information:
Rebecca Dzombak, Lightning Had Difficulty Forming in Early Earth’s Atmosphere, Eos (2022). DOI: 10.1029/2022EO220187
C. Köhn et al, Streamer Discharges in the Atmosphere of Primordial Earth, Geophysical Research Letters (2022). DOI: 10.1029/2021GL097504



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