‘Microlightning’ in water droplets may have sparked life on Earth

Life may not have begun with a dramatic lightning strike into the ocean but from many smaller “microlightning” exchanges among water droplets from crashing waterfalls or breaking waves.

New research from Stanford University shows that water sprayed into a mixture of gases thought to be present in Earth’s early atmosphere can lead to the formation of organic molecules with carbon-nitrogen bonds, including uracil, one of the components of DNA and RNA.

The study, published in the journal Science Advances, adds evidence—and a new angle—to the much-disputed Miller-Urey hypothesis, which argues that life on the planet started from a lightning strike. That theory is based on a 1952 experiment showing that organic compounds could form with the application of electricity to a mixture of water and inorganic gases.

In the current study, the researchers found that water spray, which produces small electrical charges, could do that work all by itself, no added electricity necessary.

“Microelectric discharges between oppositely charged water microdroplets make all the organic molecules observed previously in the Miller-Urey experiment, and we propose that this is a new mechanism for the prebiotic synthesis of molecules that constitute the building blocks of life,” said senior author Richard Zare, the Marguerite Blake Wilbur Professor of Natural Science and professor of chemistry in Stanford’s School of Humanities and Sciences.

Microlightning’s power and potential

For a couple billion years after its formation, Earth is believed to have had a swirl of chemicals but almost no organic molecules with carbon-nitrogen bonds, which are essential for proteins, enzymes, nucleic acids, chlorophyll, and other compounds that make up living things today.

How these biological components came about has long puzzled scientists, and the Miller-Urey experiment provided one possible explanation: that lightning striking into the ocean and interacting with early planet gases like methane, ammonia, and hydrogen could create these organic molecules.

Critics of that theory have pointed out that lightning is too infrequent and the ocean too large and dispersed for this to be a realistic cause.

Zare, along with postdoctoral scholars Yifan Meng and Yu Xia, and graduate student Jinheng Xu, propose another possibility with this research. The team first investigated how droplets of water developed different charges when divided by a spray or splash.

They found that larger droplets often carried positive charges, while smaller ones were negative. When the oppositely charged droplets came close to each other, sparks jumped between them. Zare calls this “microlightning,” since the process is related to the way energy is built up and discharged as lightning in clouds. The researchers used high-speed cameras to document the flashes of light, which are hard to detect with the human eye.

Even though the tiny flashes of microlightning may be hard to see, they still carry a lot of energy. The researchers demonstrated that power by sending sprays of room-temperature water into a gas mixture containing nitrogen, methane, carbon dioxide, and ammonia gases, which are all thought to be present on early Earth.

This resulted in the formation of organic molecules with carbon-nitrogen bonds, including hydrogen cyanide, the amino acid glycine, and uracil.

The researchers argue that these findings indicate that it was not necessarily lightning strikes, but the tiny sparks made by crashing waves or waterfalls that jump-started life on this planet.

“On early Earth, there were water sprays all over the place—into crevices or against rocks, and they can accumulate and create this chemical reaction,” Zare said. “I think this overcomes many of the problems people have with the Miller-Urey hypothesis.”

Zare’s research team focuses on investigating the potential power of small bits of water, including how water vapor may help produce ammonia, a key ingredient in fertilizer, and how water droplets spontaneously produce hydrogen peroxide.

“We usually think of water as so benign, but when it’s divided in the form of little droplets, water is highly reactive,” he said.