It is long hypothesized that meteorites delivered some of the organic molecules that helped to kickstart life on Earth. That’s because these extraterrestrial rocks were thought to have collided with the early Earth, bringing crucial organic molecules and water, which would then get trapped in fractures in the planet’s surface. But a new study performed by UO scientists has upended this theory, showing that even if it were true, such collisions would not produce enough organic material to account for all of life as we know it.
Scientists investigated how much radiation from space would penetrate into seawater and crustal rock using NASA’s Cassini spacecraft data. The team found that even if meteorites did make it to Earth, they would not have been enough to stave off the harsh conditions of the early atmosphere.
“If you took 10 million tons of meteorite material and melted it down, you’d still be left with only half a million tons of organic matter,” said lead author Robert T. Paine, a UO professor of geological sciences. “The organic matter collected on Mars may also have come from meteorites, but not enough to explain how life got started. The molecules that can be formed in space include those that can repair damaged DNA, but they are not the kind of complex bio-molecules needed for life.”
The study appears this month in the journal Astrobiology. Paine co-wrote the paper with UO doctoral student Erika Elswick and former UO graduate student Michael Callahan.
While scientists have long thought that meteorites may have seeded Earth with life, Paine said it’s also possible that organic molecules were delivered to Earth by a comet or an asteroid. Either way, he said there’s nothing on Earth or in meteorites that can directly produce life.
In their study, Paine said his team measured gamma rays from the sun and other sources, such as the center of our galaxy. They then applied these data to the ocean and crustal rocks to determine how much gamma rays would penetrate into them.
The team then mixed up ocean water samples with different amounts of rock and did the same gamma-ray measurements to ensure their results were accurate. The amount of radiation passing through was enough to destroy DNA and other molecules but not enough to kickstart biological processes, particularly photosynthesis.
“We found that meteorites could not provide enough of the building blocks of life to make it possible,” Paine said. “One mystery is why Mars did not produce more organic material from meteorites because there was a lot more material falling on Mars than on Earth.”
Paine said the team’s findings had raised more questions about how life got started, even prompting him to ask if that process may be rare in the cosmos.
“One theory is that meteorites caused life on early Earth,” he said. “But instead of this being an important contribution to explaining the origin of life, it turns out it doesn’t really matter at all. That’s kind of a shock. We cannot know whether the chemistry that led to life on Earth was one-off or present in other places in the universe.”
The research took about a year and involved running thousands of simulations to determine how much radiation would be absorbed by ocean rocks and water. Paine estimates it would take an additional year to also apply the calculations to crustal rocks like basalts.
“We have developed a model based on our measurements, but we haven’t been able to fully map out all the processes yet,” he said. “We are working on this right now and are hoping our findings will give us more insight into what happened here on early Earth. It’s very important to understand how the earliest life formed and evolved.”
The National Science Foundation, the NASA Astrobiology Institute, and the UO Research Foundation supported this work.
The research was conducted in part at UO’s Center for Astrobiology. The center studies how microbes interact with the environment and each other, including on early Earth. Paine and Callahan worked there as postdoctoral researchers in 2014-15 and 2015-16, respectively. Elswick was a UO undergraduate when she began her research as part of her senior thesis and is now a graduate student studying ocean chemistry at Oregon State University.
